Parylene Coating Blog by Diamond-MT

Parylene C vs Parylene F

Posted by Sean Horn on Fri, Jun 03, 2016 @ 07:30 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.aiga_nm__showdown_01.jpg 

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.

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

Comparing Conformal Coatings Whitepaper

Tags: parylene, parylene C, parylene f