The Wettability Of SAC305Solder And Growth Mechanism Of Intermetallic Compounds Under Thermal Stress As Well As Electric Field

Electronic packaging has been one of the largest worldwide industries with therapid development of electronic technology. Electronic packaging technology hasalready accessed to the high-density and small-pitch era with the demand forminiaturized, portable and intelligent electronics in recent decades. One of the mostcritical processes in electronic assembly is reflowing. During reflowing, solder wets toand reacts with base metal to form intermetallic compounds (IMCs). Basically, thepresence of a thin IMC layer is essential for the bondability of the solder joints.However, too thick IMC layer may weaken the solder joints due to the brittle nature andlattice mismatch between the substrates and solder alloys. Solder joints provideelectrical, thermal connection as well as mechanical support for different componentsand substrates, hence, good wettability and proper thickness of IMC layer within theavailable time effectively guarantee the reliability of solder joints, even the wholeservice life of electronic devices. It is of primary importance to investigate the wettingbehavior as well as the growth mechanism of interfacial IMCs between solder andsubstrate systematically. In this study, the effects of different coatings (Ni, Ag, Ag/Niand Au/Ni) and temperature on wetting behaviors of Sn3Ag0.5Cu solder on Cusubstrates were studied. In addition, the morphology and growth behavior of IMCs atSn3Ag0.5Cu/Cu interface under thermal stress and current field were also beeninvestigated. The main research contents and conclusions are as below:The effects of Ni, Ag, Ag/Ni, Au/Ni coatings and temperatures on wettingbehaviors of Sn3Ag0.5Cu solder on Cu substrates after reflowing at523K,553K and583K were studied. The results showed that when temperatures are constant, Agcoating are favorable to the wetting of Sn3Ag0.5Cu alloy on Cu substrates. Because thediffusion rate of Ag itself to Sn-3Ag-0.5Cu solder is much faster than Cu, it leads tofaster IMC formation rate and better wettability because the reaction products are thediffusion frontier. The diffusion rate of Au element to Sn3Ag0.5Cu melting solder is thefastest, the rapid formation and distribution of AuSn4contribute to the rapidest wettingand spreading of Sn3Ag0.5Cu on Au/Ni/Cu substrate. While the slower diffusion of Nito molten solder and none groove-like IMC formation lead to slower growth rate ofIMC and worse wettability of Sn3Ag0.5Cu alloy on Ni/Cu than pure Cu substrates. Inaddition, an increase in temperature promotes the substrate atoms diffusion and reaction rate with Sn3Ag0.5Cu alloy, decreases the viscosity as well as surface tension ofmelting solder and contributes to better wettability significantly.The morphology and growth mechanism of IMCs at Sn3Ag0.5Cu/Cu interfaceunder isothermal aging, thermal cycling and thermal shock were investigated bymicrostructural observations and phase analysis. The results showed that the IMC layerflattened with aging time because the grooves in scallop-like IMC provide a moreconvenient access for Cu atoms to dissolve and react with solders and previous IMCs.When subjected to isothermal aging, the growth rate of Cu6Sn5was lower than that ofCu3Sn. While for thermal cycling, it was Cu6Sn5that contributed more for the growth ofIMCs. This had much to do with abundant supply of Sn and Cu atoms at the interface ofsolder/Cu6Sn5caused by recrystallization of solder and compression stress duringthermal cycling. Growth dynamics of total IMC layer thickness both under isothermaland non-isothermal aging were stated as empirical power-law relationship using anequivalent aging time parameter. And the growth time exponent depended on theevolution of grain structures of solder matrix, thermal stress and propagation of cracksduring non-isothermal conditions.The effects of isothermal aging and low density current on growth rate andmicrostructural evolution of IMC at the interface of Sn3Ag0.5Cu solder and Cusubstrates were investigated at398K. The results showed that the morphologies of IMClayer under stress of aging and current were basically same. The growth rate of IMC atanode was the fastest. That was because chemical diffusion force and electronic windacted together to drive the growth of IMC at the anode. The current density was nothigh enough, so that no obvious polarization effect or cracks along the electron flowdirection were observed. The classical mean-time-to-failure analysis was used tocalculate the lifetime of ball grid array solder joints charged with electricity. However,the calculated value was much shorter than the true value. Thus, the equation need to bemodified when it is applied in Cu interconnects and flip chip solder joints.