When going into coating of electronics, you can be confronted with some problem areas that need special attention.
These can be split up into three areas:
- PCB is not designed for coating
- Fine pitch connectors
- Open connectors
Most of the coating materials and especially those that are solvent based, are low in viscosity. This means that in most cases they are almost as liquid like water.
The flow of the material in this case becomes major importance and needs special attention.
The capillary flow can cause the material to flow in directions that don’t allow any coating as we will try to explain in the following examples.
PCB is not designed for coating
In many cases (if not all) the design of a PCB is not based on the fact that in the end conformal coating needs to be applied. What you than typically find are situations in which components that require coating are placed at the edge of an area that can absolutely not be coated. Test points and grounding areas that are surrounded by components are such an example (see picture 1 and 2)
picture 1 and 2 - not designed for coating
Fine pitch connectors
Fine pitch connectors can very easily soak up coating material. This is caused by the capillary flow of the low viscosity coating materials. During coating the material is nicely applied on the solder joints of these fine pitch connectors, but because of the capillary flow the coating material runs inside the connector polluting the contacts of the connector that should stay clear of coating. (see picture 3 and 4)
picture 3 and 4 - examples of fine pitch connectors
Open connectors
A third problem area is the open connector. The open connector has the body of the connector free of the board. The gap between the board and the body of the connector leaves space for the capillary flow of the low viscosity material to easily flow inside the connector and pollute the contacts. Below you will fin some typical examples of such connectors. (see picture 5, 6, 7 and 8)
picture 5 and 6 - examples of open connectors
picture 7 - open eprom socket and picture 8 - coating inside open connector
Ways to overcome these issues
There is always a solution to any problem. Therefore these issues can be tackled by using flexible solutions.
PCB is not designed for coating
The grounding areas can be protected by automatic applying a masking material. The disadvantage of this is that after coating this material still needs to be pealed off manually. An other solution can be applying a gel version of the coating material against the edge of the grounding areas and over the components that are placed against this edge. The gel will create a kind of dam preventing the more liquid material to flow over the grounding area. Of course such gel can be applied automatic.
Fine pitch connectors
Applying a masking material on the solder joints of a fine pitch connector of course is not a solution. Using a pencil and carefully manually applying the coating material is a way to overcome this issue. However automatic applying a gel version of the selected coating material prevents the material to capillary flow inside the connector. It also creates a dam to prevent lower viscosity material to flow inside as well. Applying coating material by means of a machine prevents any operator failures to occure and provides better repeatability.
Open connectors
Also for open connectors you cannot use the masking material, but again in this case the gel version of the selected coating material provides a very easy to apply solution which can easily be provided by a machine.
