The Impact of Temperature on Parylene Adhesion
Posted by Sean Horn
Friday, May 20, 2016 7:30
@ 7:30 AM
Basic Thermal Properties of Parylene Conformal Coatings
CVD-generated parylene combines high thermal stability with a low dielectric constant, minimal moisture absorption, and other advantageous properties which sustain its adhesion to substrate surfaces. Among the most beneficial of the parylenes’ thermal properties is their ability to function at an exceptional range of temperatures. Depending on the parylene type, they are operative at temperatures as low as -271º C, and as high as 450º C, representing an ability to perform within a span of 721º C.
Much depends upon the specific parylene type, its explicit product purpose, and the environmental conditions affecting performance. However, when parylene type and purpose are appropriately matched to the expected thermal conditions of the assembly’s operational environment, parylene conformal coatings offer superior adhesion and minimal delamination.
For instance, Parylene C can endure constant exposure to 100° C for eleven+ years, accounting for 100,000 hours of use, without appreciable delamination. In contrast, more recently developed Parylene HT is useful in high temperature applications (short-term up to 450°C), although this represents an extreme range. More generally, the parylenes can provide similar service (11.4 years of persistent adhesion) in vacuums or atmospheres free of oxygen, working through ongoing exposure to 220° C, making them an excellent choice as conformal coatings for aeronautics’ and space flight uses. Higher temperatures can shorten parylene use-life in oxygen rich environs.
For most terrestrial uses, heat-treating for three hours at temperatures of 140°C, beneficially activates longer-term adhesion and insulation. Parylene’s low thermal expansion helps it retain uniform conformal qualities through innumerable functional settings.
Further thermal properties of parylene types C, N and D are provided in Table 1.
Table 1: Thermal Properties of Selected Parylenes
|Properties||Parylene C||Parylene D||Parylene N|
point, ° C
|T5 point (where modulus = Taken from secant modulus temperature curve)||125||125||160|
|T4 point (where modulus = Taken from secant modulus temperature curve)||240||240||300|
|Thermal conductivity, 25° C||2.0||—||3.0|
|Specific heat, 25° C||0.17||—||0.20|
Optional Methods of Heat Pre-Treatment to Improve Parylene Adhesion
Heat treatments to improve parylene for adhesion may occasionally include:
- Bilayer encapsulation: Accelerated testing implemented by boiling the coated materials can confirm two distinctive outcomes pertinent to adhesion of the Parylenes N and C. (1) Glow-discharge polymerized methane applied under a thicker layer of Parylene-N in a solution of isotonic sodium chloride noticeably enhanced substrate-adhesion. (2) In comparison to a film composed solely of parylene, components immersed in phosphate-buffered saline (PBS) at 57°C demonstrate superior surface-adhesion for dually-composed of atomic-layer bilayers of deposited Al2O3 and 6 µm Parylene-C provided superior to that of parylene alone.
- Parylene-on-parylene interfaces: Interaction with hydrofluoric acid during processing, parylene heated at 140°C with no oxygen-purging for three hours exhibits substantial wet adhesion strength. Testing also shows whether oxidation from the heat-treatment also added brittleness or a susceptibility to the film’s tearing, both factors of inappropriate adhesion.
Parylene coatings exhibit dependable consistency for many applications where exposure to ongoing thermal pressure is the rule. However, in some circumstances parylene films covering component substrates become fragile and inflexible due to persistent thermal stress, reducing their usefulness as conformal coatings. Unfortunately, cases of diminished coating adhesion have particularly presented themselves for biomedical implant applications, where malfunction may be life threatening. Improving these performance conditions is a significant challenge to the development of more adaptable parylene conformal coatings.
Parylene may be annealed to increase cut-through resistance, enhance coating hardness, and improve abrasion resistance. This is the result of a density and crystallinity increase, occurring after contact with heat.
At the same time, properties of crystallinity and surface morphology generally undergo some degree of transformation during deposition and thermal annealing, affecting parylene film adhesion, as well. These conditions suggest that, with proper treatment, conformal coating properties can be adapted to specified production details. Thus, the incidence of failure due to film delamination can be limited, if processes are carefully and thoroughly implemented, according to conditions of projected product use. Typically, substrates are pretreated with A-174 silane.
However, the fact that thermal stress generated in the film during CVD processing may weaken the conformal coatings’ adhesive force may further oblige customized coating procedures, to ensure delamination or similar adhesion failure does not become an issue. For instance, in certain cases, excessive temperatures (+ 150°C), and prolonged exposure to thermal sources – longer than 20 minutes – can lead to coating degradation, particularly if the films’ thicknesses are <3-μm and exposed to the higher temperature ranges. Degraded adhesion following annealing at 150°C for 20+ minutes can impair parylene’s encapsulation properties. While heat treatments of even 80°C may initiate thermal stress — somewhat compromising the parylene conformal coating, leakage rates’ prevention — factors of diminished adhesion generally do not significantly degrade the film at 150°C for the longer service durations (11.4 years), suggested previously.
Therefore, despite some inconsistencies of performance under conditions of thermal stress, parylene’s properties generally provide good thermal adhesion and endurance, in comparison to competing coating materials, for innumerable products and purposes. In some cases of terrestrial application, higher operating temperatures may shorten parylene’s functional life; oxygen-free, space vacuums are not affected by these conditions and can operate for similar timeframes at higher temperatures (220°+ C). Testing the complete structure conformally coated by parylene under conditions closely resembling intended operating settings is recommended to verify its ability to withstand or exceed these temperature-time-atmospheric conditions.
However, parylene reliably performs under most situations — in air or a vacuum – at a wider temperature range than competing conformal coatings. It does so for a decade or more, at an extreme range of temperatures, without significant loss of physical properties, providing superior adhesion and limited delamination.
To learn more about how temperature, and other variables can affect adhesion download our whitepaper: