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Can anybody advise on repairing PWB edge gold contact finger... - Mar 20, 2001 by PeteC

Contact ... - Mar 20, 2001 by Michael Parker

Although I did directly reply to your request, I have a seco... - Mar 20, 2001 by Michael Parker

Consider the fine SMTnet Archives, in addition to the earlie... - Mar 20, 2001 by davef

We're watching more carefully at post-print and pre-reflow i... - Mar 21, 2001 by PeteC

The root cause is dependant on the type of solder "splash", ... - Mar 21, 2001 by Michael Parker

Well, it's hard to say what it started out as, a smear, spla... - Mar 23, 2001 by PeteC

You are mixing long exposed paste back into fresh? How long ... - Mar 23, 2001 by Michael Parker

Fresh paste from a factory sealed container. Should paste be... - Mar 24, 2001 by PeteC

PeteC

#5646

Gold Contacts | 20 March, 2001

Can anybody advise on repairing PWB edge gold contact fingers that get solder on them?

reply »

Michael Parker

#5657

Gold Contacts | 20 March, 2001

Contact http://www.circuittechctr.com. They have a rework process and sell the necessary materials. You can buy the complete system, or just the gold solution and use your own power supply for the electrolysis.

Be sure to completely evaluate and understand the process before deciding what to do and buy.

There is cyanide in the solution, have very good ventilation for whomever is doing the actual rework.

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Michael Parker

#5658

Gold Contacts | 20 March, 2001

Although I did directly reply to your request, I have a second thought.

What are you doing to prevent the defect? Do you know for sure what the cause may be?

Do you need help in finding the answer?

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davef

#5669

Gold Contacts | 20 March, 2001

Consider the fine SMTnet Archives, in addition to the earlier comments.

reply »

PeteC

#5670

Gold Contacts | 21 March, 2001

We're watching more carefully at post-print and pre-reflow inspections for products with the gold finger contacts. Not 100% sure the cause. SMT line people suspect primary cause is improperly wiped stencil. Do you know any other possible causes? We got 3 boards with solder on the contacts (only one contact each) out of 2,200 assembled. We use human post-print pre-reflow inspection. Thx.

reply »

Michael Parker

#5681

Gold Contacts | 21 March, 2001

The root cause is dependant on the type of solder "splash", ball, whatever, that you are seeing post reflow.

If it is smears, then cleaning the stencil underside is probably the answer. Are you using automatic wipers? How many print cycles between wipes? Are you "dry" wiping or using a cleaner agent?

Large solder balls - greater than 3 mils in dia. are probably also a cleaning problem.

Very small balls, the ones hard to see with the naked eye could come from flux outgassing too fast, blowing paste everywhere. This kind of "popcorning" has many causes. Paste control - how long is the paste open to ambient atmosphere and collecting moisture? Do you have adequate moisture control for your components? Reflow profiles should be analyzed - maybe getting too hot too fast, causing the flux to boil, outgass and blow.

Paste volume - are your stencil apertures optimized for geometry and aspect ratio? Especially be concerned with pads nearest the gold fingers, this is where the outgassing of flux can blow solder onto the fingers.

When all else fails, consider processing aids (mask the fingers).

reply »

PeteC

#5716

Gold Contacts | 23 March, 2001

Well, it's hard to say what it started out as, a smear, splash or ball cause the solder formation on the finger after reflow is spread out ya know. I looked at an archive for possible causes and one was printer operators with paste on their fingers. Possible cause. Wiping is more likely I think. We're wiping by hand w/ JnJ Wipes and alcohol sprayed on the wipe. Frequency depends on the board. Fine-Pitch boards, like the one in question, get a wipe every 4 boards. Time paste sitting on the stencil varies depending on production or any corrective maintenance going on. If too long, like hours and hours, the printer operator removes the paste and mixes in with some fresh and stirs it up. We have tight controls in place for mositure sensitive components and PWBs. Our reflow profiles get to 140C in 2 minutes (from 25C), then to 179C in just over 3 minutes and time-over-liquidous is just over 1 minute. Peak temp (220C)is reached in like 4 minutes from 25C. Cool-Down from 179C to 60C is around 2 minutes. How's that? Slow enough ya think? For paste apertures I go w/ IPC-7525 standards. We measure paste height and volume with a stand-along laser measurement machine. Using a 5mil stencil on this one. Thx.

reply »

Michael Parker

#5721

Gold Contacts | 23 March, 2001

You are mixing long exposed paste back into fresh? How long has your fresh jar been open to the air? You are probably doing yourself a disservice by scavenging the paste (it could have moisture entrapped). Your visocity has definitely changed.

You are close to the answer with the hand cleaning. You have mentioned which paste brand you use. The use of alcohol may not match well with the cleaning of the flux. Check with your supplier for recommended cleaning agent.

How much drying time after stencil cleaning are you waiting before the next print cycle? Your apertures could have residual alcohol that causes the splatter.

If your using no-clean, try another brand or make. No-clean splatters more than RMA.

IPC-7525 is a good start but you may still need to play with variables until you achieve maiximized results. Going a bit lean on paste volume at selected sites (just getting enough to avoid insufficient solder) could help reduce the splatter.

Good luck

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PeteC

#5727

Gold Contacts | 24 March, 2001

Fresh paste from a factory sealed container. Should paste be scrapped out altogether if left on the stencil for too long (length-of-time?)? Our paste is Multicore CR39. Multicore reccomended the print chamber be around 22C @ 50%-60% RH. Our plant is pretty dry, typilcally 30-40% RH and sometimes down to 20% in winter. They said too low RH was bad cause flux will dry out faster. So yer sayin too much moisture is bad too. What should be the RH inside the print chanber? Multicore says alcohol is best for the CR39 (we're in the process of goin to MP100 and/or possibly Qualitek 691). We have an air-gun at each machine, set on low pressure, to blow dry the stencil after wiping. 10-4 on the IPC-7525 thing. Thx.

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