We explain when, and why, solder-based phase change materials (PCMs) are best-suited as a means to modify a robotic mechanism's kinematic and elastomechanic behavior. The preceding refers to mechanisms that possess joints which may be thermally locked and unlocked via a material phase change within the joint. Different combinations of locked and unlocked joints can yield several one-DOF mechanisms states. One actuator may be used to control motion allowed by a first state, then a new combination of locked/unlocked joints may be set and the actuator then controls motion allowed by the new state. Compared to other thermo-rheological fluids, solders yield joints with the (i) highest strength and stiffness, (ii) fastest lock/unlock speed, and (iii) lowest lock/unlock power. Herein, we cover physics-based design insights that provide understanding of how solder-based material properties and joint design dominate/limit joint performance characteristics. First order models are used to demonstrate selection of suitable PCMs and how to set initial joint geometry prior to fine tuning via detailed models/experiments. The insights and models are discussed in the context of a joint for a crawling robot that uses a single spooler motor and three solder-locking joints to crawl and steer.