Process and Evaluation of High Reliability Reworkable Edge Bond Adhesives for Large Area BGA Applications

Abstract

Reworkable underfills and edge bond adhesives are finding increasing utility in high reliability and harsh environment applications. The ASICs and FPGAs often used in these systems typically require designs incorporating large BGAs and ceramic BGAs. For these high reliability and harsh environment applications, these packages typically require underfill or edge bond materials to achieve the needed thermal cycle, mechanical shock and vibration reliability. Moreover, these applications often incorporate high dollar value printed circuit boards (on the order of thousands or tens of thousands of dollars per PCB) hence the need to rework these assemblies and maintain the integrity of the PCB and high dollar value BGAs. This further complicates the underfill requirements with a reworkability component. Reworkable underfills introduce a number of process issues that can result in significant variability in reliability performance. In contrast, edge bond adhesives provide a high reliability solution with substantial benefits over underfills. One interesting question for the large area BGA applications of reworkable underfills and edge bond materials is the comparison of their reliability performance. This paper presents a study of reliability comparison between two robust selected reworkable underfill and edge bond adhesive in a test vehicle including 11mm, 13mm, and 27mm large area BGAs. Process development for those large area BGA applications was also conducted on the underfill process and edge bond process to determine optimum process conditions. For underfill processing, establishing an underfill process that minimizing/eliminates underfill voids is critical. For edge bond processing, establishing an edge bond that maximizes bond area without encapsulating the solder balls is key to achieving high reliability. In addition, this paper also presents a study of new high performance reworkable edge bond materials designed to improve the reliability of large area BGAs and ceramic BGAs assemblies while maintaining good reworkablity. Four edge bond materials (commercially available) were studied and compared for a test vehicles with 12mm BGAs. The reliability testing protocol included board level thermal cycling (−40 to 125°C), mechanical drop testing (2900 G), and random vibration testing (3 G, 10 – 1000 Hz).