ASSESSMENT OF ELECTRICALLY CONDUCTIVE ADHESIVES FOR SMT
Author: Klaus Feldmann Company: University of Erkmgen Date Published: 4/28/1997
Surface Mount International
Abstract: The interconnection medium for joining SMDS and printed circuit boards has to fit different requirements. These are caused by technical, economical and ecological aspects, mainly given by the Iifecycle of the specific product. In the following, the results of different tests, which should demonstrate in which way a typical electri-cally conductive adhesive (epoxy-based, silver filled) fulfils those requirements, are shown. The choice of the test product based upon a number of manifold examinations and experiences with more than ten conductive adhesives . The electrical and mechanical performance of that prod-uct was tested before and after climatic stress (hot storage, thermal cycles, humidity stress). The varied test factors were: adhesive volume, cleaning of the boards, component form (R1206, S08), component metallization (SnPb, AgPL Ni/Pd), board metallization (SnPb, Pd, Schercoat (organic solderability preservation OSP) and pure cop-per) and curing cycle. The properties of solder joints with standard eutectic solder material served as a base for the assessment of the adhesive joints. In the next step the adhesive was additionally tested concerning fine pitch suitability, reworkability and silver migration effects. Finally the test program was completed with an economical assessment. The results are very encouraging. For simple applications the examined product seems to be suitable from both a technical and economical point of view. On condition that components with palladium or platinum metallization are used, the reliability of the adhesive joints fulfils very high requirements. Especially for products where soldering is not feasible or favorable, electrically conductive adhesives are an interesting alternative to soldering. Recent consider-ations about lead taxes or even a lead ban in electronic production and also the tendency to replace Sn/Pb metallizations on components by palladium lead to a further increase of the importance of conductive adhesives in the field of surface mount technology.