UNIVERSITI TUNKU ABDUL RAHMAN
Assembly Techniques
and
Packaging
Dr. Lim Soo King
03/25/2013
Table of Contents
Page
Chapter 3 Assembly Techniques and Packaging ...........................63
3.0 Introduction .............................................................................................. 63
3.1 Assembly Technologies ............................................................................ 63
3.2.1 Electrical requirements ..................................................................................... 68
3.2.2 Mechanical and Thermal properties ................................................................ 72
3.2.3 Cost ...................................................................................................................... 75
3.3 Packaging Level Integration ................................................................... 75
3.3.1 Interconnect Levels ............................................................................................ 76
3.3.1 Interconnect Level 1 - Die-to-Package-Substrate............................................ 77
3.3.2 Interconnect Level 2 - Package Substrate to Board ........................................ 79
3.3.3 Multi-Chip Modules - Die to Printed Wire Board .......................................... 81
3.4 Assembly Techniques and Processes ...................................................... 83
3.4.1 Wafer Preparation ............................................................................................. 84
3.4.2 Die Attach ........................................................................................................... 85
3.4.2.1 Eutectic Die Attach .......................................................................................................... 86
3.4.2.2 Epoxy Die Attach ............................................................................................................. 86
3.4.3 Wire Bonding...................................................................................................... 87
3.4.4 Molding/Glass Seal ............................................................................................. 88
3.4.5 Post Mold Cure/Leak Check ............................................................................. 90
3.4.6 Solder Dip/Tin Plate ........................................................................................... 90
3.4.7 Trim/Form .......................................................................................................... 92
3.4.8 Inspection ............................................................................................................ 92
Exercises .......................................................................................................... 92
Bibliography ................................................................................................... 94
- ii -
List of Figures
Figure 3.1:
Page
Variety of Package types for both plastic and hermetic surface mount and
through-hole mount .......................................................................................... 64
Figure 3.2: A hermetic package showing the integrated circuit is decoupled from external
environment ..................................................................................................... 65
Figure 3.3: A plastic package showing the integrated circuit is not decoupled from
external environment ....................................................................................... 65
Figure 3.4: A plastic package showing the integrated circuit is not decoupled from
external environment ....................................................................................... 66
Figure 3.5: Rent’s constant for varies class of chip and system figure .............................. 67
Figure 3.6: Dielectric constant of common packaging materials ....................................... 69
Figure 3.7: Cross sectional structure for impedance control. (a) Micro-strip line, (b) Strip
line, and (c) coplanar structure......................................................................... 70
Figure 3.8: Typical types of noise; (a) cross talk noise and (b) switching noise ............... 71
Figure 3.9: Cross sectional view of multi-layer lead frame package and the heat transfer
mechanism ....................................................................................................... 74
Figure 3.10: Integrated circuit packaging level ................................................................... 76
Figure 3.11: Interconnect hierarchy in traditional integrated circuit packaging .................. 77
Figure 3.12: Wiring bonding connecting pad and lead ........................................................ 77
Figure 3.13: Typical conductance and inductance of package type and wire ...................... 78
Figure 3.14: Automated tap bonding (a) polymer with imprinted wire pattern and (b) die
attach using solder bump ................................................................................. 78
Figure 3.15: Flip-chip bonding ............................................................................................... 79
Figure 3.16: Printed circuit board mounting approach. (a) through-hole mounting and (b)
surface mounting .............................................................................................. 79
Figure 3.17: Commonly use package (1) leadless carrier, (2) DIP, (3) PGA, (4) small
outline IC, (5) quad flat pack, and (6) PLCC ................................................... 80
Figure 3.18: Parameters of various chip carriers .................................................................. 81
Figure 3.19: Ball grid array packaging; (a) cross-section, (b) photo of PGA bottom .......... 81
Figure 3.20: An avionics processor module. Courtesy of Rockwell International .............. 82
Figure 3.21: Generic electronics packaging assembly sequence for plastic and ceramic
package ............................................................................................................ 84
Figure 3.22: The basic structure of a silicon device die attach with a metal preform .......... 86
Figure 3.23: Structure of ceramic dual inline package (cerdip) showing the base, the lead
frame and a lid with sealing glass .................................................................... 88
Figure 3.34: Schematics of a multi-pot transfer-mold system showing small mold
compound tablets with each large enough to fill a few cavities containing
plastic strips ..................................................................................................... 90
Figure 3.25: Molded plastic package strip showing short between tips of the lead, tight bar
and guide pin hole ............................................................................................ 91
Figure 3.26: The formed molded plastic dip package strip shows that tight bar has not been
removed............................................................................................................ 91
Chapter 3
Assembly Techniques and Packaging
_____________________________________________
3.0 Introduction
Assembly techniques and packaging involve process of choosing the right type
of package for a particular integrated circuit type and assemble the integrated
circuit in the form of die into package that can be used for application.
3.1 Assembly Technologies
There are many assembly technologies available in today’s assembly of
integrated circuit into package device that can be used to insert into printed wire
board PWB for application. The most common assembly technologies are the
plastics and hermetic assembly technologies. The plastic assembly can be subdivided into various package style both for surface mount SM and through-hole
TH mount assembly techniques. Plastic package styles can be plastic dual inline
package PDIP, plastic quad flat package PQFP, single outline package SOP,
plastic leadless chip carrier PLCC, small outline integrated circuit SOIC etc.
The technology to assemble these package various especially the wire bond,
mold, and plating operation. The overview of the package types is shown in Fig.
3.1. Note that no all available package styles are shown.
Hermetic assembly technology is basically used to assemble high reliability
integrated circuit that are used in industrial, military, and outer space
applications. In this case, the integrated circuit is decoupled from external
environment by a vacuum-tight enclosure. Common packages that are
assembled using this technology are ceramic dual inline package CDIP, pin grid
array PGA, ball grid array BGA etc. An A typical hermetic package with a
silicon chip placed in the cavity of a ceramic-based package and wedge-bonded
to make electrical connections to the terminals on the package is shown in Fig.
3.2.
- 63 -
03 Assembly Techniques and Packaging
Figure 3.1: Variety of Package types for both plastic and hermetic surface mount and
through-hole mount
- 64 -
03 Assembly Techniques and Packaging
Figure 3.2: A hermetic package showing the integrated circuit is decoupled from external
environment
Plastic assembly technology is usually used to assembly high volume, low cost
integrated circuit. The integrated circuit or die is not decoupled from external
environment. The die is in contact with epoxy resin, whereby in long run
environment contaminant can penetrate the plastic to reach the integrated circuit
causing reliability issue. However, with today’s technology, plastic package
device begins gain acceptable for housing high reliability product. A typical
plastic package structure consisting of a silicon die, a metal lead frame, and a
plastic molding compound is shown in Fig. 3.3.
Figure 3.3: A plastic package showing the integrated circuit is not decoupled from external
environment
- 65 -
03 Assembly Techniques and Packaging
Since many of them are used in electronic application, memory device usually is
low cost. The need for low cost memory device has been driving the trend for
cheaper plastic package. A variety of SM plastic packages such as SOJ, SOP,
and thin SOP TSOP has been developed for industrial use, Except for TSOP,
these packages have typically 2 mm thick body. TSOP packages have 1-mmthick plastic body suitable for compact application. As the chip occupancy
continues to grow and the stringent requirements, this imposes have led to
considerable changes in package structures. The lead-on-chip LOC structure, in
which wire interconnection within the package are made above the die circuitry
surface is notable. In conventional packages for older-generation devices, such
as that these shown in Fig. 3.2 and Fig. 3.3, the interconnection is made only in
the periphery and outside the die area. Exploiting an additional area for the
interconnections and reducing wire length. A typical lead-on-chip LOC package
is shown in Fig. 3.4. Here the tips of the lead frame extend over the chip
surface, and Au wires are stitch-bonded to the lead frame tips to connect them
with the chip bonding pads, which are located in the interior of the chip area.
The LOC structure increases the chip occupancy to more than 70% of the
package area. The structure provides die design flexibility because it allows the
pads to be located on the chip in almost any position. It also allows the
placement of substitutes for additional inner lead portions or bus-bars that work
as alternatives to power and ground Al metallizations on the die circuitry. The
bus-bars are numerous in some high-speed memory designs, since they can
enhance the electrical performance of electrical performance of the device
without increasing die size.
Figure 3.4: A plastic package showing the integrated circuit is not decoupled from external
environment
- 66 -
03 Assembly Techniques and Packaging
3.2 Packaging Design
Integrated circuit package plays a fundamental role in the operation and
performance of a component. Besides providing a mean of bringing electrical
signal and voltage supply via wires in and out of the silicon die, it also removes
heat generated by the circuit and provides mechanical support. It also protects
the integrated circuit against environmental conditions such as humidity and
heat. Furthermore the package has a major impact on the performance and
power dissipation of the integrated circuit like the microprocessor and signal
processor. This influence is getting more pronounced as technology scaling
down progressed due to reduction of internal signal delays and on-chip
capacitance. Up to 50% of the delay of a high-performance computer is due to
packaging delay caused by inductive and capacitive parasitic from packing
material. The increasing complexity of circuit integrated into a single die also
translates into a need for ever more input-output pins. This is because the
number of connections is roughly proportional to the complexity of the circuitry
on the chip. This relationship was first observed by E. Rent of IBM, who
translated it into an empirical formula called Rent’s rule. This formula relates
the number of input/output pins P to the complexity of the circuit as measured
by the number of gates.
P = kG
(3.1)
where k is the average number of I/Os per gate, G the number of gates, and 
the Rent exponent.  varies between 0.1 and 0.7. The value is strongly
dependent on the application area, architecture, and organization of the circuit,
as shown in Fig. 3.5.

0.12
0.45
0.50
0.63
0.25
Chip/System
Static memory
Microprocessor
Gate array
High speed computer - chip
High speed computer -circuit
K
6.00
0.82
1.90
1.40
82.0
Figure 3.5: Rent’s constant for varies class of chip and system figure
It is clearly shown that microprocessors display a very different input/output
behavior compared to memories. The observed rate of pin-count increase for
integrated circuits varies between 8% to 11% per year and it has been projected
that packages with more than 2,000 pins will be required by the year 2010. For
all these reasons, traditional dual-in-line, through-hole mounted packages have
- 67 -
03 Assembly Techniques and Packaging
been replaced by other approaches such as surface-mount, ball grid array, and
multichip module techniques. It is useful for the circuit designer to aware of the
available options, and their advantages and disadvantages.
Owing to its multi-functionality, a good package must comply with a large
variety of requirements namely the electrical, mechanical, thermal, and cost
requirements.
3.2.1 Electrical requirements
As the speed of integrated circuit increases, their noise margin decreases. Thus,
electrical design for package must be carefully considered. The pins should
exhibit low capacitance - both inter-wire and to the substrate, resistance, and
inductance. Large characteristic impedance should be tuned to optimize
transmission line behavior and observe that intrinsic integrated circuit
impedances are high. Packages with a design geometry larger than the silicon
can significantly affect the electrical performance of the packaged chips.
Several electrical performance criteria are important such as minimum signal
delay, signal-reflection control, and noise reduction, including simultaneous
switching noise and cross talk. These criteria, often discussed in PWB design,
which is also applied to the package. They are mutually dependent and require
trade off.
Signal Delay
The signal delay time td is defined by equation (3.2).
td =
l
l

 c / r
(3.2)
where l is the length of signal line, v is the velocity of signal, c is the velocity of
light, and r is the dielectric constant of the surrounding material.
High-speed operation requires smaller td. The ratio of td to the cycle time
usually dominates the system performance. In package construction, a short
signal line including bonding wire length and lead length in small dielectric
material typically polyimide resin is preferred. Table 3 lists the dielectric
constants of common packaging materials. An excessively small dielectricconstant of the surrounding material induces signal reflections that degrade
operating speed. Hence, an optimum dielectric value is required. The dielectric
constant of the common packaging material is shown in Fig. 3.6.
- 68 -
03 Assembly Techniques and Packaging
Materials
Al2O3
AlN
Mold compound
Polyamide
Si
GaAs
SiO2
Glass ceramic
Glass epoxy
Dielectric Constant at 1.0MHz
9.6 – 10.2
8.7
3.9- 4.3
3.5
11.7
12.9
3.9
3.9- 7.8
4.2
Figure 3.6: Dielectric constant of common packaging materials
Signal Reflection
A mismatched impedance causes signal reflections when a signal is transmitted
from a driver to a receiver through a transmission line. In CMOS VLSI/ULSI
devices, multiple reflections occur at the driver and receiver ends when the
output impedance of the output buffer is smaller than that of the transmission
line. These reflections cause a ringing phenomenon that may slow down
operation or cause the circuit to malfunction. These reflections cannot be treated
lightly when the relationship exists as shown in equation (3.3).
l
ct r
0.35  r
(3.3)
where l is the signal line length, c is the velocity of the light, r is the dielectric
constant of the surrounding material, v is the critical frequency, and tr is the
signal fall) time. The equations show that a shorter signal line l is required for
high-speed operation that needs a smaller package size. Larger package
constructions have longer signal lines that cannot be dealt with as lumpedelement circuits but it must consider distributed-element circuits. Longer wires,
longer via hole connects that have larger impedance-mismatch potentials,
should be avoided if possible or matched-impedance designs should be used
instead. Leading edge packages such as the multi-lead frame plastic packages,
and some other multi-layered packages containing strip, micro-strip, or coplanar
constructions provide better impedance matching. Figure 3.7 illustrates the strip,
micro-strip, and coplanar structures used for impedance control.
- 69 -
03 Assembly Techniques and Packaging
(a)
(b)
(c)
Figure 3.7: Cross sectional structure for impedance control. (a) Micro-strip line, (b) Strip line, and (c) coplanar structure
The characteristic impedances Zo of strip, micro-strip, and coplanar structures
are expressed by equation (3.4), (3.5), and (3.6) respectively.
Zo 
Zo 
Zo 

60 
4b

ln 
 r  0.67W(0.8  t / W) 
87
 5.98h 
ln 

 r  1.41  0.8W  t 
strip
(3.4)
micro-strip
(3.5)
 1  S /( 2W  S) 

ln  2 
 ( r  1) / 2  1  S /( 2W  S) 
o
coplanar
(3.6)
where r is the dielectric constant of the dielectric material, t and W are the
thickness and width of the conductor, and h and b are the thicknesses of the
dielectric material beneath and surrounding the conductor respectively.
For a typical coplanar structure such as a symmetrical double-strip, where
o (equal to 120) is the characteristic impedance of free space. S is the gap
between the two lines and W is the width of the line. Assumptions are that the
conductor lines are infinitely thin (t = 0), the dielectric material is infinitely
thick (h = ), and the gap widths range is 0.173 < S/(2W + S) < 1.
Noise
Two typical types of noise are cross-talk noise and simultaneous switching
noise (I noise) are found in integrated circuit. Cross-talk noise as shown in
Fig. 3.8(a) occurs when a line is undesirably affected by another line that is
placed very close to it because of the electromagnetic coupling between the two
lines. The noise, coupled by Cm, Lm, or both (m: mutual) between the two lines,
increases in proportion to the signal-voltage or current gradient and the accoupling strength. Cross-talk noise is a more serious problem in VLSI/ULSI
- 70 -
03 Assembly Techniques and Packaging
packaging intended to handle higher speeds, larger signal counts, and the
resulting narrow signal-line spacing. Major counter measures in package design
are shorter parallel signal runs, closer ground or power planes, and lower
dielectric-constant materials.
(a)
(b)
Figure 3.8: Typical types of noise; (a) cross talk noise and (b) switching noise
Simultaneous switching noise, one of the most practical electrical design
problems, particularly in CMOS ASIC devices, occurs when many output
- 71 -
03 Assembly Techniques and Packaging
buffers switch simultaneously. Figure 3.8(b) illustrates the mechanism. When
an output buffer switches from high to low, transition current i flows from the
power line VCC into the load capacitance Cl inducing the noise voltage given by
equation (3.8).
Vn  L g
di
dt
(3.8)
where Vn is the induced voltage, Lg is the inductance of the power lead, and
di/dt is the derivative of current with respect to time.
In addition, when a line switches from low to high, an electric charge
stored in the load capacitance flows into the ground line through the
transmission line, inducing the same noise voltage as shown in equation (3.9). If
j line is switching simultaneously then Vn is given by
Vn   L g
j
di
dt
(3.9)
3.2.2 Mechanical and Thermal properties
Package should be designed in such the heat removal rate should be as high as
possible. Mechanical reliability requires a good matching between the thermal
properties like the coefficient of thermal expansion CTE of the integrated circuit
and the chip carrier. Long term reliability requires a strong connection from die
to package as well as from package to printed circuit board.
As the power consumption of integrated circuits rises, it becomes
increasingly important to efficiently remove the heat generated by the die. A
large number of failure mechanisms in integrated circuit are accentuated by
increase of temperature. Examples are leakage in reverse biased diodes, electromigration, and hot electron trapping. To prevent failure, temperature of the die
must be kept within certain ranges. The temperature range for commercial
graded devices is between 0° and 70°C. Military parts are more demanding and
require a temperature range varying from -55° to 125°C.
The cooling effectiveness of a package depends upon the thermal
conduction of the package material, which consists of the package substrate and
body, the package composition, and the effectiveness of the heat transfer
between package and cooling medium.
- 72 -
03 Assembly Techniques and Packaging
As for the microprocessor device and other high performance device such
video device used in graphic card of a computer, thermal interface material TIM
is used to reduce thermal impedance between the device and heat sink. TIM is a
type of conductive paste used to fill any void or irregularity between the device
and heat sink.
Standard packaging approaches use still or circulating air as the cooling
medium. The transfer efficiency can be improved by adding finned metal heat
sinks to the package. More expensive packaging approaches, such as those used
in mainframes or super computers, force air, liquids, or inert gases through tiny
ducts in the package to achieve even greater cooling efficiencies. As an
example, a 40-pin DIP has a thermal resistance of 38°C/W and 25°C/W for
natural and forced convection air. This means that a DIP can dissipate 2 watts (3
watts) of power with natural (forced) air convection, and still keep the
temperature difference between the die and the environment below 75°C. For
comparison, the thermal resistance of a ceramic PGA ranges from 15° to
30°C/W. Since packaging approaches with decreased thermal resistance are
prohibitively expensive, keeping the power dissipation of an integrated circuit
within bounds is an economic necessity. The increasing integration levels and
circuit performance make this task nontrivial. An interesting relationship shown
in equation (3.9) has been derived by Nagata. It provides a bound on the
integration complexity and performance as a function of the thermal parameters.
N G T

tp
E
(3.9)
where NG is the number of gates on the chip, tp the propagation delay, T the
maximum temperature difference between chip and ambient environment,  the
thermal resistance between them, and E the switching energy of each gate.
Fortunately, not all gates are operating simultaneously in real systems. The
maximum number of gates can be substantially larger, based on the activity
coefficient in the circuit. For instance, it was experimentally derived that the
ratio between the average switching period and the propagation delay ranges
from 20 to 200 in mini- and large-scale computers.
Nevertheless, equation (3.9) demonstrates that heat dissipation and thermal
concern present an important limitation on circuit integration. Design
approaches for low power that reduce either E or the activity coefficient are
rapidly gaining importance.
- 73 -
03 Assembly Techniques and Packaging
Let’s consider from the aspect of the thermal resistance ja of a package as
shown in Fig. 3.9.
Figure 3.9:
Cross sectional view of multi-layer lead frame package and the heat transfer mechanism
The thermal resistance of a package is defined
 ja 
Tj  Ta
(3.10)
P
where ja in 0C/W is the junction to ambient thermal resistance, Tj is the
average chip or junction temperature, Ta is the ambient temperature, and P is the
power dissipation.
Figure 3.9 is also a simplified heat-transfer model of a packaged die, where
heat is transferred from the die to the surface of the package by conduction and
from the package surface to the ambient by convection and radiation. In most
applications, the temperature difference between the case or the package surface
and the ambient is small. Thus, radiation can be neglected. Conduction heat
transfer through the package terminals can be significant, particularly in highI/O VLSI/ULSI packages. However, if one neglects it for simplification, the
overall thermal resistance in this model can be considered as the sum of two
thermal components, which are jc and ca and is defined
 ja   jc  ca 
Tj  Tc
P

Tc  Ta
P
(3.11)
where jc is the junction to case thermal resistance, ca is the case to ambient
thermal resistance, and TC is the average case temperature. jC is relatively
- 74 -
03 Assembly Techniques and Packaging
insensitive to the ambient and is mainly a function of package materials and
geometry. ca depends on the package geometry, the package orientation in the
application, and the conditions of the ambient in the operating environment
whether heat transfer is free or by forced-convection. Heat transfer is classified
into three categories conduction, convection, and radiation. We shall not discuss
here.
3.2.3 Cost
Cost is always one of the most important requirements. Ceramic package has a
superior performance over plastic package but it is also substantially more
expensive. Increasing the heat removal capacity of a package also tends to raise
the package cost. The least expensive plastic package can dissipate up to 1.0W
of heat. More expensive but still cheap plastic package can dissipate up to
2.0W. Higher heat dissipation requires more expensive ceramic package. Chips
dissipating over 50.0W require special heat sink attachment. Extreme
techniques such as fans and blowers, liquid cooling hardware, or heat pipes, are
needed for higher dissipation levels.
Packing density is a major factor in reducing the cost of the printed circuit
board. The increasing pin count either requires an increase in the package size
or a reduction in the pitch between the pins. Both have a profound effect on the
cost of package.
3.3 Packaging Level Integration
As circuit integration proceeds on the die, it also proceeds in the packaging
through interactions among several levels of packaging. Generally, packaging
exclusive of the final system construction is classified into three levels, as
shown in Fig. 3.10. Final system requirements determine a specific selection of
the packaging method or how to combine the levels. Types 1 through 4 show
the major methods that have been used in the industry. Type 1 is the most
common choice. In type 1, the die is first packaged as a single chip and then
packaged at the third level, typically at the PWB level. Types 2 and 3, usually
called multi-chip-module MCM technologies, are used in high performance
systems, typically in mainframe computer. In type 2, the die is single-diepackaged as in type 1, and the packaged dices are then packaged at the second
level onto a smaller substrate; this forms a functionally larger and
geometrically smaller unit and utilizes the finer multilayer wiring of the
substrate. The substrate is attached to a larger mother board in the third-level
packaging. Type 3 is similar to type 2 in that it uses a smaller substrate as an
- 75 -
03 Assembly Techniques and Packaging
intermediary stage. However, bare chips are attached directly to the substrate,
usually providing a superior electrical and geometrical performance but with
some disadvantages, such as more difficult testability, increased cost, and lower
yield. Type 4 is the simplest way of packaging, where bare dices are attached
directly to the system board. The brief descriptions of the various type
packaging level integrations are mentioned in the sub-sections below.
Figure 3.10:
Integrated circuit packaging level
3.3.1 Interconnect Levels
The traditional packaging approach uses a two-level interconnection strategy.
The die is first attached to an individual chip carrier or package. The package
body contains an internal cavity where the chip is mounted. These cavities
provide ample room for many connections between chip and leads. The leads
compose the second interconnect level and connect the chip to the external
interconnect medium, which is normally the printed circuit board. Complex
systems contain even more interconnect levels, since boards are connected
together using backplanes or ribbon cables. The first two layers of the
interconnect hierarchy are illustrated in the drawing of Fig. 3.11.
- 76 -
03 Assembly Techniques and Packaging
Figure 3.11: Interconnect hierarchy in traditional integrated circuit packaging
The interconnect techniques used at levels one and two of the interconnect
hierarchy are shown here.
3.3.1 Interconnect Level 1 - Die-to-Package-Substrate
Traditionally wire bonding is the technique of choice to provide an electrical
connection between die and package. In this approach, the backside of the die is
attached to the substrate using glue with a good thermal conductance. Next, the
chip pads are individually connected to the lead frame with aluminum or gold
wire. An example of wire bonding is shown in Fig. 3.12. Although the wirebonding process is automated, it has some major disadvantages.
Figure 3.12: Wiring bonding connecting pad and lead
Wire must be attached serially one after the other. The lead time is longer with
increasing pin counts. Larger pin counts make it substantially more challenging
to find bonding patterns that avoid shorts between the wires. Bonding wire has
inferior electrical properties such as a high individual inductance (5nH or more)
and mutual inductance with neighboring signals. The inductance of a bonding
- 77 -
03 Assembly Techniques and Packaging
wire is typically about 1.0nH/mm, while the inductance per package pin ranges
between 7.0 and 40.0nH per pin depending on the type of package as well as the
positioning of the pin on the package boundary. Typical values of the parasitic
inductances and capacitances for a number of commonly used packages are
summarized in Fig. 3.13.
Package Type
68 pin plastic DIP
68 pin ceramic DIP
256 pin grid array
Wire bond
Solder bump
Capacitance pF
4
7
1-5
0.5-1
0.1-0.5
Inductance nH
35
20
2-15
1-2
0.01-0.1
Figure 3.13: Typical conductance and inductance of package type and wire
The exact value of the parasitic component is hard to predict because of the
manufacturing approach and irregular outlay. New attachment techniques are
being used as a result of these deficiencies. In one approach is called Tape
Automated Bonding TAB. The die is attached to a metal lead frame that is
printed on a polymer film typically polyimide as shown in Fig. 3.14(a). The
connection between chip pad and polymer film is wired by solder bump as
shown in Fig. 3.14(b). The tape can then be connected to the package body
using a number of techniques. One approach is using pressure connection.
(a)
Figure 3.14:
(b)
Automated tap bonding (a) polymer with imprinted wire pattern and (b) die attach using solder bump
There are many advantages of the TAB process. Besides the process is highly
automated, the sprockets in the film are used for automatic transport, and all
wire connections are made simultaneously. The printed approach helps to
reduce the wiring pitch, which results in higher lead counts. Elimination of the
long bonding wires improves the electrical performance.
- 78 -
03 Assembly Techniques and Packaging
Another approach is to flip the die upside down and attach it directly to the
substrate using solder bumps. This technique, called flip-chip mounting, has the
advantage of a superior electrical performance as shown in Fig. 3.15. Instead of
making all the I/O connections on the die boundary, pads can be placed at any
position on the chip. This can help to address the power and clock distribution
problems, since the interconnect materials on the substrate are typically copper
Cu or gold Au that have better quality than the aluminum (Al) on the chip.
Figure 3.15: Flip-chip bonding
3.3.2 Interconnect Level 2 - Package Substrate to Board
When connecting the package to the printed circuit board PCB, through-hole
mounting has been the packaging style of choice. A PCB is manufactured by
stacking layers of copper and insulating epoxy glass. In the through-hole
mounting approach, holes are drilled through the board and plated with copper.
The package pins are inserted and electrical connection is made with solder as
shown in Fig. 3.16(a). The traditional package in this class is the dual-in-line
package DIP. The packaging density of the DIP degrades rapidly when the
number of pins exceeds 64. This problem can be alleviated by using the pingrid-array PGA package that has leads on the entire bottom surface instead of
only on the periphery. PGA package can extend to large pin counts over 400
pins.
(a)
Figure 3.16:
(b)
Printed circuit board mounting approach. (a) through-hole mounting and (b) surface mounting
The through-hole mounting approach offers a mechanically reliable and sturdy
connection. However, the setback is the packaging density. For mechanical and
sturdy reasons, a minimum pitch of 2.54mm between the through-holes is
required. Even with this pitch, PGAs with large numbers of pins would also
substantially weaken the printed circuit board. In addition, through-holes limit
- 79 -
03 Assembly Techniques and Packaging
the board packing density by blocking lines that might otherwise have been
routed below them, which results in longer interconnections. PGAs with large
pin counts hence require extra routing layers to connect to the huge number of
pins. Although the parasitic capacitance and inductance of the PGA are slightly
lower than DIP, their values are still substantially large. These shortcomings of
the through-hole mounting can be solved by using the surface mount technique.
A chip is attached to the surface of the board with a solder connection without
requiring any through-holes as shown in Fig. 3.16(b). Consequently, surface
mount increases packing density due to elimination of through-holes mounting
that provides more wiring space and mechanically strengthens the PCB. The
lead pitch is reduced and chips can be mounted on both sides of the board.
The negative effects of the surface mount are many. The connection makes
the connection of printed circuit board weak. It is also cumbersome to mount a
component on a board that requires expensive mounting equipment, difficult for
board repair, and finally testing of board is more complex because the package
pins are no longer accessible at the backside of the board. Signal probing harder
or even impossible.
A variety of surface mount packages is currently in use with different pitch
and pin count parameters. Four of these packages are shown in Fig. 3.16: the
small-outline package SOIC with gull wings, the plastic leaded package PLCC
with J-shaped leads, the leadless chip carrier LCC, and quad flat pack QFP.
Figure 3.17:
Commonly use package (1) leadless carrier, (2) DIP, (3) PGA, (4) small outline IC, (5) quad flat pack, and (6) PLCC
An overview of the most important parameters for a number of packages is
shown in Fig. 3.18.
Package Type
Dual in Line DIP
Typical Lead Spacing
2.54mm
- 80 -
Maximum Lead
Count
64
03 Assembly Techniques and Packaging
Pin Grid Array PGA
Small Outline IC SOIC
Plastic Leadless Chip
Carrier PLCC
Leadless Chip Carrier LCC
2.54mm
1.27mm
>300
28
1.27mm
124
0.75mm
124
Figure 3.18: Parameters of various chip carriers
Even surface mount packaging is unable to satisfy the quest for evermore higher
pin counts. This is worsened by the demand for power connections in high
performance chips, operating at low supply voltages, require as many power and
ground pins as signal I/O. When more than 300 I/O connections are needed,
solder balls replace pins as the preferred interconnect medium between package
and board. An example of such a packaging approach, called ceramic ball grid
array (BGA) is shown in Fig. 3.19. Solder bumps are used to connect both the
die to the package substrate and the package to the board. The area array
interconnect of the BGA provides constant input/output density regardless of
the number of total package I/O pins. A minimum pitch between solder balls as
low as 0.8mm can be obtained, and packages with multiple 1000’s of I/O
signals are feasible.
(a)
(b)
Figure 3.19: Ball grid array packaging; (a) cross-section, (b) photo of PGA bottom
3.3.3 Multi-Chip Modules - Die to Printed Wire Board
The deep hierarchy of interconnect levels in the package is becoming
unacceptable in modern complex circuit designs with higher levels of
integration, large number of signals, and performance requirements. There is a
need to reduce the number of levels.
At the meantime, attention is focused on the elimination of the first level in
the packaging hierarchy, which is eliminating die to package level by mounting
the die directly on the wiring backplanes board or substrate. It offers a
substantial benefit when performance or density is a major concern. This
- 81 -
03 Assembly Techniques and Packaging
packaging approach is called multichip module MCM. As the result, there is a
substantial increase in packing density as well as improved performance. A
number of the previously mentioned die mounting techniques can be adapted to
mount dice directly on the substrate. This includes wire bonding, Tape
Automated Bonding TAB, and flip chip, although the later two are preferable.
The substrate itself can be varying over a wide range of materials depending
upon the required mechanical, electrical, thermal, and cost requirements.
Materials of choice are epoxy substrates similar to printed circuit boards, metal,
ceramics, and silicon. Silicon has the advantage of presenting a perfect match in
mechanical and thermal properties with respect to the die material.
The main advantages of the MCM approach are; it increases packaging
density and device’s performance. An example of an MCM module
implemented using a silicon substrate; commonly dubbed silicon-on-silicon is
shown in Fig. 3.20. The module that implements an avionics processor module
and is fabricated by Rockwell International contains 53 ICs and 40 discrete
devices on a 2.2x2.2substrate with aluminum polyimide interconnect.
Figure 3.20: An avionics processor module. Courtesy of Rockwell International
The interconnect wires of the module are only an order of magnitude wider than
what is typical for on-chip wires because similar patterning approaches are
used. The module itself has 180 I/O pins. Performance is improved by the
elimination of the chip to carrier layer with variety of parasitic components, and
through a reduction of the global wiring lengths on the die, a result of the
increased packaging density. For instance, a solder bump has an assorted
capacitance and inductance of only 0.1pF and 0.01nH respectively. The MCM
- 82 -
03 Assembly Techniques and Packaging
technology can also reduce power consumption significantly, since large output
drivers and associated dissipation become redundant due to the reduced load
capacitance of the output pads.
While MCM technology offers some clear benefits, its main disadvantage
is economic. This technology requires some advanced manufacturing steps that
make the process expensive. The approach is only justifiable when either dense
housing or extreme performance is essential. In the near future, this argument
might become obsolete as MCM approaches proliferate.
3.4 Assembly Techniques and Processes
This section describes the basic assembly processes and techniques used for
assembly ceramic and plastic packaged VLSI device. The processes cover a
number of assembly steps from wafer preparation through die attach, including
wire bonding, encapsulation/molding, stabilization bake/post mold bake,
temperature cycle to tin plating/solder plating, trim/form, and inspection.
Student will learn these process steps based on the generic assembly sequence
shown in Fig. 3.21 and at the same time understanding the physics of technique
for process step.
The generic assembly process steps are mainly for ceramic and plastic
packaged integrated circuit or die. The first step is the preparation of wafer step,
which basically cuts and separates the die from the wafer. The second process
step is the die attach. It is a process step that bonding the die to the paddle of the
package. The third step is the wire bonding, which is the process of connecting
the bond pad on the die to the lead of the package. This step allows the die to be
connected to external world. Encapsulation is the step involves closing the die
from the interference of external contaminant and protecting from damage etc.
The encapsulation can be done by mean of molding for the plastic package or
glass seal the lid of the package to the hermetic ceramic package. Post mold
cure or baking process is a necessary step to cross linked the plastic resin
material to provide hardening effect. Owing to high temperature process, lead of
package would be tarnished due to heat. The next process step is either tin plate
or solders plate, which involves removing the oxide and plating the leads for
preventing oxidation and providing good solderability connecting contact to the
circuit board. The second last process step involves removing the shorting bar
from the hermetic package, or trims and forms the plastic package. The last
process step is an inspection step that involves removing of the non-compliance
device such as lead problem, package crack etc from the from the production
batch.
- 83 -
03 Assembly Techniques and Packaging
Figure 3.21: Generic electronics packaging assembly sequence for plastic and ceramic
package
3.4.1 Wafer Preparation
Thickness of the fabricated wafer is normally around 650m. It needs to be
thinned before the assembly begins. Depending on the package style that the
integrated circuit to be housed, it can be thinned to approximately 400m. The
thinning is necessary to reduce thermal stress due to mismatch of the coefficient
of thermal expansion CTE between the silicon die and the packaging material.
- 84 -
03 Assembly Techniques and Packaging
Again depending on the type of packaging material, the back-side of the wafer
may require metallization deposition consisting of multilayer metallic elements
like gold-nickel-silver Au-Ni-Ag or titanium-nickel-silver Ti-Ni-Au in order
from silicon side. This helps in thermal conduction between the silicon die and
the package, and provides the superior adhesion strength and electrical
connection.
Every integrated circuit in the wafer is probed electrically to check its
functionality. The malfunction dice are marked with a drop of red ink that they
can be sorted out during die attach or die bonding process. Different color
inking schemes may be adopted to distinguish between commercial/industrial
compliance die and military compliance die. One of the schemes is to ink the
die with green color for commercial/industrial compliance die, no color is to be
used for military compliance die, and red color for fail die.
The probed wafer is then adhesively mounted to a tape that has been preassembled to a frame using a wafer dispenser. The frame is then mounted on the
dicing machine with a diamond blade to cut the scribe line for separating the
dice. The thickness of diamond blade is typically 25m thick rotating at a speed
of 20,000rpm cuts the wafer from 90.0% to 100.0% saw-through allowing
dicing street as narrow as 60m, which is closed to the width of scribe line of
between dice. 100.0% saw-though is necessary for VLSI devices, which have
large area because it reduces the chance of chipping at the edge during die
separation breaking process. In the modern VLSI assembly, the 100% sawthrough dice allow automatic picking of good dice with the aid of optical visual
system during die attach.
3.4.2 Die Attach
It is a process of attaching the die permanently to the paddle of lead frame or
ceramic package. One of the important conditions of the die attachment process
and several other processes are the requirements of high temperature and
cooling down to room temperature. This would cause thermal stress due to the
difference of coefficient of thermal expansion CTE of the silicon die and
material of package. The results are crack on the die and metallization peeling
off. Material that has CTE close to that of silicon crystal is preferable in
package construction. Choosing the packaging materials that have CTE the
same as that of silicon would be ideal reducing thermal stress to zero. In real
situation, there is no such material that can provide perfect match with the
silicon. However, in the industrial ceramic Al2O3 substrate and Alloy 42 (42%
nickel-58% iron alloy) lead frame have been used for many years due to close
- 85 -
03 Assembly Techniques and Packaging
TCE match for hermetic-ceramic package and plastic package. However, Alloy
42 is no longer the only choice in today’s assembly. Copper alloy lead frame is
preferred today for logic and microprocessor devices because copper alloy has
approximately ten times the thermal conductivity of Alloy 42. It allows better
heat transfer from the die to package via die attach material.
3.4.2.1 Eutectic Die Attach
Figure 3.22 illustrates the fundamental aspect of a die attach. Eutectic chip die
attach is metallurgically attached from the die to substrate material typically
made from Alloy 42 or attached to a ceramic substrate (90-99.5% Al2O3).
Metallization is often required on the back of the chip so that it is wettable by
die attach perform. The perform is a thin sheet of thickness less than 0.05m
consisting of solder-bonding alloy such as 98% gold-2% silicon. The paddle and
lead of the ceramic package is usually plated with gold, while the paddle and
lead of the alloy 42 lead frame or copper alloys lead frame is plated with silver.
Figure 3.22: The basic structure of a silicon device die attach with a metal preform
During the die attach, the preform is placed on the paddle heated to about
3700C. Mechanical scrubbing is done so that the preform melts and reacts with
silicon to form an Au-Si composition bond between the backside of the die and
the substrate of the ceramic. The bonding is considered complete when the AuSi composition structure becomes rich in silicon.
3.4.2.2 Epoxy Die Attach
Silver filled epoxy adhesive is the choice of polymer based material for die
attach. The silver filler usually would flake that makes epoxy electrically
conductive and thermally conductive to allow good thermal path between die
and the rest of the package. In VLSI assembly, epoxy is fed onto the substrate
material through a multi-nozzle or single nozzle dispenser to ensure the required
- 86 -
03 Assembly Techniques and Packaging
bond line thickness is created avoiding void. The back side of the die often does
not require metallization because epoxy provides better adhesion with bare
silicon or silicon dioxide. The process time of die attach usually is 1 to 2
seconds at room temperature. The epoxy is thermosetting polymers, after the die
attach, it must be cured at elevated temperature to complete the die bond. The
cure conditions range from 1250C to 1750C and require 1 to 2 hours.
3.4.3 Wire Bonding
Wire bonding is the most common method for connecting the bond pads on the
die to the leads of the package. Aluminum or gold wires are usually the choice
because they bond well to the bond pads on the die and to the metallized part of
the package forming Au-Al and Al-Ag metallurgical diffused materials. Gold or
aluminum wire of diameter 25 to 30m is balled and wedge bonded by
thermosonic or thermocompression, where the ball is bonded to the bond pad
made of aluminum and wedge-bonded at the lead plated with either gold or
silver. The temperature of wire bonding for thermosonic ranges from 150 0C to
2500C, while for thermocompression process, the temperature ranges from
3000C to 3500C.
The metallurgical diffusion primary follows equation
X2 = Dt
(3.12)
D  D0 exp  Q / RT 
(3.13)
where X is the diffusion thickness, D is the diffusion constant, t is the storage
time, D0 is the frequency factor, Q is the activation energy, R is the gas
constant, and T is the storage absolute temperature.
Gold and aluminum form a variety of inter-metallic with Au-Al first
formed and gradually change to Au-Al4 that will degrade the bond strength.
Following equation (3.12) and (3.13), Au-Al inter-metallic growth would be
severed at elevated temperature especially the temperature of molding and
hermetic glass seal. Thus, aluminum-aluminum wiring bonding at bond pad is
preferred for high reliable products.
Besides, using gold and aluminum wires to interconnect bond pads of the
die to lead of the package, copper bond wire is also used in today’s modern
VLSI assembly due to a few obvious reasons. They are cost, electrical and
- 87 -
03 Assembly Techniques and Packaging
thermal conductivity, less inter-metallic growths, and better reliability of bond
at elevated temperature.
Copper costs 90% less than gold. It is obvious in terms of cost of assembly.
It costs less. Copper wire has electrical resistivity of 0.017x10-4-cm, which is
about 30% better than the resistivity of gold, which is 0.022x10 -4-cm. The low
electrical resistivity of copper results in better electrical performance in
particular for bonding high current device. Copper has thermal conductivity of
395m-1K-1, which 25% better than the thermal conductivity of gold, which is
316Wm-1K-1. Thus, copper wire dissipates heat within the package faster than
gold wire. Thus, it minimizes the thermal stress.
Copper has a lower tendency to form inter-metallic compound with
aluminum. Unlike gold, it forms inter-metallic compound with aluminum
especially at elevated temperature due to inter-diffusivity of gold and aluminum
(bond pad). It can create voids at the bond interface that would weaken the bond
and can lead to bond lifting as well as other wire bond problems.
3.4.4 Molding/Glass Seal
Upon completion of wire bonding, the next operation is either molding or glass
seal process, which depending on the package style used. Glass seal refractory
technology relies on glass sealing a lead between two pressed ceramics as
illustrated in Fig. 3.23 using low temperature glass. The glass used for glass
sealing is PbO-ZnO-B2O3 type. Sealing is usually done at temperature above
4000C in an oxidizing ambient to avoid deoxidizing the metallic components of
the glass that would degrade the electrical insulation. For VLSI device, sealing
at temperature greater than 4000C would cause additional thermal diffusion at
the junction of transistor that would shift slightly the electrical characteristics of
the die. Thus, choosing glass-sealing technology for sealing must be carefully.
Figure 3.23: Structure of ceramic dual inline package (cerdip) showing the base, the lead
frame and a lid with sealing glass
- 88 -
03 Assembly Techniques and Packaging
Plastic encapsulation involves a number of techniques. For an example, in globtop-coating, the post wire bonded die is coated with liquid plastic resin and the
plastic are cured for the cross liking.
In VLSI plastic packaging, a pre-molding technique is sometimes used.
This technology is the plastic equivalent of the refractory ceramic cavity
package. The package is molded together with a lead frame forming a plastic
body and cavity, whereby the die is attached and bond pads to lead are wire
bonded.
The post molding technology is a transfer molding method using
thermosetting epoxy resins to mold around the frame-die assembly to form
package body. The molding process has to be controlled carefully because this
process is relatively harsh that the die and bond wire are exposed to viscous
molding material that may cause wire sweeping or lifted wire.
The molding material that is epoxy resin is made by condensing
epichlorohydrin with bisphenol-A to produce a material called Epoxy-A. An
excess of epichlorohydrin was used to leave epoxy groups on each end of low
molecular weight polymer. Today NOVOLAC epoxy is general preferred due to
its higher functionality that makes heat resist.
The molding compound is usually pre-heated and transferred into pots of
large multi-cavity mold. After it enters the pot, the molding compound melts
under pressure and heat, flows to fill the mold cavities containing frame strips
with their attached dice. For molding of VLSI die, which has large area, longer
bond wire, the multi-pot molding as shown in Fig. 3.24 is preferred aiming to
reduce damage of wire due to viscosity of the mold compound and incomplete
mold due to partial cross linking.
- 89 -
03 Assembly Techniques and Packaging
Figure 3.34: Schematics of a multi-pot transfer-mold system showing small mold compound
tablets with each large enough to fill a few cavities containing plastic strips
3.4.5 Post Mold Cure/Leak Check
For plastic packaged device, post mold cure or baking process is a necessary
step to cross linked the plastic resin material to provide hardening effect. The
curing is normally done in an oven set at temperature 150 0C for three hours of
curing time. As for the hermetic package, cure is not necessary for the package.
For ceramic packaged device, leak test is usually done to check if there is any
glass seal problem or micro-crack of the ceramic package. The leak tests are
divided into gross leak and fine leak tests. The gross leak test is easy. It is done
by immersing the ceramic package into water like the way we check the leak of
a car wheel. Fine leakage is done by placing the ceramic devices in the pressure
compressed chamber containing radioactive source for 1 to 2 hours. The
devices are then checked for fine leak with a  particle counter. If the counter
shows count result, it means that there is fine leak due to micro-crack whereby it
cannot be visually detected by naked eyes.
3.4.6 Solder Dip/Tin Plate
Owing to high temperature process, lead of package would be tarnished due to
heat. Thus, it is necessary to remove the oxide before either solder dip or tin
plate is done depending on package type. The leads of plastic package are
- 90 -
03 Assembly Techniques and Packaging
normally solder dipped, while the leads of hermetic package is normally tin
plated or solder plated. Solder dip or tin plate is necessary to protect the base
metal of the package from oxidation in order to preserve its solderability.
For plastic device, upon removing the oxide and extra mold fresh, the short
between the tips of the lead between adjacent packages is removed as shown in
Fig. 3.25 and the package is formed to the desire shape as shown in Fig. 3.26 for
plastic dual in-line DIP package. The device strips are dipped with solder flux
and then into solder bath so that solder will cover the non-oxidized bare-leads of
the plastic device.
As for the ceramic package, it is normally tin plated or solder plated via
electrolysis. It is normal electrolysis process, whereby the hermetic devices are
hung in a bracket at the negative electrode of the plating bath filled with
electrolyte.
Figure 3.25: Molded plastic package strip showing short between tips of the lead, tight bar
and guide pin hole
Figure 3.26: The formed molded plastic dip package strip shows that tight bar has not been
removed
- 91 -
03 Assembly Techniques and Packaging
3.4.7 Trim/Form
One already knows that generally the device is assembled either in plastic
package style or hermetic package style. The leads of the device are shorted
with a tight bar with the aims to prevent lead bending and also to protect the die
from damage due to electrostatic discharge ESD so that every lead is at same
potential with no potential difference for electron discharge that would damage
the die. The short between the tips of the leads is trimmed off using puncher.
The device is then formed according to the package style such the dual inline
package DIP as shown in Fig. 3.26. Take for an example, the lead of the
package is formed into gull-wing style for single outline package SOP.
For plastic package, trim and form are performed before solder dip process.
As for the hermetic package, form is normally not necessary, while trim is done
after tin plating or solder plating process to remove the shorting bar connecting
the tips of all leads.
3.4.8 Inspection
The tight bar of the plastic device is removed to singulate device. Inspection is
done to sort out non-conformance device such as lead defect, package crack,
package chip, insufficient solder coverage etc. before loaded into tube. The tube
or tray loaded devices is then transferred to test operation for initial and final
tests.
Exercises
3.1.
Why is it necessary to measure the physical parameter of a fabricated
integrated circuit?
3.2.
Calculate the current transfer distance (li) if the contact resistivity is
2.0x10-7cm2 and the resistivity of silicon is 100/.
3.3.
If the physical contact area of the n+ diffusion region is 1.0µmx0.5µm,
using the result of question 9.2, calculate the contact resistance.
3.4.
Using a cross-bridge Kelvin structure with a 1.0µm x 1.0µm contact, the
current is found to be 10.0µA through the contact and the voltage
difference across the contact is 320µV, find the contact resistivity of this
contact.
- 92 -
03 Assembly Techniques and Packaging
3.5.
State the reason necessary to have coefficient of thermal expansion
matching materials for package and silicon die.
3.6.
Why aluminum wire bonding is preferred than gold wire bonding?
3.7.
If the Au-Al bond diffusion thickness is increased by 1.0m during 1500C
time-temperature storage operation, calculate the period of time. You
may use D0 = 2.2x10-4m2/s, Q = 134kJ/mol and R= 8.31J/mol-K to help
you in the calculation.
3.8.
For VLSI device plastic molding, state the reason why multi-pot molding
is necessary.
3.9.
State the reasons why the leads of the package are normally shorted
together in a assembly operation.
3.10. Calculate the number of gates that can be included on a logic-array chip
which is to be assembled in 120 I/O package assuming that k = 4.5 and β
= 0.5.
3.11. An integrated circuit has 800 gates, its nominal propagation delay for a
transistor is 5.0x10-15s, its junction to ambient maximum temperature
difference is 550C, and junction to ambient thermal resistance is
1000C/W. Calculate the activation energy of each gate of this circuit.
3.12. State the reason why it is necessary to have heat sink for conducting away
extra heat from the package of integrated circuit?
- 93 -
03 Assembly Techniques and Packaging
Bibliography
1.
S.M. Sze, “VLSI Technology”, McGraw Hill, 2002.
2.
Jan M. Rabaey, Anantha Chandrakasan and Borivoje Nikolic “Digital
Integrated Circuit – A Design Perspective”, 2nd edition, Prentice Hall.
2003.
C.Y. Chang and S.M. Sze, “ULSI Technology”, McGraw Hill, 1996.
3.
4.
N. H. Weste and D. Harris, “CMOS VLSI Design: A Circuits and Systems
Perspective”, third edition, Pearson Addison Wesley, 2005.
5.
M. Michael Vai, “VLSI Design”, CRC Press LLC, 2001.
- 94 -