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Solder paste handing

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Stephen- SMT engineer trainee

#46641

Solder paste handing | 13 January, 2007

We are now using solder paste(OM325) and I have some problems on the folowing process:

1. We use the solder paste softer(Malcom SP-1) to warm-up for 20 mins, when it just take from refrigerator. Is there any problems for solderpaste performance?

2. The solder paste on stencil will mix with the unused paste in the jar and stored in refrigerator again. It will be warm up by softer(Malcom SP-1) for 15mins and used for normal production in the next morning. -Will this process affect the solder paste performance and what in ratio for used and unused solder paste should be mixed in order to mininize the moisture-related problems?

3. We will not check any the envionment temperature,huminity and viscosity since it is time comsuming and realted to cost. I want to ask which items is a MUST to control and record for monitoring?

4. The solder paste should be store tip down in refrigerator. WHY?

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CL

#46662

Solder paste handing | 15 January, 2007

Good Morning Stephen, 1) Most solder paste manufacturers recommend a 2 hour temperature stabilazation period. Solder paste should not be opened and exposed to ambient air if it is still cold. 2)Manufacturers do not reccomend mixing old and new paste as this effects the rheahology of the new paste. We utilize a "Working Jar" for the solder paste that has been used. We date it as to when it was originally dispensed and continue to use it until we reach maximum time exposure. In some cases we add some new paste to the old and it refreshes the performance but we never go the opposite way. Also, "It will be warm up by softer(Malcom SP-1) for 15mins and used for normal production in the next morning" If the paste is still cold, it will condense from the air and absorb moisture. 3)We have found that RH directly effects solder paste performance. (i.e. when the RH goes up, so does the occurance of bridging) We have also found that excess heat in the SMT area can be detrimental. (causing hot slump) For us, 30-40% RH and 65-70 degrees F is optimal for our process. I can actually see defect spikes when the environment gets out of control. 4. "The solder paste should be store tip down in refrigerator. WHY?" Solder paste can separate (to some degree) in the tube leaving you with either more or less flux to metal ratio. For this reason, we store the tubes on their side so that any separation (flux at the top, solder spheres at the bottom) would still have a faily consistant ratio when it is dispensed (although, I have not seen any evidence of this happening)

Hope this helps.

Good Luck

Chris

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Earl Jr.

#46666

Solder paste handing | 15 January, 2007

We use to use a malcom mixer but found it caused problems of solder/flux separation. This may be causing you problems. Earl Jr

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#46672

Solder paste handing | 15 January, 2007

CL pretty much hit the nail on the head. But also remember that solder paste will not go "bad" if not kept cool. It is not a fruit or perishable item (sorry guys, it's not). It's just metal and flux. The only reason they want you to keep it cool is to slow the separation of metal and flux down. So, if you do rotate your stock of solder paste frequently, you can get away without refrigerating. Some do separate faster than others, it all has to do with your solder paste. But to avoid the moisture problems, it may be worth looking into.

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Stephen- SMT engineer trainee

#46690

Solder paste handing | 16 January, 2007

Thanks for you reply CL.

For Q2, I want to know that is there are any ratio of mixing new or old soldering paste? Also, can we put the "working jar" and opened unused solder paste to refrigator and soft it by solder paste softer in next time of production?

For Q3, if we warm-up a new jar of soldering paste in room temp should I need to face it tip-down?

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#46691

Solder paste handing | 16 January, 2007

Hi Stephen,

Q2) As per my understanding, no solder paste supplier or any organisation provide Mixing standard or ratio as it is not recommanded. However, I do know that some company, they allow mixing of old and new solder paste (as solder paste is very expensive). To do this, I think you need perform DOE to obtain data to support the mixing.

Q3) As far as I know you do not need to face it tip-down. But remember, do not heat it up to shorten the normalising time as this willc cause flux separation.

regards

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#46707

Solder paste handing | 16 January, 2007

Then how do we explain the poor printing performance (appears to be rheology based, paste thickening/drying, poor aperture fill, sticking to the blades, etc.) of paste that has exceeded it's shelf life? You know, the stuff that no amount of stirring prior to dispensing on to the stencil or working once on the stencil can fix.

My impression has always been that it's lost some/all of the volatiles (thinning agents?) in the flux.

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#46709

Solder paste handing | 16 January, 2007

Yes, it can evaporate. My point was manufacturers suggest cool storage to avoid seperation. I guess if the flux does seperate and get pushed to the top it could have a greater chance to evaporate. I have only seen such seperation on year + old jars of paste kept in our warehouse that's about 100 degree F year round. By then the experation date has passed anyway. And yes I did get it to work by mixing the flux back inot the paste. Just don't ask me why I had to this. It worked fine and reflowed like normal.

For dried out paste, I've thinned with ISO and got it work just as if it were a fresh jar too (paste manufaturer suggested).

Perhaps paste needs to be turned like fine bottles of wine. Man Law?

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#46710

Solder paste handing | 16 January, 2007

Gaaaahhhh! NEVER mention man law and turning bottles of fine wine (metro law) in the same statement. You'll tear a hole in the time/space continuum.

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#46714

Solder paste handing | 16 January, 2007

You "guys" crack me up. OK, how about bottles of tequila?

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#46715

Solder paste handing | 16 January, 2007

Now yer talkin'!

FTR, I've seen 7 month old (5 month shelf life in a 45� F fridge) WS based pastes in tubes turn to clay and refuse to print through anything less than about 50 mil pitch. This was stuff stored in the fridge per mfr's spec.

I suspect mfr may have something to do with how successful you are at abusing paste and getting away with it.

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#46717

Solder paste handing | 16 January, 2007

Well, I do know that different solder pastes have different lives. Sh@t, some solder pastes change characteristics from lot to lot. That's why we always restest solder pastes from different lots when doing an evaluation. Some of it has to do with the package style as well.

I'm abusing my paste? I don't think so. It's pretty tuff. It even stuck a "Calvin peeing on the competitors solder paste" sticker on the fride door!

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