Lisa J. Jimarez

Interfacial adhesion of fluoropolymer composites to commercial copper foils subjected to aqueous photolithographic processes

Three commercial copper foils containing various degrees of surface roughness and a chemically similar surface treatment were laminated to a fluoropolymer composite and also subjected to water-based photolithographic and stripping processes. Surface analysis performed on the processed foils indicated that the highly basic pH of the stripping process was effective in removing zinc oxide, a component of the surface treatments on these foils. Morphologically, however, the surfaces did not seem to change. Ninety degree peel measurements of the resulting interfaces with and without a photoresist processes showed that the resulting practical adhesion seemed to be mainly controlled by mechanical interlocking and to a lesser amount, by chemical factors. When 50 /spl mu/m wide lines were subjected to the photolithographic process and a full fabrication sequence, adhesion variability seemed to be affected by interfacial attack of the fluoropolymer/metal foil interface by the alkaline medium of stripping and other process chemicals. Reliable interfacial adhesion of narrow lines is only possible when process parameters have been optimized.

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.