Application selection depends on several criteria including areas such as material selection, volume of PCBs to be coated, budget, throughput speed, type of coverage required and ease of masking. Unfortunately, it can be a combination of factors that effect choice rather than individual factors and it is important to look at all the information collectively. For example, do not choose a material first and then select the equipment after. This could be extremely costly!

So how do SCH do this when quotations are prepared in our coating service?

Well, the first question that is asked is what material is to be used?

This can be critical since some coatings lend themselves to spray processing compared to dip and vice versa. For example, a moisture cure coating is not ideal for dipping since eventually the whole dip tank will cure without inerting precautions which can be an expensive exercise!

Other factors like volume and throughput are also important. It would be easy to choose selective robotic coating for all boards, assuming an automation process would be the most cost effective. However, cost of set up and program time against volume need to be considered. Also, some PCBs just don’t coat well on a robot or the application finish requirements are too stringent to complete with a selective spray system.

A good example is where a batch size may only be a monthly drop of 10 boards at a time and it costs more to set up the robot than to coat the board and the speed advantage has been lost. Another point with robotic systems is whether the board design is suitable for selective spray? If the board is very 3D in nature or has areas which are critical as no go for coating then it may be a difficult option and at least some of the process may have to be manual in nature.

Finally, factors such as budget are very important. Amortisation of costs other a number of boards gives’ the final cost of the project. It just may be the case that cost of capital equipment cannot be returned easily and lower cost solutions may need to be found such as aerosols or subcontracting the work out.

In the end it is the balance of factors which matters and this is the reason SCH use many different conformal coatings and a variety of application methods available since there is no perfect way to coat all boards.

Contact SCH directly or click here if you want to discuss further which conformal coating method and material is right for you.

Semi automatic dip coating is one of the most efficient methods for application of conformal coatings and is excellent for all volume production whether large or small.

The process of dipping a circuit board into a conformal coating material contained in a tank at a controlled speed of immersion and withdrawal ensures complete coverage, including underneath components and around difficult large 3D boards and there is no over spray or material wastage.

The control of the speed in and out combined with the viscosity of the material defines the coating thickness of the printed circuit board and the tolerance can be controlled extremely accurately.

With over 50 years experience in conformal coating SCH Technologies can offer advice on setting up a turnkey spray room for companies planning on running their own in house conformal coating operation.

General requirements

The coating room area should be clean & dust free since PCBs are susceptible to particles sticking to the drying coating. The temperature range should be reasonably controlled within sensible limits since the viscosity of the coating will vary with temperature. However, a more critical point than temperature is the humidity which needs to be controlled to be above 35% (for ESD reasons) and <55% due to moisture issues which can effect the coating integrity and the application method. Extraction will be required if atomised spraying. The grade of the extraction will depend on whether solvents are being used. If so, then bifurcated fans should be used to ensure that unnecessary explosions are avoided! With regards spraying solvents and electrical power in coating rooms there are HSE rules and guidelines with spraying where the zone 1 area on the front of the booth must not have live power or switching within 2m. Spray coating equipment A basic coating facility could simply be an extracted area, a UV source and the coating. However, for setting up a spray coating facility that can produce high quality results regularly there are several options that will aid the operator and ensure that the best results are achieved. A basic set up could include: Spray booth – For even, repeat application of the coating(s) using a spray gun. Drying cabinet – For storage of drying PCBs after application to ensure minimal contamination from the atmosphere. UV inspection booth – For inspection of the fluorescing coating under long wave UV to ensure coating coverage to IPC A 610 class III standards or alternatives. Solvent exposure alarm SEA 500 – For monitoring the exposure of the operators during the process to ensure that they are alerted of any issues and a permanent record of the process is created. Spray booth The spray booth should be primarily designed to extract the fumes efficiently whilst allowing coating to be applied easily and reliably. Using a high quality spray gun is a must to give the best coating finishes repeatably over long time periods. Although, low cost guns work reasonably well, over time they wear and they are inferior to the finish achieved with a high quality gun. During spraying, a UV light above the PCB aids the operator in visually ensuring good coating coverage. It also ensures that shadowing effects are avoided. A manual or automatic turntable also aids spraying again to help with 3D shadowing effects. The system shown is the CB100 conformal coating spray booth. Drying cabinet Once coated the PCBs should be stored in a cabinet whilst the coating dries. The curing cabinet has four main roles. These are to extract away the air around the PCBs to aid drying and remove dangerous fumes, ensure that the PCB is held still and the coating does not flow, the PCBs are not touched to damage the coating and that any particles in the atmosphere do not settle onto the PCBs and stick. The system shown is the CC100 conformal coating drying cabinet. UV inspection booth To inspect to IPC-A -610 standards the operator needs to use UV lighting. Also, if there are any finishing requirements to take place such as hand touch up with coating and a brush, extraction will be required. The operator has a dark, enclosed area to study the PCB with the UV lighting intensity maximised using a UV-transmitting plastic window in the ceiling. Also, ESD points need to be attached to ensure correct handling of the PCBs. The system shown is the IB101 conformal coating double inspection booth. Solvent Exposure Alarms (SEA) The SEA range of solvent monitoring detectors allows operators to be aware of any breaches in exposure limits. Also, the SEA 500 will data log the results and ensure that the operators have a clear record of their exposure. These easy to use systems give real time results without requiring timely analysis or delays. The system shown is the SEA 500 measurement, alarm, and data logging system. The system shown is the SEA aeroqual 500 solvent monitoring system. For further advice on setting up a conformal coating spray room please contact us. SCH have written a technical bulletin on this topic. Select this from the technical bulletin area of the website, click here to link directly or request a copy through sales@schservices.com.

My customer has asked me to bake my printed circuit boards for 2 hours at 90 degrees C before applying the conformal coating. What is the reason for this?

There are several answers to the question and most of them are really vague and to be honest possibly short of scientific basis. Talking to Phil Kinner, an expert in conformal coating, he came up with two reasons for possibly doing this:

To overcome moisture uptake of the boards between cleaning and coating if significant delay between the two processes. High levels of moisture can create havoc with certain solvent-based materials developing their adhesion, or lead to blushing, where coating becomes cloudy, hazy or milky in appearance. This also can lead to bubbling with moisture curing mechanisms occurring at a rate greater than the permeation of by products, eg CO2 through the film.
To help prevent excessive capillary flow or dripping on inversion with low viscosity solvent-based materials. This is if the boards is still hot from the bake process. The theory being that the heat drives the solvent-off quicker and builds the viscosity faster. This works well with primer or ‘dusting’ layers, less well with traditional conformal coating processes.

An extremely high quality of conformal coating coverage can be completed with an aerosol or dedicated spray booth with spray gun when using atomised spraying. The quality of the surface finish can be superior to all other methods when a trained skilled operator completes the process, as long as the circuit board is clean and the coating has no adhesion issues and the process is suitable for low and medium volume of production.

One of the key attributes of atomised spraying is giving excellent tip coverage to components. When conformal coatings are applied to a PCB they have a tendency to slump. The first layer of coating in the picture shows a thin edge on the corner of the component. This can be countered with a second coat through double dipping or over brushing but this is a repeat process and may not be acceptable. To counter this problem the technique of atomised spraying can be used.

Thin tip coverage for conformal coating application

Conformal coating material slumps and leaves a sharp edge exposed to the environment.

Atomised spraying is the application of conformal coating using an aerosol or a batch spray gun when applied at a distance from the PCB where the coating fully atomises. The key here is the distance. Selective robotic coating does not atomised the coating when applied at 10-13mm off the printed circuit board and therefore the coating will slump on the edges of the components. This is not atomised spraying.

Atomised spraying builds up the layers of coating allowing the material to stick where it is applied. This prevents slumping and builds successive layers on the sharp edges. This can be seen schematically in figure 2. In fact, although conformal coating is not classed as a waterproofing material but is in fact a moisture barrier, the application of several layers of atomised coating can effectively “waterproof” the circuit board and give an impressive and effective barrier to water on the board.

Note, although it should be considered that even though the tip coverage is improved with this process, the coating application may be limited due to 3D effects. Further, there is a benefit that the masking requirements when spraying the coating are more “shield” than “barrier” compared to dipping since the penetration is less effective. However, the lack of penetration can be an issue where coating is desired to penetrate under devices.

Please also note, the optimum application technique for a particular PCB depends on several criteria such as material selection, volume of PCBs to be coated, budget, throughput speed, type of coverage required and ease of masking. Unfortunately, it can be a combination of factors that affect choice rather than individual factors and it is important to look at all the information collectively. The application of a liquid conformal coating material can be applied by various methods including brushing, spraying and dipping, selectively robotic spray and selective dip coating and the choice is critical.

Atomised spraying of a conformal coating to achieve good tip coverage on sharp edges of components.

One of the key issues with dip coating is the quality of the tip coverage to components. When conformal coatings are applied by dipping to a PCB they have a tendency to slump.

The first layer of coating in the picture shows a thin edge on the corner of the component. This can be countered with a second coat through double dipping or over brushing but this is a repeat process and may not be acceptable. This has to be considered when using the dip process.

I counter conclusion here is that mulitple layers of conformal coating by the dipping or spray process can in fact effectively “waterproof” a circuit board or at least improve the performance of the product by eliminating the sharp edges which can cause shorts etc.