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62 SMT Magazine • January 2015 was about the same as the high-viscosity flux (~300 poise). Five different types and viscosities of flux were used: Type viscosity Poise Indium 3600 160 indium 676 200 alpha 353 Black <150 alpha 390 140–250 Alpha 338 250–350 The printing pressure applied was 1.5–6 kgm for a 12" blade with speeds of .12 mm/sec- ond to 150 mm/second. There was a dry wipe after each print. After printing, the boards were inspected by a solder paste inspection system. Print and Inspection results The results show, as with step mode print- ing, that the type of squeegee blade used makes a difference. • Metal squeegee blades did not apply enough downward pressure to transfer flux or solder paste through the 100 µm (4 mil) square apertures onto the recessed flip-chip pad sites • Rubber squeegee blades with a square edge were also ineffective in transferring flux or paste STEnCIL PrInTInG In PCB CavITIES continues • The pump-print rubber squeegee blade applied sufficient uniform pressure over the reservoir to achieve good transfer of flux and solder paste to the pad sites • A self-contained print head squeegee system (although not available for this testing) most likely would also provide uniform pressure over the reservoir resulting in good transfer of flux or solder paste to the pad sites One problem we encountered was that the flux was too transparent to the laser optics of the solder inspection machine to measure flux volume. It was also difficult to distinguish the pad from the flux deposit because of the flux transparency. However, we were able to con- struct a reservoir cavity 356 µm (14 mil) deep over a blank FR-4 PCB to view flux deposits on a clean surface. Examination of the boards showed: • The lowest viscosity flux (160 poise) gave the poorest results. Flux deposits were left on the flex circuit pads and there was flux bleed at the trailing edge of the squeegee stroke • Alpha 338 with a viscosity of 250–350 poise worked the best. This was the highest viscosity flux for the group of fluxes tested Figure 3: close-up of cavity with apertures. Figure 4: pump-print rubber squeegee blade. the short sCooP

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