Wednesday, 14 August 2013

Design for life—part 6

Last time we looked at spike and surge - a major cause of non-age related failure in the field. In this final episode we look at environmental affects and the various solutions available to counteract them. Around 20% of failures seen back from the field occur as a direct result of the environment they have been operated in.
High ambient temperature and or poor ventilation are obvious examples but o/en the less apparent occurrences such as early morning condensation (which is commonplace inside any cold enclosure with direct atmospheric ventilation) are not always as well considered.

Somewhat counter-intuitively, relatively pure water is not particularly troublesome from an electrical leakage point of view. It is the presence of ionic substances such as fingerprints which become weakly conductive, and salts which create a thin electrolyte film that are the most problematic.

Potting or conformal coating is the industry-wide accepted solution, but few people realize that on a molecular level moisture eventually penetrates the majority of organic coatings (with the exception of Parylene). Because of this, the most crucial step is that all surface contamination is removed prior to applying the coating through vapour degreasing or semi-aqueous washing. The coatings primary role then is to prevent the subsequent deposit of ionisable contaminants.

Conformal Coating
The coating technique plays a fundamental role in its effectiveness. There are many methods available such as brush coating, spray application, conformal dipping or selective robotic process and there are pros and cons of each.
Brush coating is tends to be limited to very low volume, repair and corrective actions. Due to its highly manual nature it is o/en susceptible to air bubbles and is subjective in terms of quality. Spray application suits low to medium volume and can result in a good quality finish but is usually limited due to 3D constraints, with poor penetration under, devices.

Conformal dipping is generally highly repeatable and suited for production volume, but great care must be given to masking. Indeed the difficulty in preventing unwanted seepage means many pcb’s are unsuitable for dipping. Slumpage around sharp edges can also be a problem, but can be minimized by double dipping or supplemental spraying. If the design can accommodate it though, dipping is usually adequately effective, especially when combined with the application of a vacuum whilst the assembly is submerged in the resin which even eliminates uncoated surfaces in interior cavities.

Selective robotic process generally involves the application of atomised spray by needle injection or ultra sonic valve. The needle or valve can move around the pcb and dispense the resin very selectively and at a highly accurate thickness. The process suffers from the same 3D limitations as other spray methods and also from the same unwanted capillary effects.

It is however the technique most suited to high volume where the design of the pcb means dipping is not possible. The selection of the conformal coating material is a crucial factor that needs to be carefully considered— not only for the anticipated environment but also for the intended application technique. There are many considerations such as the atmosphere the equipment is to be protected against, temperature range, electrical, chemical and mechanical compatibility (coefficient of expansion), ease of rework, cure times and of course price. The more common materials can be summarized as follows:

Pros: Ease of rework, simple film drying, fast cure time, moisture resistance, and high florescence.
Cons: High VOC (solvent evaporation), flammability, thickness dependent cure 'me, shrinkage (approx.

Pros: Wide temperature range (up to approx. 155’C), abrasion resistance, coefficient of expansion (well matched to pcb substrate), dielectric properties.
Cons: Ease of rework, process intensive (maintaining viscosity, complex mix ratios), chloride contamination (solvent based), high component stress during thermal cycling

Pros: Dielectric properties, moisture resistance, solvent resistance, abrasion resistance.
Cons: Moisture effects cure rate and properties, long complete cure 'me (several days), thickness dependent cure 'me, high VOC (solvent evaporation), reacts violently with water if using heat cure.

Pros: Very wide temperature range (-40’C to +230’C), flexible (provides dampening and impact protection), moisture and sunlight resistance, dielectric strength, low surface energy (good penetration under components).
Cons: requires humidity to cure, solvent resistance, abrasion resistance, short pot life, and long cure times.

Potting, or encapsulation generally offers a greater degree of protection than coating as it eliminates problems such as slumpage around sharp profiles and also has major benefits in terms of mechanical shock and vibration. The main difficulty with potting is thermal management, both in terms of absolute internal component temperatures and mechanical stress due to the thermal cycling of the potting medium. Indeed it can o/en be difficult to evaluate these stresses and this can have a major impact on product longevity.

As all the components are essentially thermally coupled, much attention has to be paid to vulnerable items such as electrolytic capacitors which may actually run much hotter than they would in free air convection. In recent years there have been several innovations in potting compounds and there are a multitude of epoxies and silicone's available with wildly varying characteristics to suit the intended application. Attention needs to be paid to fire rating, thermal conductivity, viscosity, temperature rating, and dielectric strength and cure mechanism.

Some modern silicone's offer very low stress and very high thermal conductivity with temperature ratings well over 200’C making them ideal for many electronic applications. For high volume production however, consideration must be given to the additional processes required and potential limitations in product throughput.

It is also worth bearing in mind that the serviceability of the product is generally greatly reduced with potting - indeed rework can o/en be very 'me consuming and is usually uneconomic. It’s more important than ever to make sure the design is thoroughly proven electrically and properly thermally evaluated. Get these right however and the outcome is typically an extremely resilient product with a very long service life.

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The entire series can be found by searching and joining the “Switch Mode Power Supply Repair” group.

Advance Product Services Ltd
Paul Horner is Managing Director at Advance
Product Services Ltd.

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