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Bubbles and Conformal Coating

Bubbles and Conformal Coating 

Bubbles occur when solvents or air become trapped and can’t escape 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-830 – 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 conductor, bridge of lands or adjacent conductor surfaces.

Joint Industry Standard
IPC J-STD-001F – 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 we’ve seen in the IPC standards, some bubbles are acceptable. However, if bubbles are occurring in your coating process and they exceed the IPC standards, use the following information to pinpoint the cause.

Process Related
Bubbles are occurring during or immediately after coating application. The cause could be related to the fluid system, valve, or interactions with the board. Consider the following:

  • Fluid Properties
    • Is the fluid air-absorbing? If air is absorbed into the fluid while in the valve, bubbles will be dispensed to the board. In extreme cases, the coating will be dispensed as foam.
    • What is the solvent evaporation rate? Is the right solvent blend being used, and is the surface tension optimized to accelerate the elimination of bubbles?
    • Is air being introduced during reservoir refill?

  • Coating Program
    • Is the dispensed coating material displacing air that’s trapped under components?
    • Are bubbles occurring due to pass overlap? Pass overlap disturbs standing fluid, creating turbulence that can generate bubbles and cause air to become trapped in the fluid.

  • Moisture in the Board or Components
    • Bake boards and components to allow moisture to escape before coating is applied.

  • Board Contamination
    • Ensure boards are clean and contaminate-free before coating is applied.

Curing Related

Bubbles are noticed after the curing process. The cause could be related to the flash-off time or the curing profile. Consider the following:

  • Flash-off
    • Flash-off eliminates bubbles by allowing time for excess solvent to evaporate before a coated board enters the curing oven. Consider implementing a flash-off time or extend the existing flash-off time to match the coating material requirements.

  • Curing Profile
    • A curing profile that accelerates heat too quickly or is too hot can cause “skinning” of the material to occur too soon, trapping gases. 
    • For areas where the coating material is extremely thick or where air is trapped under components, a gradual cooler temperature profile allows time for gases to escape before skinning occurs.
    • In general, an accelerated curing profile is better for water-based materials and a gradual, cooler profile is better for solvent-based materials.

For additional curing information, see Conformal Coating Mechanisms and Conformal Coating Automation: The Final Step to Coating Success.

There are many factors that contribute to the occurrence of bubbles. If you need further assistance, contact us at asktheexperts@nordsonasymtek.com.

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