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Possible to temporarily store mounted but not soldered PCB's?


Possible to temporarily store mounted but not soldered PCB's? | 3 September, 2001

Hi everyone,

I work in a development firm, that has a small prototype department. We have recently invested in a small automatic SMD pick&place machine. When mounting over 100 different components, the machine runs out of feedercapacity, and the rest have to be manually placed. Furthermore we have no automatic vision system, so every mounted PCB must be manually checked before soldering, and soldering have to be done within 24 hours. This means that the limiting factor for the speed of production is the speed of the manual placing and checking, which means that our SMD machine doesn't produce as much as it could per day. If it was possible to store mounted PCB's temporarily 2-3 days until they could be checked and soldered, then the machine could be used more effectively. I've thought about putting the mounted PCB's in a fridge and/or put them in a vacuum, so that the soldering didn't have to take place within 24 hours, but could be prolonged to maybe within 48-72 hours. Any one got any experience with that? Any advice or other suggestions?

Best regards Henrik Skou Nielsen

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Possible to temporarily store mounted but not soldered PCB's? | 3 September, 2001

We have stored PCB's in the refrigerator twice when our old pick and place died during production. One time the boards were in the frig for two days. I put the boards in a tray and covered the tray with plastic food wrap. We waited about two hours after the boards were removed from the frig before restating production, that let the boards come back up to room temp. The first time we did not use the plastic wrap and the boards had a lot water on them when they were removed from the frig. Both times the boards reflowed just fine and functioned ok. We use paste with water soluble OA flux. It was the tech from the machine manufacture that made the suggestion to put the boards in the frig.

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Possible to temporarily store mounted but not soldered PCB's? | 4 September, 2001

Moisture would be my main concern. Even though components are plastic, they can absorb moisture. Moisture is also detrimental to the the solder itself. The water droplets that are left behind (evaporated even) could cause problems (i.e. chlorides). Hirsch Metals has a long lasting paste (24 hours) chemistry contact Bill Wilson at Hirsch. or 800-521-0352 /561-638-3400

Cal Driscoll ACI USA 610-362-1200

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Possible to temporarily store mounted but not soldered PCB's? | 4 September, 2001

As the others have stated, moisture is the killer. You have to be sure no moisture is allowed to get to the paste. If it does, you'll get solderballs. I would suggest not using a vacuum. The solvents and other volatile components of the flux will eventually be pulled out because of vapor pressure thus altering the composition of the flux.

If the covered tray in the frig works, use that method.

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Possible to temporarily store mounted but not soldered PCB's? | 5 September, 2001

No arument with others comments about moisture, but taking a different tact, consider that the volitle elements of the solder paste disppear as a function of the amount of time the paste is out of the container. [Maybe as a function of solder paste production also.]

You can confirm that this happens by printing some paste on a scrap board and letting it stand for several days. After that, you will see neat little stacks of solder balls -- little flux or any of the other stuff.

Sure, cooling the partially completed assembly will slow this process, but each solder paste has different characteristics. Each board and component combination has different soldering requirements, depending on finish material, solderability, and what not.

Consider performing an experiment to determine the amount of time that your board, components, and paste can sit and, when reflowed, still produce an outstanding connection. Further, this also may help in understanding the effect of the condensed moisture that others mentioned on your solder connection.

A final comment for you to consider -- since you say you are dispensing paste in the title of your posting on the main forum, try dispensing paste only on the pads that will receive components. Then machine place and reflow those componets [no hand work], and finish the lot. Then tear-down / set-up uour P&P, dispense paste on the components that you now plan to place using the other half of your current dispense program, place, reflow, etc. This may require some thought on component selection for each process run, but not much.

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Possible to temporarily store mounted but not soldered PCB's? | 6 September, 2001

The more we talk about this, the more it seems like you should use the process we've discussed to proof your design. Immediately there after, you should use a top-flight high-mix contractor to continue the prototype run for you. If you don't have the capability to do something correctly, you should stop trying and find someone who can do the job. You are not doing your company or your customer any favors by continuing down this path.

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Possible to temporarily store mounted but not soldered PCB's? | 18 September, 2001


And thanks for the advice. I have tried your method now, and it seems to be working. By visual inspection with a microscope it was almost impossible to tell the differrence between the pcb that had been in the frig and the pcb's that were normally mounted and soldered. So I assume that the pcb will function electrically, but this has yet to be tested. Therefore it is still undecided if this method is the answer to our capacity problem. More alternatives are beeing considered.

Best regards Henrik S. Nielsen RTX Telecom A/S

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Possible to temporarily store mounted but not soldered PCB's? | 26 September, 2001

i have stored boards in the refrigerator in side a tray (with a lid) for 2 days and still had them reflow with no problems. i don't know if i would go any longer than that. i would not put any qfp's or any other device that is moisture sensative device in the fridge.

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