I. Current Rating Classification for Different Energy Storage Circuits and Corresponding Base Copper Thickness Selection Standards

Energy storage system PCBs encompass power main circuits, precharge circuits, relay drive circuits, sensing circuits, and auxiliary power circuits. The current range spans from milliamperes to thousands of amperes; therefore, the standard 1oz copper foil cannot be uniformly applied. Conventional 1oz copper foil (35μm thick) is suitable only for BMS weak signal and low-current control lines. 2oz (70μm) copper foil is appropriate for branch power circuits carrying dozens of amperes. PCS busbars and battery main positive/negative circuits—which typically operate at 100A to 500A—require thicker copper foils ranging from 3oz to 4oz. Some commercial and industrial high-power energy storage systems utilize 6oz copper foil combined with localized heavy copper plating processes.

For residential wall-mounted energy storage PCS inverter circuits with a rated current of 80A and peak currents reaching 160A, 4oz copper foil is prioritized for top-layer routing, while inner layer busbars must use a minimum specification of 3oz copper. Inner layer copper foil exhibits weaker heat dissipation compared to surface traces; under identical trace width and temperature rise conditions, the current-carrying capacity of inner layers is only 60%–70% that of surface layers. Consequently, main power busbars should be routed on the PCB surface layers to leverage air convection for reduced temperature rise.

Many engineers rely solely on experience for copper thickness selection. Under full-load operation, this often results in PCB busbar temperature rises exceeding 80°C. Long-term high-temperature oxidation increases copper foil impedance, creating a vicious cycle of heating. In extreme operating conditions, this can lead to copper foil fusing and fire—a critical safety hazard for energy storage equipment. Copper foil selection must also align with the substrate's thermal resistance. Thickened copper foil exhibits rougher edge profiles after etching; under high-current operation, current crowds at these edges, creating hot spots. High-Tg laminates offer superior thermal stability and can withstand higher operating temperatures; thus, they must be paired with thickened copper foil PCBs.

The industry-standard IPC-2221 serves as the benchmark for energy storage PCB trace width calculations. Given that internal cabinet ambient temperatures can reach up to 70°C, a design margin of 20°C–30°C should be reserved to maintain the final trace temperature rise below 40°C. Taking a surface layer with 4oz copper foil and an allowable temperature rise of 30°C as an example, the minimum trace width required for 100A current is no less than 18mm; under identical conditions, the inner layer trace width must be increased to over 28mm.

Narrow, elongated traces are strictly prohibited for high-current routing. Solid copper pours are preferred for conducting busbar currents, as they ensure uniform current distribution without local crowding, resulting in significantly lower temperature rises compared to equivalent cross-sectional area traces. For short-duration, high-current loops such as precharge resistors and discharge resistors—which do not generate prolonged heat—the trace width may be reduced by 15%–20%. Conversely, DC busbars and IGBT main circuits, which operate under continuous full load, must be designed strictly according to rated peak current; design based on transient peak data is unacceptable. When multiple parallel power traces carry the same current, their lengths, widths, and copper thicknesses must be identical to prevent uneven current distribution and subsequent overheating of individual traces.

All trace corners must employ arc transitions. Right-angle or acute-angle traces induce current crowding due to the skin effect—particularly pronounced in high-frequency SiC power loops—leading to increased equivalent resistance and localized hot spots at sharp corners. Constrictions or bottlenecks in traces are forbidden; the cross-sectional area of the entire power loop must remain uniform.

Current conduction between power planes, ground planes, and inner layer busbars relies on vias. A single standard via with a 0.3mm diameter carries only 2A–3A; relying on a few vias for a 100A-class circuit risks via overheating and fusing. High-power loops in energy storage necessitate via array designs utilizing large-diameter, plated-through vias. A recommended aperture range is 0.5mm–0.8mm, with a plated copper thickness of no less than 20μm to enhance current-carrying capacity. Vias should be arranged uniformly in a matrix surrounding power device pads and terminal pads, rather than clustered on one side.

For a 100A busbar, connecting surface layers to inner layer pours requires a minimum of 30 high-current vias (0.5mm diameter), distributed across multiple arrays. Large, single-aperture through-holes on power pads are prohibited; extensive via arrays distribute current and mitigate single-point heating. Maintain a spacing of over 0.8mm between vias to prevent thermal coupling between copper walls that could create high-temperature zones. Crucially, vias connecting surface thickened copper to inner layer copper must undergo resin plugging and subsequent copper wall thickening. Standard hollow vias operating under long-term high current risk moisture ingress, leading to plating corrosion, detachment, and a sharp increase in contact resistance, causing overheating failures. For BMS sense signal lines, minimize the number of vias—ideally one per trace—to reduce sampling circuit impedance fluctuations.

Terminals, IGBTs, MOSFETs, and power resistor pads are the most concentrated heat-generating areas in energy storage systems; conventional pad designs are prone to solder fatigue, cracking, and cold joints. High-power pads require large-area copper window openings (solder mask openings). Dense thermal vias should be placed within the pad area to connect to inner layer ground planes or power copper pours, transferring heat away to the larger copper surfaces. For bolt-down copper busbar terminals, arrange a ring of vias around the pad perimeter and increase the exposed copper area on the surface layer to facilitate the addition of thermal pads later.

To further reduce overall busbar loop impedance, densely populated signal vias and tooling holes must be avoided in main power busbar pour areas; continuous, unbroken copper surfaces maximize current conduction and minimize heat generation. For commercial and industrial energy storage equipment operating under prolonged full load, copper coin (embedded copper block) technology can be implemented within PCB cutouts to significantly enhance tolerance to transient surge currents.

Design must account for transient surges—not just steady-state rated currents. Grid-tie transients and load switching in energy storage systems generate surge currents several times the rated value. Trace width, via counts, and copper thickness must incorporate a design margin of 1.8 to 2.5 times the expected current. Implementing this comprehensive suite—covering copper thickness selection, precision trace width calculation, via array optimization, and pad reinforcement—will fundamentally resolve issues of busbar overheating, copper foil degradation, and burn-through in energy storage PCBs, thereby elevating the long-term operational stability and safety of power circuits.