The problem of bubbles in the conformal coating layers
A bubble in the conformal coating is undesirable, but not always: if bubbles do not exceed a certain size, they may be acceptable.
Bubbles in the protective layers created in the conformal coating process arise when solvents or air become trapped inside and cannot escape from the material of which the protective layer is made. The presence of bubbles can lead to long-term system reliability problems, including bridging of conductive paths, corrosion of exposed areas, and cracking of the coating due to temperature changes, shock, or vibration. However, not all bubbles cause these problems - IPC has developed standards for bubble size to help determine when they can be a problem and when not.
IPC-HDBK-830: Long-Term Reliability and Testing, Section 12.1.5 - Bubbles: The presence of bubbles in the protective coating is an air entrapment or degassing phenomenon often related to mixing and/or application method. In many cases, this phenomenon cannot be overcome. Vesicles are generally acceptable when their size is less than 50% of the distance between the conductive elements in a given area of the system and do not expose the conductive elements, bridges, or adjacent conductive surfaces.
IPC J-STD-001F Brazed Electrical and Electronic Assembly Requirements: The appearance of bubbles can be considered an indispensable attribute of the process, the actual state - not a defect - which can be attributed to variability in material properties, equipment performance, or operator work, but which does not affect the form, operational readiness or product function.
Identification of threats to the long-term reliability of the product
As the IPC standards cited above say, in some cases the presence of bubbles is acceptable. However, if bubbles occur in the coating process and exceed the IPC standards, the following set of guidelines should be used to identify the causes of their occurrence.
Potential reasons related to the application process
Bubbles appear during or immediately after the application of the coating. The cause may be related to the fluid application system, valve or PCB interaction:
Properties of the fluid
Does the fluid absorb air? If air is absorbed into the fluid still in the valve, the bubbles will be transferred to the PCB. In extreme cases, the coating will be dispensed in the form of foam. What is the evaporation rate of the solvent? Is the correct solvent blend used and is the surface tension optimal to accelerate bubble elimination? Is air being injected when filling the shell material tank?
Does the protective coating material dispense displace air trapped under the components? Can bubbles arise due to overlapping dosing valve paths? The overlapping of the routes disrupts the congealing state of the fluid, creating turbulence that can cause bubbles to form and entrap air in the fluid.
Moisture in the board or components
To allow moisture to escape before the protective coating is applied, it is desirable to anneal the PCBs and components.
Make sure the PCBs are clean and free from contamination before applying a protective coating.
Potential causes related to the curing process.
If bubbles appear after the curing process, it may be due to the evaporation time or the cure profile. Consider the following variables:
The evaporation step eliminates bubbles, simply allowing time for the excess solvent to evaporate before the coated tile enters the curing oven. Consider introducing a sufficiently long flash-off time or extending the existing time to suit the requirements of the coating material.
A cure profile in which the temperature rises too quickly or becomes too hot can cause the surface layer of the material to cure too early (a "scum" is formed) and cause gas entrapment.
For areas where the shell material is extremely thick, or where the air is trapped beneath components, a milder temperature profile allows gases to escape at lower temperatures before a surface "skin" is formed.
In general, an accelerated cure profile is better for water-based materials and a gradual, cooler profile is better for solvent-based materials.