Bga underfill cte mismatch
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Transcript of Bga underfill cte mismatch
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