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Alloying Effect of Ag and Cu in SAC Solder for Ductile and Brittle Failure at Different Mechanical Test of Solder Joint
Keywords: solder joint, Mechanical Test, ductile brittel
The reliability life of solder joints is a well-known key issue for the electronics industry. From the technological point-of-view, SMT solders guaranteed the electrical and mechanical bonding on a printed circuit board. Board level temperature cycles was the comment test method of reliability life. Recently, environmental tendencies turn to leadfree materials, the properties of which still have to be studied. The two most common compositions, referred commonly as SAC305 and SAC405, are Sn3.0Ag0.5Cu and Sn4.0Ag0.5Cu, respectively. One of the main reasons for the selection of these compositions was the peak temperature of SMT reflow profile due to eutectic composition of Sn-Ag-Cu ternary alloy system (lowest melting point). SAC305 and SAC405 have been shown better reliability life than SnPb solder in board level temperature cycle test. However, the markets of mobile products including mobile phone, MP3, PDA, smart card and so on are growing rapidly. Board level drop test has become a hot topic. It is intended to evaluate and compare drop performance of surface mount electronic components for handheld electronic product applications in an accelerated test environment. In addition, some BGA packages with missing balls were found after the packages were transported to another place. It is believed that those balls were missed due to the vibration or shock during handling and transportation. Therefore, the capability of solder joint in package level to resist the sudden impact is also becoming a growing concern. In this study, the solder balls of each alloy (Sn3Ag0.5Cu, Sn3.5Ag and Sn1Ag0.5Cu) were attached to packages with Ni-Au surface finish using standard package assembly process. The evaluation matrix included package and board level tests as well as interfacial IMC studies. The package level tests (ball shear, ball pull, and high speed shear) and IMC studies were conducted on as-soldered. Board level drop tests were performed as per JESD22-B111 test method. The ductile V brittle is one of the indicators for different solder joint mechanical test. The solder joint failure mode depends on some factors, such as solder alloy, surface finish and loading mode (shear vs. tensile). In general, the ball shear is simply used to check solder joint quality after ball placement. According to study results, ball shear and pull were not enough capability to evaluate board or package level drop test performance. High speed shear test with 4m/sec of test speed can compare the drop performance during handling and transportation.
Liang Yi Hung, Senior Engineer
Siliconware Precision Industries Co., Ltd.
Taiwan 42749,
R.O.C.


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