Developing Conformal Coatings for the 21st Century: An Ever-Changing Landscape

Humi Greenman Piece

Before we look at conformal coating in the 21st Century, it is worth looking at how our industry has changed since the 20th Century and the effect these changes have had on reliability.

30 years ago, we had:

Through-hole PCAs with axial components and wide conductor spacing (see fig. 1)
Round axial components that were easy to conformably coat
Wide conductor spacing (causing a few problems with dendritic growth)
Leaded solder
The widely adopted practice of cleaning before conformal coating

fig 1 Fig. 1

Now move forward 30 years. In that time, surface mount components have been developed that are predominantly square with sharp edges, conductor spacing has been reduced, and packing densities have increased to levels never dreamt of in the past (see fig. 2).

Add to this the introduction of Low Solids flux (commonly known as “No clean flux”) and lead-free solder, we now have a mix for potential failure.

fig 2 Fig. 2

The Issues Emerging in Modern PCBs

We have to contend with vitrified flux that can lead to a mismatch of the coefficient of thermal expansion with the conformal coating, leading to cracking of the coating (fig. 3), especially with today's thermal cycling requirements ranging between 1,000 and 5,000 cycles.

There are also potentially trapped unreacted flux residues, that can cause dendritic growth. All resulting from the high process temperatures of lead-free solder. This leads to more failures and intermittent faults and therefore added warranty costs for the manufacturer.

fig 3 Fig. 3

So how can you overcome these problems, the first thing is to go back to cleaning the PCA, but why would you want to clean when you have “no clean” flux?

No-clean flux was developed in the 1980s when track widths/spacing were a lot wider than today’s tight, surface-mount packing densities. Electronic circuit boards are now in every aspect of our daily lives, often in safety-critical applications, operating in harsh environments.

In the days before no-clean solder flux, manufacturers who wanted reliability in their product would clean and if high reliability was required would conformal coat after cleaning.

With today's printed circuit assemblies, cleaning provides the following benefits:

Removal of contaminates that can cause electrochemical migration
Good adhesion of conformal coating
No capillary action of the conformal coating when applied
Better coverage of the conformal coating
No cracking of the conformal coating during thermal cycling

Coating Dense, Modern PCBs with Sharp Edge Coverage (SEC)

In 2022, HumiSeal started the development of a new classification of materials to meet the needs of today's surface mount components: Sharp Edge Coverage.

What is Sharp Edge Coverage?

Poor or no coverage of conformal coating on the sharp edges of surface mount components leads to a range of potential failures that include electro-migration on the surface of coatings, system failure due to condensation across the surface of the PCA, and potential unrestricted growth of tin whiskers. Traditionally, SEC has been addressed by applying multiple coating layers, this can be a time-consuming process and costly.

HumiSeal had initially developed two products to address this problem: 1B59SEC and 1A33SEC. A single 80um layer of 1B59SEC has been shown to be more effective than two or more 40um layers of a conventional coating (see fig. 4 & 5).

Fig 4

Fig. 4

The SEC products can be applied in a single layer, that gives full protection to the PCA, including Sharp Edges of the surface mount components. To confirm that the new SEC coating gives the protection envisaged an immersion test was developed by the HumiSeal R&D laboratory. This test consisted of a beaker of salt water, equivalent to seawater, a 24-volt power supply, and test boards designed for SIR evaluation and condensation testing.

fig 5

Fig. 5

So, How Did We Test?

Two test boards were used one coated with a single layer of 1B59SEC at 80µm and one without coating; in turn, they were connected to the power supply, and 24 volts were applied to the onboard circuit and then immersed in the salt water. Figure 6 shows the uncoated board under test, please note the leakage current flowing, the bubbling taking place around the components, and the final corrosion on the board.

Fig 6

Fig. 6

Figure 7 shows the test board coated with 1B59 SEC under test, note the zero leakage current and no bubbles forming around the components. This development by HumiSeal will provide superior protection for printed circuit boards operating in high humidity and harsh environments, providing long-term reliability.

fig 7 final

Fig. 7

As a supplier of all major forms and chemistries of conformal coatings, Chase Corporation and HumiSeal® can help you with an unbiased approach to evaluating your application and process. We’ll show you how to maximize efficiency, minimize cost, and improve product reliability. Our outstanding manufacturing and technical support groups can provide your organization with reliable global supply, unmatched quality, and superior technical support.

Please contact us today to discuss your application.