Surface Mount International Conference Proceedings


Author: Catherine Gallagher
Company: Ormet Corporation
Date Published: 4/28/1997   Conference: Surface Mount International

Abstract: Conductive adhesives are conceptually intriguing as replacements for solder in temperature sensitive and high I/O applications. However, despite the recent incorporation of some innovative design concepts, the traditional particulate filled polymers have not yet demonstrated the characteristics required to fulfill the needs of these market segments. Recent evaluations have again confirmed that the impact strength of these materials is insufficient, with attachment to tin-lead coated surfaces being especially problematic. Additionally, stability of junction resistance remains inadequate in humid environments.

Two years ago, an alternative concept of a solder/adhesive hybrid technology was presented as a possible method to realize the unfulfilled expectations of conductive adhesives. These materials are composed of an interpenetrating polymer/metal network formed in situ from a unique lead-free conductive particle and polymer blend. The metal network forms simultaneously with the cure of the polymer by a process known as transient liquid phase sintering (TLPS). This network is metallurgically alloyed to the pads providing stable electrical conduction and the multiply redundant interconnections interspersed with stress absorbing polymer provide fatigue resistance to the joint.

Since our original presentation, this concept has been validated by comparison with tin-lead solder in reliability testing. Circuit boards with gull-wing, J-lead and Ieadless components were subjected to 7-foot drop tests. All of the components survived on both copper and tin-lead finishes. Electrical conductivity of the joints was comparable to the solder connections and did not change substantially following humidity testing. Analysis of intermittent indicated performance approaching that of solder during air to air thermal shock (-55°C to +125”C) as well.

Concurrent to this anatysis, microvia interposers comprising a TLPS paste, dielectric separator and a bond-ply adhesive have been under development and evaluation for multilayer packaging. The combination of this work with the TLPS SMT technology introduces an interesting new concept for bumpless area array attach.

Key words: TLPS, composite, hybrid, conductive adhesive, SMT, area array, bumpless

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