Reflow Cooling Rate Effects on Joints

The cooling part of the reflow profile is treated as a detail, and it is not. The rate at which a joint passes from liquid to solid determines the grain structure of the alloy, the growth of the intermetallic layer and the stress that remains in the assembly.

Most specifications state a maximum cooling rate, and some state a range, because both extremes have consequences.

What Happens During Cooling

As the alloy solidifies, the grains form and grow, and the size of the grains depends on how quickly the temperature falls. A fast cool produces a finer structure with better mechanical properties in most alloys.

A slow cool allows the grains to grow and the intermetallic layer to thicken, both of which reduce the fatigue life of the joint. The visible sign is a duller, grainy surface.

Fast Cooling and Thermal Shock

Cooling very quickly creates a temperature difference across the assembly, which produces stress in the components and in the laminate. A ceramic part with a large thermal gradient can crack.

The risk is highest on a heavy component or on a large ceramic body, which is why the cooling rate is limited rather than maximised.

Effect on the Intermetallic Layer

The intermetallic layer continues to grow while the joint is above the solidus and, more slowly, after it solidifies. The thickness is a balance between the time above liquidus and the cooling rate.

An excessively thick layer is brittle and is the site where cracks begin under thermal cycling. The cooling rate is one of the two levers that control it, the other being the time above liquidus.

Effect on Warpage

The assembly cools from the reflow temperature to room temperature with the components and the laminate contracting at different rates. The rate of cooling changes the temperature difference across the board and therefore the stress that is left in it.

A slow cool reduces the differential but leaves more time for the intermetallic to grow, so the balance has to be chosen rather than assumed. The warpage consequence is described with board warpage.

Measuring the Cooling Rate

The rate is derived from the profile between the peak and the point where the joint solidifies, and it is expressed in degrees per second. It is read from the same thermocouple that measured the peak.

The measurement should be taken on a joint rather than on the board surface, since the two cool at different rates.

How the Oven Controls It

The final zones and the exit of the oven determine the cooling, and some ovens allow the rate to be adjusted with a cooling zone or with a controlled ambient. Where the oven has no active cooling, the rate is set by the exit temperature and the ambient.

The practice described for zone verification applies here as well, since a change in the ambient or in the extraction changes the cooling without any setpoint changing.

Interaction With the Paste Specification

The paste supplier specifies both the time above liquidus and the cooling rate, and the two are set together for the alloy. Running outside either of them produces joints that look acceptable and behave differently.

The profile should be compared with both limits rather than only with the peak, which is the parameter most often quoted.

Records

The cooling rate should be recorded with the profile for each product, since it is part of the specification. Where a joint fails in thermal cycling, the recorded rate is part of the investigation.

It belongs with the process evidence described for manufacturing processes.

Process Control and Verification

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.

FAQ

Faster cooling is better for the joint, is it not? Up to a point. Beyond it, the thermal gradient across the assembly causes damage.

Which is more important, the peak or the cooling rate? Both are specified, and a profile that meets the peak while missing the cooling limit is out of specification.

Can the cooling rate be adjusted? Where the oven has a cooling zone it can be, and otherwise it follows from the exit conditions.

Why is the joint dull after a slow cool? Because the grains are larger, which is the visible sign of the slower solidification.