Questions often arise about the meaning and purpose of taking a ramp profile and a soak profile in reflow soldering ovens.
According to Mark Stansfield, managing director of SolderStar Ltd. in Lancashire, United Kingdom, all reflow profiles can be simplified as three stages: ramp, soak and spike (or reflow). SolderStar manufactures temperature profiling systems used in reflow ovens.
The ramp profile or ramp-to-spike (RTS) is a profile that ramps up from ambient temperature to peak temperature at a constant heating rate. The heating rate in an RTS profile is constant through each stage. The ramp, or heating rate, exhibited on the assembly by each zone of the reflow oven is the same, typically 0.9 to 1.8°F/sec (0.5 to 1°C/sec).
The ramp profile commonly is specified in solder-paste datasheets, but it cannot always be used. If the assembly being soldered has a number of larger thermal mass components, it may be necessary to use a soak-style profile, commonly referred to as ramp-soak-spike (RSS), which uses different heating rates in the different profile stages.
A simple RTS profile may not transfer enough heat into the assembly to equalize the temperature through all thermal masses when the final spike or reflow stage is reached. If there is a large temperature variation at the reflow stage, there will be problems with establishing peak temperature and time above liquidus parameters within their given working range.
The difference between the coolest and hottest parts of the assembly at the reflow stage is known in the profiling world as ΔT (called “delta T”). To minimize ΔT, use the ramp-soak-spike formula in which a faster heating rate usually is used in the ramp phase, then reduced in the soak phase, and increased in the spike phase.
The result is to drive more heat into the assembly in the profile’s first stage -- be careful not to exceed the maximum rates of the components -- and then soak the assembly at a slower rate to allow the assembly temperatures to equalize. Equal temperatures equal low ΔT, at which point the operator can reflow the assembly successfully within all given limits specified by the paste manufacturer.