A primary function of all conformal coatings is maintaining sufficient insulation and avoiding dielectric breakdown while protecting printed circuit boards (PCBs) and related electronic assemblies. Providing a completely homogeneous coating surface, parylene (XY) conformal coatings are exceptionally corrosion-resistant, dense and pinhole-free. Among other performance advantages, ultra-thin XY protective films offer superior dielectric properties. Dielectric substances maintain electrical insulation, simultaneously transmitting electricity without conduction. They have the potential to store energy because they support electrostatic fields that release only low levels of thermal energy.
To work effectively, a conformal coating’s breakdown voltage, defined as:
- the minimum difference in charge between two points in an electrical field,
- must NOT be achieved;
- otherwise, the insulating conformal film will become electrically conductive.
Maintaining these performance factors is necessary for ongoing PCB-operation; preventing dielectric breakdown (DB) is essential. DB results from a buildup of electrical charge within a PCB that surpasses a coating material’s dielectric strength (DS – its electrical performance limit). In such cases:
- negative- and positively-charged electrons within the assembly are simultaneously pulled in opposite directions,
- ionizing the environment,
- which is transformed from an insulator to a conductor,
- generating sparks and similar electrical disturbance,
- leading to dysfunction and breakdown.
Obviously, avoiding DB is essential for effective conformal coating, requiring suitable DS. XY films generate increased dielectric strength between conductors enabling smaller, more compact PCB design
DS is a measurement of a conformal coating’s insulation effectiveness. Parylene's lower dielectric constants in comparison to liquid conformal coatings indicate its
- enhanced ability to withstand intense electrical fields,
- significantly limiting film devolution,
- while maintaining assembly performance
- under operational conditions characterized by intense electrical activity.
Conformal coating materials demonstrating fewer extractible ionic impurities and greater hydrophobicity have superior DS. In terms of measurement, higher-valued ratings (7,000) indicate a particular coating material will resist dielectric breakdown better than one whose DS value is lower, (2,000). Compared to liquid coatings, parylene’s higher DS shows considerable advantage generating appropriate dielectric protection for PCBs. In addition, XY’s lower dielectric constants (DCs) represent diminished concentrations of electric flux, resisting the impact of current fluctuation within the assembly. One of parylene’s most significant advantages is the ability to withstand substantial electrical activity, maintaining its structural integrity and assembly performance. Both DS and DC values vary according to coating material, and can also vary within material type. Table 1 provides DS/DC values for parylenes N and C, and the major liquid coatings.
TABLE 1: Dielectrical Properties of Conformal Coatings
PROPERTIES Dielectrical strength V/ml Dielectric constant
Parylene N 7,000 2.65
Parylene C 5,000 2.95 - 3.15
Acrylic 1,500 3.25 - 4.35
Epoxy 2,200 3.30 – 4.60
Silicone 2,000 3.10 – 4.20
Urethane 3,500 3.80 – 4.40
DC values exceeding 3.0, indicate inappropriate molecular response to the alternating current/field, and
- diminished ability to perform under conditions of electrical stress and fluctuation.
XY’s DC-ratings are better than those for liquid coatings, representing enhanced performance. The same results pertain to DS; parylene ratings are better in all cases, compared to wet conformal films, whose lower values indicate excessive thermal generation, undesirable for sustained conformal coating function.
DC readings for other parylene types range between 2.25 - 3.15. In all cases, these readings respond to alterations in Hertz (Hz) value -- a unit of change-frequency for alternating current (AC). Regarding parylene types,
- N’s levels remain constant, at 2.65 for DC and 7,000 for DS, exceptionally strong and resistant;
- C’s DC varies between 2.95 – 3.15, with a constant DS of 5,000, still very effective.
Dielectric loss registers dissipation factor, for levels of internal heat within conformally coated substrates; a rating of 0.1 or less is required for longer-term maintenance of coating adhesion and performance. Coating ratings respond to alterations in Hz level. Here again, XY performs admirably:
- N’s value increases slightly, from 0.0002 (60 Hz) to 0.0006 (1 MHz);
- C’s levels actually diminish -- from 0.020 to 0.013 – at Hz levels increase (60 Hz though 1 MHz).
In both cases, XY coatings exceed professional performance standards for dielectric control. As a lower DC conformal film, parylene’s weakly-bonded molecules produce dependable buffers between a PCB and its operating environment. Polarized by electrical charges, XY
- resists electrical conduction,
- enriching its utility as a coating for high-speed electrical assemblies,
- exceeding wet coatings’ performance.
To further illustrate, the wet coatings identified on Table I each register DCs larger than 3.0. Thus, the possibility of circuit-speed variance increases, a development that can interfere with the operation of any higher frequency component. In addition, DS of liquid coatings is lower, reducing their ability to maintain performance – adhesion to assembly surfaces and consistent component protection; they are more likely to break down during prolonged contact with intense electrical activity. Unlike XY films, those composed of acrylic, epoxy, silicone or urethane are prone to dielectric breakdown and current conduction, especially with the passage of time. Parylene reliably sustains an assembly’s electric field without conducting electricity, expediating the non-static transmission of electrostatic power throughout the PCB.
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