Parylene C

The widely used Parylene C owes its popularity ptincipaHy to the room temperature volatiUty of its monomer. The Parylene C monomer, chloro-A-xylylene, has become the de facto performance standard. By comparison, the Parylene N monomer, A"xylylene itself, is too volatile and would perform better ia a sub-ambient temperature deposition system. The Parylene D monomer, dichloro-A-xyljlene [85586-88-5] is too heavy, and causes distribution problems ia larger deposition systems. 

In contrast to the extreme reactivity of the monomeric PX (1) generated from it, the dimer DPX (3) feedstock for the parylene process is an exceptionally stable compound. Because of their chemical inertness, dimers in general do not exhibit shelf-life limitations. Although a variety of substituted dimers are known in the Hterature, at present only three are commercially available DPXN, DPXC, and DPXD, which give rise to Parylene N, Parylene C, and Parylene D, respectively. 

Nxylylene system, substituents affect it only to a minor extent. AH parylenes are expected to have a similar molar enthalpy of polymerization. An experimental value for the heat of polymerization of Parylene C has appeared. Using the gas evolution from the Hquid nitrogen cold trap to measure thermal input from the polymer, and taking advantage of a peculiarity of Parylene C at — 196°C to polymerize abmptiy, perhaps owing to the arrival of a free radical, a = —152 8 kJ/mol (—36.4 2.0 kcal/mol) at — 196°C was reported (25). The correction from — 196°C to room temperature is... 

The enthalpy hberated on the VDP of parylene is real and in an adiabatic situation causes a rise in temperature of the coated substrate. For Parylene C, 229.1 kj/mol (54.7 cal/mol) corresponds to 1654 J/g (395 cal/g) whereas its specific heat at 25°C is only 1.00 J/(g-K) [0.239 cal/(g-K)] (33). In most practical situations, however, the mass of parylene deposited is dwarfed by the substrate mass, and the heat of polymeriza tion is dissipated within the coated substrate over the time required to deposit the coating with minimal actual temperature rise. 

Property Parylene N Parylene C Parylene D ASTM method... 

Fig. 8. Tensile strength of Parylene C on aging at elevated temperature. To convert Pa to psi, multiply by 0.145 x 10. ...

Fig. 9. Effect of temperature on the flexibility of Parylene C in air and vacuum. To convert GPa to psi, multiply by 145,000.

Fig. 10. Long-term effect of aging in vacuum on flexibiUty of Parylenes C, D, and N at elevated temperature. Failure = 50% loss in tensile strength.

In Parylene C, the single crystalline form observed is very similar to the d form of Parylene N. Its detailed crystal stmcture has been deterrnined (a = 596 pm, b = 1269 pm, c (chain axis) = 666 pm, /5 = 135.2°) (47). X-ray studies on the crystal stmcture of Parylene D have not been reported. 

Parylene C was included among the eadiest MIL-I-46058 (51) qualified coatings (as type XY) and has since enjoyed a reputation for superior performance in protecting and preserving the operation of electronic circuits against the detrimental effects of their operating environments. 

Fig. 13. Scanning electron microscope (sem) photographs of Parylene C-coated printed circuit conductor peeled to demonstrate the adhesion of the...

In a recent report (79), a 150—200 mg/cm Parylene C coating provided protection against moisture uptake by three-phase, polyimide, microballoons, and air, syntactic foams. In a previously reported coating of a similar foam, the stated purpose was strengthening (80). 

This polymer first appeared commercially in 1965 (Parylene N Union Carbide). It is prepared by a sequence of reactions initiated by the pyrolysis of p-xylene at 950°C in the presence of steam to give the cyclic dimer. This, when pyrolysed at 550°C, yields monomeric p-xylylene. When the vapour of the monomer condenses on a cool surface it polymerises and the polymer may be stripped off as a free film. This is claimed to have a service life of 10 years at 220°C, and the main interest in it is as a dielectric film. A monochloro-substituted polymer (Parylene C) is also available. With both Parylene materials the polymers have molecular weights of the order of 500 000. 

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