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64 SMT Magazine • January 2015 • Type 6 low viscosity (300–400 poise) dispensing solder paste also worked well for reservoir printing Figures 5a and 5b show the flux deposits on the flex circuit pads for the lowest viscosity flux (160 poise) for the forward and reverse squeegee stroke. There is flux bleed at the trailing edge of the squeegee stroke. The flux deposit definition improves at higher flux viscosities as can be seen in Figure 6, which shows results of the highest viscosity 250–350 poise flux. The stencil results also de- pended on the stencil zone and aperture size. The differ- ent zones produced vastly dif- ferent results. In Figure 6, the left side shows flux deposits for zones 1, 2, and 3 for a 2 mil (50 µm) thick stencil. The right side shows flux deposits for zones 1, 2, and 3 for a 3 mil thick stencil. The best re- Figure 5a: Flux deposits on flex flip-chip pads us- ing 160 poise viscosity flux (front-rear squeegee direction). Figure 6: Flux deposits for Zones 1–3, for 2 and 3 mil thick stencils. Figure 5b: Flux deposits on flex flip-chip pads us- ing 160 poise viscosity flux (rear-front squeegee direction). 2 mil (50um) Thick Stencil 3 mil (75um) Thick Stencil Zone 1: 100um aperture Zone 1: 100um aperture Zone 3 125um aperture Zone 3 125um aperture Zone 2 112um aperture Zone 2 112um aperture STEnCIL PrInTInG In PCB CavITIES continues the short sCooP