Developing processes for plasma-applied coatings with maximum bond strength.

Plasma-deposited primer promotes adhesion of silicone over-molding on printed circuit boards for LED displays exposed to harsh, year-round weather. Exposure to water, dust, oil, chemicals, movement and extreme temperature changes can damage circuitry. This problem is exacerbated for printed circuit boards found in outdoor devices that must withstand a range of weather conditions for decades with little or no degradation in performance.

Vulnerable devices include environmental sensors, broadcast equipment, power supplies, billboards/signs, automotive components, solar panels and outdoor lighting.

Manufacturers have looked for ways to protect electronic components to ensure reliability and stability. One technique is to apply encapsulating coatings such as epoxies, polyurethanes, acrylics, thermoplastics, and Parylene to PCBs. However, for outdoor weather conditions, a silicone over-mold is often the preferred method because of its low water absorption, wide temperature range of use (typically -50° to 204°C), thermal stability, electrical resistance, and stability to UV light exposure.

Unfortunately, the topography of a PCB means the silicone must bond to many types of materials, including polymers, metals, alloys, ceramics and the FR-4 board itself. Without proper adhesion, silicone can delaminate, not only at the edges of the PCB, but also in the form of small air pockets on, or around, components. This can lead to moisture ingress and subsequent corrosion or electrical shorts.

“From a surface chemistry perspective, having a diverse group of materials to treat can be difficult because you need to develop a process for each, and the recipes can be different,” says Kevin Lewis, Ph.D., a chemist at Quantum Silicones (QSi). “It is very difficult to find any uniform treatment that works with all the different components on a printed circuit board.”

A manufacturer of outdoor LED displays approached QSi seeking a solution for its next generation of ruggedized PCBs. The product required superior chemical bonding of the silicone over-molding because the company has a long product warranty.

QSi and the development team at PVA TePla America worked to discover a solution to improve adhesion of the silicone over-molding to the LED display’s multi-component PCBs. The goal was to develop the specific process that included a plasma-applied coating that would adhere to the components and create a monolithic surface energy to create the best bond possible.

“In terms of surface energy, the best strategy is to deposit a thin-film coating over everything so the silicone only has to bond to one surface energy,” says Dr. Lewis. “By working with PVA TePla, we hoped to find a process using plasma that could basically harmonize all of the many surfaces and turn it into one.”

The joint effort led to a multi-step plasma treatment process that converts the surface energies of each sub-component into like polar groups, which improves overall bond uniformity.

Plasma is used for PCB cleaning and deposition of coatings. Plasma treatments are often conducted in a batch process in a low-pressure vacuum chamber, or can be atmospheric for inline systems. Plasma equipment manufacturers generally fall into two categories: those that produce commodity, off-the-shelf products and those that design and engineer systems to fit the needs of a specific application and/or to resolve unique surface energy challenges.

Companies such as PVA TePla are often tasked with the latter. For this particular project, QSi dispatched two of its senior chemists, Eric Washington and Dr. Lewis, to work onsite at the PVA TePla lab in Corona, CA.

The task began by securing hundreds of samples of components on the PCB from the customer and its suppliers, so that tests could be conducted. The silicone over-molding was then applied over a variety of catalysts and primers, and tests were performed to determine the degree of delamination. In addition to conducting surface tests with a contact angle goniometer, Dr. Lewis also devised a grading system to compare the options.

The silicone formulations were altered in an attempt to obtain better interaction or adhesion to the plasma coating. The plasma coatings were also varied in type, thickness and composition.  

Dr. Lewis estimates the total number of permutations evaluated, taking into account silicone formulations, plasma coating variations, and quantity of components, included more than 4,500 samples over an eight-month period.

This led to the development of a multi-step modification process to the PCB surface, completed in a batch process in a PVA TePla plasma chamber. The initial step in the process is a precision cleaning/surface activation treatment, followed by the deposition of an inert chemical primer that serves as a tie layer for the over-molding and provides a uniform surface energy for the silicone to bond.

Each batch can process 15 to 20 boards in approximately 20 minutes before the propriety silicone formulation is applied. Although the process was performed in a batch chamber system, it can also be implemented on an inline chamber system to meet high-speed, large-volume production requirements.

“There are always applications where a customer needs improved adhesion,” says Dr. Lewis. “Building of adhesion is a function of time and temperature, but most customers are not patient with the time and prefer to run processes at lower temperatures, so we are always searching for ways to drive improvements in adhesion toward shorter times and ambient conditions.”

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