bga fea underfll study
TRANSCRIPT
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 .