Low Loss Laminate Selection

A low loss laminate is chosen when the insertion loss of a channel has become the limit on the design. The loss comes from the dielectric and from the copper, and the two contributions have different frequency behaviour and different remedies. Selecting a material without separating them can lead to paying for a lower loss dielectric while the copper surface roughness remains the dominant term.

The Loss Budget

The budget starts with the channel and the receiver. A link has a certain amount of loss it can tolerate before the eye closes, and the sum of the trace loss, the connector loss, the via loss and any cable loss has to fit inside it.

The trace contribution grows with length and with frequency. Doubling the length doubles the loss, while raising the frequency raises it faster than linearly at high frequency, which is why a material that works for one data rate may not work for the next.

The budget should be written down with the contributions separated. A design that treats the loss as a single number cannot tell whether changing the laminate or shortening the route will solve the problem.

Dielectric Loss

The dielectric loss is set by the loss tangent and by the electric field that passes through the material. A material with a low loss tangent absorbs less energy, and the improvement is proportional to the reduction in the tangent.

The loss also depends on the geometry, because the field distribution determines how much of it travels in the dielectric and how much in the air or in the solder mask. A narrow trace on a thick substrate has more of its field outside the laminate than a wide trace on a thin one.

The loss tangent changes with frequency and with temperature. A figure quoted at one gigahertz does not describe the behaviour at ten, and a figure measured at room temperature does not describe a board that runs warm.

Copper Loss

At high frequency the skin effect concentrates the current near the surface of the conductor, and the effective resistance rises with the square root of frequency. The copper loss therefore grows with frequency and with the length of the trace.

The surface roughness of the copper adds to the loss, because the current has to follow the profile of the surface. A foil that is smooth at the scale of the skin depth behaves as a flat conductor, while a rough one has a longer path and a higher resistance.

The roughness of the copper is often set by the adhesion treatment that the laminate needs. A material that requires a rough foil for the bond will have a higher conductor loss, which is why some laminates are offered with a low profile foil.

The Two Contributions Compared

At moderate frequencies and short lengths the dielectric loss usually dominates, and a lower loss laminate is the effective remedy. At high frequencies the copper loss becomes comparable and eventually dominant for a long route.

The crossover depends on the material, on the roughness and on the geometry, and it should be estimated before a material is changed. A design that replaces the laminate while the copper roughness is the larger term will not see the improvement that was paid for.

The remedy for copper loss is a smoother foil, a wider trace or a shorter route. The remedy for dielectric loss is a better material, a thinner dielectric where the geometry allows it or a shorter route. Only one of those is solved by the laminate.

Grades and Their Differences

The range runs from a standard FR4 with an improved resin through a mid loss material to a very low loss PTFE based laminate. Each step reduces the loss tangent and each step raises the cost and narrows the process window.

A mid loss material is often the best compromise for a digital channel because it can be processed on a conventional line with the same drilling and plating. The very low loss materials require a different process and they are usually reserved for radio frequency work.

The choice should be made against the budget rather than against the best available figure. A material that provides more margin than the link needs costs money that would be better spent on the connector or the via design.

Frequency and Temperature Effects

The datasheet figures are usually given at a few frequencies, and the design should interpolate rather than assume the value is constant. For a wideband design the variation across the band is what matters.

Temperature has two effects. The loss tangent rises with temperature for most materials, and the dielectric constant also changes, which shifts the impedance and the delay. A board that runs hot will have a worse loss and a different delay than the same board on a bench.

The design should therefore be evaluated at the worst case temperature. A link that has margin at twenty degrees may have none at eighty, and the measurement should be made with the board at its operating temperature.

Stackup and Routing Consequences

A lower loss material is usually part of a package of changes. The same stack can be arranged so that the field sees more of the material and less of the air, the vias can be backdrilled and the connectors can be chosen for their bandwidth.

The impedance should be recalculated for the new material because the dielectric constant differs. A design that keeps the old geometry on a new laminate will have the wrong impedance and a different loss than the calculation predicted.

The reference plane must be continuous. A split in the return path adds an impedance discontinuity that produces reflections, and no laminate can compensate for it.

Verification

The loss should be measured on a test coupon or on a daisy chained test structure that represents the real trace width and the real stack. The measurement is the evidence that the material has delivered the expected improvement.

The measurement should cover the frequency band of interest and be made with a calibrated setup. A fixture that has not been de embedded will report the loss of the fixture as well as the loss of the trace.

The results should be recorded against the material batch and compared with the design estimate. A large difference points at a material that does not meet its datasheet, at a geometry that is different from the calculation or at a measurement that is not calibrated.

Practical Rules

Split the loss budget into dielectric and copper contributions, estimate the crossover and choose the material against the budget rather than against the lowest available loss.

Record the material and the measurement data with the build records and the high frequency laminate guide, and review the signal integrity budget and the via transition design when the channel is analysed.

FAQ

Which contributes more to insertion loss? It depends on the frequency, the length and the copper roughness. The dielectric usually dominates at moderate frequency, while the copper becomes comparable at high frequency.

Can a low loss laminate fix a long route? Only partly. Shortening the route or smoothing the copper reduce the loss without the cost and the process change that a new material requires.

Why measure at temperature? The loss tangent and the dielectric constant both change with temperature, so a link with margin on a bench may have none at the operating temperature.