Static electricity and good manufacturing practice

Visiting many different customers in a variety of unrelated industries is a fascinating experience. There are lots of different projects and equally, many different approaches to working practices and quality control. It is not surprising then that the rate of warranty that returns also varies widely.

At one end of the scale we have the aerospace industries where everything is considered and specified to the hilt. Operating environment, temperature, humidity, power supplies and tolerances – not to mention the operation characteristics, are all considered at design time and cross matched with the manufacturer’s specification and tested to the hilt.  Contrast this with the smaller company, operating on a limited budget, with severe time pressure.  I won’t embarrass anyone but we all know the type of operation and the cut throat industries they work in.  Unsurprisingly, our RMA’s also reflect this difference.

ESD wriststrap

However, what exactly is it that causes this variation in reliability?

After electrical abuse, i.e. subjecting the device to incorrect voltage, short circuits or general overload, static damage can be the hidden killer.Visit any competent PCB assembly house and the protection measures taken to avoid static damage are very apparent. The whole environment will be set up to avoid any possibility of allowing a static discharge.   Even the coats or overalls worn by the operators are static disipative.

What’s it all about?

We’ve all experienced the static shock when wearing a woollen pullover over a nylon shirt or walking across a nylon carpet. Well these electric shocks can carry several thousand volts. Admittedly there is a minute current flow, so apart from the surprise element they do not cause us humans too much harm. Modern digital electronics are rather different though.

ESD from fingertipsAlthough much less susceptible than the early CMOS devices, current computer chips are still damaged by such discharges. Modern devices have their inputs and outputs protected from damage by internal diodes. These are designed to be protected against a limited discharge but can fail if subject to a powerful discharge. Worse still is where a device is only weakened by such a discharge leaving it to fail in field service, at some time in the future. This is where maximum inconvenience occurs and reputations become damaged. It’s no good blaming the supplier if a product has been abused in this way.

What precautions should be taken?

Well, to start with all work surfaces should be conductive and grounded. For safety reasons this is usually via a high value resistance in the form of a conductive mat attached at one corner to earth. Anyone touching or handling components should themselves be grounded either by a shoe strap or wrist band. Although more convenient for the user, the shoe strap needs more infrastructure in the form of a conductive floor. For the casual user located at a workbench, the wrist strap is a good solution.

ESD wriststrap and board

When do we need to use the wrist band?

A simple rule is every time we handle components, motherboards, CPU, disc drive etc. It should become second nature to wear a wrist strap whenever taking the cover off a piece of equipment. With the case intact a system should be protected from static discharge. Indeed to gain CE certification, rather extreme tests involving static generators are carried out on all the exposed connectors.

Good practice through every stage of the manufacturing process will pay rewards in the long term – even if there are minimal, noticeable effects in the short term. Making use of a wrist strap should be second nature and will protect your company’s investment and reputation in the long term.

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