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48 SMT Magazine • March 2014 deliver. Even fairly low voltage transient signals on power lines can be quite dangerous because of their current capability. Current is a better measure of EOS safety of sensitive devices since it is the current that causes actual damage (with very few excep- tions). In addition, due to complex imped- ances the current capability of some devices and boards may be limited at high frequencies; therefore, voltage measurements alone may not be a definitive indication of current injection into the circuit. Another factor in favor of current measure- ments vs. voltage is that strong transient signals on power lines and ground can easily inject cor- responding signals into oscilloscope probe ca- bles via radiated emission thus distorting volt- age measurement results. Injection of radiated signal into current probe is significantly less than into a voltage probe due to a number of factors, including lower impedance of the cur- rent probe arrangement. We will focus on mea- surements of current. A typical setup of a workbench is seen in Figure 3. A grounded metal plate is used in lieu of a PCB as the worst-case scenario. Current is measured using a Tektronix CT1 current probe 6 with bandwidth of 1GHz. This probe has con- version factor of 5mV/ma, meaning that 1mA of current would be seen as 5mV on the oscil- loscope. There are many sources of noise in manu- facturing environment. Some of them are ran- dom, such as transients from turning on and off a typical heat gun or other piece of equip- ment. Others are periodic, synchronized with the waveform of voltage on the mains (50 or 60 Hz). Periodic transient signals are caused by a variety of equipment, including heaters, bright- ness control for vision systems and countless more. For the purposes of repeatability of the data we will focus on periodic signals. An easily- reproducible noise from a common light dim- mer connected to a 60W light bulb was used in tests described below. Figure 4 depicts such transient signal on power lines and corresponding current between the tip of the soldering iron and the compo- nent in the setup of Figure 3. As seen, the peak current from the tip (19.12mA) is significantly higher than allowed by ESDA STM13.1-2000. It should be not- ed that typical transient signals on power lines in the industrial settings are often significantly higher than the ones shown in Figure 4; see earlier Figure 1. Data from previously-pub- lished sources corroborate the above data. Raytheon, in its pa- per 7 presented at the ESD Sym- posium in 2005, show transient currents at the tip of soldering iron reaching 1000mA. how Does the Noise Get on the Tip of the Soldering Iron? Although the tip of most professional-grade soldering irons is grounded quite suf- ficiently for DC and very low frequencies, at high frequencies the situation is quite different. Figure 5 shows how the solder- feaTure EOS EXPOSUrE OF cOmPONENTS IN SOLDErING PrOcESS continues figure 4: Transient on power line from periodic signal and resulting current from the tip.