Developing Conformal Coatings for the 21st Century An Ever-Changing Landscape
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
· 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
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.
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.
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, 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.
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.
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.
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.
LINK:https://blog.humiseal.com/developing-conformal-coatings-for-the-21st-century
