Summary: Electrostatic Discharge or ESD is an important issue in the electronics industry today. The integrated circuits used to create DCC decoders and other items are sensitive to static, and can be damaged or destroyed by a static discharge. Proper precautions during handling and soldering are very important. Static discharge will shorten the lifespan of electronic devices, leading to erratic operation or failure.
For vacuum tubes, ESD wasn’t an issue, later with transistors it wasn’t considered to be a problem either. MOSFETs arrived, and when their failure rates began increasing, it was discovered that static build up during handling damaged the metal oxide layer in the device, causing it to fail. In response to ESD, manufacturers of electronic equipment invested time and money to protect against the effects of static electricity. This is reflected in lower failure rates and long term reliability.
Digital Command Control devices may be sensitive to static, especially decoders. Manufacturers of DCC Decoders report that many failures are a result of ESD. Proper handling, along with an ESD Safe workbench and soldering station are important. Cold winter days are especially dangerous with respect to generation of static electricity.
What is ElectroStatic Discharge?
When dissimilar materials rub together the friction results in a positive (triboelectric) charge on one surface and a negative one on the other. The charge will remain unless it has a path through which it can flow. The resulting charge can remain in place for some time and this is "static electricity". When a path does exist current will flow and the charge will be reduced or dissipated.
The levels of voltage and current produced depend of a variety of factors. The size of the person, clothing and other materials, the amount of activity, the object the discharge is made to, and humidity. The factor which affects the voltages produced is the materials being rubbed together. Walking across a carpet can create very large voltages. Even walking on a vinyl floor can create 5 kV of potential. Working at your workbench during a DCC install could easily generate 500 V or more. You will only feel a discharge of 5kV or more, so many of these static charges and discharges wouldn't even be noticed. When the discharge occurs a large current can flow (amps!) which is why you notice it.
How Does Static Transfer Happen?
There are several ways static charges can be transferred to semiconductor devices resulting in ESD damage. Often when it is touched by an item that is charged. An classic example occurs when a decoder is on a work bench and someone walks across the floor, creating a charge, and then picks it up. The charge discharges very quickly to the decoder, with the possibility of damage. Metal screwdrivers are more conductive and will discharge even faster with higher levels of peak current. It is not necessary to touch components to cause damage to them. Plastic cups can carry a charge, and placing one near a device can "induce" a charge into it, and can damage semiconductor devices. Even air or liquids flowing can create a static charge.
Damage can occur as a result of ESD when the high level of voltage causes breakdown to occur in a component. This occurs because ESD does not destroy the device, but weakens it, leaving it at risk of failing later in its life. A discharge can damage internal gates or transistors, and each time the device experiences a discharge, the time to failure decreases. Most manufacturers consider all semiconductors static sensitive devices, and many handle passive components like capacitors and resistors as being static sensitive as well. Surface mount devices (SMD), where the dimensions are smaller than traditional components, are far more susceptible to damage from ESD.
There are a number ways in which you can protect against ESD at a reasonable cost. Not only can static cause instant failures, it can introduce latent failures, where the decoder will work on the test track, and fail later in service. There are products available enabling you to take precautions against static and its effects on electronics. Anti-static mats, wrist straps, flooring, anti-static bags and other items. By putting everything and everyone at the same potential, static can be controlled.
Creating an ESD Safe Environment
- Create an area where the DCC decoders and other electronics will be worked on.
- Components and decoders stored under conditions where they will not experience static discharges.
- People who come into contact with static sensitive devices are aware of the precautions they need to take.
- Heel strap over the heel of the shoe. (You can buy ESD footwear, but that isn't really necessary for installing a decoder).
To avoid static in the area where DCC components and decoders are being handled, the bench surface should be able to dissipate any static buildup which occurs. An anti-static mat on the bench is all you need. A conductive bench is as bad as an insulated bench, as they introduce the risk of shorting out any boards placed on them. The mat will help keep everything at the same potential.
Anti-static wrist straps ensure that any charge built up on a person is safely dissipated. This is connected to ground via the lead which incorporates a large value resistor, normally in excess of 1 Mohm. It is possible to connect your wrist band, the antistatic mat, and any other points together in a junction box. It is possible to connect the strap to the bench surface or mat which is properly connected to earth ground.
To overcome problems caused by flooring surfaces there are a wide variety of conductive or static dissipative coverings or coatings such as wax.
Synthetic fabrics used in clothing and chair coverings can develop very high levels of static during normal motion, even with the use of a wrist strap.
A cold dry winter increases the production of static charges. Dry climates and air conditioning also create the ideal environment for static. Maintaining the humidity of your workspace helps reduce static electricity. Increasing the humidity creates a layer of moisture on items in the environment, which aids in dissipating any static build up, as moisture is a good conductor. A relative humidity of 40% or better is very effective. Dry air blowing across a surface can also induce a static charge, which is also reduced by increasing the humidity.
ESD Safe Soldering Irons
A wide variety of soldering irons are available. The main requirement is that the tip of the soldering iron should be grounded. Recommended resistance tip to ground should be less than five ohms. Thermostatically controlled soldering stations should use a zero voltage switching system. This prevents large spikes caused by the switching of the thermostat from appearing on the tip of the iron.
The above link has instructions on how to check if a soldering iron is ESD safe, and verify if your ESD safe iron still is acceptable. The process is simple and requires an ohmmeter and a resistor. There is a more sophisticated process, which can be done if you have the equipment.
If your iron lacks a ground pin on its line cord, it is probably not ESD Safe.
Whenever an electronic components or assemblies are transported or stored it should be placed in suitable packing. Static bags are available to keep them protected. These can be purchases, or recycled from computer card or hard drive packaging. Conductive foam is also available. A simple check with the ohmmeter will reveal if it is conductive or not. Normally the resistance will be relatively low, a few hundred ohms at most, but this will obviously depend on the area of the test probes in contact with the foam.
When buying a decoder or other DCC accessory, keep it in the package until you plan to install it. Less handling, less chance of problems.