Thermal Pad Voiding Control in Power Assembly

A large thermal pad under a power device is a joint with a very small gap and a very large area, and the flux that has to escape from the middle of it has a long way to go. The voids that result are not a paste defect; they are a consequence of the geometry and the profile.

Why Large Pads Void

The flux in the paste turns to gas as the alloy melts, and on a small pad the gas leaves at the edge in the time the joint is molten. On a large pad the gas in the middle has to travel several millimetres through a gap that is closing.

The result is a ring of sound joint at the perimeter and a void fraction in the centre, which is also where the thermal path is shortest and where the void matters most. Our paste volume notes describe how the deposit is arranged for that geometry.

Deposit Geometry and Flux Content

The paste deposit on a large pad is normally split into a grid of smaller openings rather than one large aperture. The gaps between them give the gas a path out and reduce the distance it has to travel.

A lower flux content in the paste also reduces the gas that has to escape, at the cost of less activity and a narrower process window. The two adjustments are usually made together and verified by measurement.

Profile and the Time Available

A longer time above liquidus gives the gas more time to leave and also lets more oxide form and more flux evaporate. The window is therefore narrower than it is on a small joint, and it is established by measuring the void fraction rather than by reasoning.

A soak that brings the board to a uniform temperature before the final rise reduces the difference between the pad and its surroundings, which matters because the pad is usually connected to planes that draw heat away. Our profile notes describe how the measurement is arranged on that geometry.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/Gopcba_画板-1-副本.png" alt="Void fraction measured by X-ray” />

Vacuum Assisted Reflow

Vacuum reflow applies a reduced pressure while the alloy is molten, which pulls the gas out of the joint rather than waiting for it to diffuse. It reduces the void fraction substantially on large pads and it requires an oven that can be evacuated.

The vacuum has to be applied at the right moment, because a vacuum applied before the alloy is fully molten has no effect and one applied after it has solidified has less. The timing is part of the profile. Our X-ray notes describe how the result is measured.

Measurement and Acceptance

The void fraction is measured by X-ray on a sample, and the acceptance is a fraction of the pad area rather than a count of voids. A single large void in the centre is worse than several small ones distributed across the pad.

Where the joint carries current as well as heat, the acceptance also considers where the voids are. A void at the perimeter reduces the current path and a void at the centre reduces the thermal path, and the two are judged differently. Our joint criteria notes set out how the acceptance is written.

The pad and the component base have to be flat enough for the gap to be uniform. A component with a domed base produces a joint that is thin at the centre and thick at the edges, and the gas path is closed at the perimeter where it would otherwise leave.

The same applies to the board. A local bump in the laminate changes the gap across the pad and produces a void that moves with the board rather than with the profile.

Practical Sequence for a New Design

The sequence that works is to start with a split deposit, develop the profile on a test board, measure the void fraction, and add vacuum only if the figure cannot be reached by the profile. Adding vacuum first makes the other variables harder to see.

Recording the deposit geometry, the profile and the measured void fraction together makes the result reproducible on the next batch. A void figure without those three cannot be compared with anything.

Process Control and Verification

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Is a voided thermal pad always a defect? It reduces the thermal path, and the acceptance is a fraction defined by the design rather than a zero.

Does more paste reduce voiding? It raises the void volume because there is more flux to escape, so it usually makes the result worse on a large pad.

What does gopcb provide for thermal pad joints? We provide split deposit geometry that gives the gas a path, paste and profile developed together and verified by X-ray, vacuum reflow where the target requires it, void fraction measured against a pad area acceptance, and records that tie the result to the deposit and the profile.

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.