Pain from shock when getting out of car ...

On Sat, 13 Dec 2003 03:31:07 -0500, w_tom Gave us:

Only if it is in a faraday cage.

You go arcing static discharges into a bare PCB assembly, and you are asking for a failure.

Uh - you missed something he said: "I remember having equipment problems on a machine that had sensitive cmos switches." ^^^^

CMOS switches.

That is correct....this machine was a PCB tester....your basic bed of nails....and did need all of those items. The original post was just a single element....shoes...but since it has now gotten more detailed....here we go.... They were using a bed of nails to test innnerlayers.....now, if there is a large amount of copper on one side of a layer as opposed to the other, they will act as capacitors when stacked on the table on the side of the machine. Just picking them up will cause arcing and that is the reason for the wrist straps, anti-static mats, deionizers and ground rods...so there you have it!..... By the way, this method of testing innerlayers is now obsolete with the advent of AOI systems....(Automated Optical Inspection).....Orbotech being the main player in this field......

An in-circuit tester I presume. UUT components are always at risk which is why that room must have sufficient humidity to prevent the generation of static electric by anyone, anytime. Also some tile floor waxes are known to make static electric more likely. Just another reason why technicians running that tester would have a conductive floor - a floor mat or something equivalent - where components are handled. But first and foremost is that room must maintain proper humidity. If static is so bad that arcing is observed, then room does not even have the very first component to static electric control - humidifier.

Only then are other factors, such as girls with nylon stockings and Kmart shoe addressed. We always wore specially manufactured white lab coats made of fabrics that did not create static electricity and that were sufficiently conductive to help discharge any static. Static electric control is essential for any room where the CMOS parts are not inside the entire system - which is why I thought that is what you were discussing.

BTW earth ground rods do nothing for (are totally irrelevant to) static electric protection.

If a facility has proper static electric control, then even those Kmart shoes would not have immediately damaged CMOS semiconductors. However, the question now remains - how many more defective electronic switches are now out there because minimal static electric procedures were not implemented. Static electric also causes failures to happen long after testing and shipping has completed.

Many sites have this problem that causes CMOS failures weeks or months later. Since those failures don't appear immediately during manufacturing, a myopic management does not address even the simplest requirement for avoiding static electric problems - humidity. Your static electric damage demonstrates why management must understand how and why the work gets done. It is why management automatically demands building wide facilities are installed and maintained. Minimal requirements include building wide humidity control.

Had minimal static electric problems been addressed, then even those Kmart shoes would not create immediately detectable damage. If static electric damage was detectable that quickly, then ridiculously serious static damage has occurred previously, undetected. One must address static prevention even for all those lesser static discharges that cause CMOS failure later - in the field. Very first tool to protect from static electric generation - building wide humidifier. If any one person's clothes can cause a test detected failure, then the facility has serious static electric problems everywhere - and not just from Kmart shoes.

Orig> That is correct....this machine was a PCB tester....your basic bed of

Have you ever handled single sided fr4 material???? Just curious....Ross

Tom...you never asked what part of the country this problem occured.....Northern CA.....during the summer.....Nor did you ever ask what type of equipment it was....How can you make all these assumptions without any input....just curious once again.....Ross

If sufficient humidity, then static shocks so large as to make arcs should not occur. Posted previously:

That means conditions are too ripe for static electricity - not acceptable in any electronics assembly house. That suggests that the building was much too dry.

Humidity al> Tom...you never asked what part of the country this problem

Still making alot of assumptions here Tom....these were innerlayers on bare boards....You can't put a ground wire on every layer as it goes through the shop....and humidity will not control the static on a piece of single sided material....I ask again...have you ever handled this material??

Trying to fix the problem after a problem was created? That is what grounding those PC board does; cure a symptom rather than solve the problem. A human should never be creating visible static in the first place (no matter what he wears) - because the building is properly constructed. Only then is the wrist strap used only as secondary protection (since just like in driving a car, everything one does is confirmed or double checked). Instead, static electric generators (humans) are so powerful as to even create visible arcs. Its not the fault of human's clothes. It is a building problem. Forget about protecting any electronics. You have something directly traceable to plant management failure which is why the assembly line should be shutdown immediately. Those static electric generators (called humans) should never create static electricity that strong IF the building is properly equipped.

And so again, I keep citing the serious problem. Low humidity. Especially in summer when air conditioning lowers humidity. Low humidity is just not acceptable in any electronic assembly house. If a human creates static electricity so large as to create a visible arc, then the company top management is incompetent or technically naive; did not even install humidity control.

Now to identify the destructive circuit. Earth ground has nothing to do with static electricity. The capacitor being discharged is typically (but not limited to) a shoe. Do we discharge those shoes through each grounded layer of a PC board? Of course not.

Charges exist on both sides of a shoe. A wrist strap simply connects one side of the shoe to the other; discharging the capacitor. Earthing has nothing to do with this circuit. That static wrist strap is the second layer of protection from static electricity too small to be observed. Building is modified so that static electricity is not created up front.

What are the conductive materials? Wood. Linoleum tile. Concrete. Things not considered good electrical conductors becomes excellent conductors to static electricity. Static electricity is the generation of electricity by a human (or other) body. The 'ground' in this case may be a carpet underneath the shoe (and not earth ground). Yes, even carpet can be an electrical conductor (Of course you have no carpet in the facility due to concerns about static electric control, which is the same reasons why a static electric analysis also does not permit some floor waxes.)

I make no assumptions. The static electric control in your facility is so nonexistent, that one human even created one noticeable static electric arc. That means your facility could not be any worse for static electric protection. That means your facility has no essential humidity control - which is unforgivable in any electronic assembly house.

Forget about any grounding of PCB layers. That would be only curing a symptom. You have static electric generators 'in house' which means the entire assembly line should be shut down now, and a building wide problem immediately corrected. Your problem is so bad that the building is not even properly equipped with humidity control. If it had such humidity control, then arcs would not be visible (but might still exist). To create visible arcing, building air must be too dry. No assumption. That is basic fact. Building has a problem directly traceable to top management - because humidity control is that fundamental.

I assume you are using FR-4 PC board material. It has nothing to do with your problem nor is it a solution to your problem. After installing rudimentary static electric control (such as a building wide humidifier), then institute secondary (and necessary) protection (which also does not exist because the static electric arc was so large as to be visible). Again, grounding those PC boards has nothing to do with the problem nor is it a solution to the problem. Grounding those boards might even make problems worse! Curing symptoms never solves a problem. Your problem starts with plant maintenance - or lack thereof - and top management that did not address this all so fundamentally critical need for basic static electric control. It should have already been installed first day the company started business.

Ross Mac wrote:

You claim you are not making assumptions...OK.... Have you or have you handled single sided FR4 material in a PCB environment? You never answered that question posed twice. You don't know the manufacturer of the machine....Everett Charles Board tester....Humidity never made a difference....on cold days it was probably worse than rainy days...but the problem is still there. Slip sheeting the material does not work....It would be ridiculous to ground thousands of innerlayers. So....the way it was handled was with anti static mats, grounding, deionizing fans and wrist straps.... I would recommend you forward your comments to Everett Charles directly....perhaps you know how to keep single sided material from charging like a capacitor..... Done with this thread.....I have repeated myself and the original post of mine was just a passing comment on shoes and getting static from an automobile not a symposium on grounding on a piece of equipment that I was bought in to solve a switching card problem....have a nice day!........Ross