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.
Additional Considerations for This Build
Practical attention to barrel fill pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating barrel fill explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, barrel fill is the item that decides how the rest of the board is arranged. 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. 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.
Process Control and Verification
On a design of this kind, barrel fill is the item that decides how the rest of the board is arranged. 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.
Process Control and Verification
On a design of this kind, barrel fill is the item that decides how the rest of the board is arranged. 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.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
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.
