The X-ray coating thickness gauge achieves non-contact thickness measurement through fluorescence excitation and signal analysis. Its core principle can be divided into four key steps:
1. High energy X-ray excitation fluorescence
The instrument is equipped with miniature X-ray tubes (such as tungsten or molybdenum targets) that emit high-energy X-ray beams with adjustable energy. When the radiation penetrates the coating, the inner layer electrons (such as the K layer) of the atom are ejected to form holes, and the outer layer electrons (such as the L layer) transition to fill in, releasing characteristic X-ray fluorescence. For example, nickel coating will release characteristic fluorescence of 8.26keV under excitation, whose energy strictly corresponds to the atomic number, becoming a "fingerprint" for element recognition.
2. Coating substrate fluorescence signal separation
When the coating and substrate elements are different, there is a difference in their fluorescence energy. The instrument captures two types of fluorescence signals simultaneously through a high-resolution silicon drift detector (SDD). Taking the measurement of gold coating (Au) on copper substrate (Cu) as an example, gold fluorescence (68.8 keV) and copper fluorescence (8.05 keV) form independent peak positions in the energy spectrum, and signal separation is achieved through multi-channel analyzer (MCA). If the coating is an alloy (such as tin lead alloy), the mixed fluorescence signal needs to be deconvolved.
3. Mathematical modeling of fluorescence intensity thickness
Thickness calculation relies on two types of models:
Standard curve method: Pre measure the fluorescence intensity of standard samples of different thicknesses and establish a thickness intensity database. For example, a certain instrument measures nickel coatings with a thickness of 0.1-10 μ m. A quadratic polynomial curve is fitted using 20 sets of standard samples, with an error controlled within ± 0.05 μ m.
Thin film FP method (basic parameter method): Based on the quantum theory of the interaction between X-rays and matter, input parameters such as coating density and substrate absorption coefficient, and directly calculate the thickness through iterative algorithms. This method does not require standard samples and is suitable for synchronous analysis of multi-layer coatings (such as Au/Ni/Cu triple coatings).
4. Environmental interference compensation and calibration
Temperature fluctuations (± 5 ℃) can cause a 2% -3% change in the output intensity of the X-ray tube, which is corrected in real-time by the instrument's built-in temperature sensor. Substrate roughness (Ra>0.8 μ m) can cause fluorescence scattering, and Monte Carlo simulation algorithm is required to compensate for signal loss. In addition, pure metal standard plates (such as 99.99% silver plates) are required for energy calibration during daily startup to ensure peak positioning accuracy of the energy spectrum is better than ± 0.01keV.
Typical application scenarios
In the field of semiconductor packaging, this technology can measure the uniformity of 0.5 μ m gold plating on the surface of BGA solder balls, with a detection speed of 200 points/minute, meeting the real-time control requirements of production lines. For automotive electroplating parts, the instrument can penetrate the 0.02mm chromium coating and accurately measure the thickness distribution of the underlying 0.5 μ m nickel layer, providing data support for optimizing the electroplating process.