Re: Fun with Lead-Free soldering

Richard Henry wrote: 
"Terry Given" <my_name@xxxxxxxx> wrote in message
news:0U7De.1715$PL5.196685@xxxxxxxxxxxxxxxxxx


I often come across anti-ESD attitudes. Usually from people with a
rather limited understanding of the potential effects of ESD, the worst
of which is a noticeable reduction in lifetime - IOW failures that crop
up after some time, rather than outright deaths.

I once had a holiday job (1990) at a place that used a ceramic windowed
87C552. In one week my boss killed 10 of these parts ($200 each) because
he was in the habit of carrying them to and from the programmer (in a
carpeted office) in his hand. There were straps & mats at either end,
along with bags, he just chose not to use them.

I have also had a smps controller expire due to static - boss gave a
show-and-tell to some visiting guests, picked up my fully working smps
and waved it around for about 10 minutes, before passing it to the group
to be handled. I came back a few hours later, went to do some work only
to discover it had shat itself. Luckily I soft-started the smps (provide
external startup supply, check waveforms, slowly ramp up impedance
limited HV supply etc), so found the problem, which was the UC3842
output stage, shorted *on*

Had I just flicked the switch, ka-boom.

static damage falls in the general category of "totally unnecessary
mistakes to make"



While going to college the second time, I worked as a production test tech
in the era of discrete TTL devices, but before much recognition was given to
ESD control.  After a while, I began to recognize a common type of failure,
in which a 74LS04 inverter would got sort of half-on.  This typically
occurred where the output in question was routed directly to a card I/O
connection finger.  I found my time was most efficiently used by getting the
assembler to replace all the 04's that were connected off-board on
assemblies that had that problem.


you too huh?

I have seen exactly that problem, on videogame logic pcbs. Between my 1st and 2nd stints at Uni I spent 3 years as a video game service tech. Absolutely nobody in the industry used antistatic anything - bubblewrap was the packaging of choice. A guy I know in Wellington switched to antistatic bags & handling, and saw his PCB failure rate halve. He then replaced all the cheap shitty smps with decent 300W PC supplies, and the failure rate plummeted to near zero - 1-2 PCBs per year, cf 1-10 per week.

Once I got my engineering degree I went to work for a power electronics company. We were pretty good with antistatic, but when we moved to SMT and the volume went up, our failure rate stayed constant, and became a problem. So we went to town on static - conductive jackets, the works - and watched the failure rate fall almost tenfold. designing products to greatly reduce handling also helped a lot.

even so, my first control board was susceptible to static. We used a Samtec bottom-entry connector on the control PCB, which had the PCB pins folded over the outside of the housing. This turned out to be a convenient handle, but one pin was directly connected to the micro. The first build of about 200 units had about a 10% failure rate on this pin, due entirely to static. despite foot & wrist straps, mats etc. I added ESD protection to the next run, and the problem disappeared completely. That was before the conductive jackets though.

It was important there, because zapping a $0.30 part could easily destroy $20,000 worth of power electronics.

Note:  after removing the baddies and soldering on the new parts, the
assembler would clean the board in an ultrasonic tank full of TCE, then
bring it to me still dripping.


yegods! betcha aint so happy about that nowadays.

Cheers
Terry
.

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