Improving first time EMC test pass rates

| Environmental Testing

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With initial pass rates for EMC testing of electronic products being as low as 50%, companies can do much more to reduce their chances of failure and save large amounts of money.

The cost of failure

Meeting EMC standards such as EN 55 nnn for emissions and immunity is an essential aspect of bringing electronic products to increasingly competitive markets and yet around half of all new products which are tested for the first time fail.

For these failed products, resolving the issue could be as simple as improving enclosure earthing or as complex as going right back to the drawing board but whatever the resolution, it will take time and incur costs.

Debugging faults, redesigning, repeated testing, manufacturing process adjustments and lost time in getting the product to market all add up to substantial costs which can be avoided with some straightforward risk management.

Reducing the risk of failure

Understanding the requirements of the test directive from the initial launch of the product’s design will place the product in a better position to pass first time and by implementing a few simple pre-testing best practices, the chance of failure can be reduced significantly.

Know which EMC standards are relevant

Without considering US and UK military and defence standards or the global avionics standard, there is a plethora of European and American standards relating to EMC relating to unwanted electrical emissions from equipment or the immunity of the product from electrical emissions from a different source.

There are different European EN standards depending on the type of equipment being tested including information technology devices, household appliances and power tools, industrial equipment and scientific or medical devices. Other standards relate to motor vehicle equipment.

Consider compliance from the start

Designing compliance into a product in the first instance will always be cheaper and more effective than applying it as a “fix” at a later stage. With an understanding of the principles of EMC and the effect of design elements on the ability of the product to meet the standards, designers will be better equipped to predict likely problem areas and create a design that eliminates them.

The manufacturing process also needs to be considered at this stage. Providing production engineers with specifications of how a product is to be assembled to meet compliance will reduce problems later. Examples include:

•    Absence of paint, contaminants at earth points
•    Wire length and exposure tolerances
•    Solder chemistry specifications

The production engineer needs to create a process which ensures consistent quality of assembly to eliminate variances that could affect emissions.

For complex assemblies, use only sub-assemblies that have already achieved compliance.

Undertake pre-compliance testing

EMC testing and certification facilities have controlled environments and high capital cost equipment to minimise measurement errors or spurious emissions. Such facilities can’t realistically be replicated by product manufacturers but there are tests that can be performed simply and relatively cheaply to reduce the chances of a product failing EMC tests.

Such in-house tests don’t use antennas since they don’t provide any benefit other than in a very controlled environment which is completely free of reflected signals. Instead, probes can be used for measuring common-mode current in cables and magnetic near fields. With a spectrum analyzer and line impedance stabilisation network added to the mini laboratory, conducted emissions can also be measured on AC power cords.

The cost of pre-compliance testing using either in-house simple equipment or test house services is small compared with the high costs associated with failures at late stages of product development or even loss of market share through extended development cycles.

European directives for Electro-Magnetic Compatibility

The standards landscape for EMC is still extremely complex although it has undergone some simplification with various standard revision, particularly in the military arena where EMC standards have been boiled down to just one each for the USA and the UK.

For non-military applications, the general background standards are listed in the table along with two examples of specific technologies that have their own standards for emissions and immunity.

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