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Conformal coatings are commonly used to protect a variety of printed circuit boards (PCBs) in automotive electronics from environmental factors, particularly from debris and water.

According to Dow Corning, which offers a range of conformal coatings, moisture-curable silicone conformal coatings are often used for electronics in demanding environments, such as under-the-hood automotive applications.

However, since 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.

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

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

The company claims that the implications of these cure-time improvements are far reaching.

The procedures guarantee 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 exposures to wide temperature fluctuations, harsh vibrations and fluid contact.

These are typically protected by applying a relatively thin coating (of less than 200 microns) 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 would otherwise cause corrosion and damage.

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

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

The decision process can be influenced by the ability to maximise throughput while minimising work in progress (WIP) and production-line operational costs.

Applications used in automotives frequently require the 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 highly 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 mechanisms, each with their own advantages and considerations.

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

This may be appealing for very fast production-line speeds, although the company claims that it can be slightly deceptive as 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 carry odours.

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

Higher temperatures will reduce the required oven residence time, although usually the balance between the time to warm the parts to the oven set temperature and the required thermal energy needed to cure the coating is between 100C 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 quite sensitive to certain contaminants that can poison their cure.

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

Typically, they have the lowest material cost and a very robust cure.

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

Traditionally, most users of these materials found that 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 the cure time of conformal coatings.

A proven 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.

Therefore, 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 showed remarkable cure speeds in standard convection ovens with no added humidity.

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

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

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 thicknesses and achieved full adhesion in 22-30 minutes at an RH of 50 per cent.

While this is quite an improvement over standard silicone moisture-cure coatings, it can still represent a fairly 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 had a surprising effect: it slowed the cure substantially.

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

At higher temperatures, although the cure time did reduce substantially, early results indicated that 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’ humidity level achieves extremely fast cure times.

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

At 60C, optimal cure speed was found from 10 per cent to 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 a result of 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 very fast surface cure, while avoiding compromising the bulk or depth of cure properties.

Since conformal coatings are applied in very 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 that 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 that 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 a nearly 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 a thickness to have such an extreme occurrence.

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

Adhesion occurs in a very 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.

It can be noted that longer cure times are required to obtain full adhesion, although these can also be considerably accelerated with mild heat.

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

As with the tack-free time testing, the time required to achieve full adhesion lengthened with increasing RHs 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 truly require boards to be ‘handle-able’ before they can be moved along to subsequent processing steps.

This often means that 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 greatly 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 infrared 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.

Dow Corning provides performance-enhancing solutions to serve the diverse needs of more than 25,000 customers worldwide.

A global leader in silicones, silicon-based technology and innovation, Dow Corning offers more than 7,000 products and services via the company’s Dow Corning® and XIAMETER® brands.

Dow Corning is equally owned by The Dow Chemical Company and Corning, Incorporated. More than half of Dow Corning’s annual sales are outside the United States.

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