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Solder paste recovery

#2407

Solder paste recovery | 21 November, 2000

Does anyone have any experience with solder paste recovery? Specifically, partially emptied solder paste tubes. We are currently looking into ways to reduce and potentially eliminate solder paste waste. One of the largest contributors is partially emptied tubes that jobsetters throw away. In an ideal world we could get the jobsetter to replace the tube once it's emptied, but that's not the real world. I personally am evaluating the possibility of vacuum casting the partial tubes into a new tube. I would be interested in other opinions.

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Michael Parker

#2408

Re: Solder paste recovery | 28 November, 2000

Norman- I would be pushing to get the real world to match the ideal world, in other words get your jobsetters to do the right thing. You are looking to incur more costs to justify not having to make people do their job right. A bit of training and follow up will be more effective and cheaper in the long trm. If you still feel that this is an impossible task, ask your solder paste vendor for ideas. I would caution you from trying to mix used pastes together, you need to get the right mixture of flux with the solder spheres. Since you are trying to reclaim various quantities with unknown length of time exposure to air, you will not be able to accurately determine flux quality. You may be able to reclaim by sending the used tubes back to the vendor. They have the technology and processes to create good paste. They may give you some credit for the unused portion of solder that is returned to them.

Good luck

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#2409

Re: Solder paste recovery | 28 November, 2000

Getting people to stop sounds like a good idea, but once they get running after a lengthy set-up, you can sort of understand why they don't want to stop when they quickly run out of paste after emptying a partially filled tube.

Beyond the paste quality issues Mike mentioned, removing entrapped air is a problem with consolidating partially used tubes.

An alternative to combining tubes that you might consider is to fill syringes for hand soldering.

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#2410

Re: Solder paste recovery | 29 November, 2000

Vacuum casting (same process used to make solid rocket motors) should remove any air from the paste. We had thought of pressurizing the tubes to remove the residual paste, but that would entrap air.

The problem of tubes sitting out for different lengths of time, and therefore the flux to solder ratio being affected, will probably be the biggest problem. We realize we'll have to perform extensive tests to prove the reliability of the repackaged paste.

Being relatively new to the industry and the process in particular, I'm not familiar with the relative volatilities of the flux constituents, and therefore what specifically we are loosing from the paste over time. We have planned to include a feedback loop in our system to shut the vacuum off after the partial tube is emptied to prevent pulling a hard vacuum on the paste and removing the volatile species.

Filling syringes for hand soldering does sound like a viable alternative, thanks.

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#2411

Re: Solder paste recovery | 30 November, 2000

Tell me where I can learn more about vacuum casting and its application to solder paste, please.

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#2412

Re: Solder paste recovery | 1 December, 2000

I have over 10 years experience in aerospace materials, part of which was involved with vacuum casting inert propellant formulations. These propellants are in the range of 80 - 90% solids loaded with viscosities of 15 kP. I'm not sure where I could find information on the specifics of vacuum casting, but the process is fairly simple. As it relates to solder paste it's basically like this:

An empty cartridge is placed in a vacuum chamber. In our case we will be using a 0.5" thick walled plexiglas box. The box has a quick disconnect vacuum fitting threaded into one side. There is a valve at the top that we can thread the partially empty tube onto.

The vacuum is applied to the box. After we have achieved the desired vacuum (to be determined) the top valve is opened and the paste will be transferred to the empty tube.

When the partial tube is emptied (hopefully) we will shut off the vacuum, close the top valve, attach another partial tube, and start the process over again.

I am going to check out the process this afternoon to see if in fact we can empty the partial tubes. We'll probably have to make some tweaks to the process, but since it so similar to the propellant casting process, I hope it works.

A side benefit of the process is that it is extremely low cost. A plexiglas box (built to size) and 2 fittings are the main cost.

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blnorman

#2413

Re: Solder paste recovery | 20 December, 2000

Ideally we want to mix the same lot numbers in a repackaged tube. Once again, these are partial tubes that have been thrown away. If the ratio is right to begin with the only change would be due to air exposure. Semco makes a TC seal which, if we go with my method, will be screwed into the nipple end of the cartridge eliminating air exposure. Our solder vendor has agreed to do an analysis on the flux composition of the repackaged tubes to determine if they are still within spec limits. We also plan to send a fresh tube (unmarked) to see how it compares to a "virgin" tube.

As far as sending the residue back for reprocessing, we can't due to environmental reasons. Something about a license that would absolve us from liability once the paste has left our plant. I don't know the specifics. If the repackaging didn't work, that was my secondary idea.

The union in our plant is strong. The real world won't ever match the ideal world. Basically we have no mechanism to force employees to comply with SOPs.

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