Meaning
Structural measurement of deviations in atomic spacing within a crystalline material reveals internal strain and deformation. The XRD lattice distortion analysis uses X-ray diffraction to measure the changes in atomic plane separations caused by stress. When a solder joint or silicon wafer is under load, its crystal lattice is either compressed or stretched, which alters the angle at which X-rays diffract.
This shift in the diffraction peak allows researchers to calculate the strain state of the material.
Measurement Principle
Monochromatic X-rays directed at the specimen scatter off the regular planes of atoms according to Bragg’s law. In an unstrained material, this scattering produces sharp diffraction peaks at specific, known angles. When residual or applied stress is present, the lattice planes are distorted, shifting the peaks to different angles.
Measuring these angular shifts across various sample tilts enables the complete three-dimensional strain tensor to be calculated. The method is highly sensitive and detects lattice strains as small as one part in ten thousand.
Process Optimization
Applying this diagnostic method to electronic packages helps engineers locate regions of high stress concentration. Cooling after reflow or packaging encapsulation introduces significant stress due to different expansion coefficients of the materials. By mapping these lattice distortions, designers can adjust the material selection or change the temperature profile to lower the risk of joint failure.
This feedback loop improves the mechanical robustness of the assembly before volume production begins.
Material Limitation
The technique requires a crystalline structure and cannot measure strain in amorphous materials such as underfills or polymers. Rough surface finishes or coarse grain sizes also degrade the diffraction signal, requiring specialized sample preparation to obtain clear data.
