Electromigration　is　a　major　reliability　problem　in　composite　solder　joints.　Due　to　the　anisotropy　of　the　beta-Sn　crystal　structure,　the　Sn　grain　orientations　present　in　the　solder　matrix　dominate　the　principal　failure　mechanism　in　solder　joints　under　electric　current　stressing.　In　this　work,　the　Cu6Sn5　growth　behavior　in　Cu6Sn5-reinforced　composite　solder　joints　with　three　different　Sn　grain　orientations　was　investigated　at　current　density　of　10(4)　A/cm(2)　at　room　temperature.　Micron-sized　Cu　particles　were　added　to　Sn-3.5Ag　solder　at　2%　volume　fraction　using　an　in　situ　method.　After　current　stressing　for　528　h,　the　polarity　effect　in　the　composite　solder　joint　was　greatest　for　an　angle　(theta)　between　the　c-axis　and　electron　flow　direction　of　30　degrees,　resulting　in　higher　growth　rate　of　Cu6Sn5　in　the　solder　matrix　compared　with　composite　solder　joints　with　theta　of　60　degrees　or　90　degrees.　There　were　no　noticeable　changes　in　the　composite　solder　joint　with　theta　of　90　degrees.　The　growth　behavior　of　Cu6Sn5,　Cu　atomic　motion,　and　Cu　diffusivity　in　the　composite　solder　joints　with　different　Sn　grain　orientations　were　analyzed　in　detail.