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stepped stencils

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#39954

stepped stencils | 23 February, 2006

What are some general guidelines for the minimum proximity of apertures to a stepdown on a stepped stencil. I have a step from 7 mils to 5 (one mil step each on the top and bottom). How far does the step need to extend beyond the apertures on the 7 mil foil? How close to the step can I be and get a good print on the 5 mil foil. I need to print 25 mil SMT and 0.050" pitch BGA's nearby.

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#39958

stepped stencils | 23 February, 2006

Not sure on stepped stencil designs, very application specific. Try consulting with your stencil supplier. Most of the big suppliers have applications engineering support. You can send them your board data and they can review and advise. Also, consider solder preforms on tape and reel as an alternative to stepped stencils. Pick and place and solder preform where you need more solder.

http://www.indium.com/products/circuitboardassembly/solderpreforms.php

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KEN

#39960

stepped stencils | 23 February, 2006

Depends what is adjacent to the step.

1206 chips could reside closer than say a 16 mil fine pitch. Larger apertures are more forgiving to height variations than small apertures.

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#39971

stepped stencils | 24 February, 2006

Step stencils are not widely used, due to: * Higher cost as compared to uniform thickness stencils * Danger of solder paste smearing at least close to the steps * Difficulty of cleaning the stepped stencil underside (the steps cannot be on the upper side) * Increased wear on the squeegee due to pressure variation over its length.

That aside, IPC-7525 says words to the effect * Keyout distance from edge of step-up to edge of aperture is 36 times the amount of step-down thickness. * Keyout distance from edge of step-up to edge of aperture is 0.9mm [35.4 thou] for every 0.025mm [0.98 thou] of step-down thickness.

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#39976

stepped stencils | 24 February, 2006

Kenny,

Why are you using a step? The components you list could be done with the 7 mil stencil. I would use 6 mil myself but I assume you may need the 7 mil for other components you do not list. Jerry

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RDR

#39978

stepped stencils | 24 February, 2006

I agree with Jerry. We don't use 5 mil until we get to .4mm pitch.

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Ticky Ticky Tembo No Sarimbo Hari Kari Bushkie Perry Pem Do Hai

#39990

stepped stencils | 24 February, 2006

Stepped stencils are usually the result of some dumb customer's requirement. 2 examples I can give:

1.) Customer demanded 50% wick-up the termination of a MELF component. No way you can get that from reflow. Played around with a drastic "step up" to 12-mils, the foil was 6-mils. We got more solder volume in that area but even close to 50%

2.) Customer needed to have more solder volume on some SMD fuses for "noise purposes" (ie he was using solder joints as noise filter or antennaes).

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Chunks

#39993

stepped stencils | 24 February, 2006

I agree with Jerry. A 7 mil stencil should work just fine for 25 mil pitch parts and 0.050 pitch BGA's. I too go 6 mil, but whith proper screen printing, you can 7 mil. You can always do 5 ot 10% reductions if needed. Use the homeplate design on R's and C's to eliminate solder balls/beads. Another thing to look at is the resist on your board. Make sure there is resist (or sometimes called mask) between the pads of the 25 mil pitch and BGA pads. This will also help eliminate shorts.

Unless it's something like Ticky Ticky Tembo No Sarimbo Hari Kari Bushkie Perry Pem Do Hai brough up. Then you may be up the creek. Cosmetic solder joints are a pain and actually useless like 3TNSSuicideBushPerP-Do-H stated.

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