[time-nuts] Conducting Bench Top Material
slburris at gmail.com
Mon Jan 25 20:11:24 UTC 2010
So looking at ESD mat material at Digikey, there appears to be a
bewildering array of choices. Elastomer, rubber, vinyl,
thermoplastic, laminate, foam rubber, and polyethelene.
Any guidelines about what to choose?
On Jan 25, 2010, at 11:59 AM, Ed Palmer <ed_palmer at sasktel.net> wrote:
> I've still got a paper copy of an HP Bench Brief from 1983 that was
> one of my first introductions to the dangers of ESD. I've used a
> wrist strap and antistatic mat since then. ESD protection in the
> ICs has improved since then, but I think that the article is still
> mostly applicable today.
> Lux, Jim (337C) wrote:
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com [mailto:time-nuts-
>>> bounces at febo.com] On Behalf Of Charles P. Steinmetz
>>> Sent: Monday, January 25, 2010 10:27 AM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] Conducting Bench Top Material
>>> Bruce wrote:
>>>> Although over the years the non-conductive top has been an asset
>>>> avoiding short circuits, etc., I am concerned about static
>>>> discharges when
>>>> handling modern semiconductors. Would it make sense to spray
>>>> the Masonite
>>>> with a weak copper sulphate or similar solution so as to make
>>>> the masonite
>>>> slightly conductive, but not so conductive that 155 VAC
>>>> could not
>>>> safely rest upon it? Is there a better-suited material that
>>>> could be used
>>>> to replace the Masonite?
>> One generally looks for static-dissipative surfaces, rather than
>> conductive surfaces. 1 Megohm/square, for instance. The idea is to
>> keep everything isopotential as charge drops onto things, not to
>> rigorously establish a common voltage.
>>> I notice that many folks who have contributed on this thread use
>>> anti-static benchtops, but I have never found it necessary (and I
>>> to keep the RH in my house under 45% -- it is generally 20% or less
>>> in the winter). I've been fooling with static-sensitive parts for
>>> years and haven't lost one to static yet
>> You haven't lost one *that you know of*. It also depends on the
>> kinds of parts you're working with. There are some that are quite
>> sensitive AND which don't fail outright, but just degrade
>> performance a bit when they take a hit. It also depends on the
>> energy behind the hit, of course. An example might be the
>> MiniCircuits ERA-4 or ERA-5 (just because I happen to have the data
>> sheet handy). Take a look at the later pages in the report, and
>> you can see where the gain changes slightly as a result of 100V ESD
>> hits (see page 6, where you can see gain dropping about 1.5 dB over
>> 8 pulses, with about 0.1dB per hit.)
>> As they say at the end of the report:
>> The new amplifier ERA-4XSM shows gradual degradation in the gain
>> and the
>> device voltage. That fact is not so bad. Even with the multiple
>> stress a customer
>> would rather have gradual changes then catastrophic failure. The
>> withstands a single 100V ESD pulse, or 3 pulses at 50V.
>> When we (JPL) do site visits to vendors, lackadaisical approaches
>> to ESD handling are one of the common problems. For us, who are
>> building just one or two of something that's going to be going
>> somewhere where repair isn't an option, latent damage and gradual
>> degradation are a big deal.
>> It's really a "habit" thing that everyone has to get used to.
>> That's why even nuts and bolts come in ESD packaging (even though
>> they're obviously ESD immune): it gets people in the mindset of
>> "come in the area, put on the wrist strap". Back in the 70s, when
>> ESD processes started to be used, they would have multiple
>> categories of parts, some which needed ESD precautions (CMOS parts,
>> DRAMs,etc.) and some which didn't (resistors, capacitors). It was
>> found that workers would be working with something in one category,
>> and the habits would carry over to the others, so the industry, in
>> general, went to the "everything is ESD sensitive" approach.
>> The *worst* offenders for ESD are the engineers (like those of us
>> reading the list!), because they actually know what parts are
>> sensitive and which aren't, and tend to take shortcuts with the non-
>> sensitive parts. Which works, sort of, until they guess wrong, and
>> cook something. "Hey, why is the NF on this LNA 0.2 dB higher than
>> it was yesterday?"
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