Solder Wire vs. Solder Paste

If this is thru hole soldering their is probably an advantage to changing over to paste which can be applied thru masks. If its surface mount and the design engineer chose wire then the issue is probably overheating of the transistor. You did not mention how many of these transistors are done per hour. If the rate is quite low or just occasional then the wire is just easier than buying and setting up masks. You should probably ask the engineer who chose to use wire as to why. Who knows he just may not have had time to go in and tweak the design as new methods have become available. There is a large number of solder pastes and fluxes now available for different needs.

Depending upon the device design, wave soldering in groups may be the way to go.

Hard to say the exact reason, but since you manufacture power transistors and one would thank that your design engineers would generally have electronic backgrounds, it might be that they don't have a good grasp on the manufacturing aspect of what is most efficient. They will focus only on what they know with little input from others on the manufacturability of the product. In my last position, as an Advanced Manufacturing Engineer, I would many times end up changing their designs like solder wire vs. donuts vs. paste vs. flow, soldering guns vs. resistive hot bar welding vs. hot air guns, or even altering layouts, adding keying features to best suit the manufacturing of the product. Fortunately, I was involved with the designers during the prototyping stage and was able to do these things before it became too difficult to change. I generally try to avoid solder wire due to the time & labor wasted in manual operations or the headaches involved with the feeding systems in automated operations. If the volume can support it, then its worth trying the other options.

Check this video to understand the different https://www.youtube.com/watch?v=juBAIqXer3g