| Cold Welding | I'm presently doing a project on reflow profiling. | I came across this expression. Can someone please give me a definition. | Best Regards | Tom Mc Grath Tom, I think what you are referring to is also called cold solder. You have four distinct segments in a reflow profile. The ramp up zone, where you are getting your board up to a preheat temperature is the first segment. The board then enters the soak zone. this zone is where your flux has become active and is aggressively cleaning all metalized surfaces in contact with the flux system. The solder, if 63/37 Sn/Pb, will go liquidous at its' eutectic point of 183 degrees Celcius. The board will then enter the third segment called reflow, where you are now trying to get the molten solder to wet to the leads and cook of as many volatiles as possible from your flux system. The final segment is cool down. The ramp rate in this segment should be equal to the rate of increase in the first segment. Cold solder occurs when a substrate cannot come up to the required temperature to turn the solder deposition liquidous. The trick is to know when a solder deposition is cold and when it's actually been corrupted. There are flaws in reflow profiling and process which can cause a joint to take on the appearance of cold solder. If a noble metal is encountered in the solder joint such as silver, palladium, or gold, the joint can take on a "cold appearance". (Cold solder looks like partially melted spheres under a microscope. It is typically a dull grey in color.) To get rid of this, we bump up the reflow temperature to about 235 to 240 degrees C. Also, if cool down is too rapid, the solder joint will appear cold. this is caused by the improper alignment of the crystalline structure of the joint caused by rapid cooling. Ever have a materials class? The difference between a paper clip and a razor blade lies in the quenching process. If you took a paperclip and bent it, you'd find it to be fairly flexible. So much so that it can return almost entirely to it's preformed state. You can't do that to a razor blade because of the hardness. The steel for a razor blade is brought up to it's eutectic temperature and quenched rapidly. This rapid quench causes a very hard but brittle steel called Martensite. Martensite is what is used to make razor blades. The flexible steel needed for paperclips is cooled slowly which allows the crystalline structure to take on a flexible format. The same is essentially true of a solder joint. Hope that was of some help. Good luck, Justin Medernach Mfg. Eng. Flextronics Int'l PIC East
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Cold Welding
I'm presently doing a project on reflow pro...
- Mar 09, 1998
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