Coating printed circuit boards: Why we apply selective conformal coating instead of simply spraying it on

From the car workshop to the programmable coating robot

You could say that we have always worked with a selective coating method: in the past, a car painter sprayed our assembled PCBs. We simply masked off the components that didn't need coating with masking tape and delivered the assembled PCBs to the car painter. He sprayed them and left them to dry, and we collected the PCBs again. Also a form of selective coating, right? But today, this procedure is completely unimaginable in our industry and with our quality standards. On the one hand, the amount of work involved is too high, and on the other hand, the components are now so delicate that spraying is no longer an option for us, whether externally or in-house. There are too many components that do not tolerate coating, often connectors or other contacts. We would have to mask them individually, but they are too small for that. We also prefer the coating of the PCBs to remain in-house and to be coated according to our quality assurance standards.

Advantage 1: The method is very precise

If we simply sprayed the assemblies, we would have to mask off large parts of the circuit board. Some components are open from below, where the paint would also be absorbed into the component from below. We would have to prevent this with insulating material. Selective conformal coating, on the other hand, has sharp edges and we can draw very fine lines that do not run.

Advantage 2: Because the moulding process works selectively, it is economical and efficient

As we only coat selectively, we use less paint. If we were to spray or even dip printed circuit boards over a large area, this would result in higher material costs. We can keep the coating thicknesses as low as possible. In addition, the finely drawn coating surfaces dry quickly. Incidentally, different paints also dry at different speeds, whether due to the paint itself or the coating method. We pass these time savings on to our customers. This is another reason why we use the Asymtek SL-940, which also has a continuous drying oven. All these advantages speak in favour of selective conformal coating and against dipping or spraying processes.

„Coatings that dry faster are a big issue for our customers. We understand that: When we not only assemble printed circuit boards, but also coat them, they dry for several days. A delay that we minimise as much as possible.“

Andreas op den Winkel
Head of Unit Assembly Department
A+B Electronic

Advantage 3: No air is trapped in the casting process

Trapped air is a classic problem with coatings. However, because we cast the coating instead of spraying it, aerosols are not a problem. This prevents bubbles, craters and foaming from the outset.

Advantage 4: The method ensures strictly uniform results

Selective conformal coating with a programmed robot gives us maximum reproducibility. For example, we always achieve exactly the same coating thickness in exactly the same place. In contrast, think of a spray can: if you press for even half a second less, the mass emerging from the can is lower, the coating thickness is lower and the protective effect and durability are different.

„In times of miniaturization, selective conformal coating is almost mandatory: components are getting smaller, but more demanding. It pays off that we gained experience with this method early on.“

Andreas op den Winkel
Head of Unit Assembly Department
A+B Electronic

The most frequently asked question: "Why do I need a coating at all?"

Customers sometimes suggest the coating they want. Then there are customers who ask which coating makes sense. We then decide together with the customer. However, many customers don't even realise that a coating is useful for their future product - or even necessary. Our advice is particularly helpful when there are no binding specifications from the German Federal Aviation Authority or other authorities - then our customers often don't know exactly whether and which coating makes sense. How could they? That's what we have our experts for. In such cases, we sometimes have to do some convincing. An inadequate coating can reduce durability, while a well-chosen and optimally applied coating can significantly increase durability. The coating is an investment in higher quality and therefore ultimately more satisfied customers.

„The coating is unnecessary for the function of the assembly. But not for the long-term durability. We provide proactive advice to ensure that the project is a joint success.“

Andreas op den Winkel
Head of Unit Assembly Department
A+B Electronic

The right coating makes a big difference to durability

The coatings that we apply to printed circuit boards have different protective functions. Some protect against heat, others against fungal attack. You can find more details here. Although we are not developers ourselves, we can certainly help to improve the product together at this point. One example: We have a customer who builds radios into the helmets of police officers. It was important to him that the assembly is not susceptible to salts. This is because the emergency services regularly sweat during their work. We therefore selected a special coating that not only protects, but also quickly becomes completely odourless. This is more pleasant for the emergency services. The success of a product can therefore depend largely on the paint.