Reusing recovered solder paste

We are an EMS operating mainly in automotive, industrial and lighting electronics segments. We are assembling PCBAs with 0201 component, a few BGAs, QFPs and SMD LEDs etc.

During the paste printing process, we are wasting a lot of solder paste due to changeovers and small production runs.

Is there any way to recover the solder paste, since disposition of used solder paste is tedious task? Is it OK to re-use it after mixing of fresh and used paste in 1:1 ratio?

R.S.S.

Expert Panel Responses

It is not advisable to re-use solder paste once it is removed from the stencil. The paste can be stored in metallic cans an sold back to the supplier for a large percentage of the value of the metal in the solder paste.

Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions

I've been in the soldering materials/applications industry for 25 years. Since joining Alpha, Ive been the global product manager for preforms, wave soldering flux, solder paste and more recently the Director of the soldering materials reclaim business.

This day and age and usability of Paste you shouldn't be wasting much at all. I certainly wouldn't reuse any 'Scrap' Paste by mixing with fresh unless it is in a controlled manor.

For instance some Pastes are designed to be used over say seven days, by removing off the stencil and then adding fresh to old over that period.

Never add old to new Paste always New to old. But I must stress this needs to be controlled and only certain Pastes will offer this flexibility.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd

Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Normally solder paste suppliers do not recommend re-use of solder paste because performance degrades with use. If you wish to try re-using the solder paste then I suggest the following. Remove the solder paste from the printer and place it in a separate sealed container. Do not combine it with fresh solder paste at this point. Then perform the change over and set up the line for the next job.

As you suggest, mix the solder paste 1:1 with fresh paste and place it on the stencil. This process can be repeated throughout the work day, but at the end of the work day discard the re-used solder paste. Start again with fresh solder paste the following day.

There may be performance issues with the solder paste throughout the day. If these issues create defects, then we recommend discarding the re-used paste and starting again with fresh solder paste. Also most solder paste suppliers will not guarantee performance of re-used solder paste.

Tony Lentz
Field Applications
FCT Assembly

Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

"Reusing" paste is a common practice. When removing used paste from the stencil always put it in its own unique container. Never put it in a container of fresh/new paste.

That way when the paste is finally no longer usable, you are not ruining or contaminating good/new paste. You can mix in new paste on the stencil as needed to replenish the paste as it is consumed.

Kay Parker
Technical Support Engineer
Indium Corporation

Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.

I'd recommend to first look over the solder paste datasheet provided by the paste manufacturer and see if there is any information concerning the solder's working life. The solder type (tin-lead, lead-free, no-clean and water soluble) behave differently under different environments.

I've worked in several OEM's and Contract Manufacturers and we never threw any solder paste away. Whatever was left-over was then mixed in with fresh solder paste. Some might be against this practice but it is up to your process engineer to experiment and make the final decision.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman

Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

Mixing old solder paste with new is not a good practice. Solder paste waste has always been a valid concern especially with numerous changeovers. It is difficult to apply enough paste in your screen printer when you only need to run a few boards.

You may want to investigate the new temperature stable solder paste from Loctite called GC10. This paste can remain on the stencil for an extended period and needs no refrigeration.

Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.

Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.

Re-using paste is pretty common as the solder paste price is considerable. I would advise you to:

* Use less paste on stencil, replenish often

*Should you have to remove the paste from the stencil, keep it in a separate container as you want to have good traceability on it

* Mix 1:1 ratio with new paste and implement a viscosity check to ensure proper parameters for your solder paste mix 1:1 ratio with new paste and implement a viscosity check to ensure proper parameters for your solder paste

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

Reader Comment

We have found that Customer B does not want used paste from Customer A's job on their boards. We recycle that paste for cash.

We will reuse paste inside a job
- when we flip from one side to the other
- by removing paste from the stencil and adding fresh during the next stencil set up.

Our paste formulation has an eight hour open time, paste is removed from service and placed in the recycle bin at that time.

Scott Homan, IEC Electronics ABQ

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