Insertion Loss Budgets for PCB Channels

What Insertion Loss Includes

Insertion loss is the reduction in signal power between the transmitter and the receiver, expressed in decibels as a function of frequency. In a board channel it is the sum of several contributions: the dielectric absorption of the laminate, the resistance and the surface roughness of the copper, the reflections from vias and stubs, the losses in connectors and cables, and the reflections from impedance discontinuities along the route. Each contribution has a different frequency dependence and a different fix, which is why a channel budget is the right way to think about the problem rather than a single loss number.

Dielectric Loss

Dielectric loss is proportional to frequency and to the loss tangent of the material, which is why it dominates at high frequency and why a low-loss laminate is the standard answer for long high-speed channels. It also scales with the length of the route, so a short channel on ordinary material can have less total loss than a long one on premium material. The choice of laminate is therefore a decision about the total length at the highest frequency of interest, not about the data rate alone. Where the channel is short, spending on the material buys little; where it is long, the material is usually the cheapest place to recover margin.

Conductor Loss and Roughness

Conductor loss has two parts. The first is the ordinary resistive loss, which falls with frequency in a simple model but changes once the current crowds to the surface of the trace, above which it becomes proportional to the square root of frequency. The second is the effect of the copper surface: a rough surface forces the current to follow a longer path, which increases the loss more than the nominal resistance suggests. Foil roughness is therefore a real parameter at high frequency, and the treatments that improve adhesion tend to increase roughness, which is one of the reasons high-speed laminates offer low-profile foils. The effect becomes significant above roughly 10 GHz and matters earlier on very long lines.

Vias and Stubs

A via contributes a small loss and a discontinuity, and if it has a stub the stub creates a deep notch at its resonant frequency. A stub that is short enough to move the notch well above the band is harmless; a stub that resonates inside the band can cost more margin than the entire dielectric loss of the route. Backdrilling removes the stub, and using blind or buried vias removes it by construction. The via’s own capacitance and the antipad clearance also create a small impedance discontinuity, which produces a reflection rather than a loss; both are usually managed by keeping the antipad and the via geometry consistent along the route.

Connectors and Cables

Connectors contribute loss through their contact resistance and through the impedance discontinuity at the interface, and their contribution often exceeds the loss of several centimetres of board. Cables add the loss of their own dielectric and conductor, which for a long cable can dominate everything on the board. In a budget, the connector and cable contributions should be measured or taken from the vendor’s data rather than estimated, because they are the elements that are easiest to underestimate. Where a channel is failing its budget, a change of connector can recover more margin than a change of laminate.

Building a Budget

A budget starts from the receiver’s specification, which defines the maximum loss and the frequency range over which it applies, then subtracts the contributions of the cable, the connector, the package, the vias and the board trace. The board trace is whatever remains, and that determines the length, the material and the geometry. Where the remaining allowance is negative, something has to change: a shorter route, a lower-loss material, a better connector, a different via strategy or a slower interface with more equalisation. Building the budget before the layout is what prevents the discovery of a problem after the board is ordered.

Where to Improve First

The order of leverage is usually connectors and stubs first, material second, geometry third. A backdrilled via or a better connector often recovers more than a laminate change, and both are cheaper than a redesign. Where the budget is still short, the next levers are the trace geometry, the use of a wider line to reduce conductor loss, and the layer assignment, which decides how many vias the signal has to pass through. The least effective lever is usually increasing the drive strength, because it improves the amplitude without improving the loss, and it can make emissions worse.

Insertion Loss and the Eye Diagram

Loss is only one of the effects of the channel, and it is worth separating from the others. Loss reduces the amplitude, which can be compensated by equalisation; reflections and impedance discontinuities distort the shape of the signal, which is harder to correct. A channel with modest loss but a poorly controlled impedance produces a worse eye than a lossier channel that is smooth. This is why the geometry along a high-speed route should be consistent: a line that is 50 ohms for most of its length and 45 ohms at a via, a connector and a stub creates three reflections in a short distance, and their interaction can be worse than any one of them. The design discipline is to hold the impedance through every transition, and to verify the result on a test coupon or a characterised channel rather than on the loss figure alone.

FAQ

What dominates insertion loss? At high frequency, the dielectric loss, scaled by the length of the route and the loss tangent of the laminate.

Does copper roughness matter? Yes, above roughly 10 GHz on a long line, because it forces the current to travel a longer path along the surface.

How much can a via stub cost? At resonance, more than the dielectric loss of the whole route, which is why backdrilling or blind vias are used.

Should I change the laminate first? Usually not. Connectors, cables and stubs are often the larger and cheaper contributions to fix.

Is a wider trace always better? For conductor loss, yes, but it consumes routing space and changes the impedance unless the stackup is adjusted with it.

Conclusion

Insertion loss is the sum of several contributions with different frequency behaviour, so the useful approach is a budget built before the layout and improved in order of leverage. Measure the connector and cable contributions, remove the stubs, choose the material from the length and the frequency, and check the geometry last. Signal integrity work belongs to PCB design and layout, the materials and their loss tangents are listed under PCB capabilities, and the verification is part of PCBA testing. High-speed channels are normally characterised through prototype PCB assembly in 2026.