| IS IT BETTER FOR SOLDER PASTE TO SPREAD OUT AS FAR AS POSSIBLE ON BARE COPPER, OR FOR IT TO KEEP THE SHAPE OF THE PRINT? I'VE BEEN DOING SOME SOLDER PASTE EVALUATIONS, AND I'M TRYING TO EVALUATE THE RESULTS. I'M NOT SURE THAT BECAUSE A PASTE MAY HAVE SPREAD OUTSIDE ITS ORIGINAL SHAPE, THAT THIS IS A BAD THING. I WOULD THINK THAT YOU WANT THE PASTE TO SPREAD, BUT CAN IT SPREAD "TOO MUCH"? Maurice, I assume you are referring to an OSP coated surface mount pad. If you step back for a second and look at what's going on, you'll see that you're referring to wetting. "How good does my paste wet the solderable surfaces?" That's really what you should be asking yourself. Are you getting a decent fillet. I realize you are doing an evaluation. You need to take into consideration the ability of the paste's flux system to break down the organic barrier on the pad. It's going to be a direct reflection of the activity of your flux and how compatable that flux is with an OSP process. How can it spread too much? It's only going to cover the entire surface of the pad at most and that's what you want to see. If the solder paste only breaks down the OSP under the deposition, is that acceptable? I don't think so because with a few thermal cycles, there is no OSP left and now you have bare copper begging to be oxidized. It should also tells you that as the quality of the paste degrades, in process, your activity will drop, thus increasing the likelyhood of an unsoldered surface or "open." Regards, Justin Medernach
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IS IT BETTER FOR SOLDER PASTE TO SPREAD OUT AS FAR AS POSS...
- Jul 09, 1998
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