True Story No10

 

Wrist slaps

Nowadays it's pretty common to see one of those black plastic bags with a bit of computer inside.

The label warning about static damage is well known to most people in industry.

A wrist strap and an earthed conductive table top, although not too familiar, should always be used when assembling memory and other sensitive parts into a computer.

Some buildings seem to attract static electricity in huge amounts, whilst in many other places, it is rare to ever see its effects. At one place I worked we had nylon carpets and a fairly warm environment and was shocking! If you walked more than a few yards then sat down and touched anything, say when you pulled your metal framed chair towards the desk, a loud crack and an enormous spark would leap from your finger. This is the sort of phenomenon one must guard against when handling printed circuit boards.

Most boards carrying semiconductors are quite safe from damage.

When an integrated circuit is connected up to power rails, even when no power is applied, it is normally pretty safe from damage because most modern CMOS has gate input protection, using zeners which limit the voltages, seen by the chip, to safe levels. There are some devices however that cannot use protective circuitry for fear of introducing errors into their operation. Precision analogue switches usually fall into this category, certainly the ones that were around in 1975.

One equipment, we developed, gave us no end of trouble!

There were about 20 equipments, each using eight printed circuit boards carrying eight 8-way analogue multiplexer chips. These devices were very expensive; in those days each board probably cost us about £500. In 1975 that was 2 or 3 months wages.

Most of the boards coming from Production were faulty. Usually all the analogue channels bar one group would be OK. That is 56 channels on a particular board were OK and 8 were not. The engineer would tie a label on the board handle, identifying the faulty chip, return it to its black plastic bag and send it back to Manufacturing for rework.

When the board came back the faulty channels had been fixed.

The problem was that the other 56 were now duff.

The engineer would eventually get round to testing the board, diagnose 7 faulty chips, tie a label on the handle etc.

Bill, the Head of Quality Assurance got involved when it started to look like we would miss our factory acceptance date and we gave poor reliability of the analogue switches as the reason.

He soon discovered the fact that boards were being fixed and coming back worse than when they went for repair.

The odd thing, he also discovered was that when a board with 7 faulty chips was fixed it came back with just one faulty chip.

In fact some boards had been round the loop loads of times and no-one had really noticed. The pattern was always the same and entirely predictable. He summed it up:-

(1) A lot of boards from Manufacturing arrived with one chip faulty.

(2) Boards sent back with one chip faulty came back with seven faulty.

(3) Boards sent back with seven chips faulty came back with one chip faulty.

And by now we were having difficulty buying chips and special shipments were having to be flown over from the US.

Two engineers accompanied Bill from QA around Production.

"Let's see the manufacturing documentation", Bill asked Don, the Production Manager.

"Here you are Don said", pointing out the instructions about an earthed bench and the use of a wrist strap.

"Let's see where the boards are assembled", said Bill.

Sure enough all was in order. He was shown a nice tidy bench using a large sheet of aluminium, suitably earthed, and a proper wrist strap.

"What happens to the assembled boards", asked Bill.

"They're put into black plastic bags and sent over to be flow soldered was the retort".

We went over to look.

A board was in the process of being soldered.

One corner of the board came into contact with the solder bath first and of course it was the corner nearest to the chip that was usually faulty on new boards. Static built up on the surface of the solder bath and discharged to the new board as it neared the hot metal.

NO that was too easy!

The solder bath was well earthed and later tests showed the problem wasn't there.

Tests after the soldering process revealed the same fault as predicted at the corner of the board but it was noticed that some boards were soldered the other way round but the fault was always at the same corner.

What happened after the soldering process?

The boards were taken over to an ultrasonic cleaning machine where they were dunked in a solvent to get rid of flux.

The boards were placed in a batch and immersed in the fluid and the machine vibrated everything at a very high rate and the boards came out nice and clean.

Unfortunately they came out knackered!

A simple change to the process was suggested and we afterwards had 100% working new boards.

What about the rework problem?

We went to see Jack, the chap that removed the faulty chips.

He showed us his bench.

Not quite as pristine as the production facility but all the necessary bits were there including his special isolated soldering iron and wrist strap.

"I know all about the procedure", he said producing a well thumbed document from his desk drawer.

"Fitting of Static-Sensitive Integrated Circuits", the title announced, and inside, "When fitting and soldering a chip…….".

It was all there in black and white and he assured us he definitely followed the instructions to the letter.

"I don't get it there must be something wrong", said Bill.

"Here's a typical board, show us what you normally do".

Jack removed the board from the black bag and took it over to a second bench piled high with bits of units and tools.

Shoving everything aside to make room, and using a solder wick and a very large old soldering iron he proceeded to expertly remove the marked chip.

Then he went back to his special bench, donned his wrist strap, picked up his special soldering iron and inserted a new chip.

"There you are!", proclaimed Bill, looking somewhat relieved, "You just destroyed seven good chips", and went off to slap somebody's wrist and to re-write the newly entitled procedure. "Fitting and Removal of Static-Sensitive Integrated Circuits".

We never had any trouble after that.

Post script…I bought an LC7821 for a good quality amplifier yesterday. It's a 30 pin 8 way analogue multiplexer and it cost only £1.23. Is this reverse inflation?

 

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