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Static electricity - protecting your circuits and projects.



The first company I
worked for did not use static electricity precautions very much and the odd thing is that all the circuits worked fine.  

It was not a production environment and each project was a one-off and if it was produced it would be produced off site.  

The next company was a production company and were much more aware of static problems and so used anti-static mats and wrist straps - enforcing their use.  

Some Examples

The trouble with the first company is that it gives you the idea the you don't need static precautions as it does not seem to matter too much.  In fact I heard from a colleague that someone wanted some chips to use a few at home so he dived in and grabbed a few - no static protection.  He actually destroyed a whole batch of expensive chips!

In another company I worked for equipment located in another lab got zapped (only slightly as it was an input buffer chip fortunately!).  Walking across a nylon carpet built up charge and caused a spark - this carpet had been treated with anti-static sprays.

Too late

You are probably only aware of the effects of static electricity after walking across a nylon carpet and touching something on the bench - then a spark jumps across the gap and into your circuit.  Suddenly you realize you should have used a wrist strap! TOO LATE!  

The above event is known as ESD or an Electro Static Discharge event.

So what was happening at the first company?

The answer is that the ICs were slowly being destroyed. The effects of static electricity do not have to be immediate.  

The charge from an ESD blows a hole in the metalization mask in an integrated circuit.  This may or may not cause the device to fail depending on the size of the hole - the IC still functions normally if there is enough metal left.  What it actually does is cause the failure rate of the IC to go up as for the same current flow less metal for the current means more heating and so eventual failure.

So the effect of static electricity is seen more in a production environment where you can see the statistical effects by looking at sets of units i.e. when units are returned from your customers!

Spark or no spark

Static electricity can be present even if there is no spark.  

Generating the spark depends on your how much charge you have built up and the environment e.g. humidity levels.  You may or may not see a spark but static electricity is more than likely always present so you should always use precautions e.g. at least a grounded wrist strap.

The other way that you may be aware of ESD without the spark is that you may feel it - a tiny pain or tingling as the static electricity jumps to the circuit!

The problem is that the static electricity can cause circuits to fail starting from 2kV but you are only aware of it if it is greater than 3.5kV (tingling), greater than 5-8kV (sharp pain)
. [source www.staticsmart.com]

Typical voltages for normal actions e.g. walking across a carpet:

Typical Electrostatic Voltages at 20% Relative Humidity

Sliding across a foam cushion 10-14 kV
Walking across a vinyl floor 8-12 kV
Walking across nylon carpet 2-4 kV
Walking across computer grade carpet <2 kV
Walking across StaticSmart ESD carpet with ESD footwear <0.1 kV
[source www.staticsmart.com]

Note: If the humidity is higher then you will notice less ESD this because higher humidity provides water in the air which is a conductor of electricity (and the static electricity is just charge i.e. the same as electricity in a battery but less of it and at a huge voltage 4kV = 4000 Volts).  Humidity shorts the static electricity to the nearest ground. What it really does is not let the charge build up in the first place.  This is why you can buy humidifiers for use in static sensitive areas.

IC protection

Some ICs are 'so called' static electricity protected. Does this guarantee that it won't be blown up?

No - it just makes it less likely as there will always be at some point a static electricity event that generates more static than the protection can stop and your project will always be dependent on the weakest protected device in the circuit or the weakest input buffer.

Basically it boils down to this :

Always use static precautions

If in doubt use static precautions anyway.

IC storage

The small static protection boxes are the best way toy store your ICs when not in use.  They have a conductive foam into which you push the IC.  All this does is make sure that each pin of the IC has the same voltage and because of this no charge can flow and because of this the IC is protected.  The foam could be at 10kV and it would not matter as no charge would flow through the IC.

To remove the IC you must ensure that your hand is grounded using a conductive wrist strap otherwise at the point when you remove the IC it could be blown up.

Note : If you have not got static protection foam then in a pinch you could use silver foil - make sure that all pins connect to the foil - its not the best thing to do as it does not ensure pins are connected correctly, as foam does but it's useful if you are caught out.

Precautions

The most important thing is to buy a wrist strap that can be plugged into the EARTH connection of your electricity supply using a special plug.  This gets rid of static electricity from your hand which is the first point of contact for your circuit

DO NOT POKE WIRES INTO AN ELECTRICITY SOCKET ITS TOO DANGEROUS

ESD Wrist Strap

Whenever I start doing project work the first thing I do is put on the earthed wrist strap and the best type for this is one that looks like a metal wrist watch.  It's actually better as each metal section is sprung loaded to the next so that the whole wrist strap expands and contracts to fit the wrist.

Since the fabric wrist straps
(with internal metal strips)  are cheaper companies like buying them - the problem is a fabric wrist strap is that you have to undo them each time and redo them up again - they don't adjust to your wrist size.

Eventually you'll get tired of it and just not bother (Companies seem to like them because they are cheap but in the long run they are wasting money).

DO NOT POKE WIRES INTO AN ELECTRICITY SOCKET ITS TOO DANGEROUS




The wrist strap connects to a special plug that brings out the earth connection so you can connect the wire.  


Note the anti-static strap has a high internal resistance (probably 1M Ohm) its not just a wire - this is to protect you from any mains voltage that you accidentally contact.

It also lets the discharge current discharge slowly.
Anti-Static ESD mat

The next thing you need for ESD protection is an anti-static mat which is again plugged into the earth connection of the mains (using the special plug) it is not essential but lets you put circuit boards and ICs down without worrying whether they are going to get zapped.

Anti-static ESD bags

These are essential to protect projects from ESD when you put them aside but note that it's not enough to put a circuit in the bag and leave one end open you have to seal the bag or wrap the end over (all the way along).

All the rest

If you really want to overdo things you can get heel straps, floor mats, sprays, static detection devices, signs, carpets - anything really. Bureaucrats in companies love all this - gives them lots of form filling and pen pushing opportunities... Bitter? Me? No.

But I do remember one slightly related incident.  Working on the bench at a company I was surprised to find that my soldering Iron had disappeared. I asked a colleague and apparently it had been tested and removed as it was not working to the correct temperature - it was probably not displaying the correct temperature to that shown on its LED display!  So I was left unable to work as some tester decided the soldering Iron was bad (because of a form!) no replacement was provided!

I am not sure that this is funny but it made me laugh it must be true as it's from the bbc [opens a new window].  

Hey don't do project work in a nylon suit and a wool jumper!


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