A Practical Approach to Material Selection for High Power PCBs

Estimated reading time: 3 minutes

With AI server architectures demanding high-power converter solutions to improve power efficiency and affordability, PCB design can challenge engineers who treat material selection as a single-variable problem. Pick a laminate for thermal conductivity, and you may experience a CTE mismatch that cracks vias after 50 thermal cycles. Optimize for dielectric loss, and you might sacrifice the isolation margin your certification depends on. High-power boards require consideration of multiple variables, including electrical, thermal, mechanical, reliability, and manufacturability, which cannot be optimized alone.

The Multi-Variable Trap

Material selection often starts with a single "target" spec: Dk for signal integrity, thermal conductivity for power. This practice breaks down in high-power designs because variables interact. For example, great thermal conductivity paired with poor Z-axis stability passes initial testing but fails in the field when power cycling stresses the plated through-holes. A laminate chosen for low Df carries no assurance of adequate electric strength for a design with a high isolation requirement.

The recommended practice is to treat selection as constraint satisfaction, not optimization. Set a floor for each variable, i.e., Tg, electric strength, Df, decomposition temperature, and CTI class, then evaluate candidates against all floors at once.

A Working Example: Isola IS580G for High-power Converter

Isola's IS580G is a useful case study: a single material decision touches every domain at once.

Electrical: IS580G carries a Dk of 3.80 and Df of 0.006, flat across the frequency range typical of power conversion switching harmonics, which is useful where gate-drive and control signals share a board with high-current planes.  Dk variation can introduce impedance variation that's hard to diagnose late in the cycle.

Electric strength deserves equal attention, and it's often underweighted relative to Dk/Df.  Power converters and grid-tied equipment frequently carry reinforced isolation requirements between high voltage and control circuitry, and electric strength sets the margin against flashover or puncture under transient overvoltage as required for designs to meet UL or IEC isolation certification.

The electric strength of IS580G is specified at 63 kV/mm (1600 V/mil). Laminate datasheets often list both dielectric breakdown and electric strength, and it's tempting to treat the two as interchangeable, however, they are not the same. Dielectric breakdown is an in-plane (X/Y) measurement, reflecting resin-to-glass-fiber adhesion, while electric strength is measured through the Z-axis, the direction that matters for layer-to-layer isolation in a stackup.

For isolation-rated designs, use the electric strength value specifically; dielectric breakdown doesn't substitute for it. In IS580G, the electric strength pairs with a CTI rating of 2 (250–499 V), a good fit for enclosed, pollution-degree-2 equipment, including indoor power converters, industrial drives, telecom power supplies, and similar designs where the enclosure keeps conductive dust and condensation off the board surface. Following the CTI class 2 creepage specs per IPC-2221/9592, as well as using solder mask, will help protect when applied where the coating is qualified, and the area is truly masked to generate a robust isolation solution.

Thermal: A Tg of 205°C and Td of 385°C provide real margin above peak lead-free reflow temperatures, which is critical when heavy copper and thermal vias demand longer, hotter cycles to fully reflow the board.

Mechanical and reliability: Isola engineered the low CTE resin system for improved Z-axis expansion control, reducing barrel cracking risk in plated through-holes during thermal cycling, resulting in the avoidance of future field failures.

Manufacturability: Being halogen-free, IS580G meets environmental compliance without any fabrication penalties, therefore, fabricators can process it without requalifying press cycles or drilling parameters.

A Practical Checklist

Resist the urge to lead with a single spec. Instead:

1. Set hard floors for Tg, Td, Dk/Df, electric strength, and Z-axis CTE before opening a datasheet.
2. Cross-check candidates against all floors simultaneously, not sequentially.
3. Size creepage/clearance from CTI first; apply solder mask credit only where the coating is qualified, and the area is actually masked.
4. Validate under combined thermal-mechanical-electrical stress, not static datasheet comparison alone.

Material selection isn't about finding the "best" laminate, but rather it's finding the one where weaknesses don't intersect your design's stress points.

Drew Delaney is director of OEM Marketing at Isola Group.