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Markets requiring thermal exposure at elevated temperatures include down-hole oil drilling, semiconductor processing, medical diagnostics and a multitude of military/aerospace requirements. But a limitation of flexible circuitry has been performance at extremely high temperatures. For purposes of this discussion, the definition of “extremely high” is temperatures that exceed 150–200°C on a continuous basis or 288°C on an intermittent basis. The flexible circuit industry has made inroads improving this product feature with recent developments and significantly improved high-temperature performance, which is being accomplished with new material constructions requiring some unique fabrication processing.
The adhesive system used to bond the various layers in a flexible circuit is normally considered the weakest link when parts are exposed to high temperature or a harsh chemical environment. This weakness has driven adoption of adhesiveless base laminate technology (the word “laminate” remains an industry term but is a bit misguiding when describing adhesiveless structures as they are not generally produced with a laminating process). Multiple methods for creating an adhesiveless base laminate substrates have become common (copper sputtering, vacuum deposition, cast polyimide) and provide improved performance in both extreme thermal and chemical environments.
But the most frequently used “top side” flexible circuit insulation is generally another layer of polyimide film commonly called coverlay or coverfilm. This layer of film is coated with an uncured adhesive and positioned onto the etched circuitry pattern and then permanently fastened during a temperature/pressure lamination cycle in a platen press. Consequently, an adhesive has been reintroduced into the circuit composite, despite removing the adhesive in the base substrate. Another top side dielectric used in the world of flexible printed circuits is a photo-imaged soldermask, but this material often becomes brittle at elevated temperatures, and can fracture or flake off when bent or folded.