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70 SMT007 MAGAZINE I JANUARY 2021 Resources 1. Thomas Shoaf, Joseph Clure, Denis Jean, "Achieving excellent vertical hole fill on thermally challenging boards using selective soldering," SMTAI Proceedings 2009. 2. IPC-A-610 Rev H Supported Hole Solder Fill (a). 3. C Hamilton "A Study of Copper Dissolution During Lead-Free PTH Rework," Circuits Assembly, May 2006. 4. Brian Czaplicki, "Advanced Through-Hole Rework of Thermally Challenging Components/Assemblies: An Evo- lutionary Process," IPC APEX 2009. Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. For more information, contact To read past columns or contact Wettermann, click here. incomplete knees in the barrel. Heating of the board in an oven prior to being subjected to the wave temperatures reduces thermal shock to the assembly while limiting dwell time. In the simplest of cases a soldering technician along with the proper flux and board preheater can solve the insufficient hole fill problem via flux and a soldering iron. In cases where the boards have a large thermal mass, the board may first have to be preheated in a controlled oven. Immediately upon removal from the oven the flux and solder can be added to the hole to fill the hole and meet the specification. Through careful process troubleshooting along with an understanding how board layout can result in insufficient hole fill, the proper rework process can be chosen in order bring the barrel fill into compliance. SMT007 Sound waves can travel through different mediums and move at different speeds depending on what they're travelling through. For example, they move through solids much faster than they would through liquids or gases. The result—about 36 km per second—is around twice as fast as the speed of sound in diamond, the hardest known material in the world. Einstein's theory of special relativity sets the abso- lute speed limit for a wave at the speed of light, equal to about 300,000 km per second. However, it was not known whether sound waves also have an upper speed limit when travelling through solids or liquids. T h e s t u d y, p u b l i s h e d i n t h e j o u r n a l S c i e n c e Advances, shows that predicting the upper limit of the speed of sound is dependent on two dimensionless fun- damental constants: the fine structure constant and the proton-to-electron mass ratio. These two numbers are already known to play an important role in understanding our universe. These two fundamental constants can also influence other scientific fields, such as materials science and condensed matter physics, by setting limits to specific material properties such as the speed of sound. The scientists tested their theoretical prediction and addressed one specific prediction that the speed of sound should decrease with the mass of the atom. This implies that sound is the fastest in solid atomic hydro- gen. Hydrogen becomes a fascinating metallic solid con- ducting electricity just like copper and predicted to be a room-temperature super- conductor. Researchers performed state-of-the-art quantum mechanical cal- culations and found that the speed of sound in solid atomic hydrogen is close to the theoretical funda- mental limit. (University of Cambridge) Scientists Find Upper Limit for the Speed of Sound

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