Plastic strain in thermally cycled flip-clip PBGA solder bumps

Electron-beam moire, a technique to measure displacements on the sub-micrometer scale, was used to observe and calculate strain in an underfilled, flip-chip plastic ball grid array (PBGA) package. A cross section of the package, instrumented with 450-nm-pitch gratings, was thermally cycled ten times between -55 and 125°C. During cycling, solder bumps were observed and digital images collected from the edge, from an area one quarter of the way across, and from near the midpoint of the Si chip. A comparison of these images with respect to the initial, no-load condition revealed the change in the moire fringe field due to the thermal load, and the change due to the coefficient of thermal expansion mismatch among the contained materials that resulted in plastic deformation. Data from the images were analyzed to produce plots of displacement versus position from line traces through the observed solder bumps. The slopes of the plots from the data generated within the solder bump define the average strain in the solder bump. Strains were calculated in the solder bumps at room temperature, at -55°C, and at 125°C during each thermal cycle. The initial high-temperature strain, out of the plane of the Si chip, was greater for the inner solder bumps than for the solder bump at the edge of the chip. The magnitude of the initial low-temperature compressive strain was greater at the edge of the chip than in the interior of the Si chip. It was also found that for the in-plane strains, the solder bump was put into compression both at high and low temperatures. The expansion of the underfill and solder mask at high temperature was so great that they exerted a compressive force on the solder bumps. Strain in the solder humps changed with subsequent thermal cycles. The strains decreased in value with increasing number of thermal cycles, particularly at the edge of the chip. By the 10th thermal cycle the solder bump at the edge remained in compression even at 125°C. Also, the final room-temperature images showed the solder bumps in all three measured locations to be in a state of permanent compression with respect to the initial, no-load images.