COATING TECHNOLOGY
The process also extends surface activation lifetimes.
Whereas downstream processing and staging time was once confined to a six-hour window, it now extends several weeks, providing more flexibility in manufacturing environments. This technique opens the door to new methods of
chemically engineering surface properties of PTFE. The ability to selectively functionalise the surface with primary amines, hydroxyls and carboxylic acids means that engineers can now broaden the use of this material in medical technology.
SILICONE OVER-MOULDS Silicone over-moulding is often used to protect electronic boards from outdoor weather conditions. Silicone is preferred 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 PCB 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 is even more 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. “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 Barden.
“A process using plasma can basically harmonise all of the many surfaces and turn it into one.” To accomplish this, PVA TePla has developed a
specific process starting with 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.
PRIMERS Historically, chemical primers have been used to activate difficult polymer or metallic surfaces to promote adhesion. However, many of these primers are comprised of solvents – along with catalysts – that are toxic, caustic, and carcinogenic or are potential leachables. As an alternative, PECVD can be used to deposit thin films of silicon dioxide to the substrate as an intermediate layer to improve the adhesion between a surface and a functional/linker coating or directly to a coating of choice. Within this family, the most popular are hexamethyldisiloxane (HMDSO) and tetramethyldisiloxane (TMDSO). HMDSO, in particular, is an affordable and flexible
reagent that is commercially available in a high purity, liquid form. The volatile, colourless liquid can be plasma-polymerised to create a variety of surface coatings. Depending on the composition of oxygen to HMDSO, the property of the surface can be hydrophobic or hydrophilic. Guide wires are a good example. To ease insertion,
guide wires are often treated with proprietary surface coatings to make them more lubricious. However, stainless steel guidewires are very resistant to the adhesion of lubricious coatings and other organic thin films. This problem can be solved by first applying a thin siloxane layer via a PECVD process, followed by a second PECVD process that applies a
Silicone over-moulding is often used to protect electronic boards from outdoor weather conditions
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