Following the controlled environment testing procedures for PCBA, salt spray testing represents a more aggressive, accelerated corrosion assessment used to evaluate the effectiveness of protective coatings and finishes in harsh, saline environments. The processing and preparation of PCBA for this test have specific, non-negotiable requirements that directly impact the validity of the results.
PCBA Pre-Cleaning and Surface Preparation Protocol
The state of the PCBA surface prior to coating and testing is paramount. All boards must undergo a rigorous cleaning process to remove ionic contamination, flux residues, oils, and particulates that can undermine coating adhesion and create pathways for corrosion. A typical sequence involves an aqueous wash with a saponifier or detergent formulated for electronics, followed by multiple deionized (DI) water rinses with a specific resistivity greater than 15 MΩ·cm. This is often concluded with an optional solvent rinse or plasma cleaning for stubborn organic residues. After cleaning, boards are baked in a convection oven at a temperature below the glass transition temperature (Tg) of the PCB substrate (e.g., 60-80°C for FR-4) for a period sufficient to remove all moisture, typically 2-4 hours. Handling after cleaning is strictly controlled with lint-free gloves in a low-particulate environment to prevent recontamination.
Conformal Coating Application and Curing Specifications
The application of the protective conformal coating must be performed according to a qualified procedure. The coating material (e.g., acrylic, silicone, urethane, parylene) and its target dry-film thickness are specified based on the end-use environment. Common thickness requirements range from 25 to 125 microns. Application methods include selective spraying, dipping, or brushing, each with defined parameters for viscosity, pot life, spray pressure, and dip withdrawal rate. For sprayed coatings, the coverage must be uniform and complete, with particular attention to coating the edges of components and the board, which are high-risk areas. After application, the coating must be cured under precisely controlled conditions—specific temperature, humidity, and duration—as defined by the material manufacturer's datasheet. Incomplete curing leaves the coating permeable and soft, leading to immediate test failure. A pre-test inspection under UV light (for coatings with UV tracer) or using a coating thickness gauge is mandatory to verify continuity and thickness.
Masking, Edge Protection, and Test Fixture Requirements
Not all areas of a PCBA can or should be coated. Connector pins, test points, heatsink mating surfaces, and adjustable potentiometers often require masking. The masking material must be chemically inert, withstand the coating and curing process without leaving residue, and be cleanly removable. The interface between the masked area and the coated area is a critical inspection zone; feathering or creep of coating under the mask is a defect. Furthermore, the raw edges of the PCB substrate are highly susceptible to wicking moisture. A specific edge-sealing protocol, such as applying a bead of sealant or ensuring the coating fully encapsulates the edge, is frequently required. During salt spray testing, PCBA must be mounted on non-conductive, corrosion-resistant fixtures (e.g., glass, plastic) at an angle between 15 and 30 degrees from vertical per ASTM B117. The fixtures must not create pockets that trap solution or shield the test surface. Electrical bias may be applied during testing if simulating a powered state, requiring insulated feedthroughs and secure connections.
