Weidong Xie, Mudasir Ahmad, Cherif Guirguis, Gnyaneshwar Ramakrishna, and Jianghai Gu
Cisco Systems, Inc.
Ultra-large Ball Grid Array (BGA) components with a package size that is equal or greater than 70mm have been adopted in today’s high-end network products driven by the explosive demands for faster speed and higher bandwidth. At the same time, larger and/or heavier heat sinks (HS) are required to achieve better thermal management. It becomes more challenging than ever to maintain interconnect integrity under mechanical shock environment.
Metal standoffs are typically used as the supports of Print Circuit Board Assembly (PCBA) to the sheet metal chassis. The metal standoffs provide the necessary mechanical support, special clearance, and ground connection for the PCBA. Due to the increasing BGA body size and HS weight, it is necessary to explore means to control the strain level on BGA corners to lower the risk of interconnect failures in manufacturing, shipping, and field. This study investigates the potential for using rubber anti-vibration standoffs to replace metal standoffs as an effective mitigation for enhancing the shock performance of ultra-large BGA components. The rubber materials are well-known for their excellent shock absorption characteristics. The rubber standoffs is sandwiched two metal pieces (female and male end). The two metal ends serve the same functions as regular the metal standoffs to bolt PCBAs onto sheet metal chassis but the rubber portion effectively decouple the two metal ends and dampen the shock wave passing through it therefore significantly reducing the strain level on the PCBA.
To vet the effectiveness of the approach, shock testing has been performed both at a component and product level. The test results showed 2x improvement in terms of the failure strain for a 75mm BGA mounted on a JEDEC standard test board to allow in-situ continuity monitoring during shock testing. There was an 8% to 42% improvement in terms of BGA corner strains in system level shock with a product that has multiple large BGA components.
Finite Element Analysis (FEA) modeling has been developed that captures well the effect of rubber standoffs. The validated modeling can be helpful to guide the design parameters and optimization of the rubber standoffs based on the specific system architecture such as the component location, HS attachment, and the corresponding product end-use conditions.
A systematic study of the effectiveness of rubber gasket and rubber standoff as a shock mitigation to ultra-large network BGA components has been performed. The results showed variant percentage of improvement in terms of BGA corner strain level at product level testing. Combined with a proper choice of heat sink attachment, rubber standoffs and/or rubber gaskets could potentially reduce the strain level and improve mechanical reliability to meet the requirements in end-use environment.
Even the component-only TV demonstrated a significant reduction, up to 50%, of strain level at BGA corner joints. Rubber standoffs can be an effective solution for shock mitigation, but there are other operations challenges to be considered.
Initially Published in the SMTA Proceedings