This　paper　focused　on　the　microstructure　of　SnAgCu-rare　earth　(RE)　solder　alloy　and　its　small　single-lap　joints,　focusing　on　phases　present　and　the　distribution　of　RE　in　the　SnAgCu　solder.　Energy　dispersive　x-ray　(EDX)　analysis　was　used　to　observed　the　RE-rich　phase.　The　RE　atoms　also　tended　to　aggregate　at　boundaries　of　primary　dendrites　in　the　joints　and　form　as　a　weblike　structure,　which　surrounded　the　dendrites　and　restrained　the　dendrites　from　sliding　or　moving.　It　is　assumed　that　this　would　strengthen　the　boundaries　and　increase　the　resistance　to　creep　deformation　of　the　solder　matrix.　The　creep-rupture　life　of　joints　can　be　remarkably　increased,　at　least　seven　times　more　than　that　of　SnAgCu　at　room　temperature.　The　aggregation　mechanism　of　RE　at　dendrite　boundaries　in　SnAgCu　solder　joints　was　presented.　The　drive　for　RE　atoms　to　aggregate　at　the　boundary　is　the　difference　of　the　lattice-aberration　energy　between　the　interior　and　the　boundaries　of　the　dendrites,　which　is　caused　by　a　solution　of　RE　atoms.