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76 SMT Magazine • December 2015 counterparts. This is due to two disadvantages – reliability and process-ability – prevalent in polymer thick films. Although many polymer thick films are considered to be reliable, they do not have the typical life span of a cermet (ceramic-metal) paste. It is difficult to use lead-free solders when working with polymer materials. Frequently, the processing temperatures of the solders are greater than those of the polymers used to cre- ate the conductive pastes. This leads to the leaching of the conductive materials and the poor wetting of the solder. To solve these problems, a new polymer thick film paste has been developed that is compatible with a variety of substrates and readily accepts lead-free solder. This easy-to- process paste features low temperature curing (150°C – 200°C); offers excellent solderability with SAC305 solder; and is RoHS- and REACH- compliant. It allows conductive polymer pastes to be used in a variety of functions such as po- sition sensors, low-temperature heater applica- tions, and tempered-glass connection points. This article will present data that shows how the thick film paste was assessed under various parameters including solder acceptance, adhe- sion, and thermal and SEM analysis. raw material Selection To achieve a successful formulation, four characteristics were kept in mind. The resulting ink should be chemically durable, have high mechanical strength, achieve a conductivity of less than 35mΩ/sq/mil, and most important, be solderable with SAC305 solder. In order to attain these properties, careful selection and screening of raw materials was necessary. Polymer Selection: Phenolic resin was cho- sen as the appropriate polymer for the devel- opment of the polymer paste because thermo- setting resins are durable at elevated soldering temperatures; phenolic resin will not usually encapsulate silver in the curing process leading to better conductivity; and it has the ability to be cured at low temperatures. Silver Selection: The primary goal of the silver selection was to manage solderability and conductivity. Silvers were observed at an 80% loading in a phenolic-based vehicle. More than 20 different silvers were compared for various powder/flake combinations as well as the evalu- ation of single-silver systems. Figures 1(a) and 1(b) show the SEM images of the final silver particles chosen for the final product. A dip-soldering method was used to gauge each silver sample for its ability to accept SAC305 solder at 235°C. The substrates creat- ed for the initial solderability trials were 1-in. x 2-in. pieces of FR-4 (glass-reinforced epoxy laminate). Each substrate had a 0.5-in. x 2.0-in. section of paste applied to its center. The paste was cured at 150°C for 30 minutes and resulted in a cured thickness of 25–30 μm. LoW-TemPerATUre THIcK FILm PASTeS PermIT LeAD-Free SoLDerInG ArTiCle Figure 1a: Silver powder. Figure 1b: Silver flake.