Soak Time in Reflow: A Complete Setting Guide
Soak time is the period in a reflow profile during which the board is held below the melting point of the alloy, usually between about 150 °C and 190 °C. Its purpose is to bring the whole assembly to a uniform temperature and to let the flux act on the surfaces before the alloy melts. It is the stage that decides whether every joint on the board reaches the same condition at the same moment.
This guide explains what soak time is for, how to set it from the thermal mass of the assembly, how it interacts with wetting and voiding, and how to verify it with thermocouples on the board. It also covers the limits the paste imposes, which are often tighter than the profile suggests.
What Soak Time Is For
The soak has two jobs. The first is thermal: heat has to travel from the board surface into the interior, and the interior lags. A soak long enough to let the interior catch up removes the gradient that would otherwise persist into the melting stage. The second is chemical: the flux needs time at temperature to reduce the oxide on the pad and the powder.
Both jobs need time, and neither is served by a fast pass through the stage. A profile that rushes from preheat to peak leaves the flux with less than a minute of activity and leaves the interior of a thick board well below the surface temperature when the alloy melts.
How Soak Time Relates to the Ramp
Soak time and ramp rate are two parts of one setting. A steeper ramp leaves a larger gradient, and a longer soak is needed to remove it. A gentle ramp does part of the equalisation work itself, so the soak can be shorter. Moving one without the other changes the profile in a way that is not obvious from either number alone.
The practical approach is to set the soak first, from the thermal mass and the paste, and then set the ramp so that the soak is reached without overshooting. Where throughput pressure pushes the cycle shorter, the time should be taken from the ramp rather than from the soak, because the soak is the stage that pays for itself in joint quality.
Setting Soak Time From Thermal Mass
A thin board with little copper equalises in about 60 seconds. A thick backplane with heavy planes can need 120 seconds or more. The right figure is the one at which the temperature difference across the board falls below the limit the paste and the components allow, and it is measured rather than calculated.
The measurement uses at least four thermocouples: the largest thermal mass, the smallest component, the centre of the largest copper area and a corner of the board. The soak is long enough when the traces converge before the alloy melts. Our reflow profile notes cover the attachment method and the interpretation.
Soak Time and Wetting
Wetting improves with soak time up to a point, because the flux needs time at temperature to reduce the oxide. Beyond that point the flux is consumed and the activator is exhausted, so a longer soak produces a duller joint rather than a better one. The useful range is bounded at both ends and the paste data sheet gives the values.
The effect is easiest to see on a board with a difficult finish. Where the finish is nickel or palladium, the soak has to be long enough for the flux to act, and a short soak produces a fillet with a high contact angle. Where the soak is extended beyond the paste limit, the same joint develops a rough, grainy surface that looks like poor wetting but is caused by flux exhaustion.
Soak Time and Voiding
Voiding falls as the soak lengthens, because the flux volatiles have more time to escape before the alloy seals the joint. This is one of the strongest arguments for a longer soak, particularly on a large thermal pad where the escape path is long and the volume of gas is large.
The benefit is not unlimited. A soak long enough to remove all the volatiles also drives off the solvent that the flux needs to stay active, and the result is a dry joint. The working point is a soak that removes the volatiles without consuming the activator, and it is confirmed by measuring void area on a sample rather than by extending the soak until the profile looks good.
Paste Limits on Soak Time
Every paste has a recommended maximum soak time, and exceeding it is one of the more common profile errors. The limit is shorter for a no-clean paste, which carries less activator, and longer for a water-soluble paste with a strong flux. The figure applies to the time above a stated temperature rather than to the whole cycle.
Where the assembly needs a longer soak than the paste allows, the paste should be changed rather than the limit ignored. A paste with a longer activation window is available for thick assemblies, and the change should be qualified with the same profile measurement and the same void check. Our notes on paste volume and the guidance published by IPC both cover the interaction between flux activity and profile time.
Verifying Soak Time on the Board
Soak time is measured from the thermocouple trace, not from the oven display. The start of the soak is the point at which the trace enters the soak band, and the end is the point at which it leaves. Both are read from the coolest trace, because that is the joint that will still be cold when the others are ready.
The verification should record the soak time for each thermocouple, the delta between them at the end of the soak, and the paste limit for comparison. Where the delta is large, the soak or the ramp has to change rather than the peak. The reflow profile record should carry all of those values for the product and the oven.
Common Mistakes With Soak Time
The first mistake is counting the soak from the oven entry rather than from the point at which the board enters the band. The second is reading it from the hottest trace, which understates the time for the joints that matter. The third is extending the soak to fix a wetting problem that is actually caused by an aged finish or a weak flux.
A fourth mistake is changing the soak without re-measuring the peak and the time above liquidus. The three are linked, and a longer soak usually lowers the peak unless the setpoints are adjusted. Where the soak is extended, the thermal cycling behaviour of the assembly should be re-assessed, because the joints now see a different thermal history.
FAQ
Is a longer soak always better? No. It improves equalisation and reduces voiding up to the paste’s activation limit, and beyond that it exhausts the flux and produces a dry, grainy joint.
Can soak time replace a preheat zone? A long soak in a single zone does much of the work of a preheat stage, but the ramp into it becomes steeper and the gradient across the board grows. Two zones usually give better control for a thick assembly.
Does soak time matter for a small, thin board? Less, because the board equalises quickly and the flux has an easier job. Where the paste is no-clean and the board is small, a short soak is often sufficient.
