Journal of SMT Article

RELIABILITY OF ELECTRICALLY CONDUCTIVE ADHESIVES AS A SUBSTITUTE FOR LEAD BASED SOLDER IN MILITARY, AUTOMOTIVE, AND HIGH STRESS ENVIRONMENTS

Author: Frank Liotine
Company: SMT Engineering Company
Date Published: 10/1/2001   Volume: 14-4

Abstract: Two technologies are predominantly used in electronic applications relative to component to circuit card attachment. Specifically, lead based solders and electrically conductive adhesives have been the medium serving as the electrical and mechanical connection path between a component and respective circuit card. Applications for each, from a component standpoint, have encompassed all forms of surface mount technology (SMT) packages, including ball grid arrays (BGA), flip chips, leadless chip carriers, ceramic chip devices, to name a few; and for circuit cards, applications include everything from standard G-10 FR-4 glass epoxy boards to ceramic substrates. The basic ‘connection’ difference between solder and adhesive attachment is as follows: solder is a metallurgical medium and hence forms a metallurgical intermetalic bond between the component pad or foot and the circuit card land or pad; whereas, adhesive connections strictly rely upon adherence to form the interconnection bond. Traditionally, the most common composition of the two basic connection mediums have been SN60/40 or 63/37eutectic for lead based solder, and various silver filled epoxy polymer formulas for the adhesives technology.

Due to environmental health and safety concerns, lead based solder has been replaced with alternate materials in several countries presently, and in the relative near future, it will be replaced or strictly reduced and controlled world wide, including the United States. Essentially, the alternative substitutes are non-lead solders and electrically conductive adhesives. There are a variety of lead solder substitutes, none of which are direct drop-ins for lead based solder. All such substitutes re-flow at much higher temperatures than traditional low temperature eutectic tin-lead solder. This, of course, raises manufacturing and component long-term reliability concerns. However, in some applications, a metallurgical substitute may be the only option. For many other applications, the adhesive approach will be suitable, and in some cases, adhesives may prove more reliable as the interconnection.

Until recently, the problem with adhesive interconnections has been lower electrical performance, as compared to solder, and low mechanical elasticity, forming an interconnection joint that is subject to fracture under stress. Metallurgical attachments have out performed filled adhesives relative to electrical conductivity, as measured in ohm-cm of the raw materials. This will not be a problem in many cases as joints are becoming smaller due to much increased circuit densities, and the use of finer pitch components and devices. Smaller interconnect joints means closer contact between component pad and circuit card land, hence lower electrical losses as compared to joints forming taller stand-off heights. In cases where improved conductivity and reliability are required, newer metal filler technologies will come to serve filled polymer applications. This will include nanometals, which due to the morphology can exhibit electrical conductivities greater than traditional polymer technology. Additionally, adhesive joint stiffness has been a problem in past applications, especially for high stress environments, such as space, and when combined with conditions such as high thermal expansion (TCE) differences, e.g. high TCE delta between a ceramic BGA and glass epoxy circuit board. However, recent advances in low glass transition (Tg), hybrid polyurethane technologies have significantly improved the mechanical performance of adhesives allowing for successful applications of polymers in high stress environments.

This paper describes an application where metallurgical solder was tested and found to be impractical to apply as the interconnection medium, and where large TCE differences demanded a very flexible joint. The end application describes a product used in a high stress environment. The adhesive was required to meet NASA out gas specifications, and the product was required to pass biased humidity, and thermal shock. A silver filled hybrid polyurethane adhesive was chosen, and qualified, and to date has over a four-year history of successful performance in the field.

Key words: Polyurethane, electrically conductive adhesive, nanopowder, nanometal, solder joint, lead free solder, elastic modulus, high stress environment, surface mount technology, electronic packaging, military, space, ball grid array, flip chip, circuit card.



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