characterization of mixed alloy sac-bisn bga solder...

Characterization of Mixed Alloy

SAC-BiSn BGA Solder Joints

Part II of Report on iNEMI Project on Process Development

of BiSn-Based Low Temperature Solder Pastes

Raiyo Aspandiar

Intel Corporation

[email protected]

1

Co-authors

2

Name Company

Haley Fu iNEMI, China

Jimmy Chen Flex Corp, Zhuhai, China

Shunfeng Cheng Intel Corp , Oregon, USA

Qin Chen Eunow, Suzhou, China

Richard Coyle Nokia, New Jersey, USA

Sophia Feng Celestica, Dongguan, China

Mark KrmpotichMicrosoft Corp, Washington,

USA

Bill Hardin Intel Corp, Oregon, USA

Scott Mokler Intel Corp, Oregon, USA

Name Company

Jagadeesh

RadhakrishnanIntel Corp, California, USA

Morgana RibasAlpha Assembly Systems,

India

Brook Sandy-Smith Indium Corp, New York, USA

Kok Kwan Tang Intel Corp, Kulim, Malaysia

Greg WuWistron Corporation, Hsinchu,

Taiwan

Anny ZhangIndium Corp, Washington,

USA

Wilson Zhen Lenovo, Shenzhen, China

Members of iNEMI’s

Low Temperature Solder Process and Reliability (LTSPR) Project Team

Outline

Motivation for Low Temperature Solders

Mixed SAC-BiSn BGA Solder Joints

iNEMI Low Temperature Solder Process and Reliability

Project: Phases / Timeline / Deliverables

Characterization of Mixed Alloy (SAC-BiSn) BGA

Solder Joints

Summary

Q & A

3

Motivation for

Low Temperature Solder (LTS) Reflow

Reduced Electricity Saves >

$8,500/oven/year

Reduced Emissions saves

57 metric tons of CO2 per

oven/year

Faster Technology Scaling Energy & Emissions Process & Materials

Wave Solder Elimination

Solder Material Cost Reduction

0

0.5

1

1.5

2012 2014 2016 2018 2020

SKL-Y

20x16.5x0.91mm

I/O Density

Pkg X-Y

…

5

▪ Motivation spans multiple areas

LTS Enables System Manufacturing to

Keep Pace with Moore’s Law

4

Low Temperature Solders

5

Medium Temperature Solders

[SnAgCu+Bi,In]• melt in the 210 to 220C range

Low Temperature Solders

[Bi/Sn/X, X=Ag,Cu,Ni]• melt in the 139 to 175C range

➢There are a variety of compositions

and melting ranges for Potential Low

Temperature Solders in Electronics

Manufacturing

▪ Bi-Sn system solders selected for LTSPR Project

o Significantly larger processing and economic benefits than Medium

Temperature Solders 5

Outline






Solder Joints

Summary and Next Steps

Q & A

6


7

▪ Dual Microstructure Regions

▪ SAC sphere does not melt during reflow soldering process since Peak

Reflow temperatures are below 220 C

Bi-Sn Solder Paste Reflow Profile

Formation of Mixed SAC -Bi-Sn

BGA Solder Joints

8

BGA Package Substrate

Printed Circuit Board

SAC Sphere

BiSn LTS Paste

Un-melted

SAC Region

Bi mixed

region

Before Reflow Soldering

After Reflow Soldering

o Brittleness of BiSn alloy

SAC

region

Bi-mixed

region

Fracture

Through / above

IMC

After

Mechanical Shock or DropBi causes joint

hardening and is

prone to brittle

fractures under

mechanical shock and

drop forces

Major issue with BiSn based Solders

➢ Bismuth is inherently more brittle than Tin

Bi region of Mixed BGA solder joint

Bi region of solder joint

PCB Land

crack

Cracking in the solder and along the solder/IMC interface

crack

9

BGA Solder Joint Example

Paths to Reduce BiSn Solder Embrittlement

✓ Resin added in the solder paste cures during

reflow soldering process

✓ Such resin containing pastes are called Joint

Reinforcement Pastes (JRP)

At Package level

Cured

Resin

At Solder Joint Level

✓ Resin Applied around the corners of Package and

cured either during or post reflow soldering

Resin Reinforcement

Corner Glue

• Various alternative strategies chosen by solder paste suppliers to modify the solder metallurgy for reduction in brittleness of mixed SAC-BiSnsolder joints

Ductile Bi-Sn Metallurgy

Ductile

BiSn

based

Region

➢ Both Paths considered for INEMI LTSPR Project 10

Outline






Solder Joints


Q & A

11

iNEMI LTSPR Project Participants

12

Binghamton University

▪ 22 Participants

▪ Mix of

o EMS/ODMs

o OEMs

o Suppliers

o Universities

iNEMI LTSPR Project Phases and Timeline

13

2015 2016 2017 2018

Team Formation &

SOW Ratification

Materials Selection

and Process DevelopmentMechanical Shock

/Drop Evaluation

2019

Temp Cycling / Bend /

Other Reliability Evaluation

Product Mfg

Validation

Phases

Tim

elin

e

Solder Pastes EvaluatedCode

NamePaste Category

Board

Assembly Site

Liquidus

Temp, C

Raja Kunyit SAC 1,2 219.6

Balik Pulau

Bi-Sn Baseline

1 142.8

Chee Chee 2 139.0

Teka 3 139.0

Kan You

Ductile Bi-Sn

3 174.0

Black Thorn 2 191.4

Red Prawn 1 142.2

Red Flesh 2 179.0

Sultan 2 151.1

Horlor

JRP Resin

Bi-Sn Based

1 139.0

Golden Pillow 3 141.0

Beserah 1 139.0

Chanee 1 140.0

Distribution with Four

Categories

▪ 5 Ductile Bi-Sn

Metallurgy pastes

▪ 4 Resin Reinforced Bi-

Sn pastes

▪ 3 Bi-Sn baseline

pastes (0%, 0.4%,

1%Ag)

▪ 1 SAC paste to serve

as current technology

baseline

14

Deliverables for

Material Selection and Process Development Phase

Solder Paste Materials Selection

Design and Procurement of Component and

Board Test Vehicles

Process Evaluations

Printability

Reflow Profiles

Solder Joint Defects

Rework

SIR

Characterization of Mixed SAC-BiSn Solder

Joints15

Presented at

2017 SMTA

International

Conference

Presented

Today

Outline






Solder Joints


Q & A

16

Characterization of

Mixed Alloy (SAC-BiSn) BGA Solder Joints

Stand-off Height

Bi Mixing %

Resin Encapsulation (for resin based pastes)

Voids Content

17

Test Vehicle for Process Development-- Solder Paste Development Board (SPDB) --

18

FCBGA

Socket R4 (BiSnAg Spheres) Socket R4

(SAC Spheres)

Designation Description

PCB6” x 7”x 0.040”, 8 layers,

OSP surface finish

FC BGA

16.5 x 20mm, 0.4mm nominal

pitch, 1505 SAC1205+Ni solder

spheres of 229 mm diameter

Socket R4

(LTS)

52.6 x 44.12mm, 2066 pins,

Bi-Sn-Ag solder spheres

Socket R4

(SAC)

52.6 x 44.12mm, 2066 pins,

SAC305 solder spheres

Cross-section Locations

19

AH

BP

▪ Direct Measurement of Length adjusted by the scale factor of the Image

Solder Joint Stand-off Height Measurement

20

Lowest point on

socket paddle

FC BGA

Stand –off

HeightStand –off

Height

Socket R4

Stand-off Height for all 3 Components

21

FC BGA SKT R4 (LTS) SKT R4 (SAC) ▪ Mean Stand-

off Height of

each

component is

different

▪ Pre-SMT

Solder

Sphere

Height is one

reason

▪ Partial

Collapse of

SAC solder

spheres

during reflow

is the other

Stand-off Height for FC BGA Component

22

Paste TypeAverage % Ball

Collapse

SAC 37.7

BiSn Baseline 32.5

Ductile BiSn 28.3

JRP (Resin) 19.7

➢ Full SAC joints

have lower stand-

off heights then

Mixed SAC-BiSn

joints

➢ This indicates

partial collapse of

Mixed SAC-BiSn

solder joints

Effect of FCBGA Package Warpage on

Solder Joint Standoff Height

23

➢ Row BP has consistently larger

solder joint standoff heights than

row AH

➢ This is irrespective of the paste

type or reflow profile used for

soldering

Effect of FCBGA Package Warpage on

Solder Joint Standoff Height

24

Row BP Cross-section

Pkg substrate

PCB

Row BP Row AH

Row AH Cross-section

▪ Package Warpage Characteristics plays a

larger role in determining FCBGA solder

joint stand off heights than solder paste-

solder ball combinations or reflow profiles

➢ Solder Joints appear `squished`

➢ Solder Joints appear `elongated`

dieSubstrate

Solder joints

Stiffener

die

StiffenerSubstrate

Solder joints

Post Reflow Standoff Height Comparison between

Socket R4 with different Solder Spheres

25

Pre-SMT Solder Ball Height

Partial Collapse

Full Collapse150 mm

▪ Additional collapse of 150

microns for solder joints

for Socket R4 with

BiSnAg Spheres

▪ This is due to lower

melting temp of the

BiSnAg solder spheres

during the low

temperature reflow

process

Socket with

BiSnAg Ball

Socket with

SAC Ball

Bi effect on Solder Joint Collapse

26

Reduction due

to presence of

Bismuth in the

solder joint

▪ The presence of Bismuth

in the molten solder

results in a 50 microns

additional collapse in the

Socket R4 solder joints at

SAC reflow temperatures

▪ This is due to the

reduction in the solder

surface tension caused by

presence of Bismuth

SAC BiSnAg Socket Sphere

SAC Paste

High Temperature (SAC) Reflow Profile

Full SAC SAC-BiSnSolder Joint

Microstructure

Sta

nd

-off

heig

ht,

mic

ron

s

Effect of Reflow Profile on

Standoff height for Socket R solder joints

27

Reduction due to

higher peak reflow

temperature

➢ Standoff Height of Solder

joints is lower for BiSnAg

ball Sockets reflow

soldered at higher peak

reflow temperatures (SAC

reflow profile) vs at lower

temperatures (BiSn reflow

profile)

➢ This is due to lower

surface tension of solder at

higher temperatures

leading to more joint

collapse

➢ But difference is only ~20

microns

XS Photos comparing Socket R Solder Joints

28

SAC Mixed SAC - BiSn BiSn

Reflow

Configuration

SAC 305 Ball + SAC305

Solder Paste

SAC 305 Ball + BiSn

Solder Paste

BiSnAg Ball + BiSn

Solder Paste

Reflow

Profile

High Temperature

(SAC)Low Temperature (BiSn)

Low Temperature

(BiSn)

28

Bismuth Mixing % Measurement

29

HJoint Height

HMiddle

Hupper-rightHupper-left

Bismuth Mixing % =

Bismuth Mixing% of All LTS Solder Pastes

30

➢ Mean Bi Mixing % values

are higher for the BGA

(65%), than for Socket

R4 (35%). o difference in the solder

sphere diameters of the

two components

➢ But, a wide range of

variation from 10 to

100% was observed. o This was unexpected

Factors causing the Wide Range in Bi Mixing %

Reflow Profile Parameters

Peak Reflow Temperature / Time Above Solder Liquidus /

Superheat above liquidus temperature during reflow soldering

Printability of Solder Pastes Process Parameters

Variation in Solder Paste Printed Volume

Particularly for low (<0.66) aspect ratio stencil aperture

openings

Metallurgy (Composition) Parameter

wt% Bi content in solder paste alloy

Solder Paste Chemistry

Flux activity difference affecting wetting time of paste to solder

ball once the paste melts during the reflow process

31➢ Further Investigation on these factors is ongoing

Micrographs Indicating Hot Tearing

32

Kan You Teka

Black ThornChee Chee

Impact of Bi Mixing % on Hot Tearing

33

Hot Tearing At

Package Land

Interface

Hot Tearing

Free Range

Hot Tearing At

PCB Land

Interface

Examples of Various Levels of Bi Mixing %

34

Bi Mixed

region

Bi Mixed

region

Bi Mixed

region

> 80% Bi Mixing Levels

20% to 80%

Bi Mixing Levels

< 20% Bi Mixing LevelBlack Thorn

Chee Chee Teka

Red Prawn Balik Pulau

Measurement Procedure for

Cured Resin % Height Around Solder Joint

35

Resin Height % on Solder Joints for JRP Materials

36

▪ Distribution of Resin

Heights around solder

joints were statistically

different for the four JRP

materials investigated

▪ Higher resin heights are

expected enhance

strength of the solder

joints under mechanical

shock

▪ But, a wide range in resin

heights for some JRP

materials

Examples of JRP Solder Joints

37

Horlor Golden Pillow

Beserah Chanee

38

➢JEDEC B111A Design

➢4 SoC BGA Components per board

Component Attributes

Voids Measured in Board

Level Solder Joints Only

Test Vehicles for Voids Content Measurements

Board Test Vehicle

Voids Measurement Procedure Location of the three slices location with respect to the microstructure of

mixed SAC-BiSn solder joints

39

Slice Near

Board Land

Slice at Mid

Solder Joint

Top Slice

Bottom Slice

Center Slice

Voids present in this

region are from

incoming package

solder spheres

Near transition region

between Bismuth

mixed and SAC

regions

Voids measured at this

interface are mostly

process voids generated

during reflow soldering for

board assembly

Slice Near

Package Land

Voids Content Data

40

• 6 boards per

solder paste

• Shape of data

point unique

for each board

Majority of data are

<=5% joint area level

A few data points are at a

higher % level (>>5%)

X-Ray Images of Large Voids in LTS Legs

41

Chee Chee Solder Paste

• Mid Solder Joint Location

• 100% Bi mixing in balls

Black Thorn Solder Paste

• Near PCB Land

• Hot Tearing observed

• Large Pasty Range (~64C)

Voids Distribution

42

-- Mean of Voids % vs Number of Solder Joint with Voids --

ALL Measured Legs

▪ Chee Chee Paste Solder Joints

with large voids are outlierso High Voids % but only 2

Number of Solder Joints with Voids

Me

an

of V

oid

s C

on

ten

t, %

Jo

int A

rea

Histograms of Voids Content in Solder Joints

43

ALL LTS Solder Paste Legs Only

▪ All three distributions skewed to the

lower end

▪ Near Package land slice has the

largest number of solder joints with

voids

▪ Mid Solder Joint slice has the least

number of solder joints

▪ Voids distribution near the PCB land

slice, which is directly associated

with the LTS solder paste process, is

typical of that seen for common SAC

solder pastes.

o This implies no unusual voids

generation for all LTS solder pastes

studiesVoids Content, % Solder Joint Area

Outline






Solder Joints


Q & A

44

Summary

Solder Joint Stand-off Height for FC BGA

component

o Mixed SAC-BiSn FCBGA solder joints had less

collapse (19.7 - 32.2%) during reflow than the full

SAC solder joints (37.7%)

o For FC BGA package, stand-off heights were directly

impacted by the package substrate warpage

characteristics

45

Summary (continued)

Solder Joint Stand-off Height for Socket R4

o Mixed SAC-BiSn solder joints had almost half the

amount of collapse (~22% vs 42.5%) during reflow

than the full SAC solder joints.

o Socket R4 components with BiSnAg spheres

exhibited a higher % collapse than the Socket R4

components with SAC spheres (46.8% vs 42.5%)

when soldered with the SAC paste under the higher

temperature reflow profile due to presence of Bi

46

Summary (continued)

Bismuth Mixing %

o Mean Bi Mixing % values were higher for the FC BGA

(65%), than for Socket R4 (35%), due to smaller

solder spheres of the FC BGA

o A wide variation in the Bi Mixing % (10 to 100)

observed across the various LTS solder pastes

evaluated for each component.

o Many parameters identified for this which require

further study

47

Summary (continued)

Resin Encapsulation of Solder Joints when

using resin containing solder pastes (JRP)

o Amount of encapsulation of the solder joint by the

resin as a % of solder joint height varied from 25% to

55%

o Mean of distribution of the data for each paste leg

was significant different from the others

48

Summary (continued)

Voids Content in Mixed SAC-BiSn Solder Joints

o Voids content in mixed SAC-BiSn solder joints formed

with the 11 LTS solder pastes studied were

predominantly below 5 % solder joint area, which is

the baseline for current SAC305 solder pastes.

o No unusual process voids generation observed in the

Bismuth mixed region close to the PCB Land-to-

solder interface

49

Next Steps - Current Project Activities Phase 2: Mechanical Shock Evaluation of Solder Joints

50

➢ PoP Component Test Vehicle

• JEDEC B111A board Design (4 units

/board)

• Shock (Drop) tests have been completed

on all 13 solder paste legs

• Data being analyzed

➢ FC BGA Component Test Vehicle

• Customized Shock test Board (STB) design

(1 unit per board)

• Mechanical Shock (Drop) Parameters being

Optimized to enable differentiation between

each solder paste leg

• Phase 3: Temp Cycling Evaluation of Solder Joints

➢ Component Types and board design under discussion

• BGAs, QFN, QFPs, THM (Pin-in-Paste) to be

included

Thank You!

51

characterization of mixed alloy sac-bisn bga solder...

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