Effect of Vapor-Deposited Parylene Coating on Reliability of Sintered Silver Joints for Extreme Temperature Applications

Abstract

Abstract Silver nano-particle (AgNP) sintering has been extensively shown to be an excellent bonding method for use in the assembly of high-temperature multi-chip modules (MCM) rated above 200° C. Among the proven advantages of using this material in the assembly are the high mechanical strength of the attachment joints, resilience to thermal cycles, low resistivity, and high thermal conductivity. One of the concerns related to the reliability of sintered silver joints is silver migration. Another concern is the change in the joint's microstructure under thermal stress. Vapor-deposited high-temperature fluorinated parylene coating (Paralyne HT) may have the potential to mitigate those concerns because of its superior conformal and crevice-penetration properties. In this work, impact of Parylene HT on sintered silver joints has been evaluated from the perspective of mechanical strength. Test vehicles were subjected to thermal cycling and high-temperature aging. To understand the sintered silver joint's failure mechanisms, shear test analysis of the cross sections and fracture surfaces was performed. In addition, the effectiveness of Parylene HT as a coating to inhibit silver migration at high humidity was evaluated. Coated and uncoated sintered silver test patterns were stressed with an electrical field inside a high-humidity chamber. The silver migration progression was monitored and compared between coated and uncoated samples during the test. One of the findings was that the coating material penetrates the pores of the sintered silver joints, altering their mechanical performance under thermal stress. Analysis of the performance differences between coated and uncoated test vehicles is presented in the paper.