Challenges With Package
on Package (PoP)
Technology
Greg Caswell
Sr. Member of the Technical Staff
CTEA meeting
21 February 2012
© 2004 – 2010
Agenda
• PoP Background
• Configurations and
Examples
• PoP compared to SiP
• Assembly
• Warpage Issues
• Drop Testing Impact
• Thermal Cycles
• Reliability
• Underfill
• Root Cause
• Next Generation PoP
• Through Mold Via
Benefits of PoP
• The benefits of PoP are well known. They include
• Less board real estate
• Better performance (shorter communication paths
between the micro and memory)
• Lower junction temperatures (at least compared to
stacked die)
• Greater control over the supply chain (opportunity to
upgrade memory and multiple vendors)
• Easier to debug and perform F/A (again, compared to
stacked die or multi-chip module or system in package)
• Ownership is clearly defined: Bottom package is the
logic manufacturer, the top package is the memory
manufacturer, and the two connections (at least for onepass) are the OEM
Package on Package (PoP)
• A configuration where two packaged integrated
circuits are placed directly on top of each other
o Can also be known as stacked packages
• Interconnects are between the top package and
the bottom package and the bottom and the PCB
o Top package traditionally contains multiple or
stacked die
o Bottom package traditionally contains smaller /
thinner die
4
PoP (Stacked BGAs)
• Bottom Package
o Has land pads on top perimeter
to allow for top PoP attach
o Molded using special process to
keep perimeter clear
o Requires thin die and mold cap to
allow for top package clearance
• Top Package
o Based on conventional stacked die BGA but larger ball size
and thinner mold body
o Ball pitch and size constrained by need to clear bottom
package
• Packages must be capable of being placed on the printed
circuit board (PCB) and reflowed simultaneously to each
other and to the board
5
PoP Stacked BGAs (cont.)
• Both packages are relatively thin
o Maximum height typically 1.4 to 1.6 mm
o Focus tends to be on slimming top package
• Thinning of bottom package can be difficult
o Thinner substrate can increase warpage
o Smaller ball size can impact drop testing and temp
cycle
• Standard package sizes
o 15x15 mm, with 14x14 and 12x12 also available
o 0.65mm pitch, with 0.5mm and 0.4mm available
o Ball size can vary from 0.45 to 0.35mm
6
PoP Examples
•
Stacked Package on Package (PoP): The placement is often
arranged through a soldering operation, but can also be performed
with other interconnect technology
Example of
package on
package device
from Samsung
Example of package
on package devices,
with stacked die in
each package, from
Mitsubishi
7
PoP Examples (cont.)
Texas Instruments
8
Why PoP?
• Yield / Flexibility / Ownership
• No issues with known good die
(KGD)
• Memory can be easily
upgraded
o Also allows for multiple
sourcing
• Ownership is clearly defined
o Bottom package: Logic
manuf.
o Top package: Memory
manuf.
o Board level connection: OEM
9
Thermal Comparison
10
PoP Uses
• Dominant use
o Integration of digital logic device in bottom
package with combination memory devices (i.e.
DRAM and flash) in top package
o Top package typically stacked die
• Some pure memory PoP solutions also available
• Cameras / mobile devices are main users
o Increasing interest from high rel industries
11
PoP Assembly Process
• Assembly of PoP can be
through one or two reflows
o Most commonly single
reflow (aka, one-pass)
• Top package is typically
dipped before placement
o Flux (sticky) or solder paste
12
PoP Assembly (cont.)
• PoP can also be offered
as a two-pass assembly
o IDM assembles top and
bottom package and places
them in a carrier for boardlevel assembly
• Other assembly options include use of solder on
pad (SoP) on bottom package
13
Solder on Pad (SoP)
• Consists of solder balls
on the topside of the
bottom package
• Designed to induce a
larger solder joint collapse
to absorb package warpage
• Difficulties
o Balls must be well aligned
(limited self-alignment)
o Top package can slide off the balls
during placement or reflow,
leading to a poor solder joint or bridging
14
Design Factors Impacting Warpage
• Mold
• Die
– Die size
– Die Thickness
• Die attach
– Material property
– Thickness
© 2011 Amkor Technology, Inc.
Amkor restricted release to SMTA
– Material property
– Shrinkage
– Thickness
• Laminate Substrate
– Properties
– Thickness
– Cu ratio
– Routing
June 2011, SMTA LSMIT
Warpage
• Many technical challenges present in PoP assembly
o Improper reflow profiles can lead to solder balls dislodging
or migrating off the pad
o Excessive warpage can lead to solder ball bridging, solder
slumping, head and pillow defects, or open joints
• Number one challenge in assembly is controlling and
matching warpage of top and bottom packages
o More than 90% of the defects in PoP assembly are due to
package warpage (cit. KIC)
• Minimizing warpage is a trade off between materials,
temperature control and time
o The extent and degree of warpage is increasing as
substrates become thinner
16
Package Warpage
• Due to mismatch in CTE
between the substrate, mold
compound and die
o Die attach can also play
a role
• High Tg mold compounds
are used to balance CTE
mismatch between die and
substrate
• Effect of mold compound
becomes negligible at reflow
temperatures
17
Warpage (cont.)
•
General warpage trend at room temp.
o Inconclusive
•
Some claim bottom is smiling (positive,
concave) while top is crying
(negative, convex)
o Others claim the reverse
•
Partially dependent if CTE of mold
compound is more / less than
substrate
•
Example: Periphery of bottom
package is devoid of mold compound
o At reflow temperature, exposed
substrate could expand more
compared to substrate under the
mold compound
•
Desirable to have matching warpage
18
Warpage and Yields
19
Warpage Drivers: Die
• Thinner die and smaller die tend to minimize
warpage
o Larger / thicker die tend to drive crying at RT
20
Warpage and Reflow
Profile
Ramkumar, 2008 European Electronic Assembly Reliability Summit
21
Reliability: Drop Testing / Warpage
• Each board was dropped
200 times per JEDEC JESD11B22
o 1500g for 0.5ms
• The bottom package was
always first to fail
o Inline with other studies
• No significant differences in
top package reliability
o Reliability seemed to be
independent of yields
and warpage
22
Reliability: Drop Testing / Warpage
Test vehicle was a mechanical dummy of a cell phone
The drop-test was 3 cycles on six sides = 18 drops from 1.5m
Process Development and Reliability Evaluation for Inline Package-on-Package (PoP) Assembly (Flextronics)
23
Drop Testing / Warpage
(cont.)

Four different failure modes
observed during drop testing


Failure mode 4 was only found
on combination B
Combination B


Low yield with ENIG surface
finish
Poor warpage alignment
24
Underfill
• Typically a filled epoxy
o High modulus (>10 GPa)
o Range of coefficient of thermal expansion (CTE)
values (16ppm – 30ppm)
• Improves drop test performance
o Reduces stress on interconnect due to substrate
bending
• Improves thermal cycling robustness
o Reduce shear stress on solder
o Links die and substrate to reduce thermal
expansion mismatch
25
Underfill Design
Considerations
• Design Considerations for Package on Package
Underfill
• In PoP, the top and bottom packages are usually
the same size.
• Both levels must be underfilled for good reliability.
They also must be filled simultaneously.
• The top layer underfills more slowly than the bottom
layer because of the thermal delta between the top
and bottom levels.
• In order to underfill both levels simultaneously, the
fluid must reach the top of the second level gap.
26
Reliability: Underfill and Thermal Cycling
• Temp cycle
27
Underfill and Thermal Cycling (cont.)
28
Underfill and Temperature Cycling
• Rapid time to failure for underfills D / F / G
• Best reliability
o No underfill or underfill with Tg > 110C (A and C)
29
Reliability
• Underfill is increasingly being considered for
PoP
o Improves 2nd level reliability under drop testing
• However, increasing indications that use of
underfill may greatly reduce reliability under
temperature cycling
• Case Study (-40 to 125C)
o With underfill: 300 cycles
o Without underfill: >1000 cycles
30
Warpage Resolution
• High Density PoP (Packageon-Package) and Package
Stacking Development
• Ways around package
warpage
o Solder on pad (SOP)
• While previous PoP BLR
investigations showed a
tendency to failure at the
bottom joints we see that the
finer pitch resulted in
numerous failures on the top
joints early in the testing in
leg 3. For this reason a better
composition of top package
ball and bottom package
SOP was selected in leg4
which improved the BLR
reliability
31
Device
Dynamics
Packaging System Dynamics
Challenges
Next Gen PoP: Increased - Integration,
Miniaturization, Performance & Collaboration
Signal processing
µP integration Bband + applications - increased pin counts
µP core speed 2 – 3X w/ each node (1GHz @ 45nm)
Transition to FC accelerates from 65nm
Memory Interface
Higher speed memory interface SDRAM – DDR –> LP DDR2
Wider memory bus 16 – 32
Shared to split bus to (2 channel) architectures
Increased pin counts with size reduction requires 0.4mm pitch top and bottom
Warpage control with thinner / higher density PoP stacks
Signal integrity optimization, decoupling cap integration
Power efficiency and thermal mngmt
Si / pkg co-design for PoP to optimize for cost / performance
Processor I/O
CMOS Node
Peak Power
Ave. Die Size
400
65nm
400mW
64mm²
2008
© 2011 Amkor Technology, Inc.
Amkor restricted release to SMTA
600
45nm
800mW
50mm²
2010
800
28nm
1.2 W
50mm²
2012
June 2011, SMTA LSMIT
Thru Mold Via Technology (TMV®)
• Enabling technology for next generation PoP reqmts
– Improves warpage control and PoP thickness reduction
– TMV removes bottlenecks for fine pitch memory interfaces
– Increases die to package size ratio (30%)
– Improves fine pitch board level reliability
– Supports Wirebond, FC, stacked die and passive integration
© 2011 Amkor Technology, Inc.
Amkor restricted release to SMTA
June 2011, SMTA LSMIT
Construction and package stack-up for the TMV PoP
Test Vehicle reported at SMTAI 2008
Reference : "Surface Mount Assembly and Board Level Reliability for High
Density PoP (Package on Package) Utilizing Through Mold Via
Interconnect Technology - Joint Amkor and Sony Ericsson", Paper
© 2011 Amkor Technology, Inc.
Amkor restricted release to SMTA
June 2011, SMTA LSMIT
Viking RAMStack
© 2011 Amkor Technology, Inc.
Amkor restricted release to SMTA
June 2011, SMTA LSMIT
Summary
•
390million PoP components shipped in 2010 up from < 5
million in 2005. Forecasted to grow at same high rate as
Smartphones
• DDR2 2 channel and other new memory architectures
driving higher density PoP memory interfaces
• Amkor pioneered 1st Generation PoP (PSvfBGA) and now
leading in Next Gen high density PoP with TMV®
technology shipping in HVM
• One pass SMT PoP stacking enables optimization of supply
/ logistics and lowest total cost of ownership
• Amkor and Universal Instruments planning 14mm 620 / 200
TMV PoP SMT stacking study and industry report to
facilitate SMT yield / quality optimization
© 2011 Amkor Technology, Inc.
Amkor restricted release to SMTA
June 2011, SMTA LSMIT