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Stencil Design

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#58962

Stencil Design | 1 June, 2009

Hello there,

I have a new board, fairly heavily populated with a number of FPBGA on it. The smallest has a pitch of .4mm.I have 3 questions, 1st what is the best appeture for this small pitch. 2 Will type 3 paste work or will I need type 4, and finally what will my stencil thickness be. We usually run around 5thou but I think this might even be too thick. Electroform is the way for the stencil but not sure of the thickness

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#58977

Stencil Design | 2 June, 2009

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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#58982

Stencil Design | 2 June, 2009

Where do you getyour stencils from, ask them, I buy everything from DEK(Vector Guards) and they help me 100%. Apperture size, we always have cut 1:1 for the BGA's you can try 4mil stencils. But you have to keep in mind what the other parts you have on board as well.

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#58993

Stencil Design | 3 June, 2009

Try Fineline Stencils, they help us all the time. They are the experts at what they do, they will help you with stncil thinkness, app sizes and the ratios needed for a great print.

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#58997

Stencil Design | 3 June, 2009

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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