Stencil Design

I was hoping some-one would help you with this because I too would like more information about selecting stencil thickness. I have read some about apperature ratios, but this alone doesn't appear to be the only factor when chosing stencil thickness, ie. when you use a full apperature vs. a home-plate modified apperature. Can anyone recommend where to find more details on selecting stencil thickness? Thanks.

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Area Ratio [PCB Assembly Guidelines for 0.4mm Package-On-Package (PoP) Packages, Part II, TI Application Report, SPRAAV2-April 2008, K Gutierrez & G Coley] The relationship between the surface of the aperture and the inside surface of the aperture walls in the stencil is referred to as the area ratio (AR). The area ratio is much more suitable for shapes such as circles than the aspect ratio. For fine pitch BGA pads, a round aperture is recommended. The area ratio of >= 0.66 aperture opening to the stencil wall has been shown to provide the best transfer efficiency and repeatability of the deposited paste. Values from 0.66 to 0.8 insure a good paste release from the stencil. The biggest impact on area ratio is the stencil thickness. The equation is: AR = Ap / Aw � 0.66 Where Ap is the aperture opening area and Aw is the wall area: 0.66 >=[pi*d^2/4]/pi*d*T=d/4T Where r is the aperture opening radius and T is the stencil thickness. Table 1 shows the various values of AR for different aperture openings and stencil thickness. Table 1. Stencil||Aperture Opening (thou)||Aperture Radius (thou)||Stencil Thickness (um)||Stencil Thickness (thou)||AR 100 um||10||5||100||3.94||0.64 ||11||5.5||100||3.94||0.70 ||12||6||100||3.94||0.76 ||13||6.5||100||3.94||0.83 ||14||7||100||3.94||0.89 ||15||7.5||100||3.94||0.95 ||16||8||100||3.94||1.02 125 um||10||5||125||4.92||0.51 ||11||5.5||125||4.92||0.56 ||12||6||125||4.92||0.61 ||13||6.5||125||4.92||0.66 ||14||7||125||4.92||0.71 ||15||7.5||125||4.92||0.76 ||16||8||125||4.92||0.81 5 thou||10||5||127||5||0.50 ||11||5.5||127||5||0.55 ||12||6||127||5||0.60 ||13||6.5||127||5||0.65 ||14||7||127||5||0.70 ||15||7.5||127||5||0.75 ||16||8||127||5||0.80 6 thou||10||5||152.4||6||0.42 ||11||5.5||152.4||6||0.46 ||12||6||152.4||6||0.50 ||13||6.5||152.4||6||0.54 ||14||7||152.4||6||0.58 ||15||7.5||152.4||6||0.63 ||16||8||152.4||6||0.67

As for solder paste type selection, search the fine SMTnet Archives for threads like: http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=24034

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