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Outsourcing Plasma Treatment for Surface Modification
Continued from previous page Plasma treatments are utilized to increase
the surface free energy of the material to be bond- ed. When a substrate has a high surface energy, it tends to attract. For this reason, adhesives and other liquids often spread more easily across the surface. This wettability promotes superior adhe- sion.
Adhesion promotion can be achieved by
increasing the surface free energy through several mechanisms, including precision cleaning, chemi- cally or physically modifying the surface, increas- ing surface area by roughening and primer coat- ings. The net effect is a tremendous improvement in bonding. In some cases, bonds up to 50 times stronger can be achieved. On the other hand, substrates that have a low
surface energy, such as PTFE, are difficult to adhere to other materials, without first selectively altering the surface to increase the free energy. “When a surface is really hydrophobic, like
Teflon, it’s very difficult to bond to it,” says Barden. “If you apply a liquid or adhesive, by nature it pools and will not spread effectively across the surface.
Plasma treatments are utilized to increase the surface free energy of the material to be
bonded...In some
cases, bonds up to 50 times stronger can be achieved.
There are several plasma methods employed
to increase surface energy, including physical and chemical plasmas, along with PECVD coating sur- faces.
Silicone, another material widely used in the
industry for its low surface energy, often is used as an overmolding compound to protect electronic boards from extreme conditions.
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Socket L HD44 Male
SH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 21 22 23 24 25 26 27 28 29 30 16 17 31 32 33 34 35 36 37 38 39 40 41 42 43 44 M A T C H D A T A Z-AXIS CONTROLLER CABLE Print your company information here. Set up in Preferences / Print Options.
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SH 31 32 33 34 35 36 37 38 39 40 41 42 43 44 21 22 23 24 25 26 27 28 29 30 5 6 7 8 9 10 11 12 13 14 15 16 2 3 1-6-18 10:57 AM 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,
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. “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 ener- gy,” says Barden. “A process using plasma can basically harmonize 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 and surface activation treatment followed by the deposition of an inert chemical primer. This serves as a tie layer for the overmolding and provides a uniform surface energy for the silicone to bond. Gas plasma can also provide surface condi-
tioning of in vitro diagnostic platforms prior to the adsorption of biological molecules (protein/anti- body, cells, carbohydrate, etc.) or biomimetic poly- mers. This includes precision cleaning of the sub- strate at the molecular level, along with raising the surface energy to improve surface assimilation of the intended content. “Microtiter and multiwell plates are often
made of polystyrene, which is extremely hydropho- bic. Water will bead on it,” says Barden. “However, if you treat polystyrene with oxygen plasma it will become very hydrophilic, so liquid spreads every- where. This allows aqueous solutions containing biological content to spread and deliver biomole- cules to the surface, while providing a hydrogen bonding platform to adhere them.” However, some in vitro diagnostic substrates
require more selective surface chemistry to immo- bilize a customer’s proprietary molecules. For this, PVA TePla has recently developed methods for chemically functionalizing various polymer plat- forms for the selective adhesion promotion and conjugation of bio-active molecules. This is achieved by providing particular
Continued on page 61
December, 2019
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