# Copper Coin PCB Height: Check the Step at the Pad _Published: 2026-11-06_ Table of Contents Toggle - Define the reference surface first - What the drawing and report must contain - Calculate a signed step - Compare protrusion, recess and local tilt - Do not confuse coin flatness with global board flatness - Identify which side sets the assembly limit - Check the interface with the actual package - Treat surface residue as a separate check - Use multiple points for first-article inspection - Compare readings at the same process stage - Investigate contact before claiming thermal benefit - Send a complete measurement request - FAQ On an embedded copper coin PCB, a flat coin can still sit too high or too low relative to the surrounding pads. Specify the signed step at the finished assembly surface and the local flatness separately. The correct limit comes from the actual package, solder or interface requirements and the agreed fabrication process. ![Copper coin area beside PCB surface pads](https://www.bestpcbs.com/blog/wp-content/uploads/2026/11/embedded-copper-coin-pad.jpg)Copper coin area beside PCB surface pads. AI-generated illustration, not a measurement or production record. ## Define the reference surface first Measure relative to the pads or other datum specified on the finished drawing. A height referenced to solder mask or bare laminate is not equivalent to height referenced to adjacent finished copper. Use the same construction stage for both measurements. ## What the drawing and report must contain Use signed units and identify the exact points measured. **Field** **Unit or basis** **Reason** Coin surface height μm relative to specified datum Distinguishes protrusion and recess Local flatness μm over a stated region Captures variation on the coin itself Adjacent pad datum Finished surface and measured locations Makes the step repeatable Side and stage Top or bottom, before or after finish Prevents incomparable reports Assembly interface Soldered pad or mounted thermal interface Establishes the functional limit ## Calculate a signed step If the surrounding datum is 0.000 mm and the center of the coin is +0.040 mm, the center protrudes by 40 μm. If another location on the coin measures +0.020 mm, the two sampled heights differ by 20 μm. Those points alone do not establish full-surface flatness. ## Compare protrusion, recess and local tilt Each shape can affect a different part of assembly. **Condition** **What it means** **What to check** Protrusion Coin rises above the specified reference Stencil seating, package or heat-sink contact Recess Coin is below the reference Solder or interface gap and contact Tilt or local variation Height changes across the coin Multiple measurement points and contact footprint ## Do not confuse coin flatness with global board flatness A board bow measurement describes another scale and reference. A good global result cannot rule out a local step. Measure the small region that controls solder or thermal contact and record how the board was supported. ## Identify which side sets the assembly limit The component side may be constrained by solder seating while the other side contacts a thermal interface. Specify the two faces separately. A permitted recess on one face does not define acceptable protrusion on the other. ![Copper coin PCB beneath an optical inspection instrument](https://www.bestpcbs.com/blog/wp-content/uploads/2026/11/coin-surface-height-inspection.jpg)Copper coin PCB beneath an optical inspection instrument. AI-generated illustration, not a measurement or production record. ## Check the interface with the actual package A package thermal pad and surrounding leads or lands may need to seat together. Compare the worst-case local geometry and the approved solder process. A high-conductivity insert cannot compensate for an unsuitable joint or an unverified assembly gap. ## Treat surface residue as a separate check A surface can meet the height requirement yet still have residue or an unsuitable finish. Check cleanliness and finish over the required contact area in addition to geometry. The [PCB copper coin design guide](https://www.pcbx.com/Copper_coin.pdf) illustrates why dimensional and resin-related requirements are listed separately. ## Use multiple points for first-article inspection Agree on center and perimeter readings or a suitable surface map. Record the datum and instrument method. A single center reading can miss tilt or a localized region outside the functional surface requirement. ## Compare readings at the same process stage Pressing, finishing and any approved planarization can change the measured surface. Specify whether acceptance applies to the completed board and retain intermediate data separately for troubleshooting. Do not compare a pre-finish result with a finished-board limit without agreement. ## Investigate contact before claiming thermal benefit If the component or heat sink does not seat as intended, inspect the actual interface rather than changing the copper grade first. See the [BestPCBs copper coin PCB guide](https://www.bestpcbs.com/blog/2026/10/copper-coin-pcbs/) for the broader thermal-path and construction context. ## Send a complete measurement request Provide the coin location, side, datum, signed limit, sampling points and the interface drawing. For a construction review, contact Best Technology at [sales@bestpcbs.com](mailto:sales@bestpcbs.com) with the stackup and inspection criteria. ## FAQ ### Does a flush coin need any tolerance? Yes. Flush describes the nominal relationship, not the allowable range. Specify how far the surface may sit above or below the datum and where to measure it. Agree those limits with fabrication and assembly before using the word flush as an acceptance criterion. ### Can the solder mask surface be my datum? Only if that is the explicitly defined functional reference for the design. A mask surface and a finished copper pad can differ in height. State the material, location and process stage so the inspector does not substitute an easier but different reference. ### Can one reading prove that the coin is flat? No. It establishes the height at one point. To assess flatness or tilt, inspect enough of the functional area using the agreed method. Record the underlying points or map so a reviewer can distinguish a uniform step from a surface that varies across its width. ### Is a protruding coin always worse than a recess? Neither is universally better. The acceptable direction and magnitude depend on the package, surrounding contacts, interface and process. Evaluate the finished board with its actual mating surface and use signed limits rather than assuming any visible step is automatically unacceptable or harmless. ### Can I use the same limit on the top and bottom? Only if both interfaces support it. The component side and the heat-sink side can have different gap, load and seating requirements. Record the reference and signed limit for each face independently, and confirm how both will be inspected on the finished part. ### Does good coin height prove a good thermal joint? No. Height supports fit, but contact area, solder quality or thermal interface condition also matter. Verify the assembled heat path under the relevant test conditions. Keep geometric inspection and thermal performance evidence separate rather than treating one as a replacement for the other. ### Is a microsection sufficient to verify the entire surface? A section gives detailed information at its cut plane, not a full top-surface map. Use it for the question it can answer and combine it with an appropriate surface measurement where needed. Identify the cut and measurement locations in the report for traceability. ### Can I approve a visually flat coin without measurements? Visual inspection can identify gross defects, but it cannot establish a signed micrometer-scale step. Use the agreed measurement basis when the geometry controls fit or performance. If a report lacks the datum or measurement locations, ask for clarification before releasing the lot.