Selective-column ball grid array (BGA) for improved reliability

Spherical shaped solderballs on BGA packages have stress concentrations at the solderball-to-package and solderball-to-board interfaces. These stress concentrations can lead to fatigue cracks due to the resulting applied shear stress from the board to package coefficient of thermal expansion (CTE) mismatch. The concept discussed in this paper uses a better solderball shape to reduce the stress concentration and improve the temperature cycle reliability of these packages. Selective columnar ball grid array packages have columnar shaped solder joints at selective locations within the package. The shape of the columnar joints resembles an hourglass shape which helps to move the region of high stress from the solder ball-package pad interface to the bulk of the solder and also increase the compliancy of the solder joint by reducing the cross section area. The average joint area is reduced by about 30% which reduces the shear stresses by about 50%. A non-linear temperature cycle global model was developed in ABAQUS to assess the improvement in the reliability performance by using columnar joints. The modeling results showed about 40% reduction in the inelastic strain energy density in the worst region solder ball. The model also predicted the high stress region in the bulk of the solderball. The model was validated using displacement fields obtained from Moire interferometry. These packages were stressed for 2000 cycles of temperature cycling from -25 °C to 125 °C. Cross sections of the solder balls and dye-and-pry techniques were used to understand the failure mechanisms and locations. Failure analysis showed that the columnar balls fail at a slower rate compared to the spherical balls. Cross section of the failed joints showed that they failure location was within the bulk of the solder ball rather than at the interface.