“Process assessment PWI 101-3” (continued)
Manual process steps
As indicated in the previous section, for a complete assessment the necessary process steps (both manual and automatic) will have to be critically examined. In this section, manual process steps will be discussed.
According to IEC 61340-5-1, the electrostatic charge of personnel in an ESD Protected Area (EPA) is limited to 100 volts. So far so good, you might say, no problem, but…still.
It is not certain that this 100 volt limit provides sufficient safety for every component/process
to be able to produce without problems. First of all, it will have to be investigated what the critical values are that the ESDS (ESD sensitive device) can handle without degradation or damage.
Suppose this is indeed 100 volts. By means of a resistance measurement of the personnel (as described earlier the simplest method). From a previous study and publications in standards (See fig. 1) it is known that with a resistance to earth (Rg) of 35 Mohm the effect of the built-up electrostatic charge on persons is limited to <100 volts.
The graph shows a linear relationship for these measurements. By means of extrapolation, the so-called “body voltage” of persons can now be determined for various measurement results. Suppose that the resistance measurement yields a result of 100 Mohm, this means that the possibly built up body voltage will be limited to roughly 280 volts. If the HBM (Human Body Model) value of the products to be produced is known from the specification. Then the possible risks in manual process steps can be determined in this way. The only requirements for this measurement are a resistance measuring device, called a “resistance measurement apparatus” in the ESD standards. This is certainly not an exact determination, but a first indication that is easy to achieve. Part of the inaccuracy lies in the reliability of the graph and its linear progression.
Another method to determine the impact of personnel during manual process steps is to measure the so-called “body voltage”, the electrostatic potential on the body, directly and then compare it with the specified HBM specification (VHBM) of a certain product. If the measured body voltage is lower than the specified HBM values, the chance of damage/degradation is small. For example, if this VHBM = 1500V and the measured values on a person are 1400V, this will not cause any problems. This assessment can be performed with a CPM (Charge Plate Monitor) or a walking test kit.
An inaccuracy in the method mentioned here can be that there are series resistors, for example due to the use of poorly conductive gloves, which causes a voltage division, which reduces the ESD risk for the product. In the example mentioned here of VHBM = 1500V, there will therefore not be a problem at values higher than 1500V.
If you have any input or would like to contribute to the creation of this new process assessment document, we would like to hear from you.
Bas Grootemaat
Kingsize Consultancy
Nederlands