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72 PCB007 MAGAZINE I JULY 2018 In this case, modified means to skip the sol- vent-sweller step (either NMP or butyl/caus- tic-based) and only immerse the parts in the permanganate solution, followed by the neu- tralization step. Another critical success factor relates to the operational and chemical conditions within the electroless copper deposition process. These key aspects will now be presented in some de- tail. Much of what is detailed below are the re- sults of many DOEs. These experiments yield- ed several important pieces of information. After several DOEs and other tests, the best condition for eliminating the peeling electro- less is to implement what is known as a low deposition rate electroless copper process. The definition of low deposition is a process that deposits 1.0 to 1.5 µin of copper per minute of dwell time in the electroless copper solution. This is achieved by making several modifica- tions to the current process. First, the electro- less copper plating solution should be reduced in temperature to 80−90°F with a deposition time of 20 minutes. The target deposit thick- ness is 20−25 µin in 20 minutes. To further slow the deposition rate of the bath, the amount of palladium-based catalyst deposited on the dielectric materials must be reduced. This is accomplished by reducing the cleaner/conditioner as well as catalyst concen- trations. Of course, it helps if the cleaner/con- ditioner chemistry is designed to reduce the thickness of the catalyst layer as it is adsorbed onto the polyimide material. Reducing the cat- alyst layer thickness, and in turn the amount of palladium on the material, improves adhe- sion and reduces the tendency of the electro- less copper to blister from the substrate. Polyimide Materials It is a good idea to review the chemistry in- volved in polyimide flex materials. Polyimide materials are a condensation reaction of a di- anhydride and a diamine compound in a dipo- lar aprotic solvent, such as DMF and DMAc. Poly(amic acid) is converted to the final poly- imide product by thermal imidization. Figure 2 details the reaction showing the synthesis of polyimide. Polyimide has exceptional solvent resistance, high thermal stability, and good di- electric properties. One of the critical aspects of plating is the ability to activate the substrate to be plated. Figure 3: Synthesis of polyimide. Figure 4: Treating polyimide substrates. KOH reactions of amides.

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