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Parylene C vs Parylene F

Posted by Sean Horn

Friday, June 3, 2016 7:30

@ 7:30 AM

Parylene C is the most widely used parylene type for conformal coatings.  It is classified as a poly-monochoro para-xylene, produced from dimer material, with one chlorine group per repeat unit on its main-chain phenyl ring.  As a conformal coating, Type C can be deposited at room temperature via CVD.  The resulting film exhibits low chemical, moisture, and vapor permeability, making it particularly useful where protection is needed from corrosive gases.  C’s alliance of electrical and physical properties distinguish it uses from those Parylene F, a consequence of their different chemical composition; F has a fluorine atom on its benzene ring, in contrast to C’s chlorine atom.

In comparison to Type F, Parylene C demonstrates a lesser throw-capability, generating a commensurately lower reduction in crevice penetration-activity.  At the same time, Type C has a faster rate of deposits on substrates.  Unlike C, which depends upon standard CVD processes, Type F is deposited using the Gorham method, with the cyclic dimer octofluoro-[2.2|paracyclophane].  A problem interfering with wider scale adaptation of Parylene F is difficulty synthesizing dimer.  Low availability of F dimer inhibits it commercial viability, although researchers actively seek alternatives to dimer synthesis.

Polarizability is the tendency of an atom’s electron cloud to be distorted from its normal shape by an external electric field.  Because it is influential in establishing the degree to which a given surface interacts with a designated compound’s chemistry, polarizability is well-correlated with its molecular weight, except in the case of the fluorinated chemistries.  In contract to C, Parylene F is fluorinated, characterized by fluorine atoms on its aromatic ring; its use can significantly lower coating capacitation, reducing a coated surface’s electrical charge during operation.

In comparison to Parylene C, Parylene F has a lower dielectric constant and better thermal stability, enhancing its functionality for inner layer dielectric (ILD) applications.  This capacity makes F potentially very useful for ultra large scale integration (ULSI), wherein one million or more circuit elements are situated on a single chip; these properties would support F’s expanded MEMS and NT applications, if its synthesis were more readily achieved.

F’s superior thermal stability is attributed to its aliphatic C-F bond, compared to Type C’s C-C bond.  Possessing aliphatic -CH2– chemistry, F therefore has poor oxidative and UV stability.  At the same time, F exhibits a higher coating density and significantly more penetrating power than C, although its refractive index is slightly lower.

A further comparison of essential properties of parylene Types F and C is provided in Table 1

Table 1:  Properties of Parylene F and C Compared

Properties Under Analysis Parylene F Parylene C
     
General    
Density g/cm2 1.652 1.289
Refractive index nD23 1.58 1.639
Penetration power 30x 5x
 
Electrical
Dielectrical strength/limited duration (Volts/mil @ 1 mil) 7,000 6,800
Dielectrical constant: 60 Hz 2.20 3.12
1,000 Hz 2.25 3.10
1,000,000 Hz 2.42 2.95
Dissipation factor:  60 Hz 0.0002 0.020
1,000 Hz 0.0013 0.019
1,000,000 Hz 0.0080 0.013
Volume resistivity at 23°C 50%RH 1.1(10)17 2.2(10)15
Surface resistivity at 23°C 50%RH 4.7(10)17 6.9(10)16
 
Mechanical/Physical Properties
Tensile modulus 3.0 3.2
Tensile strength, psi 7,800 10,000
Tensile strength, MPa 55 69
Yield strength, psi 7,600 8,000
Yield strength, MPa 52 55
Yield elongation 2.4 2.9
Elongation to break % 5 – 10% 10 -39%
Rockwell hardness R80 R85
Coefficient of friction – static 0.39 0.29
Coefficient of friction – dynamic 0.35 0.29
Water absorption 0.01%/24 hours 0.06%/24 hours
Water vapor temperature at 38°C, g.mm/(m2 . j) 0.32 0.10
 
Gas permeability
Nitrogen 5.8 0.95
Oxygen 34.7 7.1
Carbon dioxide 7.7
Sulfur dioxide 11
Chlorine 0.35
 
Thermal Properties
Melting temperature 435°C 290°C
Glass transition temperature 60 – 66°C 13 – 80 °C
Continuous service temperature during 100,000 hours 200°C 80°C
Continuous service temperature during 1,000 hours 350°C 115°C

 

Linear coefficient of expansion @ 25°C, K1 4.5(10)-5 3.5(10)-5
Thermal conductivity, calories/sec 3.2 cal/sec 2.0 cal/sec
Thermal conductivity, W(m.K) 0.1 0.084

 

Both Parylene F and C are largely impervious to the effects of corrosive chemicals and exhibit low levels of trace metal contamination.  Parylene C’s faster deposition rate and lower processing cost render it readily applicable for a more extensive range of uses than other parylene types, including F, explaining C’s popularity.  However, as the evidence suggests, both F and C have their own recommended applications.  Because of their unique chemical formulation, silicone coatings need to be considered within the context of all parylene types.

At Diamond-MT, we are industry-leading experts in parylene and conformal coating services. Get in touch with us today to discuss your next project by submitting a quote form or calling us at 1-814-535-3505.

To learn more about how different types of conformal coatings compare to each other, download our whitepaper:

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