Solder Joint Voiding: Causes, Measurement and Acceptance
A void is a gap inside a solder joint where the metal did not form. It looks alarming on an X-ray image, it is present in almost every joint to some degree, and the difficult part is knowing which ones matter.
Where Voids Come From
The commonest source is flux that volatilises during reflow and is trapped as the solder solidifies around it. The gas has to go somewhere, and where the joint is enclosed by a component pad the only route out is through the still liquid solder.
The second source is outgassing from the board itself, including moisture released from the laminate and residues from the fabrication process. This is why baking boards before assembly sometimes reduces voiding.
The third is the paste itself. A paste that has been exposed, or that contains an excess of flux, releases more volatiles. So does a paste printed in a thick deposit, because there is more flux in the volume.
Which Voids Matter
A void matters when it reduces the load bearing area, the thermal path or the electrical path below what the design requires. For a mechanical joint, the load bearing area is what counts, and a void at the edge of a fillet is usually irrelevant.
For a thermal joint, the position of the void is decisive. A void at the centre of a thermal pad sits directly in the path and raises the thermal resistance, while the same void area distributed around the periphery of the pad does much less.
For an electrical connection, a void that spans the full cross section is an open circuit, and one that merely reduces the cross section is an increase in resistance. The resistance change is usually small compared with the tolerance of the joint itself.
How Voids Are Measured
X-ray inspection shows voids as lighter regions within the joint, and the software computes the ratio of void area to joint area. The result depends on the projection angle and on the thresholds used, so a percentage quoted without the method is not comparable with another.
A cross section shows the void in the plane of the cut and gives a direct measurement of its size relative to the joint. It samples a single slice, so it may miss a void or find one that the whole joint only partly contains.
Scanning acoustic microscopy images the interface rather than the bulk, and it is the better tool for finding delamination of the joint from the pad. Our inspection notes describe which technique suits which defect.
Acceptance Criteria in Practice
Standards set limits in terms of the projected void area, and the limits differ for thermal pads, for ball grid array joints and for through-hole fillets. The limits are guidance, and the governing criterion is the function of the joint.
For a joint carrying current, the criterion is that the remaining cross section carries the current with acceptable temperature rise. For a joint carrying heat, it is the resulting thermal resistance. For a joint carrying load, it is the mechanical strength.
Where a customer specification quotes a void limit, that limit applies. Where it does not, the sensible approach is to define one from the function and to apply it consistently, which is more defensible than reacting to each image.
Reducing Voiding on the Line
The reflow profile is the primary lever. A longer soak allows the volatiles to escape while the solder is still liquid, and a shorter time above liquidus reduces the opportunity for the gas to be trapped as the joint freezes.
The stencil aperture design for a large pad is the second lever, since dividing the deposit into a grid reduces the volume and provides escape routes. The same measure reduces the total flux present.
Paste selection matters, and a paste formulated for reduced voiding on large pads is worth testing where the voiding is a functional problem. Our solderability notes describe how the paste and the finish interact.
The Role of the Board and the Finish
Moisture in the laminate releases gas during reflow, and the amount released depends on how the board was stored. Baking before assembly removes it, at the cost of accelerating the growth of the intermetallic layer on the finish.
The surface finish affects how the solder spreads and thus how the gas escapes. A finish with good wetting allows the solder to flow around the gas and out to the edge, while poor wetting traps the gas under the paste. Our surface finish notes describe the behaviour of each type.
A solder mask defined pad constrains the joint to the aperture, which changes the direction in which gas can escape compared with a copper defined pad. The choice should be consistent across the board.
When Voiding Indicates Something Else
A sudden increase in voiding on a stable process usually indicates a change in material rather than in the profile. A new paste lot, a board lot with a different finish condition, or a component with a different pad geometry all produce the change.
Voids concentrated in one region of the board point at the thermal profile being uneven, which may be a board design issue rather than a process one. Large copper areas take longer to reach temperature, so parts near them see a different profile from parts in the middle of the panel.
Voids that appear only after thermal cycling were probably present as gaps at the interface before cycling and are a joint formation problem rather than a gas entrapment one.
Communicating the Result
Report the measurement method with the number, because an unqualified percentage cannot be compared with a specification. State the system, the magnification, the threshold used and whether the figure refers to a single joint or to the worst case on the board.
Where the result exceeds a limit, state which joint and where it is, since the same void in a mechanical joint and in a thermal joint have different consequences. Our solder defects notes describe the accompanying failure signatures.
Keeping the images with the lot record makes a trend visible, which is what turns a series of individual judgements into a process control.
Process Control and Verification
On a design of this kind, acceptance criteria is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
FAQ
Is any void acceptable? Yes. Small voids are present in normal joints, and the acceptance question is whether the remaining joint meets the electrical, thermal and mechanical requirement.
Does baking always reduce voiding? It removes moisture related voiding and does nothing for flux related voiding, so the effect depends on which source dominates in the particular process.
What does gopcb provide for void control? We provide X-ray void measurement with the method stated, sections that show the void in the joint, profile development where voiding is functional, and paste and stencil recommendations for large thermal pads. Where a void limit is specified, we report against it; where it is not, we propose one and explain its basis.
