solder balls on high temp.solder

Hi !

Try fresh DRY solder paste... Keep solder paste in a sealed container until the moment of use.

Humidity absorbed by the soldering paste becomes vapor violently at the moment of heating it , expelling the solder in form of balls everywhere.

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Get a profiler for your oven. To say "we are not sure" and "we think it might..." shows you are running with a WAG (Wild Ass Guess). Not a good place to be if you intend to produce anything reliable, with quality.

Run your profile and verify that you are within acceptable ranges. Too hot, too fast can create your problem.

Another basic issue to examine is the stencil apertures. Are you using any reductions from aperture size to pad size? Depending on the part being placed, there are various standards to apply to reduce the stencil aperture size, which could also eliminate your solder balling.

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The profile that you should be using, hopefully, is detailed in the paste data sheet.

Connect your thermocouples to various parts of the board, depending on layout, population etc; and then run some trials observing the profile on your oven monitor/screen.

If you haven't got a data sheet, then drag one from the web or your disty.

You may have to do this several time and maybe even sacrifice a panel to the Reflow Fairy. It is time consuming but very necessary.

Even though I have profiled my oven for about 6 profiles to cover all of our assemblies, I still attach a temp sticker to every assembly. This gives the peak temp only, but I find it reassuring.

Regards,

Chris

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I've had this problem myself, though in using water soluables. But the process to eliminate it is still the same.

There are three possible causes for this to occure. Too much humidity, to much solder, and too much heat.

Are you leaving the paste in the printer too long, is there a large back up of pre printed boards, are you using paste that is out of date?

I found that my appature sizes were too big. I reduced my appatures by 75% for chips (0805 is an example) and 50% for fine pitch and BGA.

Last, can't stress this enough, if you are overheating the solder..it will boil..with or without humidity. What's your peak temp, whats your ramp time? You need to warm the solder slowly, then let it soak, then spike it, then cool it again slowly. Can't tell you how long each segment is going to be, it depends on the board and the paste specs.

Ralph

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Your profile must be precise, your stencil aperture sizes and shapes are very important, and the size of the pads on the boards should be very close to what the part manufacturer recommends. We had some trouble with hi-temp paste, and discovered that our convection oven couldn't do the job, even though we set the profile accurately and measured with a SLIM-KIK. After a little research, we obtained a SIKAMA Falcon 5x5 conduction reflow system (www.sikama.com). Because of the focus of heat a conduction system provides, the proper reflow occurred and minimized our problems. For our stencil, we use a 'V' shape laser cut. You may find the following web link to Circuits Assembly Process Defect Clinic to be helpful: http://www.circuitsassembly.com/online/2000/02/0002willis.html Good Luck

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