Thermal Vias Under Power Devices

A power device on a surface pad has nowhere to send its heat except down through the board. Thermal vias carry it to the other side, and their number, size and construction decide how much of it arrives.

How Heat Leaves the Pad

The heat passes through the solder joint into the copper pad, spreads laterally and then moves vertically through the vias. Each step has a thermal resistance and the largest one sets the total.

The joint is often the largest term rather than the via array. Our thermal pad notes describe the joint.

Vias and Their Contribution

An open via is a tube of plating in a dielectric, so the copper cross section is small. Filling the via with copper or with a conductive plug increases the cross section substantially.

The difference is large enough that the fill should be specified rather than left to the shop. Our via filling notes describe the options.

Number and Spacing

More vias reduce the resistance up to the point where the pad can no longer be fabricated or the solder wicks away. The spacing should keep the pad flat and printable.

The count should be calculated rather than chosen. Our thermal design notes describe the calculation.

Where the Heat Goes

Vias that end in an internal plane spread heat over a large area, which is usually better than a small pad on the other side. The plane has to be connected and it has to be able to give the heat up.

A plane with no external path only delays the rise. Our metal core notes describe the alternative.

Effect on Assembly

An open thermal via absorbs paste during reflow, which leaves a void under the pad. The vias are therefore tented, plugged or filled on the top side, and the choice affects the thermal result.

The two requirements should be satisfied together. Our voiding notes describe the limits.

Measurement

The result is measured by operating the board and reading the pad temperature, or by a thermal resistance measurement with a defined boundary condition. A calculation alone does not establish the margin.

The measurement should be taken at the worst case load. Our thermal measurement notes describe the technique.

Verification

The verification is a via count and fill chosen against a calculated requirement, a construction that does not wick paste from the pad, and a measured pad temperature at the worst case load rather than at a bench setting.

Our quality notes describe how the records are kept.

Process Control and Verification

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. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. 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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

FAQ

Are more vias always better? Up to the point where the pad becomes difficult to print and the paste wicks away. Beyond that the improvement is small and the assembly risk is real.

Does a filled via cost more? Yes, because of the fill and planarization steps. It conducts considerably better than an open one.

What does gopcb provide for thermal vias? We provide a via count calculated against the required thermal resistance, a fill or plug specified rather than assumed, a construction that prevents paste wicking from the pad, and a measured pad temperature at the worst case load.