Copper Roughness and High Frequency Insertion Loss

At high frequency the current no longer flows through the whole conductor. It crowds towards the surface, and the roughness of that surface becomes part of the electrical path rather than a mechanical detail. Copper that was specified for its thickness is then also specified for its finish.

The Skin Effect and Why Roughness Matters

Skin depth falls as frequency rises, so the fraction of the conductor carrying most of the current becomes thinner as the signal gets faster. Once the skin depth is comparable with the roughness amplitude, the current has to follow a longer path along the profile.

The longer path raises the resistance and the loss, and it does so progressively rather than as a step. That is why the effect is visible as an insertion loss that grows faster than a smooth conductor would predict. Our high speed design notes cover the related geometry side.

Where the Roughness Comes From

Copper foil is produced with a treatment on the laminate side so that it adheres to the resin. That treatment is a nodular structure, and its amplitude is chosen by the foil supplier for adhesion rather than for loss.

Processing adds roughness of its own. The oxide or the bonding treatment before lamination, and the etch that follows, both change the surface the current sees. Our laminate properties notes describe how the resin choice interacts with that treatment.

Measuring Surface Roughness

Roughness is measured as an amplitude figure over a sampling length, and the result depends on the sampling. A number quoted without its sampling length cannot be compared with another, which makes foil data sheets harder to use than they appear.

The measurement is made on the foil before lamination where the specification is set, and on a section of the finished board where the question is what the process produced. The two figures are related but not identical.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/pcb15.jpg" alt="Insertion loss measured on a test coupon” />

Low Profile Foil and Adhesion

A lower profile foil reduces the loss and reduces the adhesion, so the choice is a trade that the laminator has to manage. Improved bonding treatments recover part of the adhesion on a smooth foil, and their own contribution to loss has to be measured rather than assumed.

The trade is usually settled by the loss budget. Where the design has loss margin, a standard profile foil is cheaper and more robust; where the margin is thin, the low profile foil is the item that decides whether the channel closes.

Dielectric Loss and Its Share

Conductor loss is only part of the insertion loss. The dielectric absorbs energy as well, and its contribution grows with frequency and with the dissipation factor of the resin. On a long channel the dielectric term can dominate the conductor term.

That is the reason a change of laminate often produces a larger effect than a change of copper. Our high frequency laminate notes describe how the resin families compare on that figure.

Loss Targets and Test Coupons

Where a loss target exists it is usually verified on a coupon with a defined length and a defined launch, because the measurement depends on both. A coupon that is measured with a poor launch reports a loss that belongs to the launch.

The coupon has to be made from the same stack and the same copper as the product, and it has to be measured after the same process. A coupon that is built separately measures the coupon. Our PTFE properties notes describe the materials that are normally used for that coupon.

Process Control for a Loss Critical Board

Three process variables matter more than the others: the copper weight, the dielectric thickness and the lamination cycle. The first two set the geometry, and the third sets how far the resin has flowed and therefore how closely the copper follows the profile of the foil.

Those three are held in the stack-up and the travel card rather than left to the operator. Where the loss is critical, the coupon measurement is recorded per lot and the trend is reviewed rather than the individual value.

Design Choices That Reduce Sensitivity

A wider trace reduces conductor loss, and a shorter channel reduces both terms. Where the geometry is fixed by routing, the remaining lever is the stack, and a thicker dielectric reduces conductor loss while increasing the impedance unless the width is adjusted with it.

A stack-up that is drawn rather than described removes most of the ambiguity from the quotation, and it lets the shop price the board against the dielectric and copper weights that will actually be used. It also gives the loss budget a set of numbers to be calculated against.

Process Control and Verification

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. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

FAQ

Does roughness matter below a few gigahertz? It does, and its contribution falls as the frequency falls. Whether it matters is decided by the loss margin rather than by the frequency alone.

Can a smooth foil be used with a standard resin? It can, and the adhesion has to be qualified for the lamination cycle the board will see rather than assumed from the foil data sheet.

What does gopcb provide for loss critical boards? We provide foil and laminate selection against the loss budget, stack-up with copper and dielectric tolerances stated, coupon measurement with the launch defined, process control of copper weight and lamination cycle, and records per lot.