Scattering Mechanism
An inelastic deflection of X-rays by outer-shell electrons produces secondary radiation that deviates from the primary beam path. This compton scatter occurs during automated X-ray inspection of assembled circuit boards where thick solder joints act as primary sources of deflected photons. The redirected photons create a diffuse background on the sensor.
Contrast Reduction
Secondary photons arriving at the imaging sensor create a uniform background noise that reduces the sharpness of the digital radiograph. In automated inspect systems, compton scatter decreases the density contrast between the solder alloy and the surrounding laminate. This loss of contrast conceals critical defects like head-in-pillow joints or micro-voids in ball grid array assemblies.
To counter the degradation, scanners utilize hardware collimators and mathematical correction models to estimate and subtract the diffuse scatter field. When board layouts feature metal shields, the resulting scatter intensifies. If left uncorrected, the increased noise leads to false acceptances of defective joints.
The calculation of scatter correction factors depends heavily on the average atomic number of the materials present in the scan path, meaning that lead-free solder alloys yield different scatter profiles than traditional leaded finishes.
Inspection Limit
The physical ratio of scattered photons to direct imaging photons determines the maximum board thickness that a system can reliably inspect. Heavy multi-layer backplanes with thick copper weights generate high levels of compton scatter that degrade computed tomography slices. If the secondary noise exceeds the direct transmission signal, the slice reconstruction fails to resolve the structural boundaries of internal connections.
This threshold defines the operational limit for non-destructive X-ray verification.
