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86 SMT007 MAGAZINE I JULY 2021 When it comes to photoresist applications in this industry one could be led to believe [4-6] that resists, specifically developed for photo- chemical machining, are widely in use but this has not been my experience. Most resists used in photochemical machining are also found in PWB shops. at is not to say that you will not find exotic resist species in this niche industry, even resist dinosaurs such as close cousins of what used to be Kodak's KPR. Performance criteria for chemical milling photoresists are in many ways more demand- ing than those of PWB fabricators; there is a plethora of different substrates such as Alloy 42, beryllium copper, iron-nickel alloys, molyb- denum, tungsten, Invar, and many more. e etching chemistries and etch conditions, which the resist must survive, are brutal. In addition to the more benign cupric chloride and the main- stream ferric chloride, you find mixtures of hydrofluoric and nitric acid which embrittle the resist, as well as very highly alkaline solutions of potassium ferrocyanide, which act like strip- pers on aqueous processable resists. Resist Features ere are certain resist features which are highly valued by the photochemical milling industry: • Good adhesion to a variety of metal substrates through aggressive etching or plating cycles. Since most surfaces are quite smooth and don't offer much mechanical interlocking at the resist metal interface, the chemical adhesion of the resist has to be very good. • Resist flexibility/ductility. In some appli- cations, the work piece is bent aer resist lamination, and the resist has to follow the contours of the new shape without delaminating. It was also desirable that any resist overhang resulting from under- etch will not break off during processing and cause contamination. • Conformation/flow. Many resist applica- tions involve lamination over contoured surfaces, comparable to secondary imag- ing with solder mask over a circuitized surface, an application that requires good resist flow and conformation. • "in" resist. e issue here is etching uniformity. Resist thickness affects the etch rate in narrow channels because it adds to the aspect ratio of the etch channel and impedes the fluid dynam- ics of etchant replenishment. Conversely, isolated resist features allow free etchant attack from both sides. e result is an image-pattern-dependent etch rate difference. Both can be minimized with thin resist. • Clean stripping/no residual stain. Many photochemically machined products are decorative in nature, so a stained surface becomes a functional defect, not just a "cosmetic" nuisance. In this context it becomes apparent why this industry has a preference for a resist with a bleach-out ("photo-fugitive") image aer exposure. Such bleaching upon exposure prevents dye staining on the surface of the etched metal. It also allows visual inspection of the metal feature under the protective resist coating. Surface Preparation Machine oils are the most common surface contaminant in this industry. Strongly alka- line cleaners at elevated temperatures (60°C) have proven to be very effective in removing The etching chemistries and etch conditions, which the resist must survive, are brutal.