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Solder Paste Dry out

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Solder Paste Dry out | 28 February, 2008

What happens as solder paste is used when it becomes un-usable? Does it dry out due to flux evaporation or does it absorb to much moisture from the air which changes its composition too much?

Now for the big question. I had an operator add flux to the solder paste jar to get more use of it. How bad of an ooops is that? Slap his hand bad or acceptable for small manufacturing firms.

FYI - we throw our paste out every friday. New paste every Monday.

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Solder Paste Dry out | 28 February, 2008

The flux dries out and the paste sticks in the stencil apertures. As the flux dries it also causes the solder to oxidize.

Hahaha ! I remember adding flux to paste back in the day to loosen it up. Do not do it ! I have been told by reputable industry chemist that the formulation of your paste flux is very specific. You can't just dump anything into it.

There should be no reason to throw your paste out weekly in my experience.

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Solder Paste Dry out | 28 February, 2008

Let me re-state the last part.... The throwing out part is paste dependent.

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Solder Paste Dry out | 28 February, 2008

Let me clarify we throw out used paste after a week not the whole jar. Jars get used up every other day or so.

We don't really have a procedure for used paste on the do's and don'ts so I'm in the process of creating one to avoid people adding flux to the paste or over-using dried out paste.

As far as the definition of used paste is operators keep adding new paste to what they started with on Monday. Then toss on Friday.

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Solder Paste Dry out | 29 February, 2008

We have the same problem... We are using water soluable solder paste (Alpha ws609) and it dries out very quick. I don't want to throw out the paste inside the printer, but on the other and I don't want to risk my production by adding flux or alchol on it. Could you find a solution for this problem?? Thanks in advance for your response.

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Solder Paste Dry out | 29 February, 2008

Solder paste suppliers often make recommendations on solder paste handling after removing it from the container [eg, jar, cartridge, etc]. For instance, Multicore Solder Paste Handling Guidelines, September 2004 says:

As a general rule, paste that has been in use for more than 8 hrs should be disposed of. Paste, which has been on the printer for up to 4 hrs, can be stored at room temperature for up to 24 hrs before being re-used. Always store used paste in a separate container. Do not mix fresh paste with used paste unless adding more to the printer itself. To prevent contamination of unused product, do not return any material to its original container. A simple solder balling test can quickly determine the condition of solder paste after prolonged use. Simply print a small disk of paste (around 4-5mm dia) onto a non-wettable substrate and re-flow as normal. A single solder ball in a clear pool of residue indicates good coalescing ability. Large numbers solder balls remaining in the flux residue pool could be an indication of poor coalescing ability and the paste may be unfit for use.

We have no relationship, nor receive benefit from the company referenced above, it was just convient to clip and paste since it seemed to sing to the point of the thread. Regardless, most reputable paste suppliers provide very similar information to their customers.

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Solder Paste Dry out | 29 February, 2008

Thats funny that you mention the WS609. That is the exact same paste we used back in the early 90's. That formulation is notorious for drying out and getting chunky.

That is a very dated flux chemistry. I would advise looking into some of the twenty first century formulations. Look into Indium and AIM paste for example.

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Solder Paste Dry out | 4 March, 2008

I talked to a solder paste chemsist and found out a few things.

From above (Dave F) states a test for solder paste involving reflow on a non-wettable surface. Commonly known as the solder ball test this test is performed all the time by paste manufacturers. They use a ceramic coupon and run it through teh suggested heat profiles and rate the balling effect.

Bascially one or two balls is O.K. and anything more than that is bad. It seems though that unless your doing this constantly there would be process defects before you would think of testing the paste. However I don't know that statement to be true ..just a guess.

It was also stated that if the operator uses the "rule of thumb" methhod and doesn't over apply paste the usage of the paste should last much longer. A constant monitoring of less paste will make it last much longer.

Now that being said. I found that while adding regular flux to paste is a serious mistake that there are tacky fluxes to "re-wett" your paste. These tacky fluxes are paste specific and have a shelf life of years. the only draw back is; do you want your operators to have this type of process variable at the ready? What kind of control could you have? The paste manufacturer said "a little dab will do ya". That leaves a bit of guess work in it. But if your in a position where dried out paste is becoming to expensive you may wanty to look into tacky flux to fit your paste.

The prices for my tacky flux if I choose to go that route would be $30 for a jar or $20 for a tube but the tubes have a 6 min purchase order. The shelf life of these tacky fluxes is years so an order of 6 seems cost effective.

anyway hope that helps

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Solder Paste Dry out | 5 March, 2008

You are playing with fire; you evaluate and specify paste for your products/SMT line, when you start adding tacky flux or whatever to your paste you don't really know what you are then dealing with - it osn't what you evaluated. We use solder paste for a whole shift, and replenish it as required. When paste has to be removed from the stencil, it will be placed into a jar, and if we have a job that isn't fine pitch/bga we can use the "used paste". If the new job involves fine pitch/bga then new paste must be used. Gents paste is $0.11/gm how much does rework cost?


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Solder Paste Dry out | 5 March, 2008

We had the misfortune of ending up with a batch of Kester EM907 that would start to "set up" well before it's shelf life was over.

Since the lead time for a new batch was excessive, a Kester technical guru came to our facility to help us deal with the issue. He brought with him some tacky flux which was to be used as a last resort to help the paste print.

He advised that each batch of tacky flux is actually different. If you are going to add tacky flux to a paste it should be flux from the same lot as was used to manufacture your paste in the first place.

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