Parylene and Silicone Conformal Coatings: A Comparison

One liquid coating type that rivals the use of parylene is silicone conformal coating (Type SR), which cures rapidly, is reliably dielectric and displays exceptional stability across a wide temperature range.  These properties make it parylene’s chief performance competitor, for many purposes.  Further comparison delineates their benefits and disadvantages relative to each other.

Composition

While parylene and silicone are both technically polymers, their dimer materials are very different.  Parylene dimer is composed of a hydrocarbon molecule (hydrogen + carbon), allying it chemically to virtually all other available plastics.  In contrast, silicone’s dimer is comprised of an oxygen/silicone composite, creating the most unique chemical union among conformal coatings.

Deposition

As a liquid conformal coating, silicone relies on brush, dip or spray techniques for substrate application.  It typically requires a relatively thick film application, ranging from 003″-.008”, to be effective by IPC standards.  Although the resultant surface is reliably durable for selected functions, sustaining components’ function in operating environments characterized by large-scale, rapid shifts in temperature, it is less useful for any purposes requiring thin film covering to assure product operation, limiting its adaptation for MEMS/NT purposes.

In contract, parylene’s vapor-based CVD application process can deposit the substance in thinner layers than all competing coating types, generally between .0005” to .002”, making it exceptionally suitable for all PCBs and most specialized MEMS/NT functions.  Moreover, CVD allows parylene to seep deep within substrates, conforming to all surface types, regardless of shape.

Thermal Properties

Parylene’s operational temperature range is substantial.  Parylene films:

• withstand cold to levels as low as -165°C without sustaining physical damage, and
• conversely heat as high as 200°C in a vacuum.

Perhaps more important, parylene conformal coatings offer considerable functional stability in the long-term, performing as expected at a constant temperature of 80°C for 10 years.

In comparison, silicone is cured, and functions better under exceptionally heat-intensive conditions.  While most silicone types function at a baseline operating temperature range of -55°C — 200°C, some remain functional at levels as high as 600°C, far exceeding parylene’s performance range for higher temperatures.

 

 

 

 

 

Table 2 compares the thermal properties of parylenes C, D, and N with silicone.

Table 2:  Thermal Properties of Selected Parylenes, in Comparison with Silicone Conformal Coating

Properties	Parylene C	Parylene D	Parylene N	Silicone
Melting point, ° C	290	380	420	Cured
T5 point (where modulus = Taken from secant modulus temperature curve)	125	125	160	125
T4 point (where modulus = Taken from secant modulus temperature curve)	240	240	300	– 80
Thermal conductivity, 25° C	2.0	—	3.0	3.5 – 7.5
Specific heat, 25° C	0.17	—	0.20	—

 

Physical Properties Compared

Silicone

As a conformal coating, silicone cures rapidly; its generally thicker film application also generates superior vibration dampening and thermal protection.  When applied in a thick enough coat, it can actually serve as a shock absorber for the coated item, helping to protect it against heavy impact.

These factors assure good adhesion to those PCB materials that do not require thinner film covering to ensure operation; UV and corrosion resistance are superior to most competing conformal coatings.  Featuring high dielectric strength, silicone surfaces are easily reworked if repair is necessary.

Repair is not uncommon.  Silicone is very hydrophobic, with high moisture permeability; it may not prevent excess moisture-retention within PCBs, conditions that can stimulate the component’s corrosion and metallization.  Silicone’s other functional drawbacks include:

• often considerable attention to the coating process to achieve optimal thickness,
• exceptionally poor solvent resistance, and
• inferior durability during operational conditions where abrasive or other factors stimulating surface deterioration are common.

Parylene

Parylene successfully adheres to the widest selection of substrate substances and surface geometries of all conformal coatings.  Chemically and biologically inert, it provides excellent dielectric and moisture barrier properties, generating bubble- and pinhole-free conformal coatings at thicknesses as thin as .0005”.  Paylene’s other benefits include:

• high optical clarity,
• mitigated tin whisker growth, and
• flexible conformability for adaptation to all surfaces,
• enabling film-penetration of extremely small spaces and crevices, making it very functional for MEMS/NT applications.

On the negative side, parylene’s CVD processing technologies are slow, generating only limited throughput.  Parylene removal requires abrasion-based methods.  These factors combine to increase parylene’s application costs; higher prices for raw materials, labor, and lot-volume have limited wider adaptation of parylene, compared to silicone or other liquid-based films.

Table 3 presents mechanical of selected parylene types with silicone.

Table 3:  Mechanical/Physical Properties of Selected Parylenes, in Comparison with Silicone Conformal Coating

Mechanical/Physical Properties	Parylene C	Parylene D	Parylene N	Silicone
Tensile modulus	3.2	3.0	2.8	.007
Tensile strength, psi	10,000	11,000	6,000 – 11,000	800 – 1,000
Yield strength, psi	8,000	9,000	6,100	—
Yield elongation	2.9	3.0	2.5	—
Elongation to break %	10 -39%	10%	20 – 25%	10%
Rockwell hardness	R85	R80	R85	40 – 45 (Shore A)
Coefficient of friction – static	0.29	0.33	0.25	—
Coefficient of friction – dynamic	0.29	0.31	0.25	—
Water absorption	0.06%/24 hours	< 0.01%/24 hours	< 0.01%/24 hours	0.12%/7 days
Density g/cm2	1.289	1.418	1.10 – 1.12	1.05 – 1.23
Refractive index nD23	1.639	1.669	1.661	1.43

 

 

Summary

Both materials protect substrates, but do so differently.  Therefore, coating properties and the method of film application are critical factors when determining the use of either as a conformal coating.  Parylene forms a resilient, but ultra-thin coating that sometimes lacks strong abrasion resistance.  Silicone, on the other hand, is roughly equivalent to a very soft rubber, but may be absent sufficient utility for high-profile, consistently active electronic components.