Bubbles and Conformal Coating

Avoid Bubbles in Conformal Coating

Bubbles occur when solvents or air become trapped and can’t escape the conformal coating material. The presence of bubbles can lead to long-term product reliability issues, including bridging of conductive paths, corrosion of exposed areas, and cracked coating due to temperature changes, shock, or vibration. However, not all bubbles create these issues. The IPC provides standards around bubble size to help you identify when they might pose an issue and when they won’t. Let’s take a look at the IPC standards.

IPC Standards

IPC-HDBK-830A – Long-Term Reliability and Testing
Section 12.1.5 – Bubbles

The presence of bubbles in a conformal coating is a factor in air entrapment, outgassing, mixing and/or application methods. In many cases, this phenomenon cannot be overcome. Bubbles are generally acceptable when their size is less than 50% of the distance between conductors at the location, and they do not expose the conductor, bridge of lands, or adjacent conductor surfaces.

Joint Industry Standard

IPC J-STD-001J – Requirements for Soldered Electrical and Electronic Assemblies

When bubbles or voids are present and do not meet the void criteria, they shall be considered process indicators. A process indicator is a condition – not a defect – that is attributable to variation in material, equipment operation, workmanship or processes, but that does not affect the form, fit, or function of a product.

When Long-Term Product Reliability is at Risk – Identify the Cause

As outlined in the IPC standards, some bubbles in the coating process are acceptable. However, if you notice bubbles that exceed these standards, use the following information to help identify the cause.

Process-Related Causes

Bubbles occur during or immediately after coating application. The cause could be related to the fluid system, applicator, or interactions with the board. Consider the following:

Fluid Properties

• Is the fluid air-absorbing? If air is absorbed into the fluid while it is in the applicator, bubbles will be dispensed onto the board. In extreme cases, this can lead to the coating being dispensed as foam.
• What is the solvent evaporation rate? Is the correct solvent blend being used, and is the surface tension optimized to accelerate the release of bubbles?
• Is air being introduced during the reservoir refill?

Coating Program

• Is the dispensed coating material effectively displacing air that’s trapped beneath components?
• Are bubbles forming due to overlapping passes? Pass overlap disrupts the standing fluid, creating turbulence that traps air and generates bubbles.

Moisture in the Board or Components

• Coating a PCB or components that contain moisture traps the moisture inside, causing bubbles. Ensure the PCB is thoroughly baked or dried using specialized drying methods.

Board Contamination

• Ensure the boards are clean and free of contaminants before applying the coating. Consider using plasma surface treatment, as it effectively cleans surfaces and increases the surface energy (wettability) of the boards, which helps improve adhesion and prevents bubbles.

Curing-Related Causes

Bubbles can appear after the curing process, and their formation may be linked to the flash-off time or the curing profile. Consider the following:

Flash-off

• Flash-off allows excess solvent to evaporate before the coated board enters the curing oven, which helps eliminate bubbles. If you're encountering issues, consider implementing a flash-off period or extending the current time to better suit the requirements of your coating material.

Curing Profile

• A curing profile that heats too quickly or reaches excessively high temperatures can cause "skinning" of the material too soon, trapping gases within.
• In areas where the coating material is particularly thick or where air might be trapped beneath components, a gradual cooling profile will enable gases to escape before skinning occurs.
• Generally, an accelerated curing profile is more effective for water-based materials, while a gradual, cooler profile is better suited for solvent-based materials.

Numerous factors play a role in the formation of bubbles. Understanding these influences can enhance your knowledge and ability to manage bubble-related issues effectively.

If you require further assistance, please don't hesitate to reach out to us at [email protected].