Yuan: As I understand it the key word here is "rework". Both John Guy and Gunter Grossman are correct, as far as they go. John is correct when he says that the smaller grain size will yield higher reliability via increased elasticity. As John also notes, the connection metalurgy begins to change as soon as the reflow process ends. A solder connection has always been a dynamic medium of attachment and characteristics will change throughout it's useful life. However, the finer the grain density at the beginning of that useful life, the longer the useful life will be. Gunter correctly notes that stress in a connection will be self-relieved within a brief period of time. Gunter also correctly notes that the coarse lead phases are initially noted adjacent to the pads/components, where thermal energy is extracted slowly by limited conductive cooling attributable to the huge thermal mass of the pad/component interface. Those interfaces are the points where you see connection failures, attributable in part to lead phase concentration and the combined effect of intermetalic formation. In the repair of equipment you have two things to contend with, the presence of an intermetalic before you start and the application of only localized heating. The intermetalic works against you because you need to minimize it in-so-far-as possible, while the localized heating works for you because it will allow the pwa to accept/dissipate thermal energy more quickly than a pwa which was preheated for machine soldering. During several years of running a Reliability Test laboratory for the Navy (in a former life) it was demonstrated to my own satisfaction that a connection subjected to accelerated cooling will be more reliable than one allowed to cool slowly. My advice would be to train your repair technicians well, minimize the thermal energy used during the repair, and use accelerated cooling to the maximum extent allowable by the components you are using. Regards, Jim Moffitt