evaluation of stencil foil materials, suppliers and ... - IPC Outlook

As originally published in the SMTA International Conference Proceedings.Each of these metrics can be applied to the stencil printingprocess to characterize the relationship between processinputs and outputs. In the following study, they are used toselect the best stencil options for a high volume, productionoperation.EXPERIMENTAL SETUPTest VehicleTest VehicleFigure 1. Test Vehicle (non-BGA circuitry on closeup isintentionally blurred)The PCB shown in fig 1 is a typical high-volume productionproduct. Each 32-up array measures approximately 3x7inches, and has nearly 15,000 SMT pads. Of the 14,468pads, roughly 8500 are mask-defined (SMD) BGA pads and1900 are metal-defined (NSMD) 0201 pads. The same setof 10 PCBs were used for all tests.Suppliers A & D applied the coating at their sites, prior toshipping the stencils. The same coating product was appliedto stencils provided by suppliers B & C after arriving at theVicor facility.Test MatrixFour suppliers, coded A-D, submitted stencils in a variety ofconfigurations. Materials, coded 1-5, included: Electroformed stencils (#1) Electroformed nickel foils that were laser cut (#2) Standard 301SS (#5) 304SS designed for laser cutting (#3) 301SS with modified grain size (#4)Thicknesses of the foils included 0.0045” and 0.004”. Thecurrent production standard is 0.0045” laser cut nickel foils.0.004” is under consideration because the preferred 0.0045”is not available in rolled steel.Electropolished stencils were not tested in this evaluation,because not all suppliers provide electropolishing capability,and while electropolised apertures have been reported torelease higher volumes of paste due to their roundedcorners, 2 they have also reported to produce higher rates ofvariation in volume consistency. 3For each stencil, 10 prints were taken, providing roughly85,000 BGA paste deposit measurements and 19,000 0201deposit measurements. The test prints were producedsequentially on a well maintained and calibrated 2009 DEKhorizon stencil printer using, both front-to-back and backto-frontsqueegee strokes, with an automatic dry wipe aftereach print. Print parameters were: Print speed: 15 mm/sec Print pressure: 5 kg (250mm blades) Separation speed: 20mm/secThe solder paste used in all tests was Indium 3.2 HF Type 3,water soluble, lead-free, halogen-free, lot # 37310. Freshpaste was used on each stencil. The paste was not kneaded;2 dummy prints were produced before measurements weretaken. The 27 stencils were print tested in a climatecontrolled NPI manufacturing area over 5 different runs.During the tests the climate ranged from 23.0 to 25.5 o C, andrelative humidity ranged from 32.9 to 46.9%.The PCB was supported with a flat, non-vacuum toolingplate and edge clamps. Deposit volume measurements weretaken with a Koh Young 3030VAL.StencilsEach supplier was invited to submit stencils in pairs. Onestencil was printed in the as-received condition; the otherhad a hydrophobic nanocoating applied before printing.