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Stencil Aperture Reduction Guidelines

Chris May

#8171

Stencil Aperture Reduction Guidelines | 15 December, 1999

Can people let me know what reductions are typical for stencils. This can obviously be affected by the technology of the board, but for chip caps and R's do you use 1:1 and only reduce for finer pitch stuff ?

Your help would be appreciated.

Regards,

Chris.

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#8172

Re: Stencil Aperture Reduction Guidelines | 15 December, 1999

Hi Chris,

the same reduction as for fine pitch (10-15%)has decreased the amount of solderballs enormously. Sure, with adequate pad design and good printing equipment and processcontrol you can get the same results but it does no harm as long as you have enough solder left and proper wetting angles and that�s what we still have.

M2c

Wolfgang

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#8173

Re: Stencil Aperture Reduction Guidelines | 15 December, 1999

I agree with Wolfgang. We are currently reducing appertures by approx. 10% even on the chip components. This still gives us an adequate fillet volume and very greatly reduces the probability of solder balling/beading. We generally refer to the manufacturer for any fine pitch parts that are being processed for stencil requirements.

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Mike Naddra

#8174

Re: Stencil Aperture Reduction Guidelines | 15 December, 1999

As a general guidline I would agree with a 10% aperature reduction on chip caps , resistors, and fine pitch provided correct land geometries. When soldering to HASL surface finishes. Three things you may want to consider ; the stencil thickness, the surface finish on the PCB and the solids content of the paste you are using.

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Victor Salazar

#8175

Re: Stencil Aperture Reduction Guidelines | 17 December, 1999

Weare using a Home plate design on 1210 or larger, 20% reduction on pkgs from 1206 to 1210, 10% reduction on 1206 or smaller, 0 reduction on all leaded components(sot-23, IC's) with a 6 mil stencil. For fine pitch we are using 4mil stencils. We have had very good sucess using the DEK 265lt with under stencil wipe/vacuum.

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Calvin Wong

#8176

Re: Stencil Aperture Reduction Guidelines | 17 December, 1999

One thing we found out when we did new stencil is this, you reduce the stencil aperture by 10% is actually based on the Gerber. Please note also that the PCB supplier also makes the PCB according to the Gerber and there is a reduction on the pads as well. So if the PCB house reduced the pads by 10% by the Gerber and you reduced the stencil aperture 10% by the Gerber, you end up having a 1:1. So, please check the PCB that you have and then decide how much you need to reduce the stencil aperture.

Regards.

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#8177

Re: Stencil Aperture Reduction Guidelines | 20 December, 1999

Oh yes, that�s the thing with the PCB houses, always doing such things with our precious data and or wishes of the ideal paddesign. Without any aperture reduction we would have enough troubles with the PCB�s we mostly get.

Wolfgang

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Keith Stone

#8178

Re: Stencil Aperture Reduction Guidelines | 24 December, 1999

When you get to fine pitch you might want to try reducing the aperture on the long dimension of the pad only. If you reduce the width of the pad you can get solder paste left in the stencil giving insufficient or unsoldered joints. Reducing the length of the pad reduces bridging and gives enough solder for a good joint.

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