Loss Tangent and Insertion Loss in High Speed Laminates

At low frequencies a trace behaves like a resistor. At multi gigabit rates it behaves like a transmission line, and the energy that does not reach the receiver has been absorbed by the conductor and by the dielectric. Loss tangent describes the dielectric part of that absorption, and it is one of the few laminate properties that directly limits how far a high speed signal can travel. The wider family of high speed materials is compared in this guide to high frequency laminates.

What Loss Tangent Describes

Loss tangent, sometimes called the dissipation factor, is the ratio of energy lost to energy stored in the dielectric each time the field reverses. A low value means the material absorbs little energy, so more of the signal survives the journey. It is a property of the resin chemistry, and it changes with frequency and with temperature.

The value is normally quoted at a specific frequency, such as one megahertz or ten gigahertz, and the two numbers can differ substantially for the same material. A data sheet value without a stated frequency and test method cannot be compared with another material’s figure, which is the most common source of confusion when selecting a laminate.

Dielectric Constant and Its Partner Role

Dielectric constant determines the propagation velocity and the impedance of a trace for a given geometry, and it is specified together with loss tangent in every material data sheet. A lower dielectric constant allows thinner traces for the same impedance, or the same trace width with a thicker dielectric layer.

The two properties are linked in practice. Resins modified to lower the dielectric constant often also lower the loss tangent, because both come from reducing the polarisability of the polymer. Adding fillers can change one more than the other, so the pair has to be evaluated together rather than optimised separately.

Conductor Loss and Skin Effect

At high frequency, current concentrates near the surface of the conductor, and the effective cross section becomes a thin shell. This is skin effect, and it makes the resistance rise with the square root of frequency. Conductor loss therefore grows steadily, and it becomes a significant part of the total as the rate increases.

Surface roughness adds to it. The copper foil used for lamination has a treated side designed to bond to the resin, and the profile of that treatment forces current to follow a longer path. The effect becomes important when the roughness is comparable with the skin depth, which happens in the low gigahertz range for typical foils.

Comparing Laminate Families

Standard FR-4 has a loss tangent in the range of a few hundredths, which is adequate for moderate speeds but becomes the limiting factor above a few gigabits per second. Modified FR-4 grades reduce the value, and the mid loss and low loss families sit between standard material and the specialised products.

At the low loss end are the materials built on modified polyphenylene oxide, hydrocarbon or liquid crystal polymer chemistry, and at the extreme, PTFE based laminates. Each step down in loss tangent brings a step up in cost, in processing difficulty and often in the care needed during drilling and lamination. The properties of the PTFE family are described in this guide to PTFE PCB properties.

Insertion Loss and the Frequency Budget

Insertion loss is the total attenuation of the channel, combining dielectric loss, conductor loss, reflections and any loss in the connector or the package. Dielectric loss rises roughly linearly with frequency, while conductor loss rises with the square root, so the balance between them shifts as the rate increases.

A design has a loss budget derived from the receiver’s sensitivity and the equalisation it can apply. The board’s contribution is one part of that budget, and the material selection is the main lever available to the fabricator. Where the budget is tight, the difference between two similar laminates can decide whether the link closes at all.

Test Methods

Loss is measured on a transmission line structure, either a stripline or a microstrip, using a vector network analyser to extract the scattering parameters. The result depends on the line geometry, the launch structure and the calibration, so measurements from different coupons are compared only when those are matched.

Because the geometry determines the impedance, and therefore the field distribution, a coupon cannot be treated as a generic sample. It should be built from the production stack-up, with the same dielectric thickness and the same copper, and measured with the same method each time. A dedicated coupon in the panel border is the practical way to achieve that, and the materials it characterises are documented in this guide to laminate material properties.

Roughness, Copper Foil and Loss

Copper foil treatment is chosen for adhesion, and better adhesion usually means a rougher profile. On a low loss design the roughness becomes a significant part of the total loss, and the choice of foil can matter as much as the choice of resin. Smooth foils with chemical bonding treatments are available for exactly this reason.

The trade-off is that smoother foil can be harder to bond, and it may need a different oxide or bonding treatment. Where a design is close to the loss limit, the combination of foil and resin has to be evaluated together, and the result measured rather than predicted from the two data sheets.

Cost and Process Trade-offs

A low loss laminate costs more per square metre, and it usually costs more to process as well. Some materials need higher lamination temperatures, longer press cycles and more careful drilling, and they may require different desmear chemistry. Those costs are real and should be counted alongside the material price.

The design decision should be driven by the loss budget rather than by a preference for the best available material. Where the link closes with a mid loss laminate, using a low loss material adds cost without benefit. The routing and stack-up choices that reduce the demand on the material are covered in this guide to high speed design rules.

Selecting a Material for a Design

Selection starts from the required data rate, the maximum channel length and the receiver’s tolerance, converted into a loss budget at the relevant frequency. That budget then sets the maximum acceptable loss tangent, and the candidate materials are filtered by what the fabricator can process reliably at the required stack-up.

Availability and repeatability matter as much as the data sheet numbers. A material that is difficult to source, or whose properties vary between lots, will cause more problems than one with slightly higher loss that is always available. The material decision should therefore be made with the fabricator early, not after the artwork is finished.

FAQ

Is loss tangent the same as dissipation factor? Yes, the two terms describe the same property and are used interchangeably, although data sheets sometimes quote one and not the other. Both must be read with the frequency at which they were measured, because the value rises with frequency for most laminate materials.

At what speed do I need a low loss laminate? That depends on the channel length as much as the data rate, because loss accumulates with distance. Short channels often work with standard material at rates where long ones need a low loss grade. The decision follows from the loss budget, not from the data rate alone.

Does copper roughness really affect insertion loss? Yes, and it becomes significant in the low gigahertz range where the roughness is comparable with the skin depth. It adds conductor loss on top of the dielectric loss, and on a low loss laminate it can be the largest single term in the budget if a rough foil is used.