Issue link: https://iconnect007.uberflip.com/i/609948
December 2015 • SMT Magazine 79 conductor is 3mΩ/sq (when normalized at 12 μm). The polymers within the thick film poly- mer conductors do not burn off as traditional organics in a typical thick film material. While these remaining organics are impor- tant to the physical properties of the material, creating adhesion and impacting rheology, they can create gaps in the point-to-point contact of the silver particles. This interruption of contact increases the resistivity of the polymer conduc- tor. Table 2 shows the resistivity of the polymer paste when processed on a variety of substrates evaluated during testing. By increasing the curing temperature, more desirable conductivity results may be achieved. Curing at higher temperatures tends to decrease the level of polymer in the cured film. A prop- er curing is critical to finding a balance for the most desired properties of the cured film. Figure 3 shows the difference in resistance when the polymer paste is cured at 150°C and 200°C. A considerable difference is noticed in the level and spread of resistance values at different cur- ing temperatures. Polymer Paste Solderability SAC305 (96.5Sn3Ag0.5Cu) solder was chosen for this soldering process due to its popularity in the market as a replacement for Sn63 alloys. The high melting temperature of SAC305 poses many challenges in the de- LoW-TemPerATUre THIcK FILm PASTeS PermIT LeAD-Free SoLDerInG Figure 3: Boxplot of polymer paste resistance. The spread of the resistance values are shown between the two cure temperatures of 150°C and 200°C. Figure 4: The polymer paste with SAC305 solder on fr-4. ArTiCle