Issue link: https://iconnect007.uberflip.com/i/1137649
JULY 2019 I SMT007 MAGAZINE 81 Per Table 2, the working temperature of PET films is limited to about 150°C. This is the temperature that they can be subjected to for an extended period and rules out the use of most solders that have a higher melting point. But PET films can withstand higher tem- peratures for a brief period. This opens the door for using a variety of low-tem- perature solders for assembly of com- ponents to Al-PET substrates. Most of the solders are made up of tin (41– 42%), bismuth (57–57.6%) and silver (0.4–2%) in various flux systems with a melting point of ~138°C. The high- est temperature during reflow is about 185°C but only for a very short time. Assembly Trials Made Using Surface Treatment and Related Observations Boards were made with 10-mm aluminum foil laminated onto 125-mm film of PET. The etched design was a smart tag that had com- ponents including resistors, capacitors, LEDs, QFNs, battery holders, and RFID chips. The surface finishes of these components were typ- ically plated with Ni, Pd, Au, or Sn. Special fix- tures were designed to hold the boards during processing. Figure 4 shows a partial view of the smart tag design used. The trials were done at a local PCB assem- bly house and consisted of two steps. The first step was to mount etched aluminum panels on the fixture and print surface treatment com- pound using a stencil made of stainless steel, 737 x 737 mm, 50 mm. A conveyor-based reflow oven was used to cure the surface treatment com- pound at 85°C. The second step was to use the same fix- tures to hold the panels dur- ing stencil printing of sol- der, pick-and-place loading of components, and reflow oven processing. The sten- cil used for the solder paste was stainless steel, 737 x 737 mm, 100 mm. The solder used was commercially avail- able, lead-free, low-tempera- ture, no-clean Sn/Bi/Ag with a melting point of 138°C. The reflow cycle used was as rec- ommended by the manufac- turer of the solder paste. Figure 4: Smart tag design used to test surface treatment. Figure 5: Shear test results showing failure between aluminum and PET substrate (package: capacitor 0805, shear value: 22N/mm 2 ).