The scourge of electronic devices -- electrostatic discharge (ESD) -- can be virtually eliminated by designers doing one simple task: reading their datasheets.
Experts advise to read them slowly, carefully, and all the way down to the fine print. Don't overlook a single word, they say. The result could be the elimination of functional failure, shock, or even fire in an electronic device.
"Sometimes designers move too fast and assume they know what something means," Chad Marak, director of technical marketing for Littelfuse Inc., said in an interview. "But without reading all the way down to the fine print, they may find out that the information on that datasheet doesn't mean what they think it means."
A typical IEC 61000-4-2 ESD waveform is characterized by its sub-nanosecond rise time and its short (~100 ns) duration. (Source: Littelfuse Inc.)
Unfortunately, there's no way to quantify the relationship between careful scrutiny of a datasheet and flawless ESD performance. But it exists in devices ranging from televisions and smartphones to MP3 players and blood pressure pumps, Marak said. The trick is to know and understand the relevant specs, thus making the datasheet more understandable. Equally important, designers need to respect and understand the potential damage caused by the enormous voltages and currents that can strike an electronic device, if even for a few nanoseconds.
Marak told us:
Walking across a carpet in winter, when the humidity level is low, and picking up a piece of electronic equipment, you can easily inject 15,000 volts. It can be about 30 amps. Most people would consider that substantial, even though it lasts for only about 100 nanoseconds, which is much less than the blink of an eye.
Marak suggests the following for designers who are concerned about the effects of ESD:
Read the fine print. The ESD plot on the datasheet may not apply to your design, but you won't know that without careful scrutiny. Marak said:
Manufacturers will inject an ESD pulse into the device they're selling, and then show a plot of it on the datasheet. But if you don't read the fine print underneath that plot, you as a designer could be misled. That plot may not apply to your device.
Know your standards. There are two different standards for ESD design. One, the so-called Human Body Model, applies to manufacturing environments. The other, the International Electrotechnical Commission's IEC 61000-4-2, applies to handheld devices, such as portable phones and computers. "You need to know, is the datasheet showing an IEC pulse or an HBM pulse?" Marak said. "It makes a big difference."
Is it applicable? Many protection device datasheets call out 2kV and 4kV pulses, even though the majority of product manufacturers use 8kV as a benchmark. "If I'm using 8kV, then I don't care what a 2kV plot looks like," Marak said.
Know how datasheet clamping voltages are measured. Make sure the clamping voltages referenced in the datasheet correspond to your design. "If they're stating a clamping voltage between pins one and two, then make sure you're connecting to the same pins in your circuit," Marak said.
Use the right layout. "Even if you've purchased the best circuit protection device in the world, it won't matter unless you use the proper printed circuit board layout," Marak said.
For designers, the bottom line is vigilance and experience. Experienced designers typically are better at understanding the subtleties of datasheets, but even the best can be fooled if they don't examine the sheets closely. Marak said:
Manufacturers use creative marketing to present their devices in the best light. So if you don't take the time to read the datasheets and understand the ESD characteristics, then you could mislead yourself into buying a device that's not going to give you the protection you expect.
To learn more about Littelfuse's Speed2Design site, click on the link.
This article brings back memories of ESD testing for me. At one place I worked we had a fairly sophisticated ESD tester and you could program in pin combinations and set voltage values and do just about anything you wanted. It came with different fixtures to fit different packages and you got your results stored to disk. Another place I worked at had a hand held ESD tester. You set your voltage and placed the tester head on the pin you wanted to zap (you literally put the device in a clamp and held the tester to each pin - you could program voltage and length of pulse if I recall correctly) and it was fun to use. You would then take the part for testing on your own test set. I always got a good laugh at the hand held model - it had a warning printed on the front "DO NOT USE AS A CATTLE PROD."
The author of the article has made some good points. We have come a LONG WAY in ESD protection but a designer should definately check specs to see if his application is in danger of getting zapped out of commission during normal use.
Chuck, the data sheet is important becuase it is the published detail on a product. This is what the vendor claims in public the device will do. So, as you point out, you need to read all of it. Depending on the device, they can be very long. There is also a lot of fine print because of the implications. If the customer has a problem and did not read the data sheet correctly, then it is their problem. If they read it correctly and had a problem, then it is the vendors problem.
Adding to this article, in addition to proper design application, there also needs to proper ESD handling procedures, training and equipment tests during manufacturing assembly. Improperly trained assemblers may not realize how easy it is to damage a sensitive electronic component. Also, regular ESD equipment checks need to be performed at each appropriate assembly station.
I remember one case where ESD damage was occurring even though proper foot straps and wrist straps were present. Upon further examination, it was found that the dissipative ESD floor wax was wearing thin and became ineffective (causing everyone to have a false sense of security when their foot and wrist straps were properly being used).
I think you make a good point, Greg - it's very important to enforce the procedures for ESD protection during manufacturing. Sometimes we would get some guy who would enter an area thinking that the rules didn't apply to him - and then people wondered why our RMAs showed ESD damage...
Admittedly, I was not aware of all the anti-ESD measures that need to be taken by manufacturers. But the dissipative floor wax is still a surpise to me.
Yes, the dissipative floor wax is a very expensive product and must be reapplied on a regular basis. However, its a great way to control static through ESD shoes, shoe straps or chains hanging from PCB carts.
Greg's points are the best action for dealing with static sensitive devices. But Its both smart product design, AND preventative measures during fabrications.
When PCBs are open & exposed (during manufacturing) ESD wrist straps, ESD jackets, boot straps, and grounded desk pads must be in place. Any manufacturing center dealing with open electronics & fabrication processes has all of these elements implemented on the production floor.
After finished goods are distributed to consumers, the product envelope must be sufficiently insulated to prevent that "carpet-static" and other ESD effects. That goes back to the effective design of the housing enclosures, in the first place.
I couldn't agree more. In my past experience as a design engineer, I've seen several of my peers select protection devices that ended up having double the capacitance from what they expected.
The value touted on the first page of the datasheet made for nice marketing, but that was only the capacitance of a single diode. When the protection device was connected in circuit there were 2 diodes per data line and therefore twice the capacitive load. Needless to say, they had to go back to the drawing board when their designs failed due to poor signal integrity!
You make a good point about the marketing aspects, Chad. In talking to you for this article, I realized I would have been one of the engineers who blindly trusted the numbers on the datasheet. Those numbers are, after all, data. But I suppose even data can be misleading, especially if we only present the data that puts us (or our product) in the best light.
A lot of engineers I know do not give the datasheet a full examination. Simple ESD protection is usually not on their minds at all. Just making it work seems to be the only focus. In fact, I have seen countless times where a static shock from handlers of the electronics destroy prototypes. Then everyone is head scratching over why it doesn't work.
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