Reflow Profiles for Mixed Thermal Mass: Soak, Peak and Measurement

A board with a heavy connector at one end and 0402 chip parts at the other does not have one reflow temperature; it has a range, and the range is set by the difference in thermal mass. The profile has to bring the heaviest joint to temperature without cooking the lightest component, and the measurement is what shows whether it does.

Why One Board Behaves Like Several

A connector body of a few grams absorbs heat slowly and releases it slowly, while a chip component of a few milligrams follows the oven air almost exactly. During the ramp the two are twenty to thirty degrees apart, and the difference narrows during the soak as the heavy part catches up.

That gap is the reason a soak exists on mixed assemblies and not on uniform ones. The soak holds the board at a temperature below melting until the heavy parts have caught up with the light ones, so that the whole assembly crosses the liquidus within a short window instead of over a long one. The size of that gap also depends on the board, because a thick panel with heavy copper spreads heat sideways and narrows the difference, while a thin one widens it.

Measuring the Extremes

The thermocouple is attached to the joint that will be slowest and, on a second channel, to the component that will be fastest. A heavy joint is instrumented with high temperature adhesive or a soldered bead, and a small part with a fine wire bonded to a pad, because the attachment itself changes the thermal path.

Attachment by adhesive alone adds insulation and makes the reading lag, so the couple is usually fixed with a small amount of high temperature solder or with a thermally conductive cement, and the technique is kept the same between measurements. A profile measured with a different attachment is a different profile, even when the oven settings have not changed; our profile verification notes describe the method.

Soak Zone and Equalisation

The soak is the plateau between about 150 and 180 degrees Celsius, and its length is chosen to equalise the board rather than to activate the flux alone. A soak of 60 to 120 seconds is typical for a mixed assembly, and lengthening it reduces the spread between the heavy and the light joints.

A soak that is too long dries the paste and consumes the flux before the alloy melts, which produces poor wetting on the parts that were ready early. That is the trade the soak length has to settle, and the answer is found from the two thermocouple traces rather than from the oven’s zone settings. The flux activity window is a second constraint, since a flux that activates at 160 degrees has to reach that temperature on every joint before the alloy flows, and the soak length controls that as well.

Peak Temperature and Its Ceiling

The peak temperature for a lead free alloy is usually set between 240 and 250 degrees Celsius at the joint, and the ceiling comes from the components rather than from the solder. A plastic connector rated to 260 degrees has no margin at a 250 degree peak once the local variation is added.

The peak is therefore set by the most sensitive part on the board, and the heaviest joint has to reach its own minimum within that ceiling. Where the two cannot both be satisfied, the answer is a change to the assembly rather than to the profile, such as moving the connector to the second side or to a selective process.

Time Above Liquidus

The time above liquidus is the period during which the alloy is molten, and it has to be long enough for the joint to form a fillet and short enough to limit intermetallic growth. A window of 60 to 90 seconds covers most lead free assemblies, with the longer end used for heavy joints.

A short time above liquidus produces joints that look formed but have not wetted the full pad, while a long one grows the intermetallic layer and makes the joint brittle in later thermal cycling. The value is read from the thermocouple trace rather than from the recipe, because the heavy and light channels spend different times above the line. A useful discipline is to plot both traces on the same axes and mark the liquidus, because the point at which the second trace crosses it belongs to the heavy joint rather than to the board as a whole.

Ramp and Cooling Rates

The ramp rate is limited by the components and by the paste, and a figure of two to three degrees Celsius per second is common, with a slower rate used where a large ceramic part is present. The cooling rate is limited as well, because a fast quench can crack a ceramic body or a glass sealed package.

Cooling is often the forgotten half of the profile, since the board leaves the last zone with no control at all unless the oven has a cooling section. Where the first zone after the peak is hot, the effective cooling rate is set by the conveyor speed and nothing else, and the measurement has to extend past the peak to show it.

Small Parts in a Heavy Assembly

A 0402 chip sitting beside a connector sees the highest rate of change and the highest peak, and both are what damage it. Termination cracking, tombstoning and de-wetting all appear on the lightest parts of a mixed board, and they are easy to mis-attribute to the paste. Where a light part has to sit near a heavy one, the difference can be reduced by adding a copper pad or a heat spreader to the assembly, which is a design measure rather than a profile one.

Positioning the small parts away from the hottest zones of the oven is not possible, so the protection comes from a lower peak and a gentler ramp, plus the obvious measure of not placing a heavy part directly beside a small one when the layout allows a choice.

Verification Routine

The profile is verified at first article, after any change to the oven, the paste, the board or the component mix, and on a schedule that follows the oven’s maintenance. The verification uses a populated production board rather than a bare coupon, because the components are what create the spread.

The result is compared against the window rather than against the previous profile, and the figure recorded is the joint temperature of the slowest thermocouple. Our zone settings notes describe how the oven is adjusted to move the profile into the window once the measurement says it is outside.

Records and Process Control

The record carries the oven and zone settings, the conveyor speed, the thermocouple positions, the measured peak and time above liquidus for each channel, and the paste lot. Without the positions, a profile cannot be reproduced even when the graph looks the same.

A change to the assembly that adds a heavy part is a profile change, and the record is what makes that visible before the first batch is run rather than after. Our solder joint acceptance criteria describe what the joints should look like when the profile is correct, which is the second half of the same verification.

FAQ

Can one profile serve two products? Only when the thermal masses and the component ratings are close. Two boards with different heavy parts need separate profiles or separate recipes, and the thermocouple data is what shows the difference.

Does a longer soak always improve the result? It equalises the board, and beyond a point it dries the paste and consumes the flux. The useful length is the one that brings the two traces together before the liquidus rather than the longest that the oven can deliver.

How is a heavy joint measured if it is under a connector? By instrumenting the same connector on a sample board, or by drilling a hole in a sacrificial part and cementing the couple into the barrel. The reading is then applied to the production parts, since the geometry is the same.