Welcome to Minotaur Labs’ new Blog on ESD testing. For each post we will discuss a particular topic related to ESD testing. In addition to describing some of the important aspects of ESD testing we will let you know about the latest developments in ESD testing standards. We hope you find these posts useful and interesting. Please let us know what you think.
For our first post we will discuss the Human Body Model (HBM) waveform. The HBM test is intended to simulate an electrostatically charged person discharging through an integrated circuit. If you look in the latest version of the joint JEDEC/ESDA HBM standard, ANSI/ESDA/JEDEC JS-001-2017 you will see a circuit diagram similar the one shown in Figure 1. A high voltage power supply charges a 100 pF capacitor and then with a flip of a switch, discharges through a 1500 W resistor into the device under test (DUT).
Figure 1 HBM Circuit
If the DUT is close to a short the circuit diagram implies the blue current versus time curve shown in Figure 2 for 2000 V. The instantaneous rise of the current to 1.33 A is of course not realistic; inductance, capacitance and the finite turn on time of the switch will round off the peak. The specifications for the actual current waveform in JS-001 into a short are given as:
- Peak Current: 0.667 A/kV ± 10 %
- Decay Time: 130 – 170 ns
- Rise Time: 2 – 10 ns
- Ringing: ± 15 % of I Peak
An idealized waveform, compliant with these specifications, is shown in red in Figure 2. The most obvious change is the slower rise time and rounded peak to the waveform. The 2 ns to 10 ns rise time is obviously a very wide window, but there are practical reasons for this. Delivering a fast-rising current pulse to an integrated circuit is not an easy task, and the wide range of rise times gives test equipment designers a reasonable tolerance range for equipment design. In fact, most HBM test systems have rise times nearer to the high side of the specification than the low end of the specification.
There are some subtleties to how these specifications are applied to a measured waveform, as well as criteria for a 500 W load in addition to the short, but those are topics for another post.
Figure 2HBM waveforms for 2000 V
The question that is often raised; is this waveform realistic for a real HBM event on a factory floor and if not, what should the waveform be? The original waveform captures used to establish the HBM event are certainly not as good as the ones made by Jon Barth and his co-authors in 2003 . They measured discharges from people charged to various voltages and measured the discharge current under very controlled conditions with a 6 GHz oscilloscope and a high bandwidth current sensor. A sample waveform is shown in Figure 3. At first glance this real waveform is similar to the standardized waveform. There is a rapid rise in current, followed by a slower decay. Looking into the details, the Barth team found significant difference between the standard waveform and real world HBM. In a dry atmosphere they found very fast rise times, an order of magnitude faster than the 2 ns to 10 ns risetime specification. The results were also very dependent on humidity, high humidity reduced peak heights and increased the rise time. Geometry also affected the results, flat surfaces created the fastest rise times and higher peak currents, while a pointed discharge surface also reduced peak heights and increased rise times. The current decay also differed from a pure exponential decay. The decay started out with a decay faster than the standard 150 ns time constant, but then the discharge rate slowed as the discharge progressed. These effects are all related to the properties of an arc forming in air. This is a very complex physical process and some of the details are discussed in the Barth paper.