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SMT-Jan2015

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66 SMT Magazine • January 2015 sult was produced by the 2 mil thick stencil with apertures of 100 µm square (zone 1). As the aperture size gets larger, 112 µm (zone 2) and 125 µm (zone 3), the flux spreads and shorts between sites. The 3 mil (75 µm) thick stencil exhibits missing flux deposits for both zones 1 and 2 as well as excess flux for zone 3. Results for the solder paste were very en- couraging. Solder brick deposits were made when printing at 25 mm/sec at 2 kgm pres- sure for a 3 mil (75 µm) thick stencil. The top portion showed solder deposits along the leading edge. The bottom portion showed sol- der deposits along the trailing edge which re- sulted from a higher paste deviation near the step edge. Conclusion Reservoir flux and solder paste printing can be done in one step with a 3D electroform stencil, but additional studies and techniques need to be explored to achieve the best result. Our study shows that a rubber-pump squee- gee blade is necessary for flux to transfer out of the reservoir onto the substrate flip-chip pads. Higher flux viscosities (250–350 poise) gave better flux deposit results. Solder paste reservoir printing of small apertures (100 µm) is possible with type 6 low viscosity (350–400 poise) dispersion solder paste. 100 µm (4 mil) square apertures worked best; 125 µm (5 mil) apertures allowed too much flux and solder paste bridging. We also learned that the test cavity set-up needs to be perfected to avoid the trailing edge of the squeegee blade stroke next to the reservoir wall. SMT realtimewith.com CliCk To View Video interView IPc President John Mitchell opens the HKPca 2014 show with a speech on what to expect in eMS trends in 2015. Keynote Speech: Electronic Manufacturing Service Trends for 2015 by Real Time with... HKPCA 2014 Rachel Miller-Short is vice president of sales and marketing at photo Stencil llC. To read past columns or to contact the author, click here. STEnCIL PrInTInG In PCB CavITIES continues the short sCooP

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