Underfilled BGAs for a variety of plastic BGA package types and the impact on board-level reliability

There are continuous efforts in the electronics industry to reduce electronic package size. The main force pushing for smaller package size is the desire by original equipment manufacturers to reduce their mother board size and thereby their own cost. Reducing the size of electronic packages can be achieved by a variety of means and for ball grid array (BGA) packages an effective method is to decrease the pitch between the individual balls. Reducing the pitch between balls decreases the ball size and therefore the standoff between the packages and motherboards is reduced. The reduced standoff has the negative effect of reducing board-level reliability. An experiment to understand board-level reliability improvement by underfilling the BGA will be presented for several fine-pitch plastic BGA types (fleXBGA(R) and MAP (mold array process)). Four different (snap-cure) underfill epoxies with varied coefficients of thermal expansion and elastic moduli were used so that the preferred set of epoxy properties could be determined. A full factorial numerical experiment was used to identify the optimum combination of CTE and elastic modulus. Results indicate that a significant increase in board-level reliability can be achieved by underfilling the BGA.