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78 SMT Magazine • December 2015 has begun to decompose. The TGA will reveal the decomposition temperature that correlates to the maximum cure temperature of the paste. Figure 2 shows a comparison of the solderable silver conductor with two additional silver con- ductors utilizing different resin systems. The curing temperatures established for the testing of this system were set at 150°C and 200°C. The polymer paste was initially designed for operation on FR-4, alumina, and aluminum (with a polymer insulating layer). It was print- ed onto all substrates with a 280-mesh screen and cured at either 150°C or 200°C in a forced- air convection oven. The paste was printed to a wet thickness of 38–42 μm with a resulting cured thickness of 23–27 μm. All parts considered for reliability data were hand soldered with SAC305 solder using 615 RMA flux. Adhesion was performed by attaching solder-plated copper (60/40) tin/lead leads to 80 x 80 mil pads at a 90°angle to the substrate. The polymer paste was built up in a vari- ety of thicknesses to examine the correlation of cured film thickness and spreading. Table 1 shows the spreading of the 1600 mil² pads with up to five layers of paste on both FR-4 and alu- mina substrates. Spreading is apparent and has a direct correlation to the thickness of the con- ductor paste. Decreasing the level of spreading is under further evaluation. conductivity requirements One of the most challenging obstacles to overcome when working with a thick film polymer conductor is the conductivity require- ments. The usual resistivity of a silver thick film LoW-TemPerATUre THIcK FILm PASTeS PermIT LeAD-Free SoLDerInG ArTiCle Table 1: Spreading of multiple layers of polymer paste. Table 2: Resistivity of the polymer pastes.