Head-in-Pillow Review and Reduction - LA/OC...

SMTA LA/OC Chapter Meeting Presentation Feb. 15th, 2011Meeting Presentation

Head-in-Pillow Review and Reduction

Jasbir Bath, Christopher Associates, Inc. and Koki SolderEmail: Jasbir@christopherweb comEmail: [email protected]

Lead Free SOLUTIONS you can TRUST Challenging New Technologies

Agenda

• How Head in Pillow defects occur• Typical Examples of Head-in-PillowTypical Examples of Head in Pillow• Experimental Paste Development and tests used to

investigate the defect• Test Results on tin-lead and lead-free pastes• Other areas to consider for Head-in-Pillow

C l i• Conclusions• Future Work• Acknowledgements• Acknowledgements

CHALLENGING NEW TECHNOLOGIES

Copyright (C) 2009 Koki Company Limited. All Rights Reserved.

Introduction

There has been an increase inthe rate of Head-in-Pillow component soldering defectsp gwhich interrupts the merger of the BGA/CSP componentsolder spheres with the molten solder paste during reflow.

Reasons include:Reasons include:i) warpage issues of the component or boardii) ball co-planarity issues for BGA/CSP components iii) non-wetting of the component based on contamination or

Pillow defect

) g pexcessive oxidation of the component coating.

The issue occurs not only on lead-free soldered assemblies where the increased soldering temperatures may give rise to increase g p y gcomponent/board warpage but also on tin-lead soldered assemblies.



Typical Example Showing how the Head in Pillow Typical Example Showing how the Head in Pillow Component Soldering defect occurs Component Soldering defect occurs

1. BGA bump starts to be oxidized

2. Flux starts to be oxidized

1. Flux bleeds out.

2. Further oxidation of bumps.

3. Flux activation is

1. Flux activation is consumed as solder melts.

2. Oxide film on bump surface and flux layer with almost no activation



4

consumed as solder melts.

almost no activation retards merger between bump and solder.

3. TAL is also limited.

Causes of separation of BGA bump from pasteCauses of separation of BGA bump from paste

Package

Printer Chip placer ReflowBGA/CSP

Placer

• Warpage of substrate

• Inconsistent bump

• Inaccurate x-y placement• Insufficient down-stop

• Inadequate temperature profile

• Insufficient solder paste

• Inaccurate registrationpsize → Warpage of component,

PC board• Lifting of BGA bumps due to

wetting force

• Print inaccuracy



5

Head in Pillow Defect Detection6

• Difficult to detect in production during inspection and functional level testing• There is partial contact between the reflowed solder paste and the ball sphere

but no real metallurgical bond. g• There is electrical contact so not able to be picked up in production during test• Problem can occur in 1st pass assembly or during BGA/CSP rework

• Standard 2D X-ray inspection can not typically identify it • X-ray imaging of a board by tilting can identify the issue but it requires a skilled

operator and time on the machine: sampling could only occur in regular production

After assembly and shipment of product to the field, thermal or mechanical cycling can cause an open solder joint: field failures.



Paste Development to reduce Head-in-Pillow

Work done on a set of tin-lead and lead-free solder pastes developed with flux formulations having higher heat resistance and quicker wetting to reduce the head-in-pillow defectreduce the head in pillow defect.

Tests included:1. Adjustment of flux fluidity: Flux fluidity adjusted to have the flux

t t th ld f h t d i th h t tprotect the solder more from heat during the pre-heat stage.2. Heat resistivity improvement of flux: Activation level of the flux was

increased during pre-heat by improving its heat resistivity.3. SnPb solder paste tests with chip components

4. SnAgCu solder paste tests with BGA/CSP solder ball spheres

5. SnAgCu solder paste tests with BGA/CSP components6. Nitrogen versus Air Reflow test: Evaluate the use of air vs nitrogen

reflow atmosphere to investigate oxidization of BGA component.



p g p

7. In-situ reflow studies using reflow simulator equipment

Tests on SnPb paste with chip components

• An oxidized component was placed on previously reflowed and oxidized tin lead solder paste This combination was then reflowedoxidized tin-lead solder paste. This combination was then reflowed.

• Three solder pastes tested: Control SnPb Paste A, Control SnPb Paste B and Improved Head in Pillow SnPb Paste C.Paste B and Improved Head in Pillow SnPb Paste C.

• The method looked at wetting of the solder paste to the oxidized chip component:

Post reflow solder paste Chip component



Head in Pillow test with solder paste & ball spheres

Not Good Result (No (merger of paste and sphere)

Solder ball

Solder paste

Good resultPlace solder

PC board

Hot plate

Good result (Complete merger of paste with

Place solder sphere into molten solder paste

BGA sphere)



SnAgCu solder paste test with BGA components

• To verify results of previous evaluations tests were done with actual BGA/CSP component and solder paste on a test board.

• Solder paste was printed on the test board and reflowed.

• The BGA/CSP component was then placed on top of the• The BGA/CSP component was then placed on top of the reflowed solder on the board pads and reflowed again.

• Observed melting behavior using a reflow simulator withObserved melting behavior using a reflow simulator with further inspection of the soldered joints by peeling off the BGA/CSP component from the test board after reflow.



Reflow Simulator Equipment

• A reflow simulator can also be used to investigate tin-lead and lead-free paste wetting behaviour with components in-situlead free paste wetting behaviour with components in situ



Results: Adjustment of flux fluidity

• To reduce the oxidization of the solder paste the flux fluidity can be adjusted to have the flux protect the solder more from heat during the pre-heat stageduring the pre-heat stage.

Conventional paste (During Preheat)

Conventional paste (after melting)

Conventional fluxConventional flux

Head in Pillow paste(During Preheat)

Head in Pillow paste (after melting)

New flux



Results: Heat resistivity improvement of flux

• The flux was developed so that the activation level of the flux was increased during pre-heat by improving the heat g p y p gresistivity of flux.

Flux Heating at 300CConventional flux Head-in-Pillow flux



Flux Heating at 300C

Results: SnPb paste with oxidized chip components

• The wetting of the three different SnPb solder pastes (Pastes A,B,C) during reflow with components viewed with the reflow simulator are shown:

Paste A: High Viscosity Flux(Paste tended 210ºC 215ºC

to stay underneath chip). No good contact between solder paste and chip

Paste B: Better solder contact as lower

Paste A

Paste B: Better solder contact as lower viscosity flux but more flux flow out from the solder paste (less flux around the solder particles)

Paste B

)

Paste C: Low Viscosity flux, less flux flow from the solder paste and good flux activity retention during reflow: Good Paste C



Head in Pillow/ wetting result

Results: SnAgCu solder paste with solder spheres

• Typical test results for lead-free SnAgCu spheres with the developed lead-free head-in-pillow solder paste and a control

Heatingtime 30sec. after melting 50sec. 70sec.

lead-free paste are shown

Head in Pillow Paste D (Retains flux activity for

time

Paste D

Merged completely Merged completely Partially merged

( eta s u act ty o50 sec after solder paste is molten)

Merged completely Partially merged Pillow defect

Conventional Paste E (Retains flux activity for only 30 sec after solder paste is molten)

Paste E



paste is molten)

Results: Comparison of flux retention times for Lead-free SnAgCu pastes using BGA ball spheres

•



Confirmation runs with Head in Pillow Lead-freePaste D on test vehicle

HeadHead--in Pillow defect test vehiclein Pillow defect test vehicle• Material : Glass epoxy FR-4• Surface treatment : OSP (Regular)• Stencil thickness : 0.12mm [5 mils] (laser cut)• Pad size : 0.5mm diameter• Stencil aperture : 100% aperture opening to padStencil aperture : 100% aperture opening to pad• Component (BGA): SnAgCu, 1.0mm pitch, pre-conditioned at 180ºC for

100sec.• Heat source : Hot air convection• Atmosphere : Air• Reflow profile : 150-190C soak (110sec), 235C peak, 48 sec over 220C• Procedure: 1. Reflow solder paste without BGA

2. Place BGA on pre-reflowed solder. BGA2. Place BGA on pre reflowed solder.3. Reflow it.

BGA

8090

100

Pillow defect Insufficient collapse Complete merger

60.2%

203040506070

Rat

e (％

)

0.4% Incomplete collapsePillow defect

01020

S3X58-M406-3 Conventionalproduct

After peel-off

Complete mergerHead in Pillow Paste D



Newly developed flux formulation with higher heat resistance and quicker wetting reaction, drastically reduces the head-in-pillow defect.


After peel off

Results: SnAgCu paste soldered to oxidized BGA components on assembled boards

• Test boards used to confirm the ability of the developed lead-free Head-in-Pillow developed paste to solder to oxidized BGA components observed using the reflow simulatorcomponents observed using the reflow simulator

Initial 225ºC 235ºC Cross-Section Result

Lead-free SnAgCu Head-in-Pillow Paste D



Conventional SnAgCu Paste E

Results: BGA Joint Formation for Lead-free Paste D versus Conventional Paste E after Peel Off Tests

•Head in Pillow Paste D

Complete merging

Paste D

I l

Paste E

Incomplete

PCB Sid C t

Head-in-Pillow



PCB Side Component Side

Results: Dye pry test for different assembled lead-free solder pastes with BGA components



SnAgCu paste with BGA components soldered in air versus nitrogen reflow atmosphere

• Head in pillow performance improved with the use of Nitrogen atmosphere (1,000 ppm O2) presumably because the nitrogen h l d t t i d i fl ti ithelped to retain and improve flux activity

Joint FormationNG Head in PillowIncompleteJoint Formation IncompleteComplete Merging

Head in PillowComplete

Merging

Incomplete Merging

Merging



Nitrogen Air

Tin-lead Case Studies showing head-in-pillow (Smith: SMT Magazine, 2006[Kester])

22

• 2 product types: Automotive and Telecom• 0.8 to 1% defect rate

H d i Pill d b 6 t 7C d lt T t• Head-in-Pillow caused by a 6 to 7C delta T across a component for both products – the leading edge of the component was getting hotter quickerthe leading edge of the component was getting hotter quicker

than the trailing edge of the component with the paste melting earlier at the leading edgeThi d il i h d i fl– This caused a tilt in the component during reflow

– The tilt in the component caused head-in-pillow• Longer soak temperature by reflow profile adjustment reduced the• Longer soak temperature by reflow profile adjustment reduced the

delta T• Reduced delta T made the solder paste at the component leading



p p gand trailing edge melt at around the same time

• Reduced Head-in-Pillow defect to <0.1%

Tin-lead Case Studies showing head-in-pillow (Nowland et al.: IPC APEX 2010 conf.[Alcatel-Lucent])

23

Case Study 1• 0.8mm pitch 144 I/O FBGA (Rectangular part, square die)• 10% yield (90% defect rate)• 5 mils component warpage during tin-lead reflow• 2 to 4mils extra stencil aperture opening helped to increase yield

up to 80% (some solder bridging observed)Case Study 2Case Study 2• 0.8mm pitch 484 I/O BGA (19mm x19mm)• 80% yield (20% defect rate)y ( )• Maximum component warpage during the reflow process: 8 mils• Use of a Head-in-Pillow developed solder paste, stencil print



modifications, increased solder paste inspection and process monitoring reduced the defect.

Tin-lead Case Studies showing head-in-pillow (Su et. al.: SMTAI 2010 conf. [Cisco])

24

• 0.8mm pitch 144 I/O FBGA memory component• 16,000 DPMO (Defects Per Million Opportunities) due to warpage

at corners of component causing head-in-pillow defect.5 il t 220C k t t• 5mils warpage at 220C peak temperature

• Increasing stencil aperture openings in corner areas ofcomponent by 1 to 3 mils reduced DPMO rate down to 500component by 1 to 3 mils reduced DPMO rate down to 500.

• Paste change and reduced reflow soak time also can help to g preduce Head-in-Pillow



Case studies on Lead-free Head in Pillow (SMTAI 2009 conference: Vandervoort et. al [Intel])

25

Head-in-Pillow caused by BGA Socket warpage combined with PCB warpage as analyzed by Shadow Moire Analysis and Video Analysis

Large size of socket leads to large temperature gradients Temperature gradient across socket forces different melt times: Cooler inner

balls hold component shape longer - delays outer ball contact with solder paste which causes Head-in-Pillow

Time difference between the Last ball to melt and the first ball to solidify more important than TAL to understand actual window for ball/ paste wetting

Solutions Improve heat transfer by socket design features such as cap venting. Minimize

the component collapse delay and expand the process window for all joints Increasing paste amount at reflow reduces HnP defects (reduced gap) Increasing paste amount at reflow reduces HnP defects (reduced gap)• Pastes with more robust flux activator and high activity paste able to clean

oxides in a shorter space of time also reduce HnP B ll ll iti th b f ifi i it l t d th



Ball alloy composition: the absence of a specific impurity element and the presence of another created a combination that creates a low oxide solder ball which increases wettability and reduces head in pillow

Case studies on Lead-free Head in Pillow (SMTAI 2009 conference: Shimamura et. al [Senju])

26

• Head-in-Pillow increasing in move to lead-free:– increased tin content of lead-free solder- increased oxidation, – higher surface tension- less spreading/wetting, – more tears/wrinkles on solder surface- increased oxidation/corrosion. – Higher temps- increased oxidation– Less paste volume for higher density components: less flux to remove oxidation

Thi t d PCB– Thinner parts and PCBs – more warpage

Solutions Preheat zone optimization during reflow Use of nitrogen atmosphere versus air. Improved solder paste formulations with higher heat resistance and faster

wetting times and good wetting consistency.



Lead-free Case Studies showing head-in-pillow (Nowland et al.: IPC APEX Conf. 2010[Alcatel-Lucent)

27

• 473 I/O PBGA, 84I/O FBGA, 64 I/O PBGA components (all 0.8mm pitch)

• Reduced head-in-pillow defects by monitoring solder paste out of jar exposure time– monitoring solder paste out of jar exposure time

– increased stencil printer automatic paste inspection– manual inspection of all BGA paste printed pads– use of head-in-pillow developed solder paste– Use of lazer cut/ electroformed stencils

• Increased stencil apertures increased paste volume whichIncreased stencil apertures increased paste volume which helped to reduce head-in-pillow but increased solder bridging



Lead-free Case Studies showing head-in-pillow (Oliphant et al.: IPC APEX Conf. 2010[RIM])

28

• 10 % defect rate• 0.5mm pitch BGA (Sn1Ag0.5Cu spheres) with Sn3Ag0.5Cu p ( g p ) g

paste• Reduced Head-in-Pillow defect down to zero with:

f– Use of developed head-in-pillow paste– More balanced copper in PCB design (less board

warpage)warpage)– New BGA mold cap (less component warpage)– Nitrogen atmosphere (maintains flux activity in solder g p ( y

paste)– Component sphere flux dipping with tacky flux (more flux

t i t i fl ti it f tti d i fl )



to maintain flux activity for wetting during reflow)

Other areas to consider for Head in Pillow (Component Warpage)

One of the primary factors causing the head in pillow issue is warpage of the component and/or board.

• JEDEC JEP-95 standard refers to measurements of component coplanarity/flatness at room temperature. The maximum package warpage at room temperature is 3 to 8 mils (0.075 to 0.2 mm), dependent on the p ( ) pball pitch from 0.4mm to 1.27mm.

• Component coplanarity and flatness at room temperature could be different than at SMT reflow temperatures at which HIP defects occur.

• JEITA-ED-7306 and JEDEC JEP 95 SPP-24A standards are being developed to provide guidelines on the component coplanarity and flatness specifications during SMT reflowflatness specifications during SMT reflow .

• Some of the suggested changes include having a maximum package warpage during reflow of 3 to 6 mils (0.075 to 0.15mm) dependent on the ball pitch in the range of 0 4mm to 1 27mm



ball pitch in the range of 0.4mm to 1.27mm.

Other areas to consider for Head in Pillow (Board Flatness Standards))

• IPC standards on board land coplanarity at room and reflow temperatures also need to be considered and developed.

• Current IPC standards refer to max. board flatness of 7.5mils/inch• Current standards do not scale correctly with package size and I/O

count: e g 1mm pitch 1849 I/O component allowed maximumcount: e.g. 1mm pitch 1849 I/O component, allowed maximum coplanarity/warpage along board pad diagonal be too high: 18mils

• Currently work by INEMI to measure board land coplanarity at room and reflow temperatures:

R d t it i t IPC t d d– Recommend acceptance criteria to IPC standards– Provide and discuss information on board and component

coplanarity requirements to JEITA and JEDEC so common



coplanarity requirements to JEITA and JEDEC so common standards can be developed.

Other Factors affecting Component Warpage (SMTAI Conf. 2004, Vaccaro et. al [Agere Systems])

31

Vaccaro et al. showed that large component warpage could be caused by moisture in the part and the use of higher peak lead-free soldering temperatures.

So even though the component may be qualified to a certain Moisture Sensitivity Level (MSL) rating for lead-free soldering according to IPC/JEDEC J-STD-020, it still may have issues in production due to excessive component warpage.still may have issues in production due to excessive component warpage.

Component die size could help to reduce the warpage affect: E.g. For 37.5mm square part, an increase in die size from 5mm x 5 mm to 11 x 11mm could help E.g. For 37.5mm square part, an increase in die size from 5mm x 5 mm to 11 x 11mm could help

to reduce the component warpage due to the additional stiffening affect from larger die.



Other Factors affecting Component Warpage (SMTA Pan Pacific Conf. 2008, R. Lathrop [Heraeus]; SMTA Nepcon Shanghai Conf.

32

p [ ] p g2007, Lin et al. [Amkor Technology])

Lathrop indicated that one of the main factors influencing BGA coplanarity was component laminate warpage due to underfill and over molding operations on the package side.

Lin et al. conducted a study on PoP (Package on Package) components using the Shadow Moire technique to understand the influence of package warpage during lead free reflow assembly from 25°C to 260°C and then back down toduring lead-free reflow assembly from 25 C to 260 C and then back down to 25°C.

Various factors had an influence on warpage including: die size, mold compound thickness and mold compound CTE (coefficient of thermal expansion), substratethickness and mold compound CTE (coefficient of thermal expansion), substrate material, thickness and copper layer ratio These could then be optimized to reduce component package warpage.



If Head-in-Pillow Occurs what are the typical things to investigate:

• Warpage issue of the component or board • Ball coplanarity issue for BGA/CSP componentsBall coplanarity issue for BGA/CSP components• Non-wetting of component or board: contamination or

excessive oxidation of coating• Insufficient solder paste: stencil aperture and stencil area/

aspect ratiosR fl fil ti i ti• Reflow profile optimization

• Paste related issues (for tin-lead or lead-free paste)



Conclusions

• Developed tin-lead and lead-free solder pastes have been shown to p phelp to reduce the occurrence of the head-in-pillow defect by controlling flux fluidity and improved heat resistance of the flux used in the solder paste.p

• Test methods presented showed good test results on chip and BGA/CSP components as well as ball spheres in combination with

freflow simulation experiments.

• Development of solder pastes would only be a partial solution as there also needs to be progress in reducing warpage ofthere also needs to be progress in reducing warpage of components and boards during reflow by updating/developing relevant industry standards.



Future Work

N d l i i h H l f l d• New developments are occurring with Halogen-free lead-free head in pillow solder pastes to develop new halogen-free flux formulations with improved heat resistance and pfluxing ability.

• As already mentioned more work is needed on development of standards for the component and board to understand and reduce warpage during reflowunderstand and reduce warpage during reflow.



iNEMI New Project Initiative: Qualification Criteria for Warpage

36

1A. Qualification Criteria including Substrates, Package and Board Levels

Problem StatementProblem Statement• Current standard is not adequate to predict good yield results at package and

board level assembly• Measurement methods (dimensional and test) are not common• Measurement methods (dimensional and test) are not common

Need to:• Define the qualification method and criteria (sample size, preconditioning,

material and process variations [at package and board level])• Establish measurement methods

Output• Procedure and criteria reference for OEMs EMS and suppliers



• Procedure and criteria reference for OEMs, EMS and suppliers

iNEMI New Project Initiative: Primary Factors Affecting Warpage

1B. Primary Factors Affecting Warpage Problem statement• No clear understanding of key contributors for warpage at package/ board level Need to:• Identify key material properties (core, substrate, design etc (Cu trace/via,

package assembly) and key contributors (die size, thickness) which impact package and board level assembly for different applications.p g y pp

• Identify key process parameters (Underfill, etc), reflow profile, package pitch, PCB, environmental factors (shipping, storage, moisture) impacting warpage.

• Establish understanding to control the key contributors to warpageEstablish understanding to control the key contributors to warpageOutput• Develop a set of primary parameters and a working window to control warpage

in the supply chainin the supply chain• Recommendations/ guidelines for shipping and storageMore information at:



• http://www.inemi.org/cms/projects/OrgPkg/OrgPkgSubstrates_Dec09.html

Acknowledgements

• The support of the various persons at Koki Solder who pp phelped to develop the test methods and formulated the new fluxes for the tin-lead and lead-free head-in-pillow solder pastes describedpastes described.

• Information from development work from variousInformation from development work from various companies referred to in this presentation.



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