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50 SMT Magazine • March 2014 If the voltage on the tip is the same as the voltage on the PCB or on the soldered com- ponents, then there won't be any current. At DC and 50/60Hz the grounding scheme of professional soldering irons typically works well. At high frequencies the voltage between the tip and the components is nearly impos- sible to equalize due the complex impedance of overall grounding wiring. This impedance causes, among other things, ground bounce 8 and phase shift, as well as resonance and ringing 9 . What sometimes aggravates the sit- uation is that some factories opt for a sepa- rate "ESD Ground," a different grounding network which is eventually connected to a facility ground. The long wires in these two grounding networks leading to the soldering iron and to the workbenches greatly contrib- ute to a voltage differential at high frequen- cies. Figure 6 shows the current from the tip of the iron in several situations with different distances between where the soldering iron is plugged in and grounding of the workbench/ PCB. As seen, the difference in current reach- es ~80%. ing iron and a workbench look like at high fre- quencies. Several factors are at play (in no particular order of significance): • Noise on power lines induces correspond- ing noise on ground via capacitive and induc- tive coupling, as well as via leakage currents • Switched power supplies (the ones that are used in soldering irons to convert 120/250V down to a typical 24V) can be transparent for high-frequency signals due to a number of fac- tors, parasitic capacitance being the dominant one. The noise from the mains thus can propa- gate to the low-voltage line of the iron's heating element • Switched power supply inside the solder- ing iron can be a source of noise by itself • Iron's heating element is capacitively cou- pled with the tip allowing propagation of high- frequency signals • Ground wires—from mains to the iron's supply, from the iron's supply to the iron it- self and from the soldered object to the facility ground—have complex impedance, including resistance and inductance feaTure EOS EXPOSUrE OF cOmPONENTS IN SOLDErING PrOcESS continues figure 5: noise propagation in the soldering iron.