Most discussion of X-ray inspection is about how: which modality, what settings, which defects. The underlying question is why bother at all, and the answer comes down to a single word.
Resistance.
Three kinds of resistance
Electrical resistance is a function of the material’s resistivity, the length of the path, and the cross-sectional area of the conductor. Copper has far lower resistivity than plastic, which is why one carries current and the other does not.
Mechanical stress is force divided by the area over which it acts. The same load applied across less area produces more stress.
Thermal resistance is thickness divided by area, scaled by the material’s thermal resistivity. It determines how readily heat moves from where it is generated to where it can be dissipated.
What the three have in common
Look at the form of those relationships and the same term appears in all three.
Cross-sectional area sits in the denominator of every one. Electrical resistance rises as conductor area falls. Thermal resistance rises as area falls. Mechanical stress rises as the area bearing a load falls.
That single shared term is why X-ray inspection works.
X-ray cannot measure resistance directly. It cannot read out ohms, or degrees per watt, or megapascals. What it measures is internal geometry and material distribution, which means it measures the area available to conduct current, conduct heat, and carry mechanical load.
A void inside a solder joint is not three separate problems. It is one reduction in cross-sectional area that raises electrical resistance, raises thermal resistance, and raises mechanical stress simultaneously. Finding the void finds all three.
Heavy components
Consider a board carrying a transformer and large capacitors.
The electrical question is straightforward: are the connections to the board sound. The mechanical question matters just as much, because these components have real mass, and mass under vibration and shock generates force at the joints holding it. Those joints have to maintain mechanical integrity across the service life of the board, not just at the moment of assembly.
X-ray answers both, because both depend on how much sound material is actually present in the joint.
LED boards and the thermal path
An LED panel adds the third resistance as a primary concern.
The electrical connection still matters. But these components run hot, and the board itself functions as the heat sink. Heat generated at the semiconductor junction has to travel through the package, through the solder joint, and into the board where copper can spread and dissipate it.
That path runs through the solder. A joint with significant voiding presents higher thermal resistance, which means the junction runs hotter, which accelerates degradation. The joint is not just an electrical connection. It is part of the cooling system.
BGAs and QFNs
Under a BGA the electrical question is whether connection exists between the board pads and the balls, which is invisible from outside and difficult to verify by test alone.
The mechanical question is longer term. Voids inside individual balls reduce the cross-sectional area carrying load, and stress concentrates around them. Over thermal cycles and mechanical stress in service, that concentration initiates cracks. A crack propagates, and eventually the joint fails as an open, or was never fully formed in the first place as a head-in-pillow defect.
The joint that fails in year three often looked fine in year one and contained the void that caused it on day one.
The why underneath the how
Every inspection technique, every modality choice, every discussion of resolution and magnification serves this.
Products fail when current cannot flow, when heat cannot escape, or when a joint cannot carry the load applied to it. All three depend on how much material is actually where the design assumed it would be, and that is a geometric question about the inside of a sealed assembly.
X-ray is how that question gets answered without destroying the thing being asked about.
