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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


John Thorup

#11368

QFP solder paste volume | 21 May, 1999

We are running a number of boards with 8-10 25 mil PQFPs on board. Our process is mature with at least 3 years experiance and statistically few opens on the QFPs. Our beloved purchasing department has chased the almighty $ and changed FAB vendors with questionable results. Since the new vendor was properly qualified and the FABs are not technically defective I am stuck with them. The problem ranges from no release to insufficient release on 1 axis caused by HASL height irregularity preventing proper contact and lowering the surface tension to a point where the release is not complete. While the great majority of these boards will print, the paste volume is lower on 1 axis resulting in an unacceptable number of opens. I first thought to increase the stencil thickness from 6 to 7 mil but concluded that this would just exacerbate my surface tension problem. My stencils are electropolished chem etch. Does anyone havea comments about lazer cut stencils with a trapezoidal shape? other suggestions? (other than what to do with purchasing) Thanks

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JohnW

#11369

Re: QFP solder paste volume | 23 May, 1999

| We are running a number of boards with 8-10 25 mil PQFPs on board. Our process is mature with at least 3 years experiance and statistically few opens on the QFPs. Our beloved purchasing department has chased the almighty $ and changed FAB vendors with questionable results. Since the new vendor was properly qualified and the FABs are not technically defective I am stuck with them. The problem ranges from no release to insufficient release on 1 axis caused by HASL height irregularity preventing proper contact and lowering the surface tension to a point where the release is not complete. While the great majority of these boards will print, the paste volume is lower on 1 axis resulting in an unacceptable number of opens. I first thought to increase the stencil thickness from 6 to 7 mil but concluded that this would just exacerbate my surface tension problem. My stencils are electropolished chem etch. Does anyone havea comments about lazer cut stencils with a trapezoidal shape? other suggestions? (other than what to do with purchasing) Thanks | John,

The laser cut one's are probably goning to give you better release altho there have been problems in the past with rough edges in the appetures with laser cuts, but on the whole these have been sorted. But to me it sound's like your trying to get your process more robust (which is alway's a good thing) to solve a vendor problem. I have to say that even on their best day's purchasing folk aint my favorite people..unless they're buying nice pressies for me to play with..anyway's. Even if this vendor is qualified it's maybe worth having a poke around the fab plant , how long has it been since your last qualification audit..? Even if it was last month your getting quality problems cos he cant level off his boards properly, either he's not got the air knife settings right or his hasl pot aint right. You may also what to look at your fab spec's..are they tight enough for the 25 thou devices anyway??? My fist call would be to the fab house and get them to check out their process..also let purchasing know the agro this is causinf. As for stencils..generally you should get better result's from the laser..but I'm sure Earl, Dave and co will keep me right..

all the best

John

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#11370

Re: QFP solder paste volume | 24 May, 1999

| We are running a number of boards with 8-10 25 mil PQFPs on board. Our process is mature with at least 3 years experiance and statistically few opens on the QFPs. Our beloved purchasing department has chased the almighty $ and changed FAB vendors with questionable results. Since the new vendor was properly qualified and the FABs are not technically defective I am stuck with them. The problem ranges from no release to insufficient release on 1 axis caused by HASL height irregularity preventing proper contact and lowering the surface tension to a point where the release is not complete. While the great majority of these boards will print, the paste volume is lower on 1 axis resulting in an unacceptable number of opens. I first thought to increase the stencil thickness from 6 to 7 mil but concluded that this would just exacerbate my surface tension problem. My stencils are electropolished chem etch. Does anyone havea comments about lazer cut stencils with a trapezoidal shape? other suggestions? (other than what to do with purchasing) Thanks | John: John W made some good comments. Several things come to mind (feeble tho that may be on Monday morning, geez it's afternoon!! See I told ya!!!):

1 My choice: 1a Short term: Send the boards back to the fab and re-level the pads for a more consistent solder thickness. But even if your buyer pays, most likely, your company will be a head from a total cost of production. Either way, the buyer will not be happy camper. But, this may not be a good choice is the re-leveling will load-up your through holes with too much solder, which shouldn't happen, but it's a good red herring. 1b Long term: Work with your supplier to establish requirements for control and consistency. 2 I'm not real excited about increasing stencil thickness, because that may not solve your problem, as you mentioned, and may create other problems else where on the board. But if the over/under solder coating is consistently on the same row of pads (which wouldn't suprise me), either: 2a Re-etch your stencil to open-up the pads that have light HASL deposits. OR 2b Make another stencil for that fab suppliers boards with wider pads that match-up with the light HASL coating locations. 3 Purchase a new stencil made of a material with better gasketing / release. 3a Laser cut trapezoidal shape aperture stencils that you mentioned is a good alternative. 3b Plastic stencils (that've been discussed on several previous SMTnet threads) has better gasketing than metal. Remember that people using plastic creates ESD problems. 3c Electroformed stencils is another alternative. AMTX is no longer in the business, but Chepaume Industries (6001 Airport Rd. Oriskany, NY 13424 (315) 768-7001 or at (315) 768-7002 Fax 0270 Melissa chepaume@borg.com) can help. 3d A chem-etch stencil salesman was chattering on SMTnet within the past 6 months about the superiority of the release of his stencils. (Check the archives) Who knows? Depereate times call for desperate measures, actions, or something. 4 Finally, check the layout of your pads. Your previous supplier may have been the hack and you just never noticed. Hey, stranger things have happened.

Good luck

Dave F

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