bga fea underfll study

Don Blanchet 3B Associates 03 April 2016

¨  There are literally thousands of studies that have been performed to try to predict the fatigue life of solder ball attachments on BGA component packages.

¨  Many have been published. ¨  Presented here is a simplified first order

approach.

2

¨  Thermal cycle definition single cycle 20C –> 100C -> -45C –> 20C

¡  No pwb glass transition effects

¨  ¼ symmetry used to limit model size

¨  Linear solution ¡  Constant material properties over temperature

¨  Ball processing collapse included in the models

¨  Failure Criteria used : Cumulative Strain energy density – @ crack initiation

¡  From publish test data sources ¡  Solders SN63 and lead free SAC305

¨  All under fill materials bulk modulus of elasticity, Poisson’s ratio = 0.4 determined by

similarity to other organic materials.

Typical crack initiation

Expansion direction X

Expansion direction Y

Fixed center point

MLB , 1.5 x 1.5 x .100

Very large high CTE Ceramic BGA

MLB Allowed

bend

Solder ball field

Solder Ball

detail

Under fill With fillet

High CTE ceramic component

High mesh density added

In highly stressed Corner balls

part

Body Material

Designated Under fill

Under fill Thickness, mm

BGA

High CTE

ceramic

Ablefill UF8807

0.40

BGA

High CTE

ceramic

Loctite 3568

0.40

BGA

High CTE

ceramic

Mereco 1650

0.40

BGA

High CTE

ceramic

No Under fill

Ball height 0.40

Material Property

Pwb FR-4 30% cu

Eutectic solder SN63

SAC 305

Lead free

solder

(flexible)

(flexible)

(rigid)

BGA Body

ceramic

BGA Body

Plastic BT

epoxy

BGA Body Flip chip

Silicon

CTE

ppm/ C

14

24.7

23.5

250

40

28

6.7

13

3

Elastic

Modulus E, psi

3.5e0

6

2.03e06

2.50e06

250

200

1.5e06

32.0e06

3.5e06

16.3e06

Poisson’s

ratio

0.3

0.4

0.4

0.4

0.4

0.4

0.22

0.3

0.28

Under fill Materials

Board bending ~0.003 inches shown at 50X BGA & under fill removed for clarity

Expansion shown At 75X

CTE Stretch Induces solder

tensile stress

CTE mismatch Induces solder

shear stress

Ref Undistorted

ball

Likely point Of crack

formation

Joule/cu-meter

As strain energy exposure accumulates then cracks can form.

part

Body

Material

Designated Under fill material

BGA

High CTE

ceramic

Ablefill UF8807

300

2000

BGA

High CTE

ceramic

Loctite

3568

125

700

BGA

High CTE

ceramic

Mereco

1650

180

1200

BGA

High CTE

ceramic

No

underfill

200

1400

¨  For the specified temperature range per cycle the predicted solder ball fatigue life ranges from: ¡  125 to 300 cycles for SN63 solder ¡  700 to 2000 cycles for SAC 305 solder

¨  The predictions are based on published test data for

measured cumulative strain energy density in similar BGA’s.

¨  FYI : Published data for vibration testing indicate

that eutectic solder exhibits a longer life for BGAs. The opposite of thermal cycling.

¨  Schematic diagram of SN63 solder stress/strain behavior derived from Solidworks FEA simulation.

¨  Stress strain behavior vs. temperature and strain

rate ¨  Hysteresis data from references

20C

100C

-45C

Yield point

Yield point

Area is proportional to strain energy

Per thermal cycle

Take away Faster rate stiffer

Higher temp softer Much Like taffy candy

Note: the simulation performed was a non-linear visco-plastic model including creep .