When a flux-cored wire is fed on a soldering iron tip and a component is soldered for ten seconds and then the soldering iron is pulled up, a solder spike (icicle) forms. Why does this happen?

Molten solder is round and smooth because its temperature is high enough to make it liquid, which, thanks to surface tension, makes it form into a round shape. Normally, flux activity lasts only a few seconds after being exposed to air at 350°. Once flux activity disappears, the surface of the molten solder begins to solidify. Liquid on the inside but solid on the outside, there is no longer surface tension available to maintain the round shape. Thus, when the solder iron pulls away, a solder spike is formed. There is, of course, more to it than that, as my experiments involving flux activity show.

This article originally appeared in Global SMT & Packaging magazine 6.4 - Apr 2006.

Download the full article (free) in PDF format.

Yoshinobu Anbe is president of Anbe SMT Co, manufacturer of fine and sheet thermocouples. Prior to forming Abe SMT, Yoshinobu Anbe was chief researcher for Tamura Seisakusho Co, Ltd., where he developed highly thermo-conductive ICs and analyzed and created solutions for various defective solder joint problems, such as the Manhattan effect. Prior to that, he was engaged in the manufacturing engineering of PCBs with Oki Electric Co.’s development department. He is a graduate of the Nagoya Institute of Technology, Faculty of Industrial Chemistry.

Keywords : solder, solder spikes

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