The elimination of lead from most electronic assembly applications has had a number of repercussions. The low temperature alternatives suffer from detrimental performance and cost limitations. The high tin alloys have high process temperatures and a tendency to form tin whiskers, both of which have an impact on the reliability of the product. Low process temperature conductive adhesives have been developed, but do not offer the mechanical strength, thermal conductivity or electrical conductivity of metallurgically alloyed joints. Proposers offer an innovative approach that combines the low process temperature of conductive adhesives with the reliability and performance of solder. In the proposed approach, particles of copper and tin-based alloys are dispersed in an adhesive-flux binder to form a paste. The particles microsolder at standard tin-lead processing temperatures to form an alloyed metallic mesh encased in polymer adhesive. The alloy particles also form metallurgical bonds to the leads and pads. The encasing adhesive reinforces the matrix and acts as a stress sink. The hybrid approach consumes most of the tin in the composition into intermetallic structures, thus substantially reducing the potential for whisker formation. A fully alloyed matrix may thus be obtained at process temperatures below 200C.
Benefit: It is anticipated that the proposed effort will result in a hybrid solder-conductive adhesive material that can be used in applications requiring long-term reliability and for components that cannot withstand the high reflow temperatures of the high tin alloys. Markets for such materials would include military and aerospace due to the long life cycles of these devices as well as the long redesign cycles associated with these applications. Other market opportunities for these materials include low-end, high volume circuits such as RFID tags in which the substrate materials may not withstand the reflow environment. The nature of the the proposed materials also makes them ideal candidates for step soldering operations and for assembly of LED arrays to preserve the lifespan of the components.
Keywords: Hybrid, Alloyed Metallurgical Network, Composite, Conductive Paste, Reworkable, Low-Process-Temperature, Tin Whisker Mitigation
