Folks,
I am working on a book with my Indium Corporation colleagues on tackling soldering defects in electronics. It is a follow up to our The Printed Circuits Assembler's Guide to Solder Defects. The new effort is ably led by Matt Gruber and Adam Murling. In working on the chapter on reflow soldering, I thought I would share some of the initial efforts.
Ramp-to-peak (RTP) or ramp-to-spike (RTS) have become the favorite reflow profiles. To achieve a RTP profile, simply maintain a constant heating slope (delta temperature/delta time). A ratio of from 1 to 3°C/sec has been shown to be effective. This preferred ratio is dependent on the conditioning of the flux vehicle to achieve solder paste and heat uniformity between the smallest and largest mass of components populated on the PWB. A typical RTP reflow profile is shown in Figure 1.
Figure 1. A typical RTP reflow profile.
If such thermal uniformity cannot be achieved, a soak zone can be added, referred to as a ramp-soak-spike (RSS) profile. A RSS profile allows the largest thermal mass components to reach an equilibrium with the smallest thermal mass components. A typical RSS is shown in Figure 2. Since RSS would typically be used when large thermal mass components are on the PWB, to assure that all solder joints experience adequate reflow, thermal couples should be placed on a few solder joints of the largest thermal mass components.
Figure 2. A typical RSS profile.
Figure 3 shows a solder paste specification with the temperatures and times that are consistent with both RTP and RSS reflow profiles.
Figure 3. A solder paste specification showing the temperatures and times for both the RTP and RSS reflow profiles.
Cheers,
Dr. Ron