Aluminum thick-metal-backed circuits: conductive adhesive technology

Thick-metal-backed printed wiring boards are commonly used in high frequency wireless circuit applications requiring significant thermal dissipation, such as power amplifiers. The thick metal backer (TMB) typically functions as a ground plane, heat sink, and in some cases, part of a shielded housing. Electrical interconnections between RF/microwave printed wiring boards (PWBs) and TMBs have traditionally been accomplished using sweat soldering, plated through holes, or mechanical interconnections. More recently, conductive adhesive technology has been investigated to mechanically and electrically interconnect the PWB to the thick metal ground plane, due to perceived advantages in cost and performance. However, providing mechanically and electrically stable interfaces between conductive adhesives and large area metal surfaces has proven to be a significant challenge, particularly in the case of aluminum-backed circuits. Aluminum, while a cost effective material for these applications, is prone to hydrolytic processes induced by moisture and accelerated by temperature, active ionic species, galvanic effects, and other environmental factors. These processes can compromise both the electrical and mechanical stability of the bond, and thus the performance and reliability of the packaging structure. This paper describes work performed at IBM Microelectronics to understand the interfacial resistance between conductive adhesives and typical metal bonding surfaces for TMB printed circuits, and will discuss how reliable and functional adhesive interconnections were achieved for commercial designs using appropriate adherend pretreatment, mechanical design, and adhesive material.