Matching a reflow profile to a solder paste spec is a required skill for an SMT process engineer. In light of this, let’s do an example. Assume we want to start using Indium Corporation’s Indium8.9HF Pb-Free Solder paste. The solder paste reflow specification is shown in Figure 1 below.
Figure 1. The Solder Paste Reflow Spec.
Furthermore, let’s assume that we have a reflow profile that we have used in the past, as seen in Figure 2.
Now, let’s take each reflow profile requirement individually:
Ramp Profile: From Figure 2, the oven goes from room temperature to liquidus in 3.1 minutes (186 seconds); this gives a slope of (219 – 25)°C/(186 - 0 ) sec = 194/186 = 1.04 °C/sec. This result is just over the recommended spec, but within the acceptable spec as seen in Figure 1.
Figure 2. The Reflow Profile
There is no “soak” in this profile, so we can ignore this part of the solder paste spec.
From Figure 2, the time above liquidus is 4.4 – 3.1 minutes (78 Seconds), right within the recommended range of the solder paste spec. The peak temperature is 250°C, again within the specification recommended range as seen in Figure 1.
The cooling ramp rate, from Figure 2, is (250-90)°C/(5.7-3.8) min = 160°C/(114 sec) = 1. 40°C/sec, which is within the acceptable rate of the solder paste spec.
So, our reflow profile does match the solder paste specification.