Commercial encapsulants are actually a suspension of particles in liquid, which affect the rate of flow as well as the distribution of encapsulant around the solder bumps. Although, the fluid mechanics community has studied the suspension of solids in liquid for a long time, the application of classic studies to micro electronics is lim ited, said Lehmann. The challenge is that particles are no longer small in relationship to that gap. The researchers are looking at these and other factors to see how the variance affects speed of flow. By creating a database of these fundamental parameters, a model can be constructed to predict fluid behavior under various industrial conditions.
Industry is concerned with increasing the speed of flow, while maintaining the quality of the fill.
Currently the underfilling process takes several minutes, an expen sive prospect, considering the 20.5 seconds it takes to roll an attached chip and board off of the assembly line. According to Peter Borgesen, manager of the Direct Chip Attach Consortium for Universal Instruments Corp., only a handful of manufacturers now use the process, including IBM and Motorola. Borgesen predicts during the next five years, another 15 to 20 companies in the United States will use encapsulants, depending on the outcome of the Underfill Flow Group's research and that of other groups studying the problem. They are bridging a very important gap between fundamental science and in dustry.