Highfields Amateur Radio Club
Constructors Corner.

In constructing a new piece of kit or even just upgrading the shack computer you may come across the two labels shown on the right along with the words 'Static Sensitive' or 'ESD Precautions Required' or other such dire warnings. You may know that you should be grounded whilst handling the item so marked but just what is ESD and why should you protect aginst it?

In this article I hope to clarify a few points and perhaps help you to prevent damage to your expensive components.

Let me start at the begining.

Everyone has heard of static electricity, but surprisingly not so many people actually understand just how harmful it can be to delicate electronic circuits or how it is 'made'.

Static electricity can be 'made' when 2 dis-simillar materials are rubbed together. The materials should be as far apart in the Triboelectric Series (a selection is shown lower down on the right) as possible for 'best' results.

The Triboelectric Series is used to show how well some common materials will absorb, or give up the 'free' electrons within them, and therefore becoming negatively or positively charged respectively. The table shows how some materials become charged relative to each other, from most positive to most negative. The ability of the material to hold this charge is mainly dependent on the humidity of the surrounding air and the smoothness of the material. A balloon will hold it charge for much longer than some fur because the individual points (the hairs) of the fur will cause corona discharge.

For an example an inflated balloon rubbed on your jumper collects enough static charge to stick to the wall, lift hairs or small pieces of paper, but sticking a cat to the wall requires more than just stroking it. (I do NOT recommend trying to stick a cat to the wall, it is just an example (& they have sharp claws)!)

Yes it is electricity that is doing that! But how does it get there?

According to the law of conservation of energy, energy cannot be created or destroyed, it can only be changed from one form to another. When you apply a charge to your handhelds' battery, the energy is stored chemically in the cells. When the battery is full and cannot hold more, the energy is converted to heat, a sure sign that the battery is fully charged. So if you apply energy, it will always go somewhere.

Now, when you apply friction energy to the balloon, electrons are dragged about and moved into places they would not otherwise be found. This electron displacement results in a negative charge on the balloons' surface. To dissipate this energy we would need a conductor to allow the electrons to return to a position of equilibrium. Without the conductor, then the electrons are going nowhere and the charge will stay put ie. static. The stored voltage can vary widely from just a few tens to many thousands of volts, it depends upon what materials generated the charge. Skin and fur, for example, are quite simillar, which is partly why stroking the cat does not usually electrocute the cat nor allow it to stick to the wall, but there are times when, given the right conditions, stroking the cat can generate small sparks of just a couple of thousand volts.

As you can see by looking at the chart, skin and nylon are seperated in the scale. This is bad news for the manufacturers of delicate electronic components who use women on the production line as the static build up could discharge through the component as it is made, in some cases causing damage that would not become apparent until some time after the component was used in a piece of equipment, causing early failure of that equipment.

The Triboelectric Series.
+ + + Most Positive	Dry Human Skin.
	Leather.
	Rabbit Fur.
	Glass.
	Quartz.
	Mica.
	Human Hair.
	Nylon.
	Wool.
	Lead.
	Cat Fur.
	Silk.
	Aluminium.
	Paper.
Neutral or 0v		Cotton Steel.
	Wood.
	Amber.
	Hard Rubber.
	Nickel & Copper.
	Brass & Silver
	Gold & Platinum.
	Polyester.
	Styrene (Styrofoam).
	Saran Wrap.
	Polyurethane.
	Polyethylene.
	Polypropylene.
	Vinyl (PVC).
	Silicon.
Most Negative - - -	Teflon.

Electronic Components & ESD through time.

Nylon was invented in about 1940, but at that time we were still using valves, or vacuum tubes. These could easily withstand 100,000 volt static discharges.

In the 1950s we saw the germanium transistor. These could withstand low current discharges and voltages of up to 1000 would not always kill them.

The first commercially available logic chips were Transistor-Transistor Logic (TTL). Whilst having an operating voltage of only 5 volts, the outputs can sink up to 100mA of current, and input logic levels are higher than 1mA. This means that the odd 150 volts of static may only 'tickle' them a little.

The later generation of low-current devices use a pair of Field-Effect transistors in a complementary output stage, Complementary Metal Oxide Silicon (CMOS). CMOS logic circuits can be damaged with just a few tens of volts, but today most of them are protected by diodes connected between inputs and the power supply rails.

Today we have Metal Oxide Silicon Field Effect Transistors (MOSFETs) which can be destroyed by less than 10 volts.

If you are seeking weak VHF signals, then will no-doubt have come across Galium Arsenide Field Effect Transistors (GaAsFETs). GaAsFETs are probably todays most static-sensitive electronic component.

Ok, now you've seen how easy it is to generate Electrostatic Charges and seen that even a small voltage Electrostatic Discharge could 'toast' one of todays delicate devices we'll see how to prevent a discharge.

You can protect your own components by taking a series of simple precautions. For example, never wear nylon stockings when handling static sensitive components, and that goes for the women readers as well!
Seriously though, you should always ensure that anything that comes in contact with a sensitive component is at the same DC potential as the component. Damage only occurs when current flows through a sensitive device, and if everything is at the same potential then there will be no current flow.

1. Always leave static sensitive devices in their storage packaging until they are required for use.
   Some field-effect transistors are supplied with a wire, or ring, wrapped around the legs to prevent current flow through the device.
   CMOS devices are usually supplied with their legs embedded in a conductive foam, or insterted in an anti-static tube. Never store a chip on ordinary plastic foam, cardboard or similar.
   
   
2. When moving a static sensitive component from one place to another ensure that both locations are at the same potential.
   If you are to hand a chip to someone, then make body contact before handing over the component. The body contact will allow the static charges between the both of you to become equal.
   If you are to place a component on a conductive surface, install on a circuit-board, or fit to a computer then you must make contact with the surface or unit before the device does. Only then will you, the unit and the device you are holding all become the same potential.
   
   
3. When working on boards containing static-sensitive components, use an anti-static wrist-strap, or bracelet.
   An anti-static wrist-strap is cheap enough, but the strap MUST worn in direct contact with your skin, and the other end earthed, or grounded via a resistor. Why a Resistor? Well if you accidently touch a live conductor or build up a large voltage charge the resistor limits the current flowing to a safe level so it won't seriously hurt you, the resistance is usually "built in", either in the strap or the connecting lead.
   There is no rule to say that an anti-static wrist-strap shall be worn on the wrist. If you want to work with free hands, then you can wear it on your ankle.
   
   
4. Use an anti-static mat (a rubber mat upon which you place your project).
   It is impregnated with carbon so it is conductive.
   It too is usually grounded via a resistor, this time to limit the possibility of a short circuit.
   
   
5. Use a grounded tip soldering iron.
   It is senseless to take all the other precautions only to zap the component as you solder it!
   
   
6. Importantly: From time to time check your wrist (ankle) strap and its lead for integrity, a broken strap or lead is no protection at all.
   I use a proper wrist strap and lead tester that I purchased, second hand and quite cheaply, at an on-line "auction house".
   I tested the unit to ensure that it was working before first use, but a multimeter with a high resistance range (about 20 MΩ) could suffice.
   Club members who wish to test their wrist straps and leads can let me know and
   I will bring the tester along to a meeting for them to check their strap & lead.