Solder Joint Voiding Causes And Acceptance Limits

A void is a bubble of gas trapped inside a solder joint, and it is present in a large fraction of the joints on a production board. In most positions it is harmless, because the joint is still electrically continuous and mechanically adequate. In a small number of positions, on a thermal pad or in a high current path, the same void changes the behaviour of the circuit and becomes a defect.

This article explains where the gas comes from, why the thermal path is the case that matters, how the measurement is made, and how an acceptance limit should be arrived at rather than inherited.

What A Void Is And How It Forms

The gas inside a void comes from the materials that are present when the solder is molten. Flux contains solvents and activators that vaporise, the paste contains a vehicle that has to escape, and the surfaces being joined may carry moisture or an adsorbed film. Some of that gas escapes through the molten solder, and some of it is trapped as the joint solidifies.

The amount that is trapped depends on how quickly the joint solidifies and on how easily the gas can reach the surface. A large area joint under a thermal pad solidifies from the edges inward, and the last region to freeze is in the middle, which is where the gas collects. The geometry therefore determines whether a given quantity of gas becomes one large void or is dispersed as many small ones.

Sources Of Volatiles

The paste is the largest contributor and the easiest to control. A paste with a high solvent content produces more gas, and a profile that ramps too quickly vaporises the solvent faster than it can escape. The preheat stage exists to drive the volatiles off before the solder melts, and a preheat that is too short or too cool leaves the work to the reflow stage where the solder is already liquid.

The board and the components contribute as well. A laminate that has absorbed moisture releases it during reflow, and a plated through hole that has been left in a humid atmosphere carries an adsorbed film. The moisture sensitivity of the components is a related subject, and the same reasoning about absorbed water applies, as described for moisture sensitivity. The solder mask also releases volatiles if it is not fully cured before assembly, which is one reason the cure schedule matters.

Why Thermal Vias Change The Picture

A thermal pad on a power device is usually perforated with vias that connect it to an internal or a bottom plane. Those vias are open holes through the paste, and they give the gas somewhere to go instead of being trapped under the pad. A pad with an adequate via pattern often shows much less voiding than one without, which is why the via pattern is part of the thermal design rather than only a thermal feature.

The vias have to be handled carefully in the stencil, because an open via wicks paste away from the pad and leaves a starved joint. Vias that are tented or plugged on the underside trap gas, while vias that are open allow it to escape but take paste with it. The compromise is to keep the vias small and closely spaced, and to control the paste volume so that a small amount is lost into each. The design of those features is described for via in pad.

Measuring Voiding By X-Ray Inspection

Voids are found by X-ray inspection, because the solder attenuates the beam more than the gas does and a void appears as a brighter region in a transmission image. The software calculates the total void area as a percentage of the joint area, and it may also report the largest single void and the number of voids.

The measurement has limits. A two dimensional image of a three dimensional joint cannot distinguish a void near the top from one at the bottom, and a void that is not in the beam path of the slice being examined is missed. The percentage that the software reports is therefore a projection rather than a volume, and two systems can report different values for the same joint. Any limit has to be tied to the equipment and the technique that produced it.

Acceptance Limits And Their Basis

An acceptance limit should be derived from the function of the joint rather than adopted from a table. For a thermal pad the question is how much the void reduces the thermal path, and that can be answered by measuring the temperature rise of a device at different void percentages. For a current carrying joint the question is how much the void raises the resistance and the local temperature.

For a joint that carries neither heat nor a significant current, the limit can be generous, because the void does not affect the function. Setting a single percentage for the whole board is a common practice and it is a blunt instrument: it rejects good joints in positions where the void is irrelevant and it may pass a joint where the void matters. A position based limit is more work to define and it is the correct approach for a product that has to be reliable.

Reducing Voiding In Practice

The first lever is the profile. A preheat that is long enough to drive off the volatiles before the solder melts removes most of the gas before it can be trapped, and a soak stage that holds the board just below the melting point is the standard way to achieve it. The second lever is the paste, since a formulation with a lower solvent content and a more gradual flux activation produces less gas.

The third lever is the geometry. A pad that is broken into smaller areas by mask bars, or that has a well designed via pattern, gives the gas fewer places to be trapped. The fourth is the stencil, because the correct paste volume leaves the joint full without an excess that has to escape as gas. The placement of the part over the paste is also relevant, and the mechanisms that move a component during reflow are described under component shift and in the acceptance framework of quality characteristics.

Additional Considerations for This Build

Practical attention to flux volatiles pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating flux volatiles explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, thermal via is the item that decides how the rest of the board is arranged. 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.

FAQ

Is a voided joint always a defect? No. Most voids have no effect on the function of the joint. It becomes a defect when the void reduces a thermal path or a current path enough to change the behaviour of the circuit.

Why does the same joint measure differently on two X-ray systems? The reported percentage is a projection of a three dimensional feature, and it depends on the resolution of the system and on the technique used. The limit has to be tied to the equipment.

Do more vias in a thermal pad always reduce voiding? They give the gas a path out, but they also wick paste away. The via size and the stencil have to be balanced, or the joint becomes starved instead of voided.