Solder Voids in Thermal and BGA Joints
What a Void Is
A void is a cavity inside a solder joint, filled with gas or with a vacuum, where solder should be. It appears in an X-ray image as a lighter region inside a dark joint because there is less material in the beam path. Voids are formed during reflow, when gas that is generated or trapped inside the joint cannot escape before the alloy solidifies. They are a normal feature of the process rather than an exotic defect, and the engineering question is not whether voids exist but how much of the joint they occupy and where they sit.
Where the Gas Comes From
Three sources dominate. Flux volatiles are the most common: as the paste heats, the solvent and the activators vaporise, and if the joint solidifies before they can migrate out, they leave a cavity. Entrapped air is the second, introduced by a poor paste deposit, by a component placed with a void under it, or by a via that outgasses into the joint. Moisture is the third, absorbed by the board and released as steam during reflow, particularly where the laminate is wet or where the paste has been exposed. The kinetics matter as much as the quantity: a fast ramp that drives gas out quickly is often worse than a slower profile that lets the volatiles leave before the alloy freezes.
Why Thermal Joints Care More
In a signal joint a small void reduces the cross section slightly and changes nothing measurable. In a thermal joint the void sits directly in the path of the heat, and because the void has essentially no thermal conductivity, the effective contact area is reduced by the area of the void. A thermal pad under a power device with forty percent voiding has lost forty percent of its contact area to the board, and the junction temperature rises accordingly. The position also matters: a void near the edge of a pad is less damaging than one in the centre, where the heat is concentrated and the paths are shortest. This is why the acceptance criteria for thermal joints are tighter than for signal joints and are usually expressed per joint rather than per board.
Accepted Limits
The industry standard sets a general limit on void area in a ball or a joint, but the useful number is the one the design needs. For a purely electrical joint, a large fraction of the area can be voided without an effect. For a thermal pad, the limit is typically much lower, and for a joint that carries both current and heat, the limit follows the function. The specification should state the limit per joint type and the measurement method, because a limit expressed as a percentage of the projected area in a two-dimensional image is not the same as a percentage of the volume.
Measuring Void Area
Two-dimensional X-ray reports the projected void area, which is the largest cross section through the joint in the direction of the beam. That is a conservative measure for a spherical void but can understate the total, because a void that is not in the beam path is invisible. Computed tomography reconstructs the actual volume and gives the true fraction, at a much higher cost and much slower throughput. The practical approach is to set the acceptance limit in the terms the production measurement produces, and to use CT for failure analysis where a 2D result is ambiguous. Whichever method is used, the threshold and the angle of view should be recorded with the result.
Reducing Voiding
The levers are paste, profile and geometry. A paste with a lower flux content and better outgassing characteristics produces fewer and smaller voids. A profile that allows a longer preheat and a slower ramp gives the volatiles time to leave before the alloy solidifies. Splitting a large thermal pad aperture into a grid, so that the paste is printed as many small deposits rather than one slab, gives the gas many short escape paths and is one of the most effective single changes. Vacuum reflow, where the chamber pressure is reduced during the molten phase, removes most of the voids and is used where the thermal requirement is severe. The board and paste should also be dry, since absorbed moisture is a direct source of steam.
Voids That Are Not Defects
Some features that look like voids are not. A via under a pad, or a filled via, appears as a lighter region in the X-ray because the fill material absorbs less than solder. A component with a recess in its termination, or a solder ball with an internal shrinkage cavity from the original sphere, also shows as a discontinuity. The inspector needs to know which vias are filled and where they are, otherwise a normal structure will be recorded as a defect. Including the via-in-pad locations on the inspection documentation removes most of this ambiguity.
Voiding and Reliability
A void is rarely a direct short or open, so its effect on reliability is indirect but real. A joint with a large void has less metal to carry current, so its temperature rises under load, which accelerates fatigue. A void near the interface between the solder and the pad reduces the bonded area and gives a crack a starting point under thermal cycling. And a void that reaches the surface of the joint changes the geometry that a mechanical shock has to pass through. For these reasons the acceptance limit should be set by the joint’s function and by the environment, not only by a house number. Where a product is expected to see many thermal cycles in service, a joint with significant voiding that would be acceptable on a benchtop product becomes a reliability risk, and the sample-based destructive analysis that measures real void fractions is worth the effort before the design is released to production.
FAQ
What is a solder void? A gas-filled cavity inside a solder joint, formed when volatiles or entrapped air cannot escape before the alloy solidifies.
Are voids always a defect? No. Small voids are normal and harmless in signal joints, and the limit comes from the function of the joint.
Why are thermal pads held to a tighter limit? Because a void removes contact area directly from the heat path, raising the junction temperature.
How do I reduce voiding? Use a paste with better outgassing, split large apertures into a grid, extend the preheat, keep the board and paste dry, and consider vacuum reflow.
Is a shadow under a via a void? Usually not. A filled via or an open barrel shows as a lighter region and should be identified on the inspection documentation.
Conclusion
Voids are a process outcome, and the right question is how much of the joint they occupy and whether the joint is in a thermal path. Set the limit from the function, measure it with a method you can repeat, reduce it with paste, aperture geometry and profile, and document the structures that merely look like voids. The reflow and printing steps are covered by SMT PCB assembly, the inspection that measures the result by PCBA testing, and the thermal layout that decides how critical a void is by PCB design and layout. Power assemblies are normally qualified through prototype PCB assembly in 2026.
