Exposure to water, dust, oil, chemicals, movement and extreme temperature changes can damage circuitry. This problem is exacerbated for multi-component printed circuit boards (PCBs) found in outdoor devices that must withstand a wide 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.
This has led manufacturers to look for ways to protect electronic components to ensure reliability and stability in the field. One technique is to apply encapsulating coatings such as epoxies, polyurethanes, acrylics, thermoplastics such as ethylene-vinyl acetate and deposited hydrocarbons like Parylene to PCBs. However, for outdoor weather conditions a silicone over-mould is often the preferred method due to its low water absorption, wide temperature range of use (typically -50C to 204C), thermal stability, electrical resistance and stability to ultraviolet 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, all of which have unique surface energies and chemistries. Without proper adhesion, silicone can begin to delaminate, not only at the edges of the electronic board 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. It is very difficult to find any uniform treatment that works with all the different components on a printed circuit board,” said Kevin Lewis, a PhD chemist at custom silicone formulator Quantum Silicones (QSi). QSi is headquartered in Richmond, VA and manufactures a wide selection of silicone formulations for use in mould-making, potting and encapsulating, silicone gels and coatings.
Promoting adhesion to LED display PCBs
In a real world example, a longtime customer and manufacturer of outdoor LED displays approached QSi seeking a solution for its next generation of ruggedised PCBs. Unlike prior products that relied on mechanical adhesion of silicone, the next generation product required superior chemical bonding of the silicone over-moulding because the company offers one of the longest product warranties in the industry.
To meet the requirements of the customer, QSi embarked on a mission with the development team at PVA TePla America to discover the best solution to improve adhesion of the silicone over-moulding to the LED display’s multi-component PCBs. The Corona, California-based company designs plasma systems for surface activation, functionalisation, coating, ultra-fine cleaning and etching. The shared customer was already familiar with PVA TePla, having used its plasma equipment for other applications.
The goal was to co-develop the specific process that included a plasma-applied coating that would adhere to all 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,” said Dr. Lewis. “By working with PVA TePla, we hoped to find a process using plasma that could basically harmonise all of the many surfaces and turn it into one.”
The joint effort ultimately led to a multi-step plasma treatment process that converts the surface energies of each sub-component into like polar groups, which significantly improves the overall bond uniformity.
The use of plasma is already well established for PCB cleaning (plasma desmear and etch-back by plasma) and deposition of coatings including hydrophobic and super-hydrophobic barrier coatings on PCBs without the use of wet chemicals.
Plasma is a state of matter, like a solid, liquid or gas. When enough energy is added to a gas it becomes ionised into a plasma state. The collective properties of these active ingredients can be controlled to clean, activate, chemically graft and deposit a wide range of chemistries.
Plasma treatments are often conducted in a batch process in a low pressure vacuum chamber, or can be atmospheric for inline systems.
Roughly speaking, plasma equipment manufacturers 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 of Corona, California are often tasked with the latter. In many ways, the application of plasma to meet unique surface requirements is the domain of chemists and other scientists. This is reflected in the accumulation of experts at the company, which includes three Ph.D scientists and surface, polymer, physical, bio and organic chemists, as well as engineers, plasma physicists and metallurgists.
When companies present PVA TePla with a challenging surface chemistry problem they are encouraged to visit to their lab in Corona. This gives an opportunity to brainstorm with their technical team and run experiments together. It is during these technical customer/supplier meetings that many of the best experimental matrices and ideas are produced. In addition to designing and manufacturing plasma systems, the company also serves as a contract manufacturer and so has the in-house equipment to run parts and conduct experiments, with full customer involvement.
For this particular project, QSi dispatched two of its senior chemists, Eric Washington and Dr. Lewis to work on-site at the PVA TePla lab with Dr. Barden.
The task began by securing hundreds of samples of the components on the PCB board from the customer and its suppliers so that tests could be conducted individually and together. The silicone over-moulding was then applied over a variety of catalysts and primers and tests 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.
As part of the process, 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 that the total number of permutations evaluated considering silicone formulations, plasma coating variations and quantity of components resulted in over 4,500 samples being evaluated 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 plasma chamber designed and manufactured by PVA TePla.
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-moulding and provides a uniform surface energy for the silicone to bond.
Each batch can process 15-20 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.
Even with the process now firmly established for the existing customer, Dr. Lewis still continues to make frequent visits to PVA TePla about once a quarter to expand his knowledge base and to develop new methods to improve the adhesion between QSi’s different silicone chemistries and various substrates.
“There are always applications where a customer needs improved adhesion,” said 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.”