High Frequency Laminate Selection
Why the Material Matters at Frequency
At low frequencies the laminate is nearly an ideal insulator and its properties are almost irrelevant. As the frequency rises, the dielectric’s losses become significant, the conductor’s losses grow with the frequency, and the material’s dielectric constant decides the impedance and the propagation delay. The choice of laminate therefore moves from a manufacturing detail to a design parameter that affects the link budget, the impedance, the delay and the phase. Where a design has a loss or a delay requirement, the material is part of the engineering rather than a purchase.
Dielectric Constant
The dielectric constant, or relative permittivity, sets the capacitance per unit length of a transmission line and therefore the impedance and the delay. It varies with frequency, with temperature and with the resin content, so a value quoted at one frequency is not necessarily correct at another. For an impedance controlled design the value used in the calculation has to match the material and the frequency of operation, and the fabricator’s process affects the resin content and therefore the value. A material with a low dielectric constant also allows a wider trace for a given impedance, which reduces the conductor loss and affects the impedance and the routing.
Loss Tangent
The loss tangent measures how much energy the dielectric absorbs, and it rises with frequency. At high frequency it becomes the dominant loss in a long line, ahead of the conductor’s resistance. A material with a low loss tangent is therefore chosen wherever the signal has to travel or the link budget is tight, and it is the property that most distinguishes a high frequency laminate from a standard one. The loss also generates heat in the dielectric, which matters at high power, and it changes with the temperature and the moisture content. The loss tangent should be specified at the frequency of interest rather than taken from a general statement.
The Copper and the Surface
The conductor is part of the loss. At high frequency the current crowds toward the surface, so the conductor’s surface roughness increases the effective resistance, and a smoother foil reduces the loss. The copper’s thickness matters less than the surface at high frequency, which is the opposite of the low frequency case. The surface finish also contributes, since a lossy finish adds resistance, and the finish should be chosen for the frequency as well as for the solderability. Where the design is sensitive, a profilometry measurement of the foil’s roughness is part of the material’s specification.
Thermal and Mechanical Properties
High frequency materials are not only electrical. Their glass transition temperature, their coefficient of thermal expansion, their moisture absorption and their dimensional stability all affect the fabrication and the reliability. A material with a high expansion is harder to process and can stress the plated barrels; a material with a high moisture absorption needs more careful baking before assembly; a material with poor dimensional stability makes the impedance control harder. Those properties should be reviewed alongside the electrical ones, because a laminate that meets the loss requirement and fails during assembly is not a solution.
The Cost Trade
High frequency laminates cost several times what a standard FR-4 costs, and the difference grows with the performance. The design should therefore use the material only where it is needed, which may mean a mixed stackup in which only the layers carrying the high frequency signals use the special material and the rest use a standard one. That approach reduces the cost but adds the complexity of a hybrid construction, which the fabricator has to be able to build. Where the whole board can use a standard material with a careful design, the saving is large and the decision should be revisited before the material is fixed.
Specifying and Verifying
The material should be specified by its manufacturer and part number rather than by a generic description, since two materials with a similar description can have different properties. The impedance should be verified with a test coupon that the fabricator measures, and the coupon should be designed to represent the product’s stackup and geometry. The measurement should be made at the frequency of interest and by a defined method, and the results should be recorded with the lot. Where the material’s dielectric constant is used in the design, the measured value should be compared with the assumption, because a difference changes the impedance and the delay.
Moisture, Baking and the Assembly Process
A high frequency laminate often absorbs more moisture than a standard one, and a board that has been stored in humid air and not baked will release that moisture during reflow. The result is voiding, delamination or a blow at a via, and the problem appears as a process defect rather than a material property. The baking schedule should be taken from the material’s data sheet rather than from the general practice, because the temperature and the time that suit FR-4 may not suit a filled high frequency material. The storage and the handling between baking and assembly are then part of the process control.
FAQ
When do I need a high frequency laminate? When the dielectric loss or the delay matters, which depends on the frequency and the length of the line.
What is loss tangent? The measure of the dielectric’s energy absorption, which rises with frequency and becomes the dominant loss.
Why does the dielectric constant vary? It changes with frequency, temperature and the resin content, so the value must match the operating conditions.
Does the copper roughness matter? Yes, at high frequency the current crowds to the surface, so a smoother foil reduces the loss.
Can a mixed stackup reduce the cost? Yes, using the special material only where it is needed, at the cost of a more complex hybrid construction.
Conclusion
A high frequency laminate is chosen for its dielectric constant and its loss tangent, and it should be specified by part number and verified with a coupon. Use it only where the requirement demands. Material selection belongs to PCB manufacturing, the impedance and delay design is part of PCB design and layout, and the options are described under PCB capabilities. High frequency stackups are first built during prototype PCB assembly in 2026.
