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.
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.
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!
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.
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.
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...
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).
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.
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.
The question of whether engineers could have foreseen the shortcut maintenance procedures that led to the crash of American Airlines Flight 191 in 1979 will probably linger for as long as there is an engineering profession.
More than 35 years later, the post-mortem on one of the country’s worst engineering disasters appears to be simple. A contractor asked for a change in an original design. The change was approved by engineers, later resulting in a mammoth structural collapse that killed 114 people and injured 216 more.
If you’re an embedded systems engineer whose analog capabilities are getting a little bit rusty, then you’ll want to take note of an upcoming Design News Continuing Education Center class, “Analog Design for the Digital World,” running Monday, Nov. 17 through Friday, Nov. 21.
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