Faster curing makes conformal coatings more viable

Conformal coatings are used to protect a variety of printed circuit boards in automotive electronics from environmental factors such as debris and water.

Moisture-curable silicone conformal coatings are often the protective coating of choice for electronics used in demanding environments such as under-the-hood automotive applications.

However, because these materials require long cure times (they must be exposed to moisture from the air to complete their cure, a process that can take several hours), they have not always been used in high-volume, rapid line-speed manufacturing.

Newly developed cure procedures have allowed manufacturers to reduce these cure times to less than one minute.

The new procedures enable full cure of these coatings to depths of 0.005in (125 microns) with standard ovens; optimal cure speeds are obtained at 60C, 10-15 per cent relative humidity (RH) and moderate-to-high airflow rates.

The implications of these cure-time improvements are far reaching.

The new procedures promise to increase throughput and productivity for automotive electronics manufacturers and to transform conformal coating into a cost-effective processing option for volume manufacturing environments.

Many PCBs used in automotive electronics must withstand harsh and demanding environments with exposure to wide temperature fluctuations, harsh vibrations and fluid contact typically protected by applying a relatively thin (<200 micron) coating that conforms over the entire board as it cures.

Conformal coatings seal over the PCB and low architecture components to form a tough barrier to environments that otherwise would produce corrosion and damage.

Many types of protective coating are available, including acrylics, urethanes, parylenes, silicones and others.

Each has its own unique blend of application, performance and economic factors that must be balanced when making the best product selection.

Maximising throughput while minimising work-in-progress (WIP) and production-line operational costs can weigh heavily in the decision process.

Applications used in automotive frequently require advantages of silicones.

Application equipment can range from the material-inefficient but low capital and high-throughput dip tank designs to more efficient flow and spray coatings and to the efficient but more costly selective spray equipment.

Regardless of how the coatings are applied, some form of cure process must occur immediately afterwards so that the boards may be handled without fear of damaging the coating.

Silicone conformal coatings are formulated with three main types of cure mechanism, each with its own advantages and considerations.

Ultraviolet (UV) cure coatings can often be fully cured in less than 30 seconds of exposure to a strong UV light source.

This has great appeal for fast production-line speeds, but can be deceptive since only those areas of the coatings that have direct line-of-sight exposure to the UV light will cure in this timeframe.

Areas that are in the shadow of or under large components will remain liquid until a secondary moisture cure can bring about full adhesion (which can take hours or even days).

In addition, these materials typically carry a hefty price and often have disagreeable odours.

Heat-cure silicones require ovens to bring the coated boards up to temperatures typically of at least 100C for 10-20 minutes.

Higher temperatures will reduce the required oven residence time, though usually the balance between the time to warm the parts to the oven set temperature set against the required thermal energy needed to cure the coating finds an optimal balance between 100 and 115C.

These materials still have a relatively fast cure and are less costly than the UV cure coatings.

However, they often have relatively short room temperature shelf life/working life and they are sensitive to certain contaminants that can poison their cure.

Moisture cure products are the most commonly used silicone conformal coatings.

They typically have the lowest material cost and have a robust cure that is nearly foolproof.

Once these materials are exposed to humidity in the air they begin to cure.

Traditionally, most users of these materials found they needed to allow 30 - 60 minutes of open-air time to achieve enough cure to allow handling.

The automotive industry is frequently looking to reduce processing times.

To do so, one option is to reduce cure time of conformal coatings.

One way to accomplish this is to use mild oven warming of up to 60C (higher temperatures can cause significant bubbling to occur in these coatings), but the moisture content of standard convection ovens is quite low.

Since these products require atmospheric moisture to cure, low humidity conditions were not considered appropriate and the standard recommendation and practice was to install expensive controlled-humidity ovens.

Thus, users desiring the practical and economical advantages of these moisture-cure silicone coatings had to either incorporate large WIP to accommodate the long room-temperature cure time or incur a large capital expense to install and a continuing (and sometimes high) maintenance cost to operate a controlled-humidity oven.

Recent testing conducted by automotive application engineers with fast moisture-cure silicone conformal coatings has shown remarkable cure speeds in standard convection ovens with no added humidity.

In the tests, the conformal coating was spray-applied onto blank PCBs at 25, 125 and 250 microns (0.001, 0.005 and 0.010in) thickness.

These coated boards were then immediately placed in environmentally controlled chambers at 25, 40 and 60C with the humidity controlled at 15, 45 and 75 per cent RH.

Cure was monitored by measuring the time for the coating to become tack-free and for adhesion to develop.

Cure times were as expected under ambient conditions: the relatively rapid moisture cure coating used in the testing became dry to the touch within 16-24 minutes over the range of applied thickness and achieved full adhesion in 22-30 minutes at an RH of 50 per cent.

While this is an improvement over standard silicone moisture cure coatings, it can still represent a rather high WIP for large volume manufacturing lines.

At room temperature a wide zone of humidity levels will achieve fast cure times.

Increasing the humidity above 50 per cent RH slowed the cure substantially.

At 75 per cent RH and room temperature, the coating required up to an hour or more to fully cure.

At higher temperatures, the cure time reduced substantially, but early results indicated an optimal zone; a 'sweet spot' for the cure was going to be found not at the higher humidity conditions but rather at much lower RH levels.

At elevated temperatures a more narrow sweet-spot of humidity level achieves extremely fast cure times.

At room temperature, optimal cure speeds were found in a zone of relative humidity extending from about 10 to 40 per cent; boards were dry to the touch, with even the 250-micron thick coatings dry in as little as 12 minutes.

At 60C the results were even more impressive: optimal cure speed was found from 10-20 per cent RH and thin coatings could be handled in about half a minute, while even the 250-micron thick coating only required 1.5 minutes.

Coincidentally, this optimal range of humidity is exactly what most users would achieve by taking normal room air (about 50 per cent RH) and warming it to 60C - no addition or reduction of humidity is needed to obtain optimal cure times in a 60C oven.

The optimal condition of warm temperatures and low humidity to create a zone of very fast curing is due to the complex chemistry balance that occurs between the various formulation ingredients in these products and only applies to relatively thin coatings.

Moisture curing silicone products are formulated to ensure a balance between very fast surface cure while not compromising the bulk or depth of cure properties.

Since conformal coatings are applied in such thin layers, their entire cure is essentially a surface cure and therefore both their formulations and their cure profiles can be adjusted to take the highest possible advantage of surface-cure characteristics.

Cure times rapidly drop off as the coatings are warmed with no added humidity.

It is common in automotive electronics manufacturing to run line speeds as fast as possible.

With moisture-curable silicone conformal coatings the fullest extent of cure is not required before moving parts to subsequent steps - the cure will continue on its own to completion.

It is often only required to achieve a relatively tack-free surface to ensure the coating is no longer in a liquid state during an accelerated curing process.

The previous discussion and figures illustrate the cure time required to ensure the surface of the coating is tack-free.

The cure for these products proceeds from the outer surface inwards towards the board surface.

Therefore it is possible with very thick coatings to have an almost fully cured surface of the coating while the material in contact with the board remains liquid and uncured.

In practice it is uncommon to apply coatings at such thickness to actually have such an extreme occurrence.

However, it is common to have a coating appear to be fully cured and yet not have its adhesion fully developed.

Adhesion occurs in a thin zone between the surface of the board and the bottom few molecular layers of the coating.

Since this area is the furthest from contact to moisture in the air, it is also the last to fully cure.

Therefore an adhesion test can normally be used to identify the time to a full and complete cure of the entire coating.

Full adhesion was obtained with a 25-micron thick coating in as little as two minutes at 60C and 10-15 per cent relative humidity.

As with the tack-free time testing, the time required to achieve full adhesion lengthened with increasing relative humidity and with decreasing temperature.

While some manufacturing lines may require full cure with full adhesion before moving boards past the cure oven, many processes only require boards to be handle-able before they can be moved along to subsequent processing steps.

This often means the coating must be tack-free and dry to the touch, but it may not need to have obtained full adhesion.

Moisture-cure silicone coatings can be flash-cured in a low temperature oven and then shuttled along to other processes while their adhesion completes full development.

In this way cycle time can be significantly shortened to increase throughput.

Testing under controlled temperature, humidity and coating thickness for a fast moisture-cure silicone conformal coating found that cure times could be reduced from 20-25 minutes down to one minute or less when the coated boards were exposed to 60C with no added humidity.

This enhanced speed can substantially reduce WIP and increase throughput, thereby lowering part manufacturing costs, which is critical in the automotive industry to remain competitive.

In addition, it allows the utilisation of rapid heating IR ovens to reduce cure times to the bare minimum and achieve results that begin to approach the speed of UV cure coatings without the disadvantages that that type of product can bring.

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