Effect of substrate warpage on the second level assembly of advanced plastic ball grid array (PBGA) packages

Abstract

Shadow Moire technique was used to study the out-of-plane displacements that represent warpage of advanced Plastic Ball Grid Array (PBGA) packages. Two types of PBGA packages, die-up and cavity-down configurations, were studied. The latter type is usually known as Enhanced Plastic Ball Grid Array (EPBGA) package due to the advantage it offers for thermal dissipation. Shadow Moire studies indicated that cavity-down package substrates warp in a concave configuration. Die-up package substrates warp in a convex configuration. Increase in temperatures up to 240/spl deg/C had an insignificant effect on the warpage of cavity-down substrates, whereas the die-up package substrate warpage improved with increasing temperature. However, the die-up substrates reverted to their original warpage upon cooling. The sum of the warpage and the ball diameter variation account for almost all the non-planarity. The packages with worse coplanarities showed invariably worse warpages. Substrate warpage of 4 mils or less is required for the successful assembly of PBGA packages. Since cavity-down packages do not show any improvement with increasing temperature, package non-planarity should be less than 7 mils by the seating plane method to ensure less than 4 mils of warpage. The assembly processes can tolerate higher levels of coplanarity for die-up packages as warpage improves at reflow temperatures. It is recommended that the coplanarity be measured by the seating plane method.