Voiding in Solder Joints
What a Void Is
A void is a pocket of gas trapped inside a solder joint. It forms because the flux volatilises during reflow, because the surfaces release moisture or contamination, or because air is entrapped in the paste when it is printed or dispensed. The gas has nowhere to go once the alloy solidifies around it, so it remains as a discontinuity in the metal. Voids are normal to a degree; the practical question is their size, their position, and whether they affect the function the joint has to perform.
Where the Gas Comes From
Most of the gas originates with the flux. As the paste heats, the solvent and the activators vaporise, and if the vapor cannot escape before the alloy solidifies it becomes a void. The rate of heating controls this: a slow, even rise allows the volatiles to escape through the still liquid paste, while a rapid rise seals the surface and traps them. Surface contamination adds another source, since an oxide, a plating residue or a fingerprint decomposes under heat and releases gas. Paste that has absorbed moisture behaves the same way. Where the voiding is severe, the source is usually one of these rather than the alloy itself.
Profile and Atmosphere
The reflow profile is the most controllable factor. An adequate soak before the peak gives the volatiles time to leave, and a ramp that is not too fast keeps the surface open long enough for them to escape. The time above liquidus should be sufficient for the gas still present to rise but not so long that the flux burns and generates more. Where nitrogen is used, the reduced oxidation allows a slightly lower peak and a more forgiving window, which often reduces the void fraction, though the effect depends on the paste. The cooling rate matters too, since a joint that solidifies quickly can trap gas that a slower one would have released.
Paste, Stencil and Print
The paste design contributes its own volatility, and a paste with a high solvent content will void more than a low residue formulation. The print decides how much air is introduced: a deposit that is aerated by a fast separation, or one that has been reworked, carries bubbles into the joint. On a thermal pad, the aperture design is the standard control, and a grid of smaller openings with a controlled open area produces far less voiding than one large opening, because the gas from each small deposit escapes independently. The stencil thickness and the aperture area together set the paste volume, and a thicker deposit voids more than a thin one.
Component and Surface Effects
The surfaces that the solder wets are part of the story. A pad finish that decomposes at temperature, a component termination with an organic coating, or a nickel layer with poor wetting all release gas or fail to wet fully, and the unwetted area becomes a void when the solder flows around it. A solder mask that outgasses under the profile, particularly a mask that was not fully cured, releases volatiles into the neighbouring joints. The bare board’s own moisture content matters as well, so a board that has been stored in humid air and not baked will void more than a dry one.
When a Void Matters
Not every void is a defect. In many joints a void of modest size has no measurable effect on the mechanical or electrical behaviour, and the acceptance criteria recognise this. A void matters when it reduces the area of the joint below what is needed to carry the current, when it sits at the interface between the solder and the pad or the ball, when it forms a continuous path that weakens the joint under thermal cycling, or when the product’s reliability requirement does not allow it. The location is as important as the size, which is why a specification that quotes a single percentage without reference to position is hard to apply.
Reducing Voiding in Practice
The effective measures in order of impact are usually the profile, the paste and the aperture design. Extending the soak and slowing the ramp cost nothing and often remove most of the problem. Reviewing the paste against the application, and adjusting the aperture on a thermal pad to a grid, address the next largest share. Beyond that, the surface condition, the board’s dryness and the stencil’s cleanliness become the variables to check. Where the voiding remains after all of these, the measurement method itself should be verified, since a different threshold or a different viewing angle can produce a very different number for the same joint.
Measuring and Reporting
The void content of a joint is expressed as a percentage of the joint area, and it is measured from an X-ray image by a threshold that separates the dark solder from the lighter void. Changing the threshold changes the number, so the method, the equipment and the settings should be recorded with the result. Where a specification gives a limit without a method, the two parties can measure the same joint and disagree, which turns an engineering question into a commercial one. Agreeing the measurement before the limit is applied is the step that keeps the discussion technical.
FAQ
What causes voids in solder joints? Trapped gas from flux volatiles, surface contamination, moisture or aerated paste, sealed in when the joint solidifies.
Is any void acceptable? Yes. Modest voids are normal; the concern is size, position and the function the joint performs.
How do I reduce voiding quickly? Slow the ramp and extend the soak so the volatiles can escape before the alloy solidifies.
Why do thermal pads void so much? The large paste volume traps gas; a grid of smaller apertures lets it escape.
Can two inspectors measure the same void differently? Yes, the result depends on the threshold and the viewing angle, so the method must be fixed.
Conclusion
Voiding is a gas escape problem, so control the heat, the paste and the deposit volume before judging the joint. Specify the limit by size and position together. Reflow control belongs to SMT PCB assembly, the joint itself sits within PCB assembly, and the measurement is part of quality management. Voiding control for a new assembly is developed during prototype PCB assembly in 2026.
