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. that are the main cause. The effects on electronic devices may be invisible to the naked eye, but just because we cannot see the damage does not mean it does not exist. Degradation caused by ESD to electronic components may not show up on test. All IC’s are sensitive to static discharge, it is only the threshold at which damage occurs which varies.
Back in 1984 independent American research estimated in the electronics industry losses directly attributed to ESD amounted to 18 billion dollars. This may have been a conservative estimate as the figure was based on only 40% of the industry sales and were projected on a survey of around 50 electronic companies.
We cannot afford to ignore the problem, assuming a 0.5% damage to IC’s (remember that’s unseen damage which may be degradation or catastrophic failure) with an average of 20 to 30 IC’s per PCB this equals 10% defective PCB’s, assuming an average of 5 PCB’s per system that equals 40% defective systems. Every year other industries report loses of millions of pounds due to ESD causing plant and production losses. ESD also causes industrial equipment to fail; electrostatic sparks cause fires, explosions and personnel injury. Many industries have suffered including NASA in 1964 when scientists lost their lives. Health and safety executive’s statistics 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 are transferred from one material to the other, leaving both materials charged. Since electrons have a negative charge, the material which acquires electrons is left negatively charged while the one which surrenders electrons becomes positively charged. The triboelectric series is often used in explaining static electricity. The triboelectric chart below shows a sample of the materials that create positive and negative charges.
• Air + (Positive)
• Human Hand
• Human Hair
• Nylon
• Wool
• Fur
• Silk
• Paper
• Cotton
• Wood
• Rubber
• Rayon
• Polyester
• Polyethylene
• PVC (Vinyl)
• Silicon
• Teflon – (Negative)
Prevention is the best cure
We can hear static at between 2,000 and 3,000 volts, feel static above 3,000 volts, and see static at 5,000 volts. With relative humidity at 10% to 20%, a person walking across a carpet can generate 35,000 volts, across a vinyl floor 12,000 volts, and a worker at a bench 6,000 volts. As technology demands more, integrated circuits can be damaged by as little as 5 volts. This can happen at any time from receiving, incoming inspection, stores, kitting, PCB assembly, auto insertion, wave or reflow soldering, test, rework or shipping the final goods. By eliminating as many materials that generate static, and ensuring that you take static precautions at the workstation, during material handling, use static measurement and monitoring equipment, and most importantly train staff to be aware of the problems. ESD related damage will then be reduced to a minimum, improving product quality and increasing profitability.
Many different standards are used throughout Europe, USA, and the rest of the World. The current European standard is EN 61340 and the United States being S20.20. To obtain a copy of either standard contact in Europe: BECA 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.
