small Mosfet BGA Under fillCTE induced stress

FEA Structural Evaluation

Don Blanchet3B Associates3 Jan 2005

• 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

BGA Package

Solidworks Model

pwb

BGA

Under fillShown

transparent

CTE dimensional change vs. temperature “X&Y”

17.0 ppm/C

10.0 ppm/C

CTE dimensional change vs. temperature “Z”

100.0 ppm/C

35.0 ppm/C24.0 ppm/C

Glass transition 120 C

FEA mesh – for thermal expansion study

400,000 elements 550,000 nodes 1,200,000 degrees of freedom

Shown withUnder fillRemoved

FEA mesh detail views – for thermal expansion study

400,000 elements 550,000 nodes 1,200,000 degrees of freedom

Results – solder ball stress contours from FEA

Results – BGA solder deformation with under fill(under fill not shown)

Corner balls are under the highest stressprimarily in tension

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.

• 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