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:
Acrylic.
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.
5%)
Epoxy.
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
Polyurethane.
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.
Silicones.
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
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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entire series can be found by searching and joining the “Switch Mode Power Supply
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Advance
Product Services Ltd
Paul
Horner is Managing Director at Advance
Product
Services Ltd.
