As a person with more information and numbers thrown at me in a day than I can manage to store, I am a lover of rules of thumb. They come in very handy as an engineer when I am traveling off site and need to determine what solder to use and in assisting customers on the fly. One which I use on a daily basis and advise to all my customers to whom it applies is:
The operating temperature of a device, in Kelvin, should NOT be greater than 90% of the melting temperature of the solder used.
When the device operates at temperatures too close to the melting point of solder, many stresses are applied to the joint which negatively affect its quality. Excessive intermetallic growth is likely and interfacial cracking becomes a threat.