Within　electronic　products,　solder　joints　with　common　interfacial　structure　of　Cu/IMCs/Sn-based　solders/IMCs/Cu　cannot　be　used　under　high　temperature　for　relatively　low　melting　points　of　Sn-based　solders　(200-300　degrees　C).　However,　there　is　a　trend　for　solder　joints　to　service　under　high　temperature　because　of　the　objective　for　achieving　multi-functionality　of　electronic　products.　With　the　purpose　of　ensuring　that　solder　joints　can　service　under　high　temperature,　full　Cu3Sn　solder　joints　with　the　interfacial　structure　of　Cu/Cu3Sn/Cu　can　be　a　substitute　due　to　the　high　melting　point　of　Cu3Sn　(676　degrees　C).　In　this　investigation,　soldering　process　parameters　were　optimized　systematically　in　order　to　obtain　such　joints.　Further,　interfacial　microstructure　evolution　during　soldering　was　analyzed.　The　soldering　temperature　of　260　degrees　C,　the　soldering　pressure　of　1　N　and　the　soldering　time　of　5　h　were　found　to　be　the　optimal　parameter　combination.　During　soldering　of　260　degrees　C　and　1　N,　the　Cu6Sn5　precipitated　first　in　a　planar　shape　at　Cu-Sn　interfaces,　which　was　followed　by　the　appearance　of　planar　Cu3Sn　between　Cu　and　Cu6Sn5.　Then,　the　Cu6Sn5　at　opposite　sides　continued　to　grow　with　a　transition　from　a　planar　shape　to　a　scallop-like　shape　until　residual　Sn　was　consumed　totally.　Meanwhile,　the　Cu3Sn　grew　with　a　round-trip　shift　from　a　planar　shape　to　a　wave-like　shape　until　the　full　Cu3Sn　solder　joint　was　eventually　formed　at　5　h.　The　detailed　reasons　for　the　shape　transformation　in　both　Cu6Sn5　and　Cu3Sn　during　soldering　were　given.　Afterwards,　a　microstructure　evolution　model　for　Cu-Sn-Cu　sandwich　structure　during　soldering　was　proposed.　Besides,　it　was　found　that　no　void　appeared　in　the　interfacial　region　during　the　entire　soldering　process,　and　a　discuss　about　what　led　to　the　formation　of　void-free　joints　was　conducted.