With　the　rapid　development　of　the　semiconductor　industry,　electronic　devices　are　developing　toward　miniaturization,　integration,　and　multi-functionalization.　In　order　to　meet　this　trend　of　the　development,　the　size　of　the　solder　joints　will　be　reduced　rapidly,　and　it　can　be　foreseen　that　this　will　bring　many　reliability　problems.　At　present,　lead-free　solder　and　Cu　substrate　are　the　most　important　bump　interconnect　materials.　In　general,　the　formation　of　intermetallic　compounds　is　a　guarantee　for　the　reliability　of　solder　joints,　but　if　its　number　and　size　are　too　large,　it　will　easily　cause　the　failure　of　the　solder　joints.　Especially,　the　volume　proportion　of　intermetallic　compounds　that　in　the　solder　joint　is　increasing,　so　the　study　of　its　size　and　morphology　is　more　and　more　important.　Nowadays,　there　are　many　studies　showing　that　primary　Cu6Sn5　exhibits　various　morphologies.　However,　these　studies　mainly　focus　on　the　influence　of　the　composition　and　cooling　rate　of　the　solder　on　the　primary　Cu6Sn5　morphology,　the　reflow　temperature　and　time　in　the　actual　production　also　have　a　great　influence　on　the　morphology　of　primary　Cu6Sn5.　Based　on　the　above　reasons,　the　Cu6Sn5　phase　that　the　most　important　intermetallic　compound　between　Sn/Cu,　was　studied　in　this　paper,　and　we　will　carry　out　an　in-depth　study　of　its　growth　behavior.　The　ultimate　goal　is　to　control　the　growth　of　Cu6Sn5　phase　and　improve　the　reliability　of　interconnect　bump　interface　in　the　future.　The　experimental　material　used　in　this　experiment　was　a　composite　solder　made　of　Sn3.5Ag　mixed　with　2　vol.%　Cu　particles.　The　sample　was　about　300　μm　solder　ball.　The　temperatures　used　in　this　experiment　was　250　°C,　and　the　time　corresponded　to　each　temperature　were　30　s　-　300　s.　In　order　to　study　the　morphology　of　hexagonal　Cu6Sn5,　scanning　electron　microscopy　(SEM)　was　used　in　this　research.　Through　this　study,　we　found　that　the　reflow　time　will　have　a　great　influence　on　the　morphology　and　size　of　the　hexagonal　Cu6Sn5.　©　2018　IEEE.