certain BGA underfill can cause thermal cycling failures flexible easily repaired material tend to have high Poissons ratio and act like incompressible fluids when restricted under the BGA

1. small Mosfet BGA Under fill
CTE induced stress
FEA Structural Evaluation
Don Blanchet
3B Associates
3 Jan 2005

2. • The FET package is a 3.1 x 3.1 mm Ball Grid Array with 36 solder
balls.
• The assumed process temperature excursion is +180C , through
the glass transition temperature of the under fill material.
• The solder is lead free , 96.5 Sn – 3.0 Ag – 0.5 Cu.
• The substrate is epoxy glass MLB construction.
• The under fill material is flexible and elastomeric for easy repair.
• The objective is to demonstrate the negative effect high
expansion under fills can have on the stress and fatigue failure of
small package BGA corner balls.
Assumptions & Objectives

3. BGA Package

4. Solidworks Model
pwb
BGA
Under fill
Shown
transparent

5. CTE dimensional change vs. temperature “X&Y”
17.0 ppm/C
10.0 ppm/C

6. CTE dimensional change vs. temperature “Z”
100.0 ppm/C
35.0 ppm/C
24.0 ppm/C
Glass transition 120 C

7. FEA mesh – for thermal expansion study
400,000 elements 550,000 nodes 1,200,000 degrees of freedom
Shown with
Under fill
Removed

8. FEA mesh detail views – for thermal expansion study
400,000 elements 550,000 nodes 1,200,000 degrees of freedom

9. Results – solder ball stress contours from FEA

10. Results – BGA solder deformation with under fill
(under fill not shown)
Corner balls are under the highest stress
primarily in tension

11. Results – under fill constrained acting like a fluid
Package is slightly concave up
Under fill bulges at edges , it is almost incompressible like a hydraulic fluid.

12. • The FET BGA’s with under fill are exposed to significantly more
solder ball stress than a design without under fill.
• The peak strain in the corner balls with under fill is approximately
10X the strain for the assembly without under fill.
• The trapped under fill acts much like a hydraulic fluid and exerts
high pressures as it expands.
• Variables:
– The primary variable is the mismatch between the under fill and
solder CTE’s in the z-axis.
• The under fill glass transition is one underlying cause
• The under fill incompressibility is another significant factor
– A secondary variable is the TCE mismatch between the BGA package
and the multilayer PWB.
Conclusions