Reflow Cooling Rate and Solder Joint Structure

A solder joint solidifies in a second, and what happens in that second decides the grain structure of the alloy. A fast cooling produces a fine grain and a slow one produces a coarse grain, and the difference shows up in the mechanical behaviour of the joint rather than in its appearance.

What the Cooling Rate Controls

The alloy solidifies by nucleation and growth, and both are time dependent. A fast cooling produces many nuclei and little growth, which gives a fine structure; a slow cooling produces the opposite.

The grain size affects the strength, the ductility and the way the joint behaves under thermal cycling. It also affects the appearance, which is why a joint that was cooled slowly looks different from one that was cooled quickly. Our joint criteria notes define the appearance that the process is held to.

Intermetallic Growth

At the interface between the alloy and the copper, an intermetallic layer forms as soon as the alloy is molten. The layer grows while the joint is above the melting point, so the time above liquidus controls its thickness.

A thin layer is a good bond and a thick one is brittle and prone to cracking at the interface. The balance is set by the profile rather than by the alloy alone. Our mixed thermal mass notes describe a board where the time above liquidus differs between joints on the same panel.

Cooling Rate and the Profile

The cooling rate is set by the difference between the board and the ambient and by the gas flow. A fast ramp down increases the thermal gradient across the assembly, which is a stress on the components and on the laminate.

The practical range is a compromise, and it is normally specified as a maximum ramp down rather than as a target. Where the joint reliability is critical, the ramp is measured on the board rather than read from the oven. Our profile verification notes describe how that is arranged.

Lead free alloys have a higher melting point and a different solidification behaviour from tin lead. A tin silver copper alloy solidifies over a range, and the last liquid to freeze forms a network at the grain boundaries that behaves differently from the bulk.

That is one reason the cooling rate matters more for lead free than for tin lead. A slow cooling allows that network to become more continuous, and the joint is more susceptible to the thermal cycling failures that occur at the boundaries.

The structure is examined by polishing a section and etching it, and the grain size is measured against a reference. That is a destructive test, so it is applied to a sample or a coupon rather than to a product.

The measurement is worth having when a change has been made, because a change to the profile or to the alloy moves the structure before it moves a test result. Our thermal cycling notes describe how the change is then qualified over life.

The cooling rate is one of the few profile parameters that is often left at a default, because the joints look correct at almost any setting. Where the product will see a thermal cycle in service, the parameter deserves the same attention as the peak temperature.

A joint that is cooled too slowly is not a defect at inspection and it may fail in the field, which is the pattern that makes the parameter worth controlling deliberately.

Process Control and Verification

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. 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.

Does a fast cooling make a stronger joint? It produces a finer structure with different properties, and the effect on a specific design is measured by cycling rather than assumed.

Is the cooling rate part of the profile? It is, and it is usually specified as a maximum ramp down rather than as a target figure.

What does gopcb provide for joint structure control? We provide profile development with the cooling rate measured on the board, sections that show the grain structure and the intermetallic thickness, thermal cycling that qualifies a change, and records that tie the structure to the profile and the alloy.

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.