In　SMT,　the　investigation　on　solder　joint＇s　failure　is　always　very　important.　Thermal　fatigue　is　the　main　failure　form　for　solder　joint　in　SMT.　In　this　paper　the　failure　process　of　solder　joint　in　SMT　was　investigated　by　both　electrical　resistance　measurement　method　and　crack　observation　method　together.　The　characteristics　of　electrical　resistance　value　variation　of　lead-tin　and　lead-free　solder　(SAC305)　joint　during　thermal　fatigue　test　were　obtained.　And　at　the　same　time　the　crack　propagation　in　solder　joint　was　observed.　According　to　this,　the　failure　rules　of　lead-tin　and　lead-free　solder　joint　were　compared.　And　by　FEM,　the　relationship　between　electrical　resistance　value　variation　and　crack　propagation　of　solder　joint　during　thermal　fatigue　test　was　studied,　through　which　an　criterion　based　on　electrical　resistance　value　variation　for　solder　joint＇s　failure　during　thermal　fatigue　test　could　be　obtained　from　experience.　It　was　shown　from　experimental　results　that　the　lead-free　solder　(SAC305)　joint　had　a　higher　resistibility　from　thermal　fatigue　than　the　traditional　lead-tin　eutectic　solder　joint.　And　the　criterion　based　on　electrical　resistance　value　variation　was　built　up　from　the　experimental　and　simulation　results.　©2009　IEEE.