Dry Films

Dry films are not coated onto substrates in liquid form, but as an already (almost) solvent-free film. While the further processing is basically the same as with liquid resists, dry films off er completely new possibilities with regard to substrate selection and processing conditions.

Fundamentals of Dry Films

Differences to Liquid Resists

Due to a solvent content in the range of approx. 30 - 90%, liquid resists have a viscosity of approx. 10 - 1000 cSt, which means that they can be spin-, spray or dip coated a more or less viscous liquid, and subsequently require a softbake  to minimize the remaining solvent concentration in the resist film.
Dry films, on the other hand, are produced as solvent-free (or solvent-poor) films which can be exposed after lamination onto the substrate without a previous baking step.

Areas of Application for Dry Films

Dried films are particularly an alternative to liquid resists if the substrates to be coated have vias or generally depressions that should remain resist-free. While it is difficult to impossible to subsequently develop liquid resists out og these vias, laminating dry films automatically achieves the goal of resist-free holes.
The greater the desired coating thickness, the greater the time saved by the obsolete softbake, which is one of the most time-consuming process steps in thick resist processing.

Limits of Dry Films

Even if the achievable thickness of modern dry films is comparable to the thickness range attained with common liquid thick resists, few μm thin resist films for resolutions around 1 μm are so far only achievable with liquid resists.
Textured substrates can only be coated over the entire surface with liquid resists. For processes such as wet chemical etching or electroplating, when an optimized resist adhesion is crucial, it may be necessary to use liquid coatings with their basically better adhesion potential.