Parylene Coating Blog by Diamond-MT

Parylene Dieletric Properties

Posted by Sean Horn on Fri, Jun 05, 2015 @ 08:00 AM

 Basic Dielectrics and Conformal Coatings

A dielectric substance is an electrical insulator with the additional property of transmitting electricity without conduction.  Dielectric mediums are efficient supporters of electrostatic fields that also dissipate thermal energy at a minimal rate; thus, they have the potential to store energy.  The parameter encompassed by a substance's dielectric constant is a common source of comparison in the design of printed-circuit-boards (PCBs) and similar higher-speed electrical components. 

The lower the dielectric constant, the less the extent to which a material concentrates electric flux,  Composed of weakly bonded molecules, lower dielectric-constant substances such as parylene conformal coatings produce a reliable barrier between a substrate and its environment.  Polarized by electrical charges, they resist electrical conduction and therefore can be effectively used to coat the high-speed electrical components common to IT, biomedical and communications' media. 

Essential Dielectric Properties of the Parylene 

Parylene conformal coatings are known for their documented dielectric properties. Highly corrosion resistant, dense and pinhole free, they provide a completely homogeneous coating surface.  Dielectric properties differ according to parylene type.  For instance, the average in-plane and out-of-plane dielectric constant for parylene N is 2.65, while parylene C is 2.95.  Other parylene types range between 2.25 - 3.15.  Table 1 provides a further dielectric comparison of major commercially viable types of parylene.

TABLE 1:   Dielectrical properties of Parylene N and C

PROPERTIES

PARYLENE N

PARYLENE C

 

Dielectrical strength, V/ml

 

7,000

 

5,600

Dielectric constant 

60 Hz

1KHz

1MHz

 

2.65

2.65

2.65

 

3.15

3.10

2.95

Dissipation Factor 

60 Hz

1KHz

1MHz

 

0.0002

0.0002

0.0006

 

0.020

0.019

0.013

 

 

 

Parylene's Dielectrical Performance Relative to Other Conformal Coatings

In comparison to competing types of conformal coating, parylene's lower dielectric constants reflect its capacity to withstand intense electrical fields.  One of its advantages as a surface coating is a considerably lesser tendency not to devolve when subjected to significant electrical activity.  Competing coatings have higher dielectric constants (Epoxy, 3.3 - 4.6. Polyurethane, 3.8 - 4.4, Silicone, 3.1 - 4.2), and a lower non-conductive capacity.  Their dielectrical strengths are also lower (Epoxy, 2200. Polyurethane, 3500, Silicone, 2000), indicative of the diminished ability to provide dielectric protection compared to the parylenes.

Substances with higher dielectrical readings break down much more readily during prolonged contact with intense electrical activity.  On the odd chance that parylene does suddenly begin to conduct current and is subjected to a dielectric breakdown, the condition is temporary, subsiding with removal of the excessive electric field.  At that point, parylene reverts to its normal, non-conductive dielectric state and does not sustain permanent damage.

Advanced Applications Aided by Parylene's Dielectrical Properties  

Parylene's dielectrical capacities have extended the utility of existing products and aided in the development of new ones.  Among current applications are:

  • Integrated circuits:  Parylene polymers offer comparatively low dielectric constants (k~2.25-3.1), enhancing their utility as coatings for next generation, higher-speed integrated circuits and related electronic components.  Whereas conventional dielectrics and metals generate interconnect delays that interfere with sub-.025 µm devices, parylene's low k constant enables production of more efficient and reliable high-speed electronics, which also significantly reduce cross-talk between adjacent lines.   
  • Radiation applications:  Parylene polymers prepared in uniform thicknesses < 0.025 µexhibit their typical exceptional nonabsorbent and gas barrier properties, when used for radiation generated applications.  Parylene's dielectrical advantages are apparent in comparison to commercially available Mylar, polycarbonate, polypropylene, and similar competitive substances, when used as a conformal coating throughout the entire soft x-ray region.  
  • Graphene field effect transistors:  Parylene gate dielectrics are useful to enhanced development and fabrication of nanotech graphene field effect transistors, combining parylene back gate and exposed graphene top surfaces.  In such cases, parylene back gate stacks coating thermal silicon oxide support the use of optical reflection microscopy.  The stable neutrality point gate voltage exhibited by parylene gated devices under ambient conditions also generate lesser hysteresis, in comparison to other substances.
  • Microchip biomedical labs:  Parylene C masks have exhibited exceptional and fully biocompatible utility in the development of micropatches of biomolecules within lectrowetting-on-dielectric (EWOD) based microfluidic chips used for medical/biological assays and exploration at the cellular level.  Full spatial control of the micropatches can be achieved while sustaining the chip's essential hydrophobicity.  The fact that cells can be arranged either singly or as clusters on the chip's digital microfluidic surface demonstrates Parylene C's capacity to be adapted for additional biomedical purposes. 

Conclusion 

When tested for dielectric withstanding, at 1500 Vrms/0 Hz, using method 301 in accordance with Mil-Std-202, parylene's electrical resistance measured 10% or less of the maximum allowed requirement.  These low scores further represent the superior dielectric qualities of parylene conformal coatings.  They do not conduct electricity but nevertheless sustain an electric field, permitting the safe passage of electrostatic lines of force through protected components.  

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