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Solder particals in the oven?

Bob

#20195

Solder particals in the oven? | 4 June, 2002

Hi,

We are manufacturing a small PCB with large areas of gold plating. We experience from time to time what appear to be solder splashes on the gold areas.

I do not believe the contamination to be from "outgassing" due to the distance from the deposited paste and contamination, and it is not from bad cleaning or handling.

The profile has been adjusted and various stencil designs used which has greatly improved the issue.

My question is: - Could this contamination occur from small solder particles being carried around within the oven and occasionally landing on the board and then melting on the gold?

My thoughts were to somehow filter all the circulating air in the oven to either proove or disproove this theory.

Has anyone thoughts on this theory?

Solder type used = Alpha LR-735 Apperture size = approx 80% of pad. Oven = Vitronics XPM Prifile = gentle ramp, small soak peak.

Cheers...

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Hussman69

#20205

Solder particals in the oven? | 4 June, 2002

Hmmm...

I know the Vitronics oven had bigger motors on em', but I don't think they're blowing molten solder balls around.

You did say you changed your profile and stencil designs, which helped, but didn't exactly state what ya did. I believe this may be helpful.

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Bob

#20206

Solder particals in the oven? | 4 June, 2002

The original profile had a higher ramp rate, and the apertures were 100%.

The ramp rate is now much gentler and the addition of a small soak area around 160 � 170 degrees should burn off any excess flux. Appertures are now smaller and we get no leaching.

I don�t think for one minute that the solder is being blown about, but what are the possibilities of solder particles possibly dry, and relatively light being picked up and carried around with any burnt off flux gasses.

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Bob

#20207

Solder particals in the oven? | 4 June, 2002

Did I say leaching I meant bleeding. Doh!!

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#20209

Solder particals in the oven? | 4 June, 2002

Solder balls floating around on the air currents of a reflow oven. I sincerely doubt it.

Could the solder on your gold fingers be from: * Excess placement force during P&P? * Ejactula from solder deposits during rapid preheat ramp rates? * Ejactula moisture in the solder paste boiling-off during heating? * Inadequate under-side wiping during printing? * Poor misprint cleaning? * Operators not removing gloves during board handling?

We've talked about this previously on SMTnet. A search of the fine SMTnet Archives on ... solder and gold and finger ... gives over 130 hits.

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Bob

#20227

Solder particals in the oven? | 5 June, 2002

No.

Placement force is fine. Also boards have been inspected prior to reflow using microscope with no evidance of misplaced solder balls.

The ramp rate has been reduced to acceptable limits, typically .8 - 1.5 degrees per sec. Outgassing from either flux or moisture should be fine, as the paste is stored for above the recomended time from the refrigerator prior to use.

Wipe is fine this would also be detected using inspection pre-reflow.

When we have poor cleaning from mis-prints this gives different (definate) results.

I have simular thoughts to the theory, but I am struggling to find out where else this could be is happening.

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Hussman69

#20231

Solder particals in the oven? | 5 June, 2002

That's OK, as long as you didn't say breading, I knew what you meant.

Well, for a quick fix try placing Kapton tape over the gold. It's not a cure, but gets you product.

Next try putting a couple of non-screen printed boards thru the oven and see what you get. Then try a non-screen printed board thru with some screen printed boards. That could tell you weather it's particles blowing around in your oven. Have you opened up you oven? Maybe you have a zone curtain a cooling fan that has residue and solder balls on it. I used to have a BTU oven that would puke flux residue on the boards to let me know the PM wasn't done on it. Always dripped from the curtain that seperated the last heat zone from the cooling zone. Maybe one of your exhausts is clogged.

Try another brand of oven if possible. That would almost surely tell you if it were your oven. Call an oven manufacturer that's close by and see if you can use one of their demo ovens.

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#20237

Solder particals in the oven? | 5 June, 2002

I have seen this happen before it has been my experience that even one tiny sphere of solder form your solder paste will melt and spread out on your gold fingers. The particles are so small that they are not visible to the naked eye. If operators have ANY paste residue on their fingers what so ever, you are going to have this problem. It is imperative that your operators have clean hands when handling boards with gold fingers. I really do not think that your oven is blowing molten solder around. Although, I have had several operators try to convince me that the oven is blowing their parts off the pads.hehehe. We always tape our gold fingers with Kapton to prevent this problem.

P.S. I agree with Dave you must always beware of rogue ejactula !

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#20250

Solder particals in the oven? | 6 June, 2002

It has been my experience that the only thing(s) that will ever cause this is: 1. Operators with paste on their hands. 2. Paste on the screen printer tooling. 3. Paste on the bottom of the stencil. Even very very small amounts of paste on the gold fingers will cause this. You usually cannot even see the particles on the fingers with your naked eye. We run about 10 assemblies with gold fingers. The best way to make this problem go away is to mask the gold fingers with Kapton. Its an extra step in the process, but it seems to pay off in the long run. We have spent a lot of time in the past trying to diagnose this problem, and reworking boards. I sincerely doubt that the oven is blowing solder around, although I have seen weirder things happen!

PS: I agree with DaveF. One must always beware of rogue ejactula as well !

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#20251

Solder particals in the oven? | 6 June, 2002

Whoa ! sorry guys I thought my post from yesterday did not go through.

Anyhow they both mean the same thing.

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Brad Jakeway

#20260

Solder particals in the oven? | 6 June, 2002

What you are seeing on your gold fingers is referred to as flux spattering. This is a very commond problem with gold fingers if you have a zero flux spattering criteria for quality control. The key to eliminating this problem is in the reflow profile and the type of solder paste used. If you are currently using LR 735 you may want to switch to a more current solder paste like UP 78. Try a very slow ramp rate to soak (1 degree C per second) and then soak the board at about 160 degrees C for 90 seconds then ramp up to about 210 degrees C with a time above liquidus of sixty seconds. The solder paste you are using will make a big difference in the results you achieve. I have solved this problem with UP 78 in the past. If you want to discuss this give me a call. 614-486-7536.

Brad

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#20261

Solder particals in the oven? | 6 June, 2002

Brad your theory sounds good, but I have seen this happen at very low ramp rates .5 deg to 1.5 deg C rate of rise a sec. with the same soak duration you mentioned. If flux spattering is the case and you are using Alpha paste..switch to Indium and your problem will dissapear ! LOL Just kidding.

But seriously I have only seen this happen due to some type of paste contamination. Just some more food for thought.

Cyber

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Bob

#20266

Solder particals in the oven? | 6 June, 2002

Thanks for all the input.

It is pretty much what we have checked for, microscopic analisys before printing, assembly and reflow, but like we all seem to agree one ball of solder is hard to find.

The boards are full QWERTY keypads with a few LED's as backlighting so kapton tape is unfortunatly not an option.

I will make sure that the operators are continuing to be extra vigilant when running these boards.

Once again thanks for clarifying what I think I already believed.

Bob...

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Solder Paste Inspection