Carolyn R. McCormick

New insights in critical solder joint location

Current numerical modeling approaches to assess BGA solder joint thermal mechanical fatigue, typically identify the max DNP (distance from neutral point) solder joint as the first to fail. An experimental and numerical analysis has shown that critical solder joints can be located interior to the max DNP location. In this work, FCBGA packages were subjected to accelerated thermal cycling stress. At regular intervals, subsets of the units being tested were extracted and analyzed with conventional dye and pull failure analysis techniques to better understand the progression of crack growth. Failure analysis revealed that cracks initiated on the diagonal axis and on the periphery, interior to the max DNP location. Numerical models using an accumulated damage metric to predict the critical solder ball, indicated that the max DNP solder ball would likely be the first to initiate a crack. However, examining the peel stress (D33) component on the solder balls revealed that the diagonal max DNP solder joints are under a compressive stress. Assuming that a crack will not preferentially propagate under a compressive stress, a qualitative approach using accumulated damage and peel stress from numerical modeling was defined to identify the critical solder joint. In this paper the experimental program, numerical model validation and results, and qualitative approach to couple accumulated damage and peel stress are discussed.

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 /spl deg/C to 125 /spl deg/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.