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 .

There are several ways and no ideal solution. Basically, you can spray onto boards that are covered in paper or ESD bag material which will then earth to the turntable of the booth. Since the operator should be earthed via the ESD points or the floor then you creating good protection for the device.

However, it is important to change the material regularly since a build up of coating turns the material to an insulator fairly quickly!

The two layer mesh filtration means that the filters used are formed from two layers. A rougher, thicker, less dense filter is used on top and the coating is prayed on to this one(which is also less costly to replace) and a finer, thinner, more dense filter is used on the bottom to give you a laminate air flow.

The extraction should be fitted by a local air conditioning company to the customer. Due to the difficulties in supply of the correct extraction, which is dependent on location of booth to the exhaust chimney, local hse regulations, it isn’t practical for us in the UK to fit it.

The CFM is calculated from the fact that you need 0.7 m/s air flow through the spray booth to provide adequate extraction (this is a law in the UK). In order to work out the CFM we must multiply the face velocity by the area of the front aperture and then do the relevant conversion to get the figure into cubic feet per minute.

The Air Fed (CE) kit is a compressed air fed respirator supplied by compressed air passing via the visor. The full face breathing apparatus is supplied to EN166 and prEN1835:1995 standards.

The Air Fed (CE) system will provide protection against airborne dusts, mists, gases, and vapours. This visor offers respiratory protection to class 3 and hence offers a protection factor of up to 200. This means that it can be used in areas where concentration or contaminated in the workplace air is up to 200 times the Occupational Exposure Limit (OEL).

This system should not be used where the level of contaminated exceeds 200 x the O.E.L or where the contaminated or level is unknown. This system should be used in oxygen deficient atmospheres. This system should not be used below 0 degrees C

The full face mask option is ideal where highly hazardous conformal coatings are in use when spraying and any contact at all with the materials is dangerous.

Also, where stricter application and interpretation of HSE rules is in use at an organisation the inclusion of this particular PPE can be of help.

Each set of filters comprises of two filters, one for left and one for the right side of the conformal coating booth. They are identical shape. The top filters are green in colour and are for rough filtering. The bottom filters are white in colour and are for fine filtering.

There are 4 x set of green and 1 x set of white filters in each pack. This keeps it simple for ordering (one type of pack only!).

For usage of conformal coating spraying typically change green filters once/week approx, change white filters once/month (one pack per month).

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. This can be seen in the figure below.

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.

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 to success for atomised spraying is the distance from the circuit board to the spray gun.

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.

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.