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SEPTEMBER 2021 I SMT007 MAGAZINE 57 free soldering temperatures. e fastest rate of dissolution occurs at the knee of a plated through-hole joint. To evaluate the copper dissolution, average measurements from multiple pins on multiple boards assembled under the nominal condition of 270°C, five seconds have been used (Figure 4). Measurements from the pad and knee are shown here. It is evident from the figure that SACX Plus and SnCX Plus alloys result in re- duced levels of copper dissolution vs. SAC 305. ere is a clear trend showing that higher silver results in faster copper dissolution, especially at the knee of the PTH joint (Figure 5). Dross Formation Several studies conducted by Alpha have confirmed that the rate of dross generation is reduced as the content of silver is reduced from wave solder- ing bar solder. Although SAC dross can be recycled, there is a cost of recycling to the cir- cuit assembler. Dross for re- cycling requires processing of metal oxides to the base metal, so the selling price is approxi- mately 50% of the LME price of the metals when the dross is sold for recycling. Again, metals prices fluctuate, but as dross is removed from a wave soldering pot, it must be replaced with new bar solder. e cost of the new solder is at least two times the sell- ing price of the dross, so obviously reduc- ing dross generation reduces the total cost of ownership of the solder used in a PTH pro- cess (Figure 6). e processing variables used were pot tem- peratures of 240, 255, 270, 285 and 300°C, and contact times of three, five, and seven seconds. ermocouples were placed in predetermined locations to ascertain the temperature on the test vehicles. Figure 4: Copper dissolution vs. lead free alloys. Figure 5: Copper dissolution at the knee of plated through-hole joints.