It's Not Just the ESD (Static)

RE: It's Not Just the ESD (Static)

DAB — Fri, 19 May 2017 17:39:43 GMT

Great post John.

I sympathize with ESD issues.

Back in the 1970's the initial EPROMS were very sensitive. I could blow them up from about a meter. This was at a time when they were about 100 USD each, so they made me stay out of the lab where they were stored.

DAB

RE: It's Not Just the ESD (Static)

shabaz — Thu, 18 May 2017 23:27:31 GMT

Hi John,

What a beautifully constructed machine, inside and out. The whole project, it looks fantastic!!

Regarding the 2N7000, it might not be you, maybe the supplier isn't looking after them correctly or they are faulty in some way, e.g. faulty batch sold cheaply.

To be honest for personal-use prototypes even basic ESD precautions can be slightly relaxed, because often ESD damage won't be noticeable except over time, and as a percentage of failures in a production environment, e.g. a few percent, which would be easily noticeable in production quantities and would impact yield. Because you're just making one item (with just a handful of parts), it would be surprising to have it fail near-100% of the time, and I know you're extremely good with your work methods, so this is unexpected.

Having said that, for prototypes I try to order the assembly so that the gate-source resistors are soldered first, so that the MOSFETs have something to prevent charge building up as soon as they are soldered to the board. Another approach could be to have some copper wire strand wrapped around the gate and source (or all three pins if it is easier), and solder the MOSFET to your board (and resistors), and then unwrap the wire after the MOSFET area of the circuit is all soldered in.

I work on carpet, and even if I don't clip the wrist strap on, I just occasionally touch the nearest earthed object, and I've not had noticeable ESD issues in prototypes yet (I'm sure I am damaging some slightly, bit it's not noticeable just yet).

One other thing is that the iron is earthed here (through 1M resistance), but I don't know in the US if soldering irons have the same thing.

The temperature of your iron is about right, provided the tip isn't too large (I am guessing the tip size is just fine since the soldering looks quite neat). As a data point, the tip I generally use is about 2mm diameter, and for me I feel comfortable with the temperature set to about 330 degrees (and normal leaded solder). With a large tip the heat transferred is bigger, but the 2-3mm or so tip should be really good with through-hole parts.

EDIT: 3mm of wire between MOSFET body and the circuit board does seem a bit too short, I've not done them pushed in so much, usually I leave the body of the MOSFET/BJT higher up on the board. Also as [mention:7d5ee4caf55f4ccaa8b711eb508ecedd:e9ed411860ed4f2ba0265705b8793d05] says, the temperature-controlled irons make a big difference.

RE: It's Not Just the ESD (Static)

rachaelp — Thu, 18 May 2017 23:07:34 GMT

After having to spend so long reworking your board to replace all the broken FET's are you thinking it might be time to invest in a temperature controlled soldering iron? With the amount of stuff you build it'll probably be a reasonably good investment, and you'd be turning the negative of a nightmare build into the positive of an excuse to buy a new toy for your lab! [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-e42fe9c9-3c05-4d1e-bfad-19c16e64aad3/contentimage_5F00_2516.png:16:16]

Excellent project by the way, another great job done!

Best Regards,

Rachael

RE: It's Not Just the ESD (Static)

dougw — Thu, 18 May 2017 22:18:49 GMT

Very strange failures. I've used many of these particular FETs with no ESD precautions and no particular soldering care and I usually insert them all the way down on the board, and never seen one die. The only thing I do is form the leads prior to insertion so there is no stress on the leads. Very strange.

Congratulations on your perseverance - nice job.

RE: It's Not Just the ESD (Static)

balearicdynamics — Thu, 18 May 2017 20:21:09 GMT

Very good project!

The use of lights in conjunction of some kind of patient feedback has a lot of application I'm predictive diagnosis of some degenerative illness

Enrico