Improved reliability in small multichip ball grid arrays

The subject of this paper is a 14×22 mm ball grid array (BGA) integrated circuit assembly containing two or three chips. Three failure modes came to light in the reliability testing of this BGA package: delamination of solder resist from the top copper layer occurred in moisture resistance testing; cracking of the top layer solder resist and consequent cracking of the top copper layer occurred in temperature cycling; and cracking of the bottom layer solder resist which propagated into the bottom copper layer occurred in thermal shock. The failure analysis techniques used to disclose these failures are presented. Finite element analysis of thermomechanical stress within the multichip structure was carried out. The purpose was to find the root cause of one of the failure modes and to explore possible means of overcoming the stress damage. The characteristics of the original and modified substrate layout designs are detailed. The improved performance in reliability testing is compared with the original. All failure modes were eliminated in the final design, and the product was qualified to greatly improved reliability standards.