Brief Reflow Profiles Analysis

TYPES OF PROFILES

Two types of reflow profiles exist: soak profile, or ramp/soak/spike profile and ramp-to-peak, also called ramp-to-spike as shown below in Fig.1 and Fig. 2 for conventional Lead-free application.

Fig.1                                        Fig.2

Four Zones in Profiles

Zone 1. Preheating stage

This stage is designed to precondition the PCB assembly prior to actual reflow, removing solvent and reducing thermal shock to the PCB assembly. The ramping rate is between 0.5-2.0℃/second, the upper limit temperature residues between 150-175℃/second based on Fig.1.

Zone 2.  Preflow phase

During this stage the flux activator removes any existing surface oxide from the component leads and PWB pad finishes, as well as solder powder.

The soak temperature is controlled within a tight range (see Figure 1) for a specified time. This “plateau” in the reflow profile allows the thermal gradient across the PCB to equilibrate prior to reflow, it is useful for large PCB and diversity of component size.

Zone 3.  Reflow

The reflow of the solder alloy will melt the solder metal and form mechanical and electrical connection between PCB and component via the formation of intermetallic compound.

Two critical parameters are: peak temperature and TAL (time-above-liquidus). The peak temperature is generally 20-30°C above the liquidus and TAL are normally within 30-90 seconds

Zone 4. Cooling

The rapid cooling phase will lead to a fine grain structure. The limiting factor for the maximum cooling rate is the stress that is exerted on the solder joint if the rate is too fast due to the different CTE of components involved. A cooling rate of ~4°C/second is normal.