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
CTE dimensional change vs. temperature X&Y
17.0 ppm/C10.0 ppm/C
CTE dimensional change vs. temperature Z
100.0 ppm/C35.0 ppm/C24.0 ppm/CGlass transition 120 C
FEA mesh for thermal expansion study
400,000 elements 550,000 nodes 1,200,000 degrees of freedomShown withUnder fillRemoved
FEA mesh detail views for thermal expansion study400,000 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 upUnder fill bulges at edges , it is almost incompressible like a hydraulic fluid.
The FET BGAs 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 CTEs in the z-axis.The under fill glass transition is one underlying causeThe under fill incompressibility is another significant factorA secondary variable is the TCE mismatch between the BGA package and the multilayer PWB.Conclusions