Flat Bump Package
Liang Zhizhong, Tao Yujuan
Jiangsu Changjiang Electronics Technology Co., Ltd., Jiangyin Jiangsu 214431,
China
Email: lzz@cj-elec.com, tyj@cj-elec.com, Phone: 86-510-6856383
Abstract
Consumer market and new applications worldwide have demanded the
semiconductor package to be designed smaller in size, better performance and
more reliable. FBP, developed by JCET, not only meets the requirements but
excels the performance of several existing packaging technologies in comparison.
In this paper, the special structure and excellent performance of FBP are
introduced, described and illustrated graphically.
1. Introduction
As the semiconductor manufacturing technology advances, the packaging
technology is driven to develop correspondingly to meet the stringent requirements
such as high thermal/electrical performance, low resistive interference and better
reliability. FBP (Flat Bump Package) is just designed to meet those requirements,
with smaller and thinner package than TSSOP, SOT, SOD, and QFN; together with
additional advantage of the bump leads----they are easier to joint firmly with PCB.
2. The Comparison between FBP and QFN
2.1 Structure Comparisons
FBP
The side lead is
unexposed.
Fig. 1
QFN
Fig. 2
The exposed side lead is
easy to cause burr and
delamination issue.
2.1.1 The Substrate and the Lead Frame
The substrate of FBP is strong enough to choose the copper material in high
purity, such as A151, A1100, with more than 90% of conductivity. The copper is
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purer, the resistive interference is lower, and the heat-dissipation is faster. From
the point of material selection, FBP is intrinsically better in its excellent
thermal/electrical performance and low resistive interference. In addition, FBP can
finish the whole assembly process without the expensive tape adhering on the
back of the substrate; this effective processing technology reduces material cost
and avoids contamination issue.
QFN chooses through-etched lead frame. The structure is too soft to select
the copper material in high purity to improve its thermal/electrical performance and
reduce the resistive interference. Furthermore, the lead frame of QFN needs
adhering tape on its back, and the tape will contaminate the die and bonding pads,
especially in high temperature treatment processes.
2.1.2 The Plating Layer on the Inner Leads
The surfaces of the inner leads in FBP package are plated with gold. Gold
does not oxidize and plasma cleaning can be eliminated. In addition, compared
with silver, gold adheres to the molding compound better and the delamination
issue does not happen in FBP package.
2.1.3 The Plating Layer of the Outer Leads
In FBP package, the outer leads (Fig. 3) are also plated with gold, whose
characteristics ensure the storage life of FBP products is expected much longer
without any issue, even in humid environment.
In QFN package, the outer leads (Fig. 4) are plated with tin, whose
characteristics ensure that the outer leads are easy to oxidize; and worst yet can
even melt especially in Package Sawing process.
FBP
Fig. 3
QFN
Fig. 4
2.1.4 The Structure of the Outer Leads
The bump-lead design is the most obvious characteristic of FBP. Due to the
outer leads are bulgy to the molding compound, the solder paste can adhere to the
bottom and around of the bumps firmly without short circuit issue. FBP has good
heat sink ability built-in: heat dissipates not only from the die pad to PCB but also
from the gaps between every two bumps. So FBP improves its thermal
performance once again by its special structure, in addition to its copper material in
high purity. Since good thermal/electrical performance and low resistive
interference are correlated with each other, the application of FBP can be the best
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choice for power IC and RF IC packaging, especially for high-end product.
In QFN package, since the outer leads are on the same level of the molding
compound, the solder paste can only adhere to the bottom surface in SMT and the
joint strength is weak; furthermore, the soft solder paste is easy to cause short
circuit when being pressed. Once the resin bleeding issue happens, the bad
coplanarity will affect surface mounting and electrical transferring. As to the
thermal performance, QFN can only dissipate heat from the die pad to PCB
because the flux used in SMT is easy to fill the left area.
Therefore, FBP products are easier to joint with PCB more firmly with higher
thermal/electrical performance.
Molding Compound
The Bump Leads of FBP
Solder Paste
The Leads of PCB
PCB
Fig. 5
2.1.5 The Side Leads
After package sawing, the side leads of FBP are protected by the molding
compound, avoiding many reliability issues. (Fig. 1) While the side leads of QFN
are exposed, they are easy to oxidize or delaminate because of environmental
factors. (Fig. 2)
2.2 Process Comparisons
2.2.1 Die Bonding
There are three kinds of bonding technologies: epoxy, soft solder and eutectic.
All the technologies can be used in FBP package, while QFN can only choose
epoxy due to the tape on the back of the lead frame. The temperature of soft solder
or eutectic is too high for the tape’s temperature rating. Compared with QFN, FBP
can also be applied to back-metaled chips and lots of power devices.
2.2.2 Wire Bonding
FBP substrate can provides stable stands for wire bonding, similar to the
traditional operation and easy to control. The bonding joints, especially the second
bonds, will not be soft or broken.
The lead frame of QFN is through etched and the tape on its back will be
softened in high temperature Wire Bonding process. The bonding stands will be
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shaken but stable, resulting in the bonding joints to be soft or broken, and this is
one of the main defects of QFN.
2.2.3 Molding
The copper substrate of FBP can avoid resin bleeding completely no matter
how strong the transfer pressure is. Furthermore, the transfer pressure is higher,
the filling density of the compound is higher, and the adhesion between the
compound and the substrate is better, which helps to improve the reliability of the
products. (Fig. 7, Fig. 8)
On the contrary, the resin-bleeding issue usually happens in QFN packages,
which reduces the package yield, affects the reliability of the products, and causes
a lot of reworking. In molding, when the transfer pressure applied increases, it is
easier to cause resin bleeding or the bleeding area would be larger. Considering
the factor, the transfer pressure in QFN package is not very high and the
compound with low filling density is easy to absorb moisture. Consequently, at the
molding station, FBP has the advantages of higher assembly yield, better reliability
and no reworking.
Fig. 6 QFN Resin-bleeding Area
Fig. 7 FBP SAT before Precondition
Fig. 8 FBP SAT of IR Reflow 260℃ after Precondition
2.2.4 Package Sawing
Micro-cracking and copper burr usually happen in Package Sawing process.
FBP provides a solution to minimize this defect since only the molding material,
with low tensile stress, is being sawn, instead of through multiple materials.
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In QFN package, the molding compound and copper are sawn at one time,
which would cause such issues as copper burr, micro-cracking or delamination.
(Fig. 2)
In brief, FBP provides the perfect solutions on all the defects of QFN. The
thermal/electrical performance is better, the interference is lower, the reliability is
better, the assembly yield is higher, and the joint strength with PCB is stronger. All
these ensure that FBP can replace QFN packages on the basis of pin to pin. In
addition, FBP can increase its function leads by multi rows of outer leads. As for
QFN, it is hard to reach, especially under the condition of high lead-count and fine
pitch.
With all these advantages, a new nomenclature for FBP structure with multi
rows leads can be Heat Bump Grid Array or HBGA for short. HBGA is similar to
BGA and CSP from the outside seeing, but they are quite different from the
structure and process.
3. The Comparison between HBGA/FBP and BGA
HBGA: Heat Bump Grid Array
Fig. 9
BGA: Ball Grid Array
Fig. 10
3.1 Substrate
HBGA uses copper substrate, whose production cost is low and the thermal
performance is excellent.
BGA uses FR-4 plastic substrate, whose production cost is about three times
higher than that of the copper substrate of HBGA.
3.2 Reliability
In HBGA package, the copper with high purity has a good adhesion with
related materials, and the products can pass MSL-3 at least, even under the
condition of 260℃ IR reflow.
In BGA package, the plastic substrate has a worse adhesion with related
materials, and the products can pass MSL-3 at most under the condition of 240℃
IR reflow.
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3.3 Thermal Performance
In addition to the copper material in high purity, HBGA dissipates heat not only
from the die pad to PCB but also from the gaps between every two bumps, so the
thermal performance of HBGA is excellent.
As to the plastic substrate of BGA package, the heat is transferred layer by
layer, and too much heat focuses in the substrate, which would cause substrate
delamination and EMI issues. As for the die, its life and performance will be
affected.
3.4 Coplanarity and the Joint Strength with PCB
In HBGA package, the special structure and process can ensure the
coplanarity completely, since the bottom of the outer leads is also the one of the
original copper material. Furthermore, the bump leads are easy to joint with PCB
firmly.
In BGA package, the process is much harder to control. Especially, the outer
solder balls usually happen to be with different size, missing or oxidized. The bad
coplanarity will affect the surface mounting process, the joint strength, and the
electrical performance.
Compared with BGA, HGBA is of shorter and easier process, lower production
cost, higher thermal performance and more stable reliability. Compared with CSP,
HBGA still has those advantages. In addition, HBGA can own multi-die-pads and
do multi-die or stack-die package. (Fig. 11 & Fig. 12)
Single Package:
Fig. 11 Multi Chip Pads; Multi Chips
Fig. 12 Single Chip Pad; Stack Chips
3. FBP Reliability Test
Moisture Sensitivity: MSL-Ⅲ
PCT, 121℃/100%RH/2atm: 96/168/336hours
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Temp. Cycle, -65℃~150℃: 100/500/1000cycles
HAST, 130℃/85%: 100hours
HTSL, 150℃: 168/500/1000hours
Thermal Shock, -55℃~125℃: 100/500/1000cycles
Vibration Test: 20~2000 hz/20g
Drop Test: 30kg/1000mm/2times
FBP products have passed all the tests above.
4. Conclusions
With all the data and experimental results, FBP/HBGA can show your
products to the package field with high assembly yield, no resin bleeding, and no
reworking. FBP can replace QFN, MCM, BGA, CSP, SiP, providing you economical
solutions. JCET can also design special packages to meet your requirements.
References
1. Rao R. Tummala, “Microelectronics Packaging Handbook (Second Edition)”,
Publishing House of Electronics Industry, 2001, Beijing, pp100-036.
2. Guo Jialong, “Semiconductor Packaging Engineering”, ISBN 957-21-2704-7
Biography
Jerry Liang serves as CTO of JCET (Jiangsu Changjiang Electronics Technology
Corp., Ltd.) at present, in charge of investment planning, product development,
process engineering, equipment maintenance, etc.
Vivian Tao, born in 1981, graduating from Nantong University in 2004, serves as
engineer of JCET (Jiangsu Changjiang Electronics Technology Corp., Ltd.) at
present, engaged in research and design of semiconductor packaging technology.
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