How can engineers design in reliability?

Designing electronics that works on the bench is one thing, but lets put these electronics in a harsh environments, which is dirty, where the temperature changes from hot to cold, perhaps rapidly. Maybe the equipment is going to be mounted in a helicopter and experience extreme vibration or perhaps in an environment that subjects the circuits to steam?

Every designer should have a kind of risk assessment in their heads of the factors every design has to face and one of these without a doubt is the expected lifetime of the product or equipment. For example, equipment that only needs to operate only a few hours for a race needs to be thought of differently to the reliability expected for a board operating on an aircraft without fail for 25 years.

The key to knowing how to design the electronics is to have a good knowledge of the environment the equipment is going to operate in. What are the temperatures that it’s expected to operate in? Vibration and mechanical shock are also issues as well as humidity and moisture. The possibilities can be daunting – but if a few simple things are done, the risks around many

Of these issues can be reduced or eliminated.

Temperature

The first thing to remember about temperature changes, is that everything moves. If warming up then all the materials expand, when cooling they contract. If the circuit  undergoes rapid heating or cooling or both the board is going to expand and contact. While this isn’t a big issue in the x and y axis of the board, the z axis is different.

The values of thermal expansion of PCB materials is typically greater in the z axis, but this is widely disregarded as the distance is so short. But if a high degree of change takes place then via’s can fracture, often creating a very irritating failure that will be hard to find and harder to fix.

Reducing the total thickness of the board can help with this if cheaper materials need to be used. Using materials that are designed to operate in higher temperatures can be very effective as these have a reduced coefficient of thermal expansion. The extra cost of these enhanced FR4 materials is often minimal, it only get higher when considering much higher grade materials, the this cost is mostly lost in the processing of the PCB’s.

Another cost effective solution is to ask the fabricator to enhance the via wall thickness. This is covered more in the next article.

Lastly the right components need fitting to the board. If the electronics are going to be operating in an 80C environment, then 50C rated components aren’t going to be good enough. Make sure the components are rated for the task, or it’s likely that something weird will start happening at the operating extremes.

Mechanical Shock – Vibration

This is perhaps the hardest external factor to try and de-risk. Making sure that the pads are large enough, especially for components with a large mass is about all a designer can do. Only when carrying out drop and vibration testing can a designer or engineer get an idea of how well the electronics will hold together and survive in use.

Humidity & Moisture

However undesirable, humidity and moisture can be a big issue for electronics operating outside the home or office. Cars, planes, industrial equipment can all experience this. Where heat can create steam, any electronics subjected to steam are going to have a really tough time, especially if any other chemicals are in the steam as well. External factors like enclosing the electronics or conformally coating them can be used but add extra cost which can be undesirable in some cases. The best alternative is to monitor the moisture the board is subjected to, perhaps allowing extra features to shut down the circuit or warn the end user.

The question is how to do this cost effectively. Adding a conductive test coupon to act as a sensor to any kind of surface pollution is the first step. Adding it to either of the surface copper layers and leaving it exposed and coated in the same finish as the rest of the board will cost very little. Coupling this to a spare op-amp and digital I/O could mean that the only extra expense is the cost of designing it in and testing it.

Designing in reliability is more about thought and understanding than cost. More can be achieved using cheap and simple techniques before spending a lot more money – it just depends where the electronics going and how important it is that it keeps on going.

© Circuit Mechanix 2016

Contact: circuitmechanix@gmail.com

 

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