Plastic strain in thermally cycled flip-chip PBGA solderballs

Electron-beam moire was used to observe and calculate strain in underfilled flip-chip PBGA packages. A package cross section, instrumented with 450 nm pitch gratings, was thermally cycled ten times between -55 and 125°C. During cycling, solder balls were observed and digital images collected from the edge, from an area one quarter of the way across, and from the chip midpoint. Comparison of the images with respect to initial no-load conditions revealed the change in moire fringe field due to the thermal load, and due to CTE mismatch among the materials that resulted in plastic deformation. Image data was analyzed to produce plots of displacement versus position from line traces through solder balls. Plot slopes from data generated within the balls define average strain. Strains were calculated at room temperature, -55°C and 125°C during each thermal cycle. Initial high-temperature strain, out of the chip plane, was greater for inner solder balls than at the chip edge. Initial low-temperature compressive strain was greater at the chip edge than in the chip interior. It was also found that for in-plane strains, the ball was under compression at both high and low temperatures. Underfill and solder mask expansion at high temperature was so great that they exerted compressive force on the solder balls. Strain changed with further thermal cycles, decreasing with increasing thermal cycles, particularly at the chip edge. By the 10th thermal cycle, the edge solder ball remained in compression even at 125°C. Also, the final room temperature images showed solder balls in all three measured locations to be in permanent compression