Лако-красочные материалы — производство

The development of polyimide-based conductive die attach adhesives resulted from the attractive properties exhibited by this generic class of materials. The high Tg value (typi­cally, 100°C higher than epoxies) offers superior performance during high-temperature processing, such as wirebonding, overmolding, soldering, and lid sealing. Other properties of interest include low ionic contamination levels and a low CTE.

There are several disadvantages to using polyimides that limit their usefulness and increase manufacturing costs. In general, polyimide adhesives are formed by the thermally induced imidization of a polyamic acid precursor. The polyimide precursors are dissolved in a solvent such as и-methyl pyrrolidinone that must be removed before curing can begin. Most cure reactions generate water as a by-product which must also be removed carefully to prevent void formation. Finally, very high temperatures (>250°C) are required to cure the material fully. High residual stresses can result from a combination of the elevated process temperature, Tg, shrinkage during cure, and high modulus of the imidized resin.

Several major advances in polyimide chemistry have reduced some of the process difficulties. By end capping the polyimide precursor with acetylenic unsaturation, by-product free addition cures can be achieved. The development of a thermoplastic polyimide eliminates the high-temperature cure requirement [50]. Both of these modifications alter other characteristics, such as lowering of the high-temperature resis­tance over a traditional polyimide. The incorporation of dimethylsiloxane block segments into the polyimide backbone has also been reported [49]. This type of modification leads to an adhesive matrix with a lower modulus of elasticity and lower Tg value than those of unmodified polyimides and has been associated with lower-stress die attach assemblies.