A Guide to Parylene Temperatures

A specialized chemical vapor deposition (CVD) process attaches conformal coatings composed parylene (XY) to substrates.  CVD uniformly encapsulates all exposed substrate surfaces as a gaseous monomer; completely eliminating wet coatings’ liquid phase and need for post-deposition curing.  Synthesizing in-process, CVD polymerization requires careful monitoring of temperature levels throughout.

Beneficial thermal properties of XY protective coatings include reliable performance through an exceptional range of temperatures.  Parylene is available in variety of material formats, prominently Types C, N, F, D and AF-4.  Each has a particular range of properties that determine its optimal uses.  Types C and N exhibit faster deposition rates than other parylenes, making them useful for a wider range of coating functions.  However, operating temperature is a significant determinant of use:  Much depends on chemical composition.

• Used more frequently than other XY varietals, Parylene C is a poly-monochoro para-xylene.  It is a carbon-hydrogen combination material, with one chlorine group per repeat-unit on its main-chain phenyl ring.  In oxygen-dominated atmospheres, C conformal films regularly provide reliable assembly security at temperatures of 100° C (212° F/water’s boiling point) for 100,000 hours (approximately 10 years).  C is suggested for use in operating environments reflecting these temperature conditions.  Chemical, corrosive gas, moisture, and vapor permeability remain consistently low.  C generates exceptional vacuum stability, registering only 0.12% total weight-loss (TWL) at 49.4° C/10^-6 torr (1 torr = 1/760 SAP (standard atmospheric pressure, 1 mm Hg).   C can also be effective at temperatures below zero, to -165º C.
• With a completely linear chemical format, Parylene N is the most naturally-occurring of the parylene series.  Used less regularly than Type C, N is highly crystalline; each molecule consists of a carbon-hydrogen combination.  N’s melting point of 420° C is greater than most other XY types.  Vacuum stability is high, registering TWL-levels of 0.30% at 49.4° C, and 10^-6 torr.  These properties encourage higher temperature applications.  Compared to other XY varietals, N’s low dielectric constant/dissipation values also recommend uses with assemblies and parts subjected to higher levels of unit vibration during operation.  N’s electrical/physical properties are not noticeably impacted by cycling from -270º C to room temperature, adding to its versatility.
•  Parylene F has fluorine atoms on its aromatic ring.  Possessing aliphatic -CH₂– chemistry, F’s superior thermal stability is attributed to this aliphatic C-F bond, compared to Type C’s C-C bond.   Better thermal stability, and reduced electrical charge/dielectric constant expand its use for ILD (inner layer dielectric) applications, such as those for ULSI (ultra large-scale integration), where a single chip can incorporate a million or more circuit elements.   F is a good choice for many microelectromechanical systems (MEMS)/nanotech (NT) solutions.

• Originating from the same monomer as Type C, Parylene D’s chemical composition contains two atoms of chlorine in place of two hydrogen atoms.  Like Type C, D conformal films can perform at 134° C (273° F), dependably securing assembly performance in oxygen-dominated environs for 10 years, at a constant 100° C.  Parylene F resists higher operating temperatures and UV light better than C or N.
• Parylene AF-4’s melting point is greater than 500° C.  It survives at higher temperatures/UV-exposure better than other parylenes for long durations because it possesses CF₂ units, situated between its polymer-chain rings.

Because oxygen-free environments prohibit oxidative degeneration, XY’s operational temperature range increases significantly if used in inert atmospheres, characterized by absence of air. Depending on XY type, parylene can function at temperatures as low as -270º C, or as great as 450º C, representing total sustained operation within a thermal span of 720º C.   CVD for higher temperature range XY types are costlier.  Table I provides temperature data pertinent to using major parylene types.

Table I:  Significant Temperatures for Selected Parylenes

Parylene 

Properties                Type C               D            N            F            AF-4                  

Melting point, ° C             290             380          420          —           > 500                  

Continuous service

temperature (O₂/inert

environments, ° C )        80/230           —         60/220       200           350                   

Short-term service

temperature (per 1,000

hours use, O₂/inert

environments ° C)         110/320          —         85/265       250           450                  

T5 point, ° C

(modulus = 690 MPa)       125             125          160          —              —                      

T4 point, ° C

(modulus = 70 MPa)         240             240          300          —              —                       

Thermal conductivity,

25° C                                    2.0          —                 3.0        —              —                       

Specific heat, 25° C               0.17         —                0.20      —              —                    

 

To learn more about parylene, download our whitepaper now: https://blog.paryleneconformalcoating.com/parylene-101-guide/