What the coefficient of thermal expansion actually decides on a high-frequency board, with reference values for the common Rogers grades and the reason FR-4 fails in the Z axis.
The coefficient of thermal expansion is one of those parameters that attracts little attention in design review and then explains a surprising number of field failures. It describes how much a material grows and shrinks with temperature, and on a printed circuit board it decides whether the plated through-holes stay connected after a few hundred thermal cycles. Rogers materials are known for CTE behaviour that is both low and well matched to copper, which is the reason so much high-reliability RF hardware uses them.
This article works through what CTE measures, what the common RO4000 and PTFE series laminates actually deliver, how that compares with ordinary FR-4, and why the difference shows up in product lifetime rather than in a bench measurement.
CTE is the rate at which a material expands or contracts as temperature changes, expressed in ppm/°C. A lower value means greater dimensional stability across a thermal cycle. On a PCB the figure that matters is not the material’s CTE on its own but the difference between the material’s CTE and copper’s.
Copper expands at about 17 ppm/°C. Anything bonded to it that expands faster is being stretched on every heating cycle and compressed on every cooling cycle, and the stress accumulates where the structure is weakest. On a plated through-hole that weakest point is the interface between the plated barrel and the dielectric, which is why CTE matching is ultimately a through-hole reliability question.
Different grades are optimised differently, but they share two characteristics that come directly from the way the materials are constructed.
Expansion in the X and Y axes is held low so that the board surface does not grow significantly at high temperature. That keeps circuit patterns in the position they were designed for, which matters more at high frequency than it does on a digital board, because a transmission line that moves is a transmission line whose impedance has changed.
Copper foil expands at approximately 17 ppm/°C, and RO4003C and RO4350B both have in-plane CTE values close to that number. Bringing the two materials into alignment reduces the internal stress created by thermal mismatch at its source, rather than managing the consequences later. The dimensional behaviour of a rolled-copper laminate is examined further in this article on the advantages of RO3000 rolled copper material.
The figures below are the ones that come up in design reviews. Note that the in-plane values cluster near copper, while the Z-axis values are where the materials differ most both from copper and from each other. The behaviour of a specific PTFE grade is documented in this article on RT/duroid 5880 parameters.
| Material | X-Y Axis CTE | Z Axis CTE | Note |
|---|---|---|---|
| Copper foil (reference) | ~17 ppm/°C | — | The figure every laminate is being matched against |
| RO4003C | 11 ppm/°C | 46 ppm/°C | Popular high-frequency material with exceptional dimensional rigidity |
| RO4350B | 13 ppm/°C | Around 50 ppm/°C | Balances RF performance with manufacturability; suited to multilayer structures |
| RT/duroid 5880 | ~12 ppm/°C | Around 50 ppm/°C | Built for extreme high-frequency work; supports phase and signal stability |
| Standard FR-4 | 16 to 18 ppm/°C | Over 200 ppm/°C above the glass transition | In-plane is close to copper, but the Z axis dominates the failure mode |
The comparison is stark, and it is worth understanding why, because FR-4’s in-plane figure looks perfectly acceptable at first glance. FR-4 expands at 16 to 18 ppm/°C in the X and Y axes, which is close enough to copper that in-plane stress is not the problem. The problem is the Z axis: above its glass transition temperature the expansion can exceed 200 ppm/°C, and the difference between that and copper’s 17 ppm/°C creates the failure known in the industry as zippering, where the plated barrel of a through-hole tears away from the pad during reflow.
Rogers materials address both directions. In-plane expansion sits close to copper, and the Z-axis figure is deliberately optimised to around 50 ppm/°C rather than being left to the resin system. The result is far less stress on the plated vias, and thermal cycle endurance that FR-4 cannot match. On a high-reliability product that difference is not a performance improvement; it is the difference between a lifetime measured in cycles and a lifetime measured in failures.
The benefits below are not abstract. Each one corresponds to a defect that appears when CTE is not controlled, which is why material knowledge is inseparable from process capability; the same argument is made from the manufacturing side in this article on high-frequency PCB materials and processing expertise.
Consistent expansion across all layers is what prevents delamination and warping during lamination and soldering. A laminate that expands evenly keeps its layer-to-layer registration; one that does not will, over a production batch, produce boards that warp in the reflow oven and are scrapped for mechanical rather than electrical reasons.
An optimised Z-axis figure directly protects the vias. Even after hundreds of thermal cycles, plated through-holes in a Rogers board remain highly resistant to barrel cracking and pad separation. This is the clearest single benefit of CTE matching, and on multilayer high-reliability designs it is usually the reason the material was chosen in the first place.
At very high frequency the dimensional accuracy of a transmission line decides its impedance and its phase. Structures such as microstrips are defined by their geometry, so low thermal drift and dimensional stability mean the circuit performs the same way at 85°C as it did at 25°C. That consistency is what allows a phased array to hold its beam and what keeps a filter’s passband where the design put it. The measurement side of this is covered in these steps for impedance control.
Three environments make CTE a first-order requirement rather than a specification detail.
CTE is not a datasheet formality. It decides whether a board still works after a thousand temperature cycles, and on high-frequency designs it also decides whether the transmission lines keep the dimensions that give them their impedance. A low figure and a close match with copper provide both benefits at once, which is exactly why the material class that offers them dominates demanding RF work. When a design is pushed towards its thermal limits, CTE deserves to be treated as a specification rather than a footnote.
Frequently Asked Questions
The rate at which the laminate expands or contracts with temperature, expressed in ppm/°C. What matters in practice is not the value alone but how far it differs from copper's 17 ppm/°C, because that difference is the source of the stress that damages plated holes.
Because the Z axis acts in the same direction as the plated through-hole. In-plane expansion is usually close to copper on both FR-4 and Rogers materials, whereas Z-axis expansion is where FR-4 reaches over 200 ppm/°C above its glass transition and Rogers materials stay near 50 ppm/°C.
RO4003C expands at 11 ppm/°C in the X and Y axes and 46 ppm/°C in the Z axis. RO4350B is 13 ppm/°C in-plane with a Z-axis figure around 50 ppm/°C, which suits multilayer structures.
The failure that occurs when the dielectric expands far faster than copper in the Z direction during reflow. The barrel of the plated through-hole tears away from the pad, and the connection is open or intermittent depending on how much damage accumulated.
Yes, at high frequency. Transmission lines are defined by their geometry, so a laminate that changes dimensions with temperature changes impedance and phase with it. Low thermal drift keeps a filter's passband and a phased array's beam where the design put them.
Send us your operating temperature range, layer count and cycling requirements and we will confirm which laminate keeps your through-holes reliable.
Get a Fast Quote Fast Reply on WhatsApp