Topic

Humidity protection

Very few bare components can withstand a humid environment without some kind of protection, and similarly with modules and assemblies. Relatively quick breakdown may happen at comparatively small voltage gradients if the surface is full of moisture, has some ionic contamination, and a mobile metal species is available – watch those dendrites grow!

So how can we improve insulation resistance, high-voltage breakdown, and protect against electromigration? The answer has to be some kind of humidity protection. Sometimes this will take the form of an enclosure, though few enclosures are sufficiently hermetic to protect a board against the affects of hot, humid air.

For most applications, whether individual components, modules or complete boards, resin protection of some type is the most commonly used, the compound covering the part to be protected and perhaps even the whole structure. For small components, such as semiconductors, transfer moulding is the preferred process, but a number of other processes are possible, many of which can be grouped together under the terms ‘embedding’ or ‘encapsulation’, in which resin completely encloses the part to be protected. Another approach, which finds favour for board-level applications, in particular with automotive and military isuers, is to provide the board with a conformal coating.

Protection for smaller components

For smaller components, transfer moulding is the process most commonly used. This is described in our paper Making mouldings. Unfortunately, not every type of component can be moulded in this way, either because of the shape of the internal element, or its ability to withstand the process. There are also issues of throughput and cost to consider, especially for smaller components.

Where the component has leads, or there is some other way of catching hold of the part, dip encapsulation becomes a possibility, and this generally works well with components that are irregular in shape. A typical application is for the ‘tantalum bead’ axial capacitor. The dipping process is normally carried out with ‘100% solids’ liquid epoxy resin systems, but phenolic resin systems with a solvent carrier (such as DurezTM) have also been used. Care has to be taken to control the viscosity of the material in the bath, and resins compounded for dipping generally contain specific anti-slump additives for this purpose and require tight control of viscosity.

A significant limitation of the liquid dip encapsulation process is the length of time for which the resin bath is exposed to air. An alternative, which is conceptually similar, is ‘powder coating’. In this the heated component is dipped into a fluidised bed of resin powder. The resin partially fuses and adheres to the surface, and the coating is subsequently cured. Provided that the component has sufficient thermal capacity to create a coating from the powder, the process is clearly less wasteful than dipping in liquid, and there is less tendency for the coating to slump after application.

Dip encapsulation is not the only process by which a liquid resin system can be used for encapsulation. Systems which are ‘100% solids’, with no solvents present, whether one-part or two-part materials, can also be used for purposes such as impregnation, casting and potting.

Impregnation fills the interstices of a component (such as a coil or motor windings) with a low-viscosity resin system in order to consolidate the structure. To improve penetration, the resin is often warmed and vacuum assistance used.

In casting, a polymer material is heated so that it is sufficiently fluid, then poured into a mould, and cured without pressure. Curing is carried out at room or elevated temperature depending on the resin used. For heavily filled materials, and castings with small clearances around embedded components, there is always a danger of entrapping air, leaving voids in the casting. To minimise this, it is common to preheat the mould and embedded components, to ‘outgas’ the casting resin, to fill in several stages, and to apply a vacuum to the filled mould while the resin is still fluid.

Advantages of casting are low mould cost, its ability to produce large parts with thick sections, the good surface finish on parts, and the fact that few finishing operations are required. The disadvantages are that the process is limited to simple shapes and is slow. Most casting polymers are thermosets, although thermoplastics such as nylon have been used.

Potting is very similar to casting, except that in the potting operation the mould remains a part of the final product. Ranges of standard moulds are available, typically as ‘empty nylon boxes’, and this approach is common for small quantity production of modules which need environmental protection. Potting compounds can be made in a range of viscosities, depending on application, filling small gaps requiring a less viscous material than filling large cases.

Potting involves adding a different material with yet another coefficient of thermal expansion, and can therefore add more stresses to the assembly. In addition, the potting process itself, which usually involves dispensing the polymer, cannot be relied upon to provide 100% fill or repeatable results, even when the casting precautions given above are taken. Therefore potting tends to be used only in the cases where it is absolutely essential for environmental reasons.

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Conformal coating

Conformal coating is the process of coating the assembled PCB and components in a thin layer of protective ‘varnish’, which ‘conforms’ to the profile of the assembly. Mainly used in the assembly of systems for harsh environments, such as automotive, aerospace and military applications, these coatings protect the boards from a variety of environmental problems such as:

Vacuum-applied solder masks provide enough protection to the PCB in moderately aggressive environments, but fail to protect components and solder pads.

Although coatings would seem to be a solution to many potential problems, they do add difficulties of their own, and a conformal coating is generally applied only if the application demands it. The first issue is cost, as an extra process is involved, but there are also yield and reliability issues:

Conformal coating materials

The coating type must be chosen to meet the requirements of the application such as solvent and chemical resistance, ease of application and the possibility of repair. A wide variety of resins is used, including acrylics, epoxies, silicones, and urethanes:

Table 1: Comparative properties of conformal coating resin families
epoxy urethane acrylic silicone
more sensitive to the action of moisture generally good moisture performance
most suitable for applications requiring protection against mechanical stress offer the best protection against occasional high humidity
significant differences between products supplied by different manufacturers no significant differences between products from different manufacturers
function only to 125°C work up to 200°C
best products for hardness/adhesion good hardness and adhesion worst adhesion good adhesion
no improvement in insulation characteristics improved insulation characteristics
marked Q reduction after immersion test no effect on high frequency parameters some Q reduction after immersion test no effect on high frequency parameters
correct thermal shock performance, but lifting and darkening in moisture resistance tests

pass thermal shock and moisture resistance test, but slight iscoloration

no failures on moisture test excellent moisture resistance
  ageing caused severe discoloration some pores caused by adhesion defects in thermal shock test some small blisters on ageing
non-flammable or self- extinguishing   mostly non-flammable non-flammable
difficult to repair; cannot be removed with a soldering iron poorly repairable easily repaired with a soldering iron easily repaired with a soldering iron
not removable solvent removable
Source: Cavero 1990

Concerns about conformal coatings

Conformal coatings can exert a hydraulic force between component and substrate during cure or subsequent thermal cycling, and this stress may be increased as a result of differences in the CTEs of the coating, components and board. This has been known to cause glass diode breakage in through-hole constructions, and in surface mount assemblies can fracture solder joints.

Test results on sensors have confirmed that conformal coating can produce permanent physical stress and that the level of stress is related to the thickness of the coating and differences in material CTEs. Effects on devices are most marked at low temperatures, and some materials demonstrate a hysteresis effect after temperature cycling.

Circuit failure has also been reported to be caused by moisture penetration to uncoated areas under surface mount devices. However, trying to remove this failure cause by increasing the coating thickness, so that it virtually encapsulates the on-board components, introduces further problems:

Applying conformal coatings

There are four methods by which conformal coatings of conventional polymers are applied to assemblies:

The last three processes are all used in volume production, and their repeatability depends on controlling the viscosity of the material used and selecting material and process to reduce the extent of runs and slump in the coating.

The ultimate in thinness and evenness of coating is produced by vapour deposition, a process where a vapour condenses onto the boards to form an adherent coating. The material most frequently cited is Parylene™, whose reactive monomer (paraxylylene) polymerises onto a cold substrate to form even, adherent pinhole-free layers over a range of thickness from 0.1µm to >100µm. Unlike liquid resin materials, the coating is the same thickness at the edges or over protrusions as it is on flat surfaces or in corners. Also, because the deposition takes place at ambient temperature, no stresses are induced. The process is, however, restricted in its commercial application as it uses specialised vacuum deposition equipment and takes a substantial time – the 25µm typically applied on a PCB takes five hours to deposit onto only 3.5m2 of assembly surface area.

Supplementary information

A wide variety of materials is available, and several different application methods. Much more information in the Concoat paper at this link.

 

Process visuals to follow

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