Ramp is defined as a change in temperature over time, expressed in degrees per second. A common value is 4°C/sec, though many component and solder paste manufacturers insist on 2°C/sec. Who ever said reflow was easy?
Change in temperature (Δ t) with Respect to your Components
Many components have a specification where the rise in temperature should not exceed a specified temperature per second, such as in the 2°C per second example already given. The solder paste manufacturer will require comparable upper limits. Tinned components (containing solder) and solder, in this case, behave in a similar ways. Rapid evaporation of the flux contained in the solder paste can lead to defects, such as, but not limited too, lead lift, tombstoning and solder balls.
Possible Moisture in your Components
Rapid heat introduction to a component can lead to steam generation within the component if the moisture content is too high. Microcracks will likely appear under these conditions.
In the soak segment of the profile, the solder paste will approach a phase change. The amount of energy introduced to both the component and the PCB will approach equilibrium. In this stage, most of the flux should be evaporated out of your solder paste, but this can also depend on the type of paste.
The duration of the soak varies by solder paste manufacturer’s specs. The mass of your PCB also factors into the required duration of the soak. Again, a balance of heat transfer and flux evaporation must be achieved. Too rapid of heat transfer can cause solder splattering and the production of solder balls, bridging and other defects. If the heat transfer is too slow, the flux concentration will remain too high and adversely impact the wetability of the bond pads, resulting in cold solder joints, voids and incomplete reflow.
After the soak segment, the profile enters the ramp to peak segment of the profile, which is a given temperature range and duration of time that exceeds the melting temperature of the alloy, as determined by the solder paste spec. A successful profile will typically range in temperature up to 30°C higher than liquidus, which is ~183°C for eutectic and ~ 217°C for lead-free. It is VERY important throughout the peak segment to consider the temperature tolerance of your components.
The final area of the RSS Profile is the cooling section. A typical specification for the cool down is not to exceed -6°C/sec (falling slope). Check the spec of your paste, but don’t forget to also check the specs of your components. Certain packages may be sensitive to rapid cooling.