Reflow Profile Design Guide

A reflow profile is the temperature history that a board experiences as it passes through the soldering oven. The profile has to melt the solder at every joint, activate the flux without burning it, and complete the alloy without cooking the components. It is a curve with four zones, and every one of them is a compromise between the parts that heat quickly and the parts that heat slowly.

What the Profile Has to Achieve

The profile has to bring every joint above the melting point of the alloy for long enough to form a proper intermetallic layer, and it has to do so without exceeding the temperature limit of any component on the board.

It also has to activate the flux. The flux removes the oxide from the surfaces and keeps it away while the solder melts, and it does that within a temperature window of its own. A profile that crosses that window too fast leaves oxide, and one that lingers too long consumes the flux before the solder melts.

The third requirement is the health of the board itself. The laminate, the mask and the finish all have a temperature limit, and a profile that is too hot will discolour the mask or drive moisture out of the laminate and cause delamination.

The profile is therefore a compromise between the coldest joint on the board and the most sensitive component, and the spread between them is what the profile has to accommodate.

The Four Zones

The first zone is the preheat, where the board is brought up from room temperature to around a hundred and fifty degrees. The ramp rate is limited by the components, because a fast ramp causes thermal shock and can crack a ceramic capacitor.

The second zone is the soak, where the board is held for a period while the flux activates and the temperature equalises across the assembly. The soak is where the difference between a heavy connector and a small resistor is reduced.

The third zone is the reflow, where the temperature rises above the liquidus of the alloy and the solder melts. The peak is reached at the end of this zone, and the time above liquidus is measured across it.

The fourth zone is the cooling, which is often neglected. A slow cool gives a coarse grain structure and a dull joint, while a fast cool gives a bright joint and a finer structure. The rate is limited by the thermal shock that the components can take.

Soak and Flux Activation

The soak is the zone that is most often shortened to increase throughput, and it is the zone that causes the most defects when it is wrong. Its purpose is to bring the whole assembly to a uniform temperature before the solder melts.

A soak that is too short leaves the heavy parts cold when the small parts are already at the peak. The heavy part then sees a shorter time above liquidus and forms a poor joint, while the small part is overheated.

A soak that is too long consumes the flux, which is a volatile mixture of solvents and activators. Once the activator is gone the oxide reforms and the joint becomes dull, grainy and weak.

The correct soak is found by measuring the profile on a board that carries the real thermal mass, not on a bare coupon. The measurement should be repeated on the heaviest and the lightest area of the board, since those two are the extremes that the process has to satisfy.

Peak Temperature and Time Above Liquidus

The peak temperature is set by the alloy: a lead free alloy melts near two hundred and seventeen degrees and a tin lead alloy near a hundred and eighty three. The peak is normally between twenty and thirty degrees above the liquidus, which gives a margin for the coldest joint on the board.

The time above liquidus is the period during which the solder is molten, and it has to be long enough for the alloy to wet the surfaces and to form the intermetallic layer. A time that is too short gives a cold joint, and one that is too long grows the intermetallic layer until the joint becomes brittle.

The peak is limited at the top by the components. A plastic connector, an electrolytic capacitor and a battery each have a maximum temperature that is often below the peak that the solder requires, and those parts have to be excluded from the reflow or protected.

The solder defect guide lists the joint appearance that corresponds to each of these deviations, and it is the quickest way to read a profile from the finished board.

Thermal Mass and Component Variation

Thermal mass is the reason a single profile cannot be perfect. A large ground plane, a heavy connector and a metal shield all take longer to heat than a small resistor, and the difference can be tens of degrees at the moment the solder melts.

The designer can reduce the spread by balancing the copper across the board, by placing the heavy parts away from the light ones, and by using thermal reliefs on the pads that connect to planes. A board that is well balanced is easier to reflow than one that is not.

The fixture or the pallet that carries the board also affects the profile, because it absorbs heat and shadows the board from the convection. A pallet that is used for one product and not for another will give a different profile for the same oven settings.

The ovens themselves vary along the belt, so the profile is measured at the position where the product runs. A profile taken on the centre line and a profile taken on the edge can differ enough to change the process window.

Measurement and Troubleshooting

The profile is measured with a thermocouple attached to a real board with a high temperature adhesive, and the board is run through the oven with the production belt speed. The measurement is repeated whenever the product, the oven or the pallet changes.

The common faults are readable from the curve. A cold joint with a grainy surface points to a peak that is too low, a dull joint with burnt flux points to a soak that is too long, and a warped board points to a cooling rate that is too fast.

The mask and the finish interact with the profile as well, since a board with a HASL finish starts with a different surface than one with an ENIG finish, and the flux has to cope with that difference.

Where a defect is found, the AOI check tells where on the board it occurs, and the position is often the clue: a defect that follows the heavy parts is a thermal mass problem rather than an oven setting.

FAQ

Why does the soak matter if the solder melts in the reflow zone? Because the soak equalises the temperature across the assembly and activates the flux. Without it the heavy joints are cold while the light ones are overheated.

How long should the time above liquidus be? Long enough to wet the surfaces and form the alloy, and short enough to keep the intermetallic layer thin. The vendor of the paste quotes a window, and the board should be measured inside it.

Can one profile cover several products? Only if the thermal mass and the component limits are similar. A different board usually needs its own measurement, even in the same oven.