Evaluation of a Stacked Package Utilizing LGA Devices

Abstract

A Land Grid Array (LGA) is an area array component similar to a Ball Grid Array (BGA), but without solder balls attached to the bottom surface of the package. Due to the absence of solder balls, the LGA package has a lower thickness as compared to an equivalent BGA package. Accordingly, this reduces the thickness of the module assembly. This is a critical consideration especially for modules using stacked packages. On the other hand, the lower standoff height of the solder joints could have an impact on the performance of the package during temperature cycling. For high density modules, along with ‘minimizing’ the thickness, it is desirable to avoid increasing the package size in the length and width directions. This allows maintaining the same number of Inputs/Outputs (I/Os) as a comparable BGA device and accommodates the package on existing Printed Circuit Board (PCB) designs. For this reason, the LGA package used for the current research was identical to the BGA device that is currently used for the stacked package, but without solder balls on the bottom side. The stacked package utilizing these LGA devices and the modules were assembled using Sn3.8Ag0.7Cu lead-free solder. The reliability of the assembly was assessed using thermal cycling test. Based on the results, the first failure was recorded at the end of 1250 thermal cycles, whereas the comparable stacked package using BGA components was able to withstand 1500 cycles without failures. Dye & pry analysis was also performed on the failed stacked packages as a part of failure analysis.