Solder Void Formation and Vacuum Reflow
Voids in a solder joint come from gas that cannot escape before the solder solidifies. The gas can come from the paste, from the plating or from the surfaces, and the way to remove it is to make it easier to leave.
Where the Gas Comes From
The flux in the paste volatilises during the ramp. Most of it escapes through the paste itself, and some is trapped under the component where the gap is small and the escape path is long.
Moisture absorbed by the parts and by the board becomes steam at reflow temperature. Outgassing from the plating and from the mask adds to it. Our component storage notes describe the moisture that should have been removed before the process.
Where the Voids Sit
A void at the edge of a joint reduces the visible fillet. A void in the middle of a thermal pad reduces the area that carries heat, and its position matters more than its total volume.
A large single void under a thermal pad is worse than the same volume distributed as many small ones, because the heat path is blocked rather than merely narrowed.
Design Measures
The escape path can be improved by splitting a large pad into apertures with gaps between them, so that the gas does not have to travel across the whole pad.
Where a via opens into the pad, the paste can wick into it and the gas can escape the same way. The via treatment should be decided with the paste volume. Our via fill notes describe the options.
<img src="https://www.gopcba.com/wp-content/uploads/2026/05/smart-logistic-PCBA.jpg" alt="X-ray image of voids under a thermal pad” />
Paste and Profile Measures
The paste choice matters because the flux chemistry and the metal load decide how much gas is produced and how easily it escapes. A paste with a higher metal load produces less residue and less gas.
The profile decides whether the gas has time to leave. A slower ramp and a longer time above liquidus allow more escape, at the cost of more oxidation and more intermetallic growth. Our profile notes describe how the balance is measured.
Vacuum Reflow
Vacuum reflow applies a reduced pressure while the solder is molten, so that the trapped gas expands and rises out of the joint. It is applied for a short period after the liquidus and before solidification.
The result is a large reduction in void area and it requires equipment and a recipe per product. The vacuum must be released before the solder freezes, or the joint is disturbed. Our thermal notes describe how the result is measured by thermal performance.
Measurement of Voids
The measurement is made by X-ray, and it is usually expressed as the void area as a percentage of the joint area. The method of measurement should be defined, because the figure changes with the threshold and the image processing.
Where the criterion is a percentage, the distribution of void sizes should be reported as well, because an average hides a single large void. Our X-ray notes describe what the image can resolve.
Verification
The verification is the X-ray measurement on a sample from the production process, expressed as a distribution rather than a single figure, and the thermal performance of the finished assembly.
Where the void level rises, the paste batch, the profile and the component storage should be checked in that order. Our quality notes describe how the result is recorded.
Process Control and Verification
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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.
Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
FAQ
Is a void free joint achievable? Not reliably. The aim is to keep the void area below the level at which the thermal or mechanical performance is affected.
Does vacuum reflow suit every product? No. It costs cycle time and equipment, and it is justified where the void level affects the function, such as under a power device.
What does gopcb provide for void control? We provide paste selection for metal load and flux chemistry, aperture splitting to shorten the escape path, via treatment decisions, profile development within the flux window, vacuum reflow with a product specific recipe, and X-ray measurement reported as a distribution against a defined criterion.
