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46 SMT Magazine • March 2014 Main source of voltage on ground is tran- sient signals leaked from power lines. Transient signals can come from a number of sources, such as switched power supplies, thyristor con- trol, servo motors, equipment commutation and so on 1 . These signals can reach significant magnitude. Figure 1 shows a transient signal on a power line caused by turning on ubiquitous heat gun. As seen, the peak signal reaches 8.7V and this is not the highest magnitude found in manufacturing environment where plenty of high-current equipment is operating. By virtue of neutral and ground being even- tually connected together at some point, and because of leakage currents (parasitic currents between power lines and ground) present in al- most all equipment and, to a much higher de- gree, in manufacturing equipment, these tran- sient signals are also present on ground. Cur- rent leakage at high frequencies is significantly higher than often-specified leakage at power line frequencies. This is due to much-reduced impedance of parasitic capacitance coupling at higher frequencies. With the complexity of ground- ing network and increased leakage at high fre- quencies in the soldering iron itself, there is a strong possibility of current spikes between grounded iron tip and grounded PC board with components. What is acceptable and Safe? There are a number of standards and rec- ommendations limiting signal on the tip of a soldering iron. The ESD Association's STM13.1- 2000 2 sets current limit at 10mA and voltage limit at 20mV. While the test set-up in this doc- ument implies mains (50/60Hz) signal, there is no stated limit of properties of the signal. It should be noted that the current limit in this document is about 15 years old (it takes at least three years to finalize and to release a document within the standards organizations); the current limits now should be substantially lower to re- flect higher sensitivity of today's components. Now-obsolete MIL-STD-2000 3 and its asso- ciated military standards specify no more than 2mV RMS voltage on the tip. RMS values may be very misleading for transient signals. 2mV RMS may translate into quite high peak-voltage of transient signal; the voltage spikes can be very narrow (i.e., have very short duty cycle). Figure 2 shows the difference between peak and RMS values of a transient signal, and from turning on the same heat gun on a workbench where the time base was spread to the degree where typical multimeter can measure it. 761mV peak translates into only a 15.8mV RMS signal, a 48x ratio in this case. For this type of waveform, a 2mV RMS signal would translate into 96mV peak signal. Obviously, RMS value is not the feaTure EOS EXPOSUrE OF cOmPONENTS IN SOLDErING PrOcESS continues figure 1: Transient signal on power line caused by turning on heat gun. Figure 2: rMS and peak values of a transient signal typical on power lines.