Hello Again! The last time we discussed bubbles in conformal coating, I wrote how to resolve process related root causes. There are other simple ways to reduce the amount of bubbles:
- by varying the types of solvents used in a product
- by using a product that contains a surface tension modifier, hence increasing the rate at which bubbles burst.
If you recall, most of the bubbles are basically solvent or air entrapped in the coating. As you might expect, the air and solvent try to find their way up through the liquid to escape. The problem is that the surface dries first, creating a thin skin that prevents those bubbles from escaping. Therefore, the solution is to eliminate the bubbles in the coating prior to the skin formation. In other words, use a solvent set and other surface tension reducing chemistries to accelerate elimination of bubbles.
Surface Tension
Let me start by explaining what is surface tension of a liquid. Surface tension is a property of liquid to "resist" an external force, similar to bubble entrapment. If an external force (a bubble) tries to break the surface of a liquid, it needs to be greater than the surface tension of that liquid. An additional important note here, surface tension is related to the density of the liquid. By incorporating the proper solvent, this will help to reduce the surface tension of the coating therefore improving bubble dissipation.
Product selection along with lower density solvent are the fastest and the simplest ways to reduce bubbles. For example, HumiSeal 1B78 was specifically formulated with solvents and additive package, which in experiments show much faster reduction of bubbles at the surface. Conformal coating process engineers who work with solvent-borne coatings, have always encountered bubble formation related to issues independent of their process. Adjustments to coating viscosity or evaporation rate must be made to improve product quality. Some of the product related root causes and solutions are listed below.
Non-process related bubble sources:
- Coating skins over too fast entrapping bubbles
- Coating does not contain surface tension modifiers entrapping bubbles
- Surface tension of coating is too high not allowing for bubbles to “pop”
- Solvent evaporates too fast creating bubbles at the surface
- Not enough solvent creating high surface tension of the coating
Product related solutions for bubble reduction:
- Select product that best suits your production and ambient conditions
- Reduce surface tension by addition of solvent
- Slow down your solvent evaporation rate
- Use product that contains surface tension modifiers
Bubbles in conformal coating is one of the most common technical help request which customers present to us. No one seems to be immune to this simple yet frequent problem. HumiSeal continues to improve its products to help the all customer achieve maximum process efficiency with minimal defect rates. 1B78 is just one example of product improvement. For more information about HumiSeal’s latest products and technical help please contact us.
