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December 2015 • SMT Magazine 77 Silver Coating Selection: Silver coatings also played a role in the final selection of the silver used in the polymer paste. Fatty acids and hydrophilic coatings were assessed to de- termine optimal properties and compatibility. Overall, saturated fatty acids seemed to be most appropriate for the solderability and conductiv- ity requirements of the polymer paste. Coating amounts ranged from 0.30 – 4.00% with the most desirable level being 0.40 – 0.60%. rheology and Printability After requirements for solderability and con- ductivity were achieved, the pastes containing the preferred silvers were evaluated by screen printing. Using a 280 mesh, 0.5-mil-emulsion, stainless-steel screen, each paste was printed onto a 2-in. x 2-in. piece of FR-4 substrate. However, the addition of various additives was needed to achieve the most desirable printabil- ity and line definition. Choosing the proper additives for a polymer paste is more critical than for a high-temper- ature firing paste. Usually, traditional cermet paste's inorganic materials will remain in the film after the burn-out process, but organic additives will be removed. When considering polymer thick film, both organic and inorgan- ic materials will remain in the final film after processing. Thixotropic compounds added to any formulation of a polymer conductive paste will remain in the cured film and can diminish properties such as adhesion, solderability and conductivity. Thermal Analysis When defining the optimal curing tempera- ture for a polymer conductor, a thermogravi- metric analysis (TGA) must be considered. All volatile solvents must be removed from the polymer conductor's cured film. Furthermore, it is important to ensure that none of the polymer LoW-TemPerATUre THIcK FILm PASTeS PermIT LeAD-Free SoLDerInG ArTiCle Figure 2: A comparison of the solderable silver conductor with two additional silver conductors utilizing different resin systems.