Technology Today

2010 Issue 2

To Preserve and Protect:

Ultrathin Environmental and Electroactive Polymer Coatings

GVD's Exilis electrically con-ductive polymer

GVD Corporation deposits conformal, "shrink-wrap" coatings of ultrathin polymers onto substrates that have complex topologies. For example, open-cell foam coated with GVD's Exilis electrically conductive polymer is shown here. GVD's vapor deposition coating process preserves the foam's open-cell structure.

Many Raytheon electronic compo­nents and systems must survive harsh environmental conditions for long periods. Environmental condi­tions such as humidity, bias, temperature cycling, and ionic contamination can cause de-lamination and migration of the metal­lic interconnects. In an effort to reduce or eliminate this type of damage, Raytheon is investigating the use of GVD Corporation’s polytetrafluoroethylene coating (PTFE, also known as Teflon®) as an alternative can­didate for the board-level environmental protection of active electronically scanned arrays (AESA).

Conventional Wet Coatings—Challenges

Applications ranging from aerospace struc­tures to radio frequency (RF) electronics to microelectromechanical systems (MEMs) re­quire ultrathin polymer coatings tailored to meet customer needs. Commonly used wet-coating methods are often complicated by the need to blend in and then remove sol­vents to ensure proper coating uniformity. Solvent purchase, processing, extraction, and disposal add to manufacturing costs and/or production time.

Coatings with uniform thickness may be dif­ficult to achieve with many wet processes, especially when very thin coatings are needed. Further, solvent-substrate incom­patibility may damage the part being coated or prevent adequate wetting; the latter also contributes to poor coating uniformity.

Coating of nano- or micron-scale surface roughness is required in many emerging applications (e.g., some flat-screen televisions have tiny moving mirrors that require a lubricating coating). But wet processes are often not adequate to achieve the required coating coverage, consistency, smoothness and thickness. Non-uniformity is exacerbated when the part being coated has a complex topology. In many cases, small features (microns or below) are obscured or overcoated when the solvent is driven off. Further, on drying, the strong liquid surface tension forces of wet coatings tend to cause small particles (e.g., carbon nanotubes, ceramic fillers) to aggregate, producing incomplete or uneven coverage. Hence, there is a great need for an all-dry, low-cost approach to depositing functional polymer coatings.

GVD Corporation's Novel, Vapor-Deposited Coatings

To address this need, GVD Corporation provides the Exilis™ line of ultrathin, sol­vent-free polymer coatings. Exilis coatings are based on novel chemical vapor deposi­tion (CVD) technologies developed at the Massachusetts Institute of Technology by Prof. Karen Gleason.

These technologies accommodate a wide range of off-the-shelf monomers and pre­cursors, and parts being coated need not have any special surface chemistry. Exilis coatings can be used in applications re­quiring environmental protection (circuit boards), RF transparency (radomes), optical transparency (lenses, displays), lubrication or release (composite molding tools), anti-stiction (MEMs), or electrical conductivity (electromagnetic interference [EMI] shield­ing, resistive heating, energy storage). Raytheon has applications in almost all of these areas.

GVD currently produces coatings based on intrinsically conductive polymers (e.g., PEDOT [Polyethylenedioxythiophene], a polythiophene), silicones, and PTFE. PTFE is a widely used polymer with unique proper­ties, including a tendency to repel water, a non-stick surface that minimizes friction, and unsurpassed chemical resistance.

Ultrathin, Conformal Coatings

Exilis coating thicknesses in the 25 nanometer to 10 micron range are typical. Deposition rates of up to 1 micron/minute or more are achievable for coatings based on PTFE, GVD's most mature product offering. Exilis PTFE coatings are ready to use right after deposition; no post-processing (drying, curing) is required. GVD's coatings are highly conformal to simple substrates and those with complex topologies, including molds, nanoparticles, foams, membranes and nanofibers (see figure).

For example, when an Exilis polymer coating is to be deposited on a porous substrate, the reactive monomer vapors infiltrate the substrate's pores, forming a thin polymer coating on contact and "shrink-wrapping" the porous structure. The open porosity of the substrate is thus preserved. Indeed, uniform "shrink-wrapping" of geometries as small as individual carbon nanotubes has been demonstrated.

Raytheon's Potential Use of GVD Coatings

When fully populated, Raytheon's AESA panel array boards have 128 TR channels. GVD's coating process facilitates lower-cost, near-room-temperature, conformal coating of the entire panel array board with proper masking. GVD's process is so gentle that even facial tissue can be coated.

Exilis coatings show significant promise in protecting Raytheon components, such as those in AESAs, that are subjected to harsh environments. For example, Exilis coatings may effectively shield circuit boards from corrosive salt water. These coatings (1012 – 1013 Ω resistance range) have survived under physiological saline soak and DC electrical bias for greater than four years. No coating cracks, pinholes or other failure manifestations have developed over this time period. For AESAs, the GVD coating can be tailored to provide exceptional dielectric performance as well as superior moisture barrier properties. Raytheon IDS is currently verifying the performance GVD coatings against required metrics.

Erik S. Handy, Ph.D.,
GVD Corporation, Cambridge, Mass.