Dielectric Loss Tangent Selection Guide
The loss tangent of a laminate is the fraction of the energy in the field that is turned into heat as the signal passes. It is quoted as a small number, it changes with frequency and temperature, and it is the property that separates a general purpose board from a high speed one. Choosing a material by its loss tangent is a cost decision as much as an electrical one, because the low loss materials are several times the price of a standard laminate.
What Loss Tangent Means
A dielectric material stores energy in the electric field and loses a fraction of it as heat on every cycle. The loss tangent is the ratio between the energy lost and the energy stored, and it is also called the dissipation factor.
The loss appears in the transmission line as an attenuation that grows with frequency, and it is the dominant term on a long line at a high data rate. It is quoted at a frequency, because the value is not constant.
The property belongs to the resin rather than to the glass, so two laminates with the same glass style have different loss tangents if their resins differ. The choice of resin system is therefore the choice of loss.
The laminate properties guide lists the value alongside the dielectric constant, and the two together determine the loss of a line.
Frequency and Temperature Dependence
The loss tangent of a resin rises with frequency in the range that matters for a digital link, so a material that is quoted at one gigahertz has a higher value at ten.
The temperature dependence is also significant. A resin that is near its glass transition loses more energy, so a board that runs hot is more lossy than the same board at room temperature.
The two effects combine in a product that runs warm and carries a fast signal, which is why a link that passes on the bench can fail in the enclosure.
A data sheet that quotes a single value without a frequency is not a specification. The value has to be read at the frequency of interest and, where possible, at the temperature of the application.
Material Families
A standard FR4 has a loss tangent around 0.02, which is adequate for a slow design and marginal for a fast one. It is the cheapest option and the most widely available.
A modified FR4 with a different resin system reaches around 0.01 and is used for a moderate data rate. It processes like a standard material and does not require the changes in drilling and lamination that a low loss material needs.
The low loss materials based on a hydrocarbon or a polyphenylene ether chemistry reach around 0.005 and below. They are used for the longest and fastest links, and they cost several times as much as FR4.
PTFE based laminates reach the lowest values, and they bring their own process requirements. A material with a very low loss tangent is not simply a better FR4.
How to Read a Data Sheet
The first thing to check is the frequency at which the value is quoted. A figure given at one megahertz is not useful for a link at ten gigahertz.
The second is the test method, since different methods give different values for the same material. A value measured on a resonator and a value measured on a transmission line are not the same number.
The third is the tolerance. A material that is quoted as a typical value with no spread is a material whose consistency is unknown, and the consistency matters as much as the average for a controlled impedance line.
The fourth is the environment, because a value measured at room temperature and dry is not the value in a humid enclosure. The high frequency laminate guide compares the materials on a common basis.
When the Choice Matters
The choice matters when the dielectric loss is a large part of the total loss. On a short line the conductor loss and the vias dominate, and a low loss material buys almost nothing.
A useful rule is to compare the dielectric loss with the budget at the data rate and the length of the link. If the dielectric term is a small fraction of the allowance, the material can be a standard one.
The choice also matters for a long backplane, where the length is measured in tens of centimetres and the loss accumulates. That is the application for which the low loss materials were developed.
The high speed design rules give the framework for the comparison, and the decision should be made with the budget in hand rather than by habit.
<img src="https://www.gopcba.com/wp-content/uploads/2026/08/img1-2.webp" alt="Insertion loss curves of two laminate materials compared” />
Cost and Availability
A low loss laminate costs several times a standard FR4, and the price difference is largest for a small quantity because of the minimum order. The cost of the material is only part of it.
The processing also costs more. A low loss material often needs a different drilling recipe, a different lamination cycle and a higher press temperature, and the shop may add a surcharge for the handling.
The availability is the third factor. A specialised laminate may come from one supplier with a long lead time, which is a supply risk for a product with a long life.
The cost reduction review should therefore treat the material choice as a system decision, including the processing and the supply chain, not as a line item on the laminate.
Practical Rules
Start with the loss budget and the length of the link, and choose the material that meets it with margin rather than the lowest available value.
Ask for the loss tangent at the frequency of interest and at the operating temperature, and ask for the spread rather than the typical value.
Keep the same material on both sides of a symmetric stackup, because a difference changes the balance and can warp the panel.
Record the material, the resin system and the coupon result with the design, so that a later change can be evaluated against the same numbers.
Process Control and Verification
On a design of this kind, laminate is the item that decides how the rest of the board is arranged. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
FAQ
What is a good loss tangent? Below about 0.005 for a long high speed link, around 0.01 for a moderate one, and around 0.02 for a standard FR4. The value has to be at the frequency of interest.
Does a low loss laminate help a short trace? Very little, because the conductor loss and the vias dominate the budget on a short line. The material change is a waste of money there.
Why does the value change with temperature? Because the resin softens as it approaches its glass transition and absorbs more energy. A board that runs hot is more lossy than the data sheet suggests.
