ASTM B568 measures how thick a metallic coating is without cutting the part. X-rays excite the coating and the substrate beneath it, and the ratio between the two signals gives the thickness. Gold over nickel over copper on a connector, tin on a lead frame, zinc on a fastener: all measured in seconds, on the part, with nothing destroyed. MatX Lab runs XRF thickness measurements on plated components, connectors, lead frames, and coated stock, aligned to ASTM B568 and ISO 3497.
What Is ASTM B568?
ASTM B568 is the standard test method for measurement of coating thickness by X-ray spectrometry. A collimated X-ray beam strikes the surface, and the elements in both the coating and the substrate fluoresce at their own characteristic energies.
Two effects give you the thickness. Emission from the coating rises as the coating gets thicker. Emission from the substrate falls, because a thicker coating absorbs more of the substrate signal on its way out. Comparing both against calibration standards of known thickness converts the count rates into a number.
Multi-layer stacks work the same way. A gold-over-palladium-over-nickel connector finish reports all three layers in one measurement, provided each element is distinguishable from the others and from the base metal.
ASTM B568 Scope and Applications
The method is suitable for measuring metallic coatings on both metallic and non-metallic substrates, provided the coating and substrate elements produce separate X-ray lines.
Typical applications include:
- Plating line control. Measuring coating thickness at specific points on production parts, with results available quickly enough to make adjustments before an entire shift’s production is affected.
- Incoming inspection. Checking plated components against the required purchase specification before they move into assembly.
- Precious metal control. Monitoring gold and palladium coating thickness, where even a tenth of a micrometer can make a significant difference in material cost, especially when dealing with high production volumes.
- Failure investigation. Checking whether a solderability or contact resistance failure traces back to thin or missing plating.
Where the coating and substrate share similar atomic numbers, the method struggles, and a different technique applies. Chromium on steel and nickel on iron are the common problem pairs. Magnetic and eddy current gauges suit some of those cases, and microscopic cross-section under ASTM B487 settles any dispute destructively. MatX Lab reports measured values and does not certify products or act as a standards body.
ASTM B568 Test Procedure
Calibration is the whole game. An XRF thickness reading compares the sample to standards, and a reading taken with the wrong calibration is precise but wrong.
| Step | What happens |
| Part review | We confirm the coating stack, substrate alloy, and expected thickness range, since all three determine the correct calibration. |
| Calibration | We calibrate the instrument with certified standards that match your coating and substrate combination across the expected range. |
| Collimator selection | We choose a spot size that fits the feature. Small features need a fine collimator, which costs counting time. |
| Part positioning | Position the part at the correct measuring distance and perpendicular to the beam, avoiding curvature and edges. |
| Measurement point selection | Points are chosen and recorded so repeat measurements land in the same place, which matters on parts with thickness variation. |
| Acquisition | Counts are collected for a set time. Longer counting improves precision, and thin precious metal layers need more of it. |
| Calculation | Thickness is calculated per layer from the intensity ratios against the calibration. |
| Replication | Measurements are repeated at the specified number of locations, and the spread is reported alongside the mean. |
Limitations: The coating and substrate need to produce separate X-ray lines for the method to work reliably. For example, nickel on iron and chromium on steel can be difficult to measure accurately, regardless of what an instrument brochure may suggest.
Every coating also has a thickness limit. Once the coating becomes thick enough, the substrate signal is no longer detected, so increasing the thickness further does not change the reading. This upper limit varies by coating and substrate combination. The coating composition can also affect calibration. For instance, using a pure tin calibration to measure a tin-lead coating can give an incorrect thickness result. Surface condition matters as well.
Curved or rough surfaces can change the measurement geometry used during calibration. Small-radius surfaces are a common reason why a supplier and customer can get different thickness readings from the same component. Spot size is another limitation.
If you need to measure a small feature, such as a 100 µm pad, the instrument needs suitable optics and a sufficiently small measurement spot, which not every system can provide. Finally, any underlying layers that were not included in the measurement setup can affect the result without being obvious. This is why you need to know the full coating structure when setting up and interpreting a coating thickness measurement.
ASTM B568 Specimen Requirements and Test Conditions
| Parameter | Typical requirement |
| Coating types | Gold, silver, palladium, nickel, tin, zinc, copper, chromium on suitable substrates |
| Thickness range | Roughly 0.01 µm to tens of micrometers, coating and substrate dependent |
| Spot size | Down to tens of micrometers with fine collimation or capillary optics |
| Surface | Flat, clean, free of oil and oxide at the measuring point |
| Measurement points | Specified by you, commonly three to five per part |
| Calibration standards | Certified, matched to the coating and substrate combination |
| Parts required | Enough units to cover the sampling plan, plus spares |
Tell us exactly where on the part the coating thickness specification applies. Plating thickness can vary with current density, so a contact area and a shielded recess on the same connector may have significantly different coating thicknesses, sometimes by a factor of two. Measuring the wrong location can therefore lead to disagreement rather than useful data.
ASTM B568 Test Results and Reporting
The coating thicknesses for each coating at each sampling location are provided along with the average and variation from locations. The report states the calibration standards used and their certified values. The report includes the counting time and collimator used, along with the test results. If the measurement points were specified using a drawing or photograph, we also include that reference so the same locations can be used for repeat measurements. If a reading is close to the saturation limit for a particular coating-substrate combination, we make this clear in the report.
ASTM B568 FAQs
Can you measure a coating on a plastic or ceramic substrate?
Yes, provided the substrate produces a usable signal or the coating alone can be quantified. Tell us the substrate at quotation, since a non-metallic base changes which calibration approach applies and sometimes rules the method out altogether.
Why does my supplier's reading differ from yours?
Usually calibration or location. Different calibration standards, a different assumed coating alloy, or measurement at a different point on the part will each produce a real difference between two correct instruments. Comparing the measurement locations first resolves most of these disputes.