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March 2016 • The PCB Magazine 63 as preferential adsorption of the wetting agent to the fibers (Figure 8). The objective of this study has been to develop conditioning systems to provide required wetting and hydrophilicity of PI laminate as well as resin/glass with rigid inner layers of polyimide based laminates. Re- sults from the flexible substrate are discussed here. Rigid-flex, which may need an additional glass conditioning compound, is not discussed here. Experimental The experimental strategy was to formulate a low temperature novel conditioning system for polyimide substrate that is compatible with the initiator and long life catalyst for DMS-E process. Three key focus areas of the formula- tion work were to identify optimum bath pH, sweller, and reducer compounds that perform well at low operating temperature. Each of these components plays a critical role in preparing the PI substrate for the subsequent step, which is to absorb MnO 2 , which is a vital part of the DMS-E process. The sweller compound swells the surface of the PI substrate resulting in a larger surface area and higher surface roughness. This makes the substrate more susceptible to the other addi- tives. An alkaline pH cleaves the polymer end groups and creates functional groups which are more receptive to the reducer compounds. The function groups of the PI substrate are modified Table 3: screening experimental matrix. Figure 8: Conditioning of glass surfaces. by the reducer which allows an optimal reduc- tion and therefore a better formation of MnO 2 by the permanganate in the initiator. A series of statistically designed experi- ments were conducted to develop a robust pro- cess that included formulation and process pa- rameters optimization. The initial screening ex- periment focused on the formulation part and included two different types of sweller, two dif- ferent types of reducer and two different types of caustic for pH adjustment. Table 3 shows the initial screening experimental matrix. Dif- ferent types of flex substrates were included in the evaluation to gage the effectiveness of the conditioner. Test Vehicle The test vehicle used in this experiment was a simple double-sided flex substrate. Depending on supplier, the substrate also had drilled holes. Prior to the DMS-E process, each test coupon direCt metallization system for flexible printed CirCuits