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February 2014 • SMT Magazine 15 FINe-TuNING THe STeNCIL MaNuFaCTurING PrOCeSS continues both front-to-back and back-to-front squeegee strokes, with an automatic dry wipe after each print. Print parameters were: • Print speed: 7 mm/sec • Print pressure: 8 kg (250 mm blades) • Separation speed: 20 mm/sec • Wipe sequence vacuum/dry/vacuum The solder paste used in all tests was Indium 3.2 HF Type 3, water soluble, lead-free, halogen- free, lot numbers PS52867 and PS54561. Fresh paste was used on each stencil. The paste was not kneaded; two dummy prints were produced before measurements were taken. The 17 sten- cils were print tested in a climate controlled NPI manufacturing area over 9 different runs. Dur- ing the tests the room temperature ranged from 22.0–25.3°C, and relative humidity ranged from 36.3–42.9%. The PCB was supported with a flat, non- vacuum tooling plate and edge clamps. Deposit volume measurements were taken with a Koh Young 3020VAL using a Bare Board Teach to set the reference plane. Test Matrices All the experimental stencils were pro- duced by the same supplier. Their thickness was specified at 4 mil. The laser cut stencils were all produced on the same cutter within a two-week period. The first-generation nano- coating, Nano1, (DEK NanoProTek) was ap- plied to the specified stencils at the supplier's site; second-generation coating, Nano2, (Acu- lon NanoClear) was applied at the test site. The designs of the individual experiments are listed in Tables 1–4. results aperture Measurements To calculate actual transfer efficiencies and area ratios, the stencils' apertures and thick- FEATUrE Table 1: laser cutting parameter experiment. Table 2: New stencil materials experiment. Table 3: MicrobgA aperture shape experiment. Table 4: Under wipe experiment.