• Preventing leakage currents and catastrophic electrical failures due to moisture and contaminants.
• Preventing corrosion of solder, copper, and other metal surfaces.
• Preventing or reducing tin whisker growth, metal migration, and conductive anodic filament (CAF) formation.[3]
• Reducing or preventing arcing and corona discharge.
• Providing mechanical support and improving the fatigue life of solder joints and components during mechanical shock or vibration.
Coatings belonging to the generic families of acrylics, silicones, p-polyxylylenes, epoxies, and polyurethanes are specified in, and some have been qualified to, MIL-I-46058C or IPC-CC-830A. In most PWAs, the devices are already packaged in hermetically sealed or epoxy-molded packages, then solder attached to the board. In such cases, the purposes of the conformal coating are to prevent moisture, salts, and other contaminants from corroding the exposed solder joints and conductor traces and to reduce the amount of moisture that may be absorbed by the board material itself which results in degradation of electrical insulating properties. The coating must be of high purity and of a molecular structure that its insulation resistance will be high and will not significantly degrade under humidity-temperature exposures. The ten-day humidity-temperature cycling test specified in MIL-I-46058C or MIL-STD-202 is widely used to compare and qualify coatings for printed wiring boards.
Generally, the insulation resistance of the coating drops on initial exposure to moisture during the first day, then levels off. High-purity coatings such as fluorocarbons, silicones, and parylenes have shown better insulation properties under humid conditions than some epoxy coatings (Fig. 4.2). Decreases in insulation resistance as much as nine decades have been reported for uncoated laminates, but with a suitable protective coating, the decrease may be only one to three decades under the same conditions[4] (Table 4.1). Coatings afford similar protection to epoxy-glass laminates under even more severe stresses of combined temperature, humidity, and applied voltage[5] (Table 4.2).
The sharp and rapid decrease in insulation resistance of most polymer coatings occurs during the first humidity cycle with some recovery occurring in subsequent cycles. It has been postulated that coatings absorb water quickly during the first cycle and, accelerated by the applied voltage, partial hydrolysis of the coating constituents occurs. This then results in a breakdown of the coating in the region of the hot conductors and causes the insulation resistance to drop. However, because of differences in their moisture absorption and permeability properties and in their inherent purity, not all coatings undergo a sharp initial drop. Figure 4.3 shows a significant difference between a coating qualified to a military specification and a commercial-grade coating.[4]
Besides the beneficial effects on insulation resistance, conformal coatings reduce or prevent arcing and corona discharge in closely-spaced conductors that are exposed to high voltages. Conformal coatings allow the design engineer greater freedom in designing closer conductor line spacings at higher voltage capabilities. The increased electrical insulation afforded by an epoxy coating on a G-10 laminate board with 62-mil conductor spacing was reported by McGuiness.[6] In this report, flashover voltage for the coated board was 5,000 volts compared with 1,600 volts for the uncoated board. The effectiveness of the coatings in allowing closer spacings for various voltages is shown in Table 4.3. In more recent high-density multilayer PCBs, where bare-chip devices are used, for example, Chip-onBoard (COB), improved barrier properties and compatibility with devices
are required.
The conformal coating of high-voltage power supplies and dc-dc converters is essential in preventing dielectric breakdown and arc over. It is critical that any high-voltage circuit be thoroughly clean and dry prior to conformal coating. With the trend towards using water-based fluxes and water cleaning, there is an increased risk of water absorption and retention within the board, hence vacuum baking or other means of drying becomes necessary.