screen thickness

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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If you failed math like I did. A 0.005 inch stencil would work fine for those parts. Most stencils are 0.004in for fine pitch 1005s 201s. 0.005in is probably the most common generic stencil. 0.006in generally is if you need extra solder for big parts.

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