Life of a Solder Stencil

IPC-7525 "Stencil Design Guidelines" �9 END OF LIFE, states "Stencils should be inspected periodically for damage that would conribute to decreased printing performance. Refer to Section 7 for inspection guidelines." Or words to that effect.

Now before you get yourself in lather buying IPC-7525, Section 7 contains no quantative measures. It's more like "make sure the wheel in Alvin & The Chipmunks' cage is oiled before asking them to sing" kinda stuff.

Which is just what we do. It's too BIG a pain, for such a small pay-off to do otherwise. Just set-up an inspection schedule based on annual production volume and inspect the stencils using a 12 point rating scheme [using the nice things you talked about or those in IPC-7525]. Any stencil with less than 57 points gets MRBd. That'll shut 'em up, I'd bet.

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I've never tracked the life of a stencil, but if your seeing an decrease in the life of your stencil you may want to look at the printer itself. What kind of printers are you using? I'm more familiar with MPM screen printers and I have seen were certain parameters are out of spec for a .006" stencil the printer will wear the stencil out in no time.

You may want to check and see if the amount of pressure being placed on the stencil is correct or close to what the equipment manufacture recommends when the machine is set at factory specs. If its not you may want to look at the set up of the print head. May need rebuilt or be out of calibration. Also if your machine is using a sensing device to tell the machine where the stencil is you may want to make sure it is functioning properly and is set up correctly also.

Good Luck, Jones

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we met the same problem with audit, and i make a tooling log for each stencil and in tooling log , set up some criteria for checking such as tension , mechanical damage, printing result ect. The stencil will be checked periodicly by process engineer, and the cycle is based on the quantity of products per year and all these will be used mainly for audit, but i am still not satified with this, so are there any other good ways of record and track the stencil life?

is it necessary to record the check result after production for each time?

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By far the easiest way is to track solder paste measurement on your board - or solder defects after reflow (key word- easiest). Why spend 2 weeks examining a stencil when it clearly doesn't make defects, or IS making defects. I find using 2-D data from the screen printer is an effective way to tell when a stencil is going bad or when it's been damaged. 3-D is a bit better, but always tends to lead to the dark side of data collecting and should be avoided, or at least dealt with under strict common sense supervision! I've seen new stencils, with no wear, cause solder defects cause they where etched wrong - and I've seen old stencils that shined like chrome cause of the wear but caused no problems (no fine pitch mind you). So I don't think counting strokes on your stencils will get you any closer to determining wear on them or tell you when you're about to have a defect problem (which is the ultimate goal of your boss who asked the silly question, I'm sure).

So I wouldn't spend a lot of time staring at your stencils, cause they won't tell ya much (but very therapeutic). Use your smarts and look at the real reason anyone would ask you to look at stencil wear - which is defects. Then recommend alternative common sense approaches to determining solder paste related defects and ways to measure them (2-D, 3-D and soon to come 4-D!). If you MUST give a number, say 38,560 alternating strokes will wear a common stencil - less if the strokes are in the same direction, by.....oh say 32%! If anything else, just blame the oven profile for all your problems!

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Rather than meausre the stencil, why not measure it's output. The stencils function is to gage the right amount of solder in the right location. If you are measuring paste volume and you are tracking via SPC, you will begin to see a degradation when the stencil begins to wear.

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