FEATURE CONTRACT MANUFACTURING & PCBS
PROMOTING ADHESION OF CONFORMABLE coatings for multi-component PCBs
Michael Barden head of research & development for PVA TePla America explores how a plasma- deposited primer is promoting adhesion of silicone over-moulding on printed circuit boards for LED displays exposed to harsh, year-round weather. Barden designs and develops custom surface coatings for the medical device and electronics industry. He manages both internal and collaborative R&D as well as contract processing. Here’s more on the latest advances in PBC processing
E
xposure 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. 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 -50°C to 204°C), 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.
PROMOTING ADHESION TO LED DISPLAY PCBS In a real world example, a manufacturer of outdoor LED displays approached Quantum Silicones (QSi), a manufacturer of silicone formulations, looking for a solution for its next generation of ruggedised PCBs. To meet the requirements of the customer, QSi
embarked on a mission with the development team at PVA TePla America, a system engineering firm that designs plasma systems for surface activation, functionalisation, coating, ultra-fine cleaning and etching, to discover the best solution to improve adhesion of the silicone over-moulding to the LED display’s multi-component PCBs. 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.
16 OCTOBER 2016 | ELECTRONICS
plasma) and deposition of coatings including hydrophobic and super-hydrophobic barrier coatings on PCBs without the use of wet chemicals. 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, a grading system was devised 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. The estimated 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. Building of adhesion is a function of time and
Figure 1: PVA-QSI blue silicone gel pouring
Figure 2: PVA-QSI cured blue silicone gel
Figure 3: PVA-QSI optically clear silicone potting
The joint effort ultimately led to a specialised,
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
temperature, but most customers are not patient with the time and prefer to run processes at lower temperatures so the company are always searching for ways to drive improvements in adhesion toward shorter times and ambient conditions.
PVA TePla America
www.pvateplaamerica.com www.pvateplaamerica.com
/ ELECTRONICS
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