Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Home plate or bow tie?

#3970

Home plate or bow tie? | 27 May, 2000

Subject line pretty much says it all. We're going to try some stencil design changes to reduce midship solder balls.

Anyone have some comments on pros or cons for either?

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Micah Newcomb

#3971

Re: Home plate or bow tie? | 27 May, 2000

Steve, I use homebase apertures as a standard for all chip parts and have no complaints, no solder beading, no issues. I have had to replace almost all stencils ordered previous to my arrival as they all caused defects (7-9mil thickness and 1:1 apertures caused beading/balling). The homebase aperture has worked well for me, though you will have to experiment to see what works best for your applications. Good luck Micah

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Sal

#3972

Re: Home plate or bow tie? | 28 May, 2000

Can't agree more than moving over to Homebase apertures, But ensure that the cut back angle is correct,to acute of and angle will leave you with an inadequate contact area fot the terminations on the chip components and you will start seeing insufficients, also make sure that the apertures are central to the pads, if either of the the aperture is sat to far forward or to far back you will see unequal wetting and get the tombstone effect. Double check your Gerber before sending it out .Good Luck.

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#3973

Re: Home plate or bow tie? | 10 October, 2000

I agree that homplate is a good design, but also this could be caused by the stencil thickness and aperture size, if you are at 7-9 mils thick stencil and a 1:1 ratio on aperture size then 2 things i know of will happen, one a squishing out of past onto the masking, causing solder ball formation during reflow, and with that amount of paste it is possible to get outgasing during reflow, blowing paste onto boards in the masked area and agin creating beads of paste to form. One thing to check is your aspect ratio make sure you are greater than 1.5 and one way to find this out is to take your fine pitch aperture size and divide it by your stencil thickness, if you get 1.5 or higher you will be good, if lower you may want to try a thinner stencil. Say 5 or 6 mils Ex. 25 mil piich parts with a .o12" aperture and a .006" thick stencil would = an aspect ratio of 2.0 which wuold be good or a .005" stencil = 2.4 even better so just going to homeplate or bowtie may not be the best solution if you have other process problems.

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