Durability of anisotropically conductive adhesive joints in surface mount applications and in flexible to rigid board interconnection

Anisotropically conductive adhesives are well established in LCD interconnection, and they are also considered to be a possible solution to high density packaging in surface mount assembly. Such an application requires joint resistances of milliOhms instead of Ohms, with negligible drift during the lifetime of the product. With many adhesives this is still a point of concern, especially when exposed to heat and moisture. This paper describes some of our efforts to reveal the influence of adhesive properties on electrical performance. Quad flat packages (QFPs) and polyimide flex foils have been bonded to printed circuit boards using a selection of commercially available anisotropically conductive adhesives. QFPs and corresponding boards were supplied with different metallizations; hex to rigid board interconnection was tested only with flash gold platings. Reliability of the adhesive joints was evaluated in terms of contact resistance before and after exposure to hot storage, damp heat, and rapid change of temperature conditions. Failure analysis of adhesive joints has been assessed by cross-sectioning and microscopic examination. The results show that durability of thermosetting adhesive bonded QFPs is excellent for solder-plated components and boards. This may be attributed to the reflowing of the metallizations. In general, performance of different adhesives seems to get better with increasing glass transition temperature. Results are also influenced by parameters such as particle size and composition, incorporation of additional fillers and resin chemistry.