0.35mm pitch wafer level package board level reliability: Studying effect of ball de-population with varying ball size

Board level reliability studies have been performed on wafer level packages (WLP) of various die sizes with 0.35mm ball pitch, SAC405 solder alloy, and two solder ball sizes. The 0.35mm pitch test vehicles included 5mm × 5mm and 6mm × 6mm package sizes. All test vehicles had areas of depopulation in the ball array, but used the same design rules regardless of ball size. The parts were assembled at 3 different suppliers. All of the WLPs studied passed the minimum drop shock requirements of 30 drops. For board level temperature cycle, a minimum of 500 cycles was targeted as a passing result. The 220um ball size on 5mm × 5mm 188 pin WLP package failed at both the corner most solder balls and solder balls near the corner at all suppliers. Only one of three suppliers' corner balls passed using the 230um ball. For interior balls, the 220um ball size on 5mm × 5mm 188 pin WLP package failed temperature cycle at two of three suppliers, while two of three suppliers passed with 230um ball. For the larger 6mm × 6mm test vehicle, the corner most solder balls were also very sensitive to temperature cycle fatigue failures. The 220um ball size failed at the corner at all three suppliers. For interior balls, the 220um ball size failed temperature cycle at one of three suppliers, while two of three suppliers passed 500 cycles only marginally. This study shows that depopulated WLP ball arrays of 5mm × 5mm and 6mm × 6mm are sensitive to temperature cycle fatigue failures at 0.35mm pitch with SAC405 alloy.