PARYLENE EXPLANATION FROM WIKIPEDIA

Parylene is the trade name for a variety of chemical vapor deposited poly(p-xylene) used as moisture barriers and electrical insulators. Among them, Parylene C is the most popular due to its combination of barrier properties, cost, and other manufacturing advantages.

Parylene is a , which is self-initiated (no initiator needed) and un- terminated (no termination group needed) with no or catalyst required. The precursor, [2.2]paracyclophane, yields 100% monomer and initiator and does not yield any by-products.

Parylene C and to a lesser extent AF-4, SF, HT (all the same ) are used for coating printed circuit boards (PCBs) and medical devices. There are numerous other applications as parylene is an excellent moisture barrier. It is the most bio-accepted coating for stents, defibrillators, pacemakers and other devices permanently implanted into the body.[citation needed]

Parylenes are relatively soft (0.25 GPa) except for Parylene X (1.0 GPa) and they have poor oxidative resistance (~115 oC) and UV stability, except for Parylene AF-4. However, Parylene AF-4 is more expensive due to a three-step synthesis of its precursor with low yield and a poor deposition efficiency.

Parylene N is a polymer manufactured from di-p-xylylene, a synthesized from p- xylylene. Di-p-xylylene, more properly known as [2.2]paracyclophane, is made from p- xylylene in several steps involving bromination, amination and elimination. [1]

Parylene N is an unsubstituted molecule. Heating [2.2]paracyclophane under low pressure (1.0 mTorr- 1.0 Torr) conditions gives rise to a diradical species[2] [3] which polymerizes when deposited on a surface. Until the monomer comes into contact with a surface it is in a gaseous phase and can access the entire exposed surface. It has a variety of uses. In electronics, chemical vapor deposition at low pressure onto circuit boards produces a thin, even conformal polymer coating.

Other derivatives

There are a number of derivatives and isomers of parylene including: Parylene N (hydrocarbon), Parylene C (one group per repeat unit), Parylene D (two chlorine groups per repeat unit), Parylene AF-4 (generic name, aliphatic flourination 4 atoms), Parylene SF, Parylene HT (AF-4, SCS product), Parylene A, Parylene AM (one methylene group per repeat unit), Parylene VT-4 (generic name, atoms on the aromatic ring), Parylene CF, and Parylene X (a cross-linkable version, not commercially available). Characteristics and advantages

• Hydrophobic, chemically resistant coating with good barrier properties for inorganic and organic media, strong , caustic solutions, gases and water vapor • Low leakage current and a low constant (average in-plane and out-of- plane: 2.67 parylene N and 2.5 parylene AF-4, SF, HT)[4] • A biostable, biocompatible coating; FDA approved for various applications • Dense pin-hole free from 14 Å thickness [5] • Thin highly conformal transparent coating • Coating without temperature load of the substrates as coating takes place at ambient temperature in the vacuum • Highly resistant • Completely homogeneous surface • Oxidatively stable up to 350°C (Parylene AF-4, SF, HT) • Low intrinsic thin film stress due to its room temperature deposition • Low coefficient of (AF-4, HT, SF) • Very low permeability to gases Typical applications

• Dielectric coating (e.g. cores/coils) • Hydrophobic coating (e.g. biomedical hoses) • Barrier layers (e.g. for filter, diaphragms, valves) • Microwave electronics • Sensors in rough environment (e.g. automotive fuel/air sensors) • Electronics for space travel and military • Corrosion protection for metallic surfaces • Reinforcement of micro-structures • Abrasion protection • Protection of plastic, rubber, etc. from harmful environmental conditions • Reduction of friction (e.g. for guiding catheters, also acupuncture needles) • Dissolving deuterated polyethylene for making nuclear targets