Thermal Relief Design and Its Effect on Soldering

A thermal relief is a connection to a plane that is narrower than the pad it serves. It exists to limit how quickly heat leaves the pad during soldering, and it is the reason some joints form easily and some do not.

What the Relief Does

The copper plane under a pad conducts heat away faster than the soldering process can supply it. The pad stays below the melting point while its neighbours reach it.

The relief reduces the cross section of that connection, so the heat has to travel through a narrower path. The pad then reaches temperature with the rest of the board. Our thermal mass notes describe the same effect at board level.

Where It Is Needed

A pad connected to an internal plane needs a relief. A pad on a signal trace does not, because the trace is already narrow and it does not conduct enough heat to matter.

A pad connected to a plane through a full connection on a thick board is the difficult case: it needs higher temperature, longer time or a preheat that brings the plane up. Our profile notes describe how the requirement is established.

Relief Geometry

The relief is described by the number and width of the spokes and the width of the gap between the spoke and the plane. More spokes conduct more heat and resist soldering more.

The gap must be wide enough to be etched reliably and narrow enough to be covered by the mask. Our mask notes describe the dam that has to cover the gap.

Electrical Consequences

The relief adds inductance, which matters on a fast signal. It also raises the thermal resistance of the connection, which matters on a pad that carries heat away from a device.

The two requirements conflict with the soldering requirement. On a ground pad under a power device, the thermal performance matters more than the soldering ease, and the profile has to be adjusted instead. Our thermal via notes describe the arrangement that results.

Relief on Through-Hole Pads

A through-hole pad connected to a plane behaves the same way in a wave or a selective solder process, and the relief is what allows the barrel to fill.

Where the barrel does not fill, the joint is weak and it is often not visible from the top. Our hole copper notes describe the barrel condition that a good fill requires.

Where No Relief Is Used

Where a pad is connected for electrical or thermal reasons and no relief is used, the design should state that the joint will require a different profile.

The joint should then be verified on a first article, because the assumption that a full connection solders acceptably is often wrong on a thick board.

Verification

The verification is the joint appearance on the relieved and the unrelieved pads on the same board, and the thermal performance where the relief was removed for that reason.

Where the relieved joint is acceptable and the unrelieved one is not, the design choice is confirmed and the profile does not need to change. Our joint criteria notes describe the criteria applied.

Process Control and Verification

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. 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.

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.

Verification and Records

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.

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

FAQ

Do all plane connections need a relief? Only the pads that will be soldered. A pad that carries no joint can be connected fully.

Does a relief weaken the connection? It reduces the thermal and electrical connection, and it is a deliberate trade. The current it must carry should be checked.

What does gopcb provide for thermal relief design? We provide relief rules by pad type and process, spoke and gap geometry that the mask can cover, electrical and thermal assessment where the relief is a compromise, first article verification of relieved and unrelieved joints, and profile development for the pads that are connected fully.