“Soldering irons, solder extractors, and other devices that are in direct electrical contact with sensitive components can inject large amounts of energy into these devices. Specifically, the metal contact between the tip of the soldering iron tip and the pins of the component is a path for high current flow that can cause serious equipment damage.
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Soldering irons, solder extractors, and other devices that are in direct electrical contact with sensitive components can inject large amounts of energy into these devices. Specifically, the metal contact between the tip of the soldering iron tip and the pins of the component is a path for high current flow that can cause serious equipment damage.
Where does the tip of the soldering iron get its voltage? After all, it should be grounded, like a PCB with components, so in theory there should be no voltage difference, so there should be no harmful current flow between the tip and the device. However, this may be true only for DC or very low frequencies such as mains (50/60Hz). For high frequency signals, it can be quite different.
Transient signals as a source of EOS
Assuming the tip of the iron is properly grounded, the voltage on it can be reached primarily through the ground connection, and to some extent through capacitive coupling between the heating element and the tip.
Ground itself does not produce any signal. However, the ground wire connects the entire plant, and once some stray electrical signal enters the ground network, that signal can travel great distances.
The main source of voltage on ground is transient signals leaking from power lines.Transient signals can come from a variety of sources, such as switching power supplies, thyristor controls, servo motors, equipment commutation, etc.[1]. These signals can reach large amplitudes. Figure 1 shows the transient signal on the power line caused by turning on a ubiquitous heat gun. As you can see, the peak signal reached 13.3V, which is not the highest signal found in a manufacturing environment running a lot of high current equipment.
Since neutral and ground will eventually be connected together at some point, and since leakage currents (parasitic currents between power and ground) are present in almost all equipment, in manufacturing equipment, to a higher degree also These transient signals are present on the ground. The current leakage at high frequencies is significantly higher than the leakage typically specified at power line frequencies. This is due to the greatly reduced impedance of parasitic capacitive coupling at higher frequencies. Due to the complexity of the ground network and the increased leakage of the soldering iron itself at high frequencies, there is a high probability of current spikes between the grounded soldering iron tip and the grounded PC board with components.
What is acceptable and safe?
There are many standards and recommendations for limiting the signal on soldering iron tips. STM13.1-2000 of the ESD Association [2]Set the current limit to 10mA and the voltage limit to 20mV. Although the test setups in this document imply a mains (50/60Hz) signal, no limits are specified for signal characteristics. It should be noted that the current limits in this document are approximately 15 years old (it takes at least three years for documentation to be completed and published within a standards body); current current limits should be significantly reduced to reflect the higher sensitivity of today’s components .
The now obsolete MIL-STD-2000 [3]and its associated military standards specify that the voltage on the tip must not exceed 2mV RMS. The RMS value can be extremely misleading for transient signals. 2mV RMS may translate into a very high peak voltage of the transient signal C The voltage spike may be very narrow, i.e. the duty cycle is very short C Please refer to Figure 2 to see the difference between the peak and RMS value of the transient signal, which also comes from On the same heat gun on the bench, the time base is extended to what a typical multimeter can measure. The 761mV peak only translates to a 15.8mV RMS signal – 48 times that in this case. For such waveforms, a 2mV RMS signal will translate to a 96mV peak signal. Obviously, RMS values are not the best way to specify a signal on a soldering iron tip.
Section 2.5.33.2 of the IPC-TM-650 allows 2V peak voltage to be provided on the tip of the soldering iron, which is very high; Section 2.5.33.3 of the same document allows currents up to 1µA to be measured with a multimeter, rather than providing RMS or average scope.
Unlike the above IPC standards, IPC-A-610-E [4]It is the most basic document to control the quality of PCB assembly, providing the following instructions:
3.1.1 EOS/ESD Prevention C Electrical Overstress (EOS)
…tools and equipment need to be carefully tested to ensure that they do not generate destructive energy, including voltage spikes, before handling or machining sensitive components. Current research shows that voltages and spikes less than 0.5 volts are acceptable. However, an increasing number of extremely sensitive components require that soldering irons, solder extractors, test instruments and other equipment must never generate spikes greater than 0.3 volts.
IPC-7711 [5]Instructions for rework of Electronic circuits are provided, mimicking IPC-A-610-E.
Which measurements are important
Let’s examine the characteristics of the EOS signal caused by conducted EMI. Typically, the conducted transmit signal is a high energy signal, ie has a low output impedance and is capable of delivering high current. The reason is that generating interference on low-impedance power lines requires power, and only a truly low-impedance noise source can provide. Even fairly low voltage transients on power lines can be very dangerous due to the current capabilities of power lines.
Current is a better measure of EOS safety for sensitive equipment, as it is the current that causes actual damage (with very few exceptions). Also, the current capability of some devices and boards may be limited at high frequencies due to complex impedances. Therefore, the voltage measurement itself does not fully determine the current injection into the circuit.
Another factor that favors current measurements over voltage is that transients on power lines and ground are strong and can easily radiate a corresponding signal into the oscilloscope’s probe cable, distorting the voltage measurement. Due to a number of factors, including the lower impedance of the current probe arrangement, the injection of the radiation signal into the current probe is significantly less than the injection into the voltage probe. We will focus on the measurement of current.
A typical setup of the bench is shown in Figure 3. In the worst case, use a grounded metal plate instead of the PCB.Current using CT1 current probe from Tektronix[6]Measurements were made with a bandwidth of 1GHz. This probe has a conversion factor of 5mV/ma, which means that a current of 1mA is seen as 5mV on an oscilloscope.
There are many sources of noise in a manufacturing environment. Some of these are random, such as the transients that come with turning a typical heat gun or other device on and off. Others are periodic, synchronizing with the voltage waveform (50 or 60 Hz) on the mains. Periodic transients are caused by a variety of devices, including heaters, brightness controls for vision systems, and more. For data reproducibility, we focus on periodic signals. In the tests described below, easily reproducible noise produced by a common dimmer connected to a 60W bulb was used.
Figure 4 depicts such transients on the power line and the corresponding current flow between the tip and the assembly in the setup shown in Figure 3. As shown, the peak current from the tip (19.12mA) is significantly higher than the peak current allowed by ESDA. STM13.1-2000. It should be noted that in industrial environments, typical transient signals on power lines are often significantly higher than the signal C shown in Figure 4 see Figure 1 above.
Data from previously published sources corroborates the above data.Paper presented by Raytheon at the 2005 ESD Symposium[7]shows that the transient current at the tip of the soldering iron reaches 1000mA.
How noise is created on the tip of a soldering iron
While the tip of most professional grade soldering irons is grounded enough for DC and very low frequencies, at high frequencies things are quite different. Figure 5 shows how the soldering iron and table look at high frequencies. There are several factors at play (in no particular order of importance):
Noise on power lines induces corresponding noise on ground through capacitive and inductive coupling and leakage currents.
Switching power supplies (power supplies used in soldering irons to convert 120/250V to typical 24V) can be transparent to high frequency signals due to a number of factors, with parasitic capacitance being the main factor.Noise from the mains can therefore travel to the low voltage wiring of the iron’s heating element
The switching power supply inside the soldering iron may itself be a source of noise
The heating element of the iron is capacitively coupled to the tip of the soldering iron to transmit high frequency signals
Ground wires – from the mains to the iron’s power source, from the iron’s power source to the iron itself, and from the object being welded to the ground of the device – have complex impedances, both resistive and inductive.
If the voltage on the tip is the same as the voltage on the PCB or on the component being soldered, there won’t be any current flow. The grounding scheme of a professional soldering iron usually works well at DC and 50/60Hz frequencies. At high frequencies, it is nearly impossible to equalize the voltage between the tip and the component due to the complex impedance of the entire ground wire.Among other factors, this impedance can cause ground bounce[8]and phase shift, as well as resonance and ringing[9]. This is sometimes exacerbated by the fact that some plants have opted for a separate “ESD ground” C a different ground network that is ultimately connected to the facility ground. The long wires in these two ground nets lead to the soldering iron and the workbench, which greatly increases the voltage difference at high frequencies. Figure 6 shows the current flowing from the tip of the soldering iron in several cases with different distances between where the soldering iron is inserted and the ground of the workbench/PCB. As shown, the current difference reaches ~80%.
Soldering iron performance
Do all soldering irons create and/or reduce noise the same? How about a top soldering iron? If the tip of the soldering iron has current, does it mean that the soldering iron itself is defective or not suitable for use with sensitive components?
High frequency currents from professional grade properly installed soldering iron tips are not caused by the soldering iron itself, but by the complex device topology, wiring and operation of the device. The soldering iron is just one component of the soldering process, and no matter its quality, it cannot fundamentally solve the problem of the device itself. In short, if you have a good soldering iron, it’s doing its job. It is the user’s task to provide a secure EOS environment for the entire workbench where the iron is only one of the components.
Effects of transient signals on power lines and ground
If the source of the transient on the power line is known and can be removed without affecting the production process, the current reduction from the tip of the iron is relatively simple. However, the source is often unknown or cannot be removed. The only options left are ground management and filtering out transients on power lines and ground.
Grounding management
Rerouting ground connections and separating “noisy” grounds from clean grounds can help reduce unwanted current flow.This article[4]Recommended and explained techniques can help mitigate some noise issues. Specifically, low impedance to facility ground and separation between “noisy” and “quiet” grounds, and connecting soldering irons and workbenches to “quiet” grounds often result in lower level transients.
As shown in Figure 6, grounding as close as possible between the workbench and the soldering iron can significantly reduce current exposure during soldering.
However, ground management alone will not solve the noise problem satisfactorily because the source of the EMI cannot be eliminated, and there are still problematic signals in the soldering iron.
filter out noise
Unless the noise on the power lines and ground is greatly reduced, there is always the possibility of EOS exposure during the soldering process. There are mainly two cases:
a) Both ground and power lines have
Noise b) The noise is only present on the ground, and there is no strong noise on the power line, i.e. the noise on the ground comes from other places in the factory
Power Line EMI Filters
These filters suppress noise on the power line and provide relatively clean power to the load (in our case, the soldering iron). Some EMI filters can also suppress noise in the ground wire.
Figure 7 shows the recommended application of a power line EMI filter using a soldering iron. It is important to connect the ground of the workbench or tool to the ground terminal of the filter, not the equipment ground. The C filter creates a quiet “EMI ecosystem” at its output. Figure 8 shows the tip current of a soldering iron optimized for use with OnFILTER’s APN515LG filter, which is optimized for the soldering process, with the same noise on the power line as in Figure 4. It can be seen that the current flowing from the tip of the soldering iron is negligible.
This requires specially designed filters optimized for welding process characteristics. For details, please refer to[9].
ground filter
If there is no appreciable noise on the power line, but there is noise on the ground propagating from elsewhere, a ground wire filter may be a good replacement for a power line filter that is less expensive than a more complex power line filter. Connect a ground wire filter between your equipment ground and the workbench/tool. To provide a safe, noise-free environment, proper selection of ground wire filters is required.
in conclusion
High-frequency signals on power lines and ground can cause high currents to flow into sensitive equipment during the welding process, resulting in electrical overstress and equipment damage. Proper analysis of the soldering environment, and any environment where conductive objects come into contact with sensitive equipment, and preventive and corrective action can improve yields and reduce EOS-induced failures.
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