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80 SMT Magazine • December 2015 velopment of a low-curing, solder-accepting polymer paste. The cure temperature of the polymer paste is typically ≥50°C below the suggested reflow temperature of the SAC305 solder alloy. The melting point of SAC305 is between 217°C– 219°C. For the test procedures, the solder pot temperature was set to 235°C–250°C depending on the substrate choice. The polymer paste was originally designed to be solderable using SAC305 solder with 615 RMA flux. Solderability was determined by ob- serving 40 x 40 mil pads after a dip-soldering method of five seconds. Figure 4 shows the ini- tial solderability testing of the polymer paste on FR-4 with SAC305 at 235°C. After confirming solderability with SAC305, other Pb and Pb-free solders were tested. Included in the additional testing, the oth- er solder choices were: 62Sn2Ag36Pb (Sn62), 10Sn2Ag88Pb (10/88/2) and 95Sn5Ag (95/5). All solders were tested in the same method as described above, with the exception of temper- ature (Figure 5). Figure 5 shows the wettability of the different solders on the polymer paste when printed on FR-4 substrates. For the remainder of the substrates tested, soldering was performed with SAC305 solder and 615 RMA flux. Solderability was performed on all substrates cured at both 150°C and 200°C. The solderability of all substrates consid- ered in this experiment is shown in Figure 6. LoW-TemPerATUre THIcK FILm PASTeS PermIT LeAD-Free SoLDerInG ArTiCle Figure 5: Solderability of the polymer paste with different alloys: (a) Sn62 at 230°C; (b) 10/88/2 at 330°C; (c) 95/5 at 270°C. Figure 6: Solderability of the polymer paste on various substrates: (a) FR-4; (b) alumina; (c) silicone dielectric on aluminum; (d) epoxy/polyimide dielectric on aluminum. note: The pictures labeled "1" represent a 150°C curing and the pictures labeled "2" represent a 200°C curing.

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