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Corroding Solder Joints

RICK_EEI

#8049

Corroding Solder Joints | 27 December, 1999

Hi everyone. We use a small ultrasonic aqueous cleaner. After each PCB we hand solder, (water soluble) we place it in the cleaner immediately. Lately we have had some problems with the solder joints appearing to be corroding away. We tried lowering the temp and also the cleaning time. At first this seemed to take care of the problem but it has started back again. We tried adding a saponifier but this made it messy. Has anyone ever had this problem and able to eliminate it?? Any help would be greatly appreciated.

thanks, Rick

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jacob lacourse

#8050

Re: Corroding Solder Joints | 27 December, 1999

If it's what I think it is you have a white residue on the legs of some of your fine pitch parts. We have came across this problem some time ago the way we eliminated this problem is to make sure your w/s solder and your liquid flux is compatible. sometimes your paste and your flux can have a reaction causing the solder joints to look corroted. hope that helps, jake

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Brian W.

#8051

Re: Corroding Solder Joints | 27 December, 1999

Depending on the components you are seeing this on, one thing to check would be the solderability of the parts. Sometimes you will find the plating leeches away, and this is a component plating issue. When you solder, the plating actually leeches away from the component lead, leaving areas that look like non-wetted or corroded joints. The problem is usually poor plating on the part of the component manufacturer. You will usually see this on chip components.

Good luck. Brian W.

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Amy Castor

#8052

Re: Corroding Solder Joints | 28 December, 1999

I have tried the water soluble flux and the aqueous cleaners but I got the same results you did; horrible corrosion and a white flaky mess. I know exactly what you're talking about. We do a lot of hand soldering around here, and the magical solution for us was to move to an RMA flux (Kester 186) and use just plain old 99.5% isopropal alcohol. I have three containers and I dip the boards three different times, each time in the cleaner alcohol solution and about 30 seconds each using a brush to wipe away excess flux. It solved our problems for low volume work, but for large volumes it's a different story. Alcohol has a low flash point and it's dangerous to use a pump controlled circulation system. I have heard some use a cheap dishwasher and just put their solutions in there and filter it out for aqueous solutions, but for now, we're up in the air for a low cost, high volume board cleaning system. RMA flux definitely solved our cleaning problems though and I've never seen cleaner boards. If you can't go that route, just make sure your flux and cleaners are compatible.

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#8053

Re: Corroding Solder Joints | 28 December, 1999

Rick, Does the ultrasonic cleaning system that you use reuse the same water for each cleaning cycle? I have had a similar problem in the past using a Brandson ultrasonic cleaner and water soluable flux. The problem was the buildup of flux in the cleaning bath. (Kester 1189) The organic fluxes tend to be extremely high in acidity. In time we were essentally dropping our board into a steamy bath of acid which did corrode the solder joints as well as damage plated surfaces. A simple test would be to change out the cleaning medium and run a test lot of parts. If they come out OK, chances are that that was your problem. Hope This Helps Chris

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#8054

Re: Corroding Solder Joints | 28 December, 1999

Rick, Ultrasonics is a mechanical scrubbing action (cavitation) similar to any other scrubbing action, if you scrub long enough, you stand the chance of wearing away some of the surface material (shiny coating of a solder joint).

The shiny coat on a solder joint is the protective oxidation resulting from the soldering process. If you "scrub" long enough, whether it be with a brush or ultrasonics etc., you can wear off this protective coat and expose the raw metal (tin/lead) to water which will cause the metals to corrode.

You were on the right track before when you reduced the cleaning time. Reducing the heat will have little effect since the joints were just exposed to temperatures much hotter than boiling water. (110 -120 degrees F works best with ultrasonics)

Water washable flux should not need a saponifier. Hot saponifiers can corrode raw metals faster than plain water. Saponifiers need temperatures of 140 degrees and up, otherwise, they are "messy" as you found out.

The best chemical to use with water washable flux is simple dish washing liquid (Joy, Dawn etc.)at about 1/2% concentration (a few drops can make a big difference). This will provide better cavitation and you should be able to reduce the ultrasonic cleaning time to 30 40 seconds.

The other variable is the type of ultrasonic cleaner you are using. It must be at a frequency of 40 kHz or higher (the lower the frequency, the more aggressive the scrubbing action). Also, it should use a "sweep frequency" technology. If it was purchased new within the past 5-6 years, it most likely uses a sweep frequency. If not, the cavitation may become concentrated within one area of the bath and cause the joint surfaces to wear faster in that area.

I hope this helps. There are some technical papers on our Web Site that may also be of interest. www.smartsonic.com Click on the "Recommended Reading" button. Or, contact me directly. 1(800) 906-440-R

Best regards, Bill

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Jim

#8055

Re: Corroding Solder Joints | 28 December, 1999

Hi Rick,

Water soluble flux is heatee, deionized water. So, use DI, and keep it heated. It is also important that the water be moving, so that fresh water is continually being presented to the fluxed surfaces, and the flux-laden water is continually being carried away from the surfaces (so dissolved flux won't be tempted to jump back out of solution, and attack your connections). Frankly, a static bath like yours is not ideal. The ultrasonics don't help much, either, because you need water movement on a macro level, and u/s only gives it on a micro level. Without adequate movement, you create an ever-increasing layer of flux-laden water that surrounds your boards. If you must stay with that kind of system, be sure you change your water frequently, to avoid saturating your tank, as Chris described. Adding an underwater jet or a sparger would help.

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Jim

#8056

Re: Corroding Solder Joints | 28 December, 1999

Rick,

The first line of my message should have read as:

"The best way to clean water soluble flux is with heated, deionized water."

Don't know what happened to it in cyberspace.

Jim

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#8057

Re: Corroding Solder Joints | 29 December, 1999

Moving water will disrupt the ultrasonic cavitation. If you need to move the water, do it when the u/s are off.

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Stu Leech

#8058

Re: Corroding Solder Joints | 31 December, 1999

Intel had a similar problem with one of their military packages. They had "dull" leads. QC screamed loudly about this because the "dull" leads looked like a solder-coating problem. (Actually the solderability was equal or better than the "shinny" leads).

They used static bathes and could not use ultrasonic due to Mil-Specs.

Their solution was to switch to AQ-1401 Pak-Clean aqueous cleaner. No more problems.

Stu Leech sleech@ix.netcom.com

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