The allowable permanent mechanical load on joints soldered on PCBs is compared to the initial solder strength relatively low. The reason for that is that the solder is sensitive for creep and fatigue loads, due to the low melting point of the solder alloy. If a metal is used at a temperature range that is larger than 0.4 times its melting point (in K) the metal will creep when exposed over a certain mechanical load.
With lead-free soft-solder alloys the 0.4 times melting point will give a temperature of about 200K which is far below the ambient temperature at which the solderjoints will normally function. This means that for the permanent load at a solderjoint one must calculate with the creep strength.
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