Acoustic verification of mold compounds

摘要

A large lot of plastic-packaged ICs is on its way to surface-mounting. The components have passed electrical inspection, where those having functional defects were removed. They have passed optical inspection to find those components having external defects, and they have undergone mass acoustic imaging to find those components having internal structural defects. The survivors of these tests will be surface-mounted and perform reliably for their expected lifespans. What could go wrong? What could go wrong is an abrupt in-service failure due to an inconspicuous change in the packaging material from that used in previous lots. What typically happens is that the maker of the components has for some reason—cost, availability, processing characteristics, or perhaps even unknowingly—received a modified or altogether different encapsulant material from its suppliers to package the die. A discreet change will not show up in the electrical inspection, optical imaging, or acoustic imaging mentioned above. The changed thermal, mechanical, and adhesive properties of the mold compound, however, can lead to unexpected damage during reflow or in service. Specifically, these components will differ from those in previous lots in two material characteristics: the acoustic impedance of the mold compound and the acoustic attenuation of the mold compound. An acoustic micro imaging tool such as those in Sonoscan's C-SAM lineup can measure each of these values quickly and easily.