Static Electricity in the Electronics Industry: What Causes It?
Static is a major problem, it is also a natural phenomenon. In electronics, people are the main generators, but in processing industries it is fast moving materials, etc. which are the main cause. The effects on electronic devices may be invisible to the naked eye, but just because we can’t see the damage doesn’t mean it doesn’t exist. Degradation caused by electrostatic discharge in electronic components may not show up in the test. All ICs are sensitive to static discharge, it is only the threshold at which damage occurs that varies.
In 1984, an independent US investigation estimated losses directly attributed to ESD in the electronics industry at $ 18 billion. This may have been a conservative estimate, as the figure was based on just 40 percent of industry sales and projected in a survey of around 50 electronics companies.
We cannot afford to ignore the problem, assuming 0.5 percent damage to ICs (remember this is invisible damage that can be degradation or catastrophic failure) with an average of 20 to 30 ICs per PCB, this equals 10 percent. percent defective PCBs, assuming an average of 5 PCBs. per system which equates to 40 percent defective systems. Every year, other industries report losses of millions of pounds due to electrostatic discharge (ESD) causing plant and production losses. Electrostatic discharge also causes industrial equipment failure; Electrostatic sparks cause fires, explosions, and personal injury. Many industries have suffered, including NASA in 1964 when scientists lost their lives. Statistics from health and safety executives show 50 major incidents in the UK each year.
So what causes static?
Static electricity is generated by friction and separation of materials, which is called triboelectric generation. Electrons transfer from one material to another, leaving both materials charged. Since electrons are negatively charged, the material that acquires electrons is negatively charged while the material that delivers electrons becomes positively charged. The triboelectric series is often used to explain static electricity.
The following triboelectric table shows a sample of the materials that create positive and negative charges.
Air + (positive)
Human hand
Human hair
Nylon
Wool
Skin
Silk
Paper
Cotton
Wood
Rubber
Lightning
Polyester
Polyethylene
PVC (vinyl)
Silicon
Teflon – (negative)
Prevention is the best cure
We can hear static between 2000 and 3000 volts, feel static above 3000 volts, and see static at 5000 volts. At 10 to 20 percent relative humidity, a person walking on a carpet can generate 35,000 volts, a vinyl floor 12,000 volts, and a worker on a bench 6,000 volts. As technology demands more, integrated circuits can be damaged by as little as 5 volts. This can happen anytime from receiving, inbound inspection, stores, kit, PCB assembly, auto insert, wave or reflow soldering, testing, rework, or final product shipment. By removing as much of the static-generating materials as possible and making sure to take precautions against static at the workstation, while handling materials, use static monitoring and measurement equipment and, most importantly, train personnel to be aware of both of the problems. ESD-related damage will be minimized, improving product quality and increasing profitability.
With the installation of the proper protective products, such as heat sink mats, grounding cables and wrist straps, and ESD clothing, these problems can be avoided.
Many different standards are used in Europe, the US, and the rest of the world. The current European standard is EN 61340 and that of the United States is S20.20. To obtain a copy of any of the standard contacts in Europe: SCHOLARSHIP British Electrostatic Control Association, Heathcote House, 136, Hagley road, Edgbaston, Birmingham B16 9PN. For the United States: ESD Association, 7902 Turin road, Ste 4, Rome, NY 13440-2069.
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