When are Conductive Adhesives an Alternative to Solder?

摘要

Conductive adhesives (CAs) have been an important problem-solving class of assembly materials for decades, but primarilyrnas die attaches products, ever since they replaced metallurgical systems. Renewed and intensified interest in lead-free (L-F)rnelectronic assembly has moved CAs back into the spotlight. Although lead-free solders, both new and very old, have beenrnstudied for several years, they are a partial solution at best. One may conclude that: (1) lead-free alloys do work, (2) there isrnno drop-in replacement, and (3) their higher processing temperatures are detrimental.rnThe expected increase in soldering temperatures is cause for concern over potential damage to laminates, packages and somernsemiconductors. New alloys will also require modifications to some soldering equipment. High temperature L-F processingrncould bring significant and costly "collateral damage". BGAs could require pre-bakes, PWBs may degrade and optoelectronicrncomponents could fail or suffer reduced lifetimes. The next -generation sub-micron CPUs, with evolving low k dielectricrnlayers, may not tolerate excessive soldering temperatures. These problems may be "fixed", but not truly solved. Cost-addingrnwork-around strategies include higher performance laminates, upgraded molding compounds and in-process cooling, butrnconsiderable time and money will be needed to re-engineer, re-test and re-qualify.rnPolymer Solders (Conductive Adhesives) have provided a good alternative for temperature-limited assembly for decades.rnThese well-tested joining materials process like solder on the same equipment. Fluxing or cleaning is never required. Andrnthey run at more than 100 degrees lower than solder. Both reflow and batch ovens can be set at 110℃ to 150℃ to quicklyrnharden these polymer systems. Adhesives are used to assemble SMTs to a variety of systems including medical devices,rnmemory modules, and computers. Chances are that LEDs in your business phone and ink jet printer are assembled withrnconductive adhesives. Perhaps the flip chips driving your flat panel display are adhesively bonded.rnThis paper will compare Polymer Solders to L-F alloys to show limitations and advantages for the technology. There arernimportant restrictions, especially lower mechanical strength revealed in the drop test. But adhesives research has beenrnenergized after years of simple incremental improvements and fresh new approaches will be summarized that includernintrinsically conductive polymers (ICP) and Nanomaterials.