Protective coatings for BMS boards
Conformal coating is a thin layer that follows the contours of a circuit board, protecting the battery management system (BMS) board from humidity, condensation and conductive dust so that cell measurement and balancing remain accurate throughout the battery's life.
The BMS is the brain of the battery pack: it measures the voltage of each cell, monitors temperature, controls balancing and disconnects the circuit when a fault occurs. Even a small leakage current path caused by moisture on the board can skew voltage measurements by a few millivolts, enough for the state-of-charge algorithm to produce wrong results.
The harsh environment inside a battery pack
Electric vehicle battery packs and storage cabinets go through constant hot and cold cycles. When the temperature drops quickly, residual moisture in the air can condense on the board surface. Combined with high voltages of several hundred volts, ion migration and dendritic corrosion can easily occur between closely spaced component leads.
Common types of conformal coating
- Acrylic: easy to apply, easy to remove for rework, good moisture resistance and reasonable cost.
- Silicone: wide temperature resistance up to 200 °C, flexible, suitable for areas near power components.
- Polyurethane: better chemical and solvent resistance than acrylic.
- Fast-curing UV coatings: shorten drying time, suit high-throughput lines, and require a secondary cure mechanism for shadowed areas.
For BMS boards, many designers prefer silicone or UV-cure coatings with high dielectric strength and a dry film thickness of 50 to 200 µm.
A quality coating process
Surface preparation
Boards must be cleaned of flux residue, especially where no-clean solder paste is used in high-density areas. Residue left under the coating will absorb moisture and cause blistering.
Selective coating and inspection
Selective coating machines help avoid keep-out areas such as connectors, test points and sensors. Most materials contain a fluorescent tracer, allowing coverage to be inspected under UV light directly on the line or with AOI equipment.
Evaluating protection performance
Coated boards are typically subjected to condensation tests, damp heat and surface insulation resistance (SIR) measurements. Results show that a suitable coating helps maintain insulation resistance above 10^9 ohm after hundreds of hours of testing.
One often-overlooked point is coating thickness at the edges of component leads. A material with too low a viscosity will pull away from sharp edges, leaving thin areas vulnerable to moisture. Measuring thickness with an eddy current gauge on sample boards and periodic cross-sectioning help detect this issue early, before mass production.
Contact AMS Vietnam to select the right protective coating for your BMS boards and get support setting up coating machine parameters.
