The Effect of Mixing BGA and Solder Paste Alloys on the Formation of Voids

an Alent plc Company

The effect of mixing BGA

and solder paste alloys

on the formation of voids

September 2010

Alan Plant

Regional Applications Manager

HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company

Introduction • Reflow Profile is a known method of managing voiding in BGA’s.

– Alpha has recently uncovered the mechanism behind this and is actively formulating lower voiding pastes.

• Since 2006 there has been a trend for BGA and CSP package makers to supply components with lower silver alloys

• SAC 305 is still a primary lead free solder paste alloy – Along with SAC 405 and SAC 387

• Low silver SAC alloys are known to have better drop shock resistance

• This study also looks at voiding as a function of alloy composition


Introduction

• Reflow Profile is a known method of managing

voiding in BGA’s.

– Alpha has recently uncovered the mechanism behind

this and is actively formulating lower voiding pastes.

• This study also looks at voiding as a function of

alloy composition

• Total volume of the BGA/CSP Sphere and Paste

Deposit has a significant effect on Voiding

• Stencil Aperture design also can reduce voiding


BGA Voiding – Peak

Temperature Effect


Introduction

It is empirically observed that:

– For the same reflow profile

• Different flux formulations have different voiding performance

– For the same flux formulation

• Different profiles will yield different voiding performance

– A lower peak temperature profile will yield better voiding

results than a higher peak temperature profile


Differential Scanning Calorimetry

Differential scanning calorimetry was used to assess the

relative reaction rates of different activators with copper.

Note that there is a sharp exothermal reaction

between activator 1 and copper at 241.7º C.

Blue = Flux Weight%

Green = Sample Temp

vs. Control

Red = Sample Temp

Exothermal reaction =

Creates vapor which

contribute to voiding


Voiding Performance

Void Size Distribution

BGA256

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0-4% 4-9% 9-12% 12-16% 16-20% >20%

% of Joint area

% o

f Joi

nts

OM350 (Activator 2)HS 175C / 60 secsoak 240C peak

CVP-380 (Activator 1)HS 175C / 60 secsoak 240C peak

The expected trend is observed, as with a peak reflow temperature of

240ºC, the paste containing activator 2 has better voiding

performance than the paste containing activator 1


Voiding Performance

Also, as predicted, voiding performance improves measurably for

the same paste formulation reflowed with a lower peak temperature.


BGA256

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0-4% 4-9% 9-12% 12-16% 16-20% >20%

% of Joint area

% o

f J

oin

ts

CVP-380 240C peak temperature

CVP-380 230C peak temperature

Activator #1

Same Flux & alloy

Red = Peak 240ºC

Green = Peak 230ºC

Voiding

Red = Class II

Green = Class III


Another Example


BGA256 90%

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%

% of Joint area

% o

f J

oin

ts

Cerf St ramp 229Cp

Cerf 175-60 soak 245Cp

Cerf St ramp 1.5Cs 240Cp

Activator 2

Ramp vs. Soak profile

NOTE

Peak temperature has

a greater impact vs.

soak profiles


BGA Voiding – Alloy

Composition Effect


What effect do the package ball alloy and

the solder paste alloy have on solder joint

voids?


DOE Test Matrix

Reflow

Profile

Short

Soak

Long

Soak

Short

Soak

Long

Soak

Short

Soak

Long

Soak

SACX 0307

SAC 105

SAC 305

Sphere Alloy

Pa

ste

Allo

y

SACX 0307 SAC 105 SAC 305

Voiding Measured for all 18 Combinations

Paste Alloy / Profile / Sphere alloy


Voiding Test Procedure

• Test Method

– ALPHA PUT Procedure JC-PUT-0016

• Equipment- Phoenix Micromex-HLN

• Measure and report voiding area on 256 IO BGA Package


Cookson Electronics Test Vehicle

2 x BGA-256


Low Soak Profile

175C/60S Soak 240C Peak 60S TAL


High Soak Profile

160-180ºC/120S Soak 250ºC Peak 60S TAL


Results


SACX® 0307 Paste

Solder paste SACX-BGA256-20mil

0

50

100

150

200

250

300

0-1 1-3 3-5 5-7 >7Void size, %

Nu

mb

er o

f v

oid

s

SACX

SAC105

SAC305

SACX

SAC105

SAC305

Solid lines

•175C @ 60s Soak

•245C Peak

•60s TAL

Dotted lines

•160-180C @ 120s Soak

•250C Peak

•40s to 76s TAL


SAC 105 Paste

Solder paste SAC105-BGA256-20mil

0

50

100

150

200

250

300

0-1 1-3 3-5 5-7 >7Void size, %

Nu

mb

er o

f v

oid

s

SACXSAC105SAC305SACXSAC105SAC305

Solid lines

•175C @ 60s Soak

•245C Peak

•60s TAL

Dotted lines

•160-180C @ 120s Soak

•250C Peak

•40s to 76s TAL


SAC 305 Paste

Solder Paste SAC305-BGA256-20mil

0

50

100

150

200

250

300

0-1 1-3 3-5 5-7 >7Void size, %

Nu

mb

er o

f v

oid

s


Solid lines

•175C @ 60s Soak

•245C Peak

•60s TAL

Dotted lines

•160-180C @ 120s Soak

•250C Peak

•40s to 76s TAL


Data Analysis-Scoring Results

Solder paste SAC105-BGA256-20mil

0

50

100

150

200

250

300

0-1 1-3 3-5 5-7 >7Void size, %

Nu

mb

er o

f v

oid

s


10 5 1 0


Confirmation Run SACX 0807


BGA256

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%

% of Joint area

% o

f J

oin

ts

CVP-360 CERF ST.RAMP 0.7c_s

245cp 60 TAL

CVP-360 CERF ST.RAMP 1.5c_s

245cp 60 TAL

SAC 305 Spheres, SACX 0807

Paste- Class II Voiding


SACX 0807 Example Void Size Distribution - SACX0807 CVP-360 Paste,

SAC105 BGA 256 (20 mil Sphere)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%

% of Joint area

% o

f J

oin

ts

Ramp: 0.7 C/sec, 240 Peak,TAL - 85 s

Ramp: 1.5 C/sec, 245 Peak,TAL - 45 s

SAC 105 Spheres, SACX 0807 Paste-

Class III+


Print Deposit Effect on Voiding

• Reducing aperture size reduces voiding

• Smaller paste/sphere volumes = reduced

voiding


Reducing Aperture Size Reduces

Voiding

No Aperture Reduction


BGA256

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%

% of Joint area

% o

f J

oin

ts

438_167B19 CERF HS 160_60 SOAK 240CP

438_167B19 CERF HS 175_60 240CP

438_167B19 CERF ST.RAMP 1.5c_sec 245CP 60

TAL


Reducing Aperture Size Reduces

Voiding


BGA256 90%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%

% of Joint area

% o

f J

oin

ts

438_167B19 CERF HS 160_60 SOAK 240CP

438_167B19 CERF HS 175_60 240CP

438_167B19 CERF ST.RAMP 1.5c_sec 245CP 60 TAL

10% Aperture

Reduction


Reducing Paste/Sphere Volumes

Reduces Voiding Void % Distribution For BGA256 20Mil

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Zero <0-4 4-9 9-12 12-16 16-20 >20

% of Void Area

% o

f Voi

d Di

strib

utio

n

Void % Distribution For BGA256 15Mil

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Zero <0-4 4-9 9-12 12-16 16-20 >20

% of Void Area

% o

f Voi

d Di

strib

utio

n

20 Mil Circles (0.5 mm)


Same Solder Paste and Reflow Profile


Reducing Paste/Sphere Volumes

Reduces Voiding Void % Distribution For BGA256 15Mil

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Zero <0-4 4-9 9-12 12-16 16-20 >20

% of Void Area

% o

f Voi

d Di

strib

utio

n 15 Mil Circles (0.4 mm)

Same Solder Paste and Reflow Profile

Void % Distribution For BGA225 12Mil

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

Zero <0-4 4-9 9-12 12-16 16-20 >20

% of Void Area

% o

f Voi

d Di

strib

utio

n



Overall Conclusions

• Like alloys tend to produce fewer large voids than mixed alloy combinations

– Slight difference in melting point is one possible cause.

• Peak Reflow Temperature – Has a significant effect on voiding due to the interaction of the

flux with the Cu at key temperatures.

• Solder Paste Formulation – Has a significant effect on voiding due to the type of chemistries

used and where they become reactive with the Cu.

• Solder Joint Volume – Reduced volume of paste and/or sphere reduces voiding


For more information please

visit:

www.Alpha.Alent.com

The Effect of Mixing BGA and Solder Paste Alloys on the Formation of Voids

Technology

Transcript of The Effect of Mixing BGA and Solder Paste Alloys on the Formation of Voids