| The developing trend toward miniaturization leads to the downsizing of solder joints to micro-scale (micro solder joints), which resulted in a more serious effect on interfacial reaction from factors such as solder size, soldering process, and crystal orientation and cross-solder interaction of substrates. The service reliabilites of the micro solder joints were challenged. However, the classic "ripening theory" model of interfacial reaction cannot explain these new issues existing in the lead-free interfacial reaction under miniaturization. The present work researched the effect of solder size, soldering process, crystal orientation and cross-solder interaction of substrates on interfacial reaction of pure Sn and Sn-Ag-Cu lead-free solder joints on Cu and Ni substrates, based on atomic diffusion flux controlled kinetics and the cluster nucleation theory, the evolution mechanism of interfacial intermetallic compounds (IMCs) under the effcet of different factors were clarified, respectively; the theoretical model of concentration gradient controlled (CGC) liquid/solid interfacial reaction was established; the size effcet of interfacial reaction was interpreted from the theoretical and experimental perspective; the evolution of interfacial microstructure during the soldering process was characterized using synchrotron radiation real-time imaging technology; the function mechnisms of crystal orientation and cross-solder interaction of substrates on interfacial IMC evolution were revealed; the effect of interfacial microstructure on tensile properties of the micro solder joints was evaluated. The main conclusions are drawn as follows:1. For the size effect on interfacial reaction:the downsizing of solder joints changed the solute (Cu) atomic fluxes at the joint interfaces, and thus induced the size effect phenomenon. In detail, with the downsizing of solder joints, for Sn-xAg-yCu/Cu solder joints, the average Cu concentration in the solder increased, the Cu6Sns grain diameter increased, the thickness of interfacial IMC layer increased, and the consumption of Cu substrate decreased; for Sn-3.0Ag-0.5Cu/Ni-P solder joints, the decrease rate of average Cu concentration in the solder was accelerated, and consequently the interfacial IMC transformation process from needle and rod type (Cu, Ni)6Sns to polyhedral (Cu,Ni)6Sns, and finally to needle and bulk type (Ni,Cu)3Sn4 was accelerated; for Cu/Sn-3.0Ag-0.5Cu/Ni-P solder joints, (Cu,Ni)6Sns grain diameter increased and Cu consumption decreased on Cu side, and the interfacial IMC maintained needle type (Cu,Ni)6Sns with thickness increased on Ni side. Based on atomic flux, a concentration gradient controlled (CGC) interfacial reaction theoretical model was established, and the calculated results matched well with the experimental ones; interfacial Cu concentration gradient was revealed to be the root cause of the size effect. The concentration gradient controlled (CGC) interfacial reaction theoretical model can also be used to explain the size effect on Ni substrate.2. For the effect of soldering process on interfacial reaction:the top of Cu6Sn5 grains dissolved into the solder at a rate of 0.11μm/s at the heating stage, kept scallop shape and grew slowly at the temperature holding stage, and rapidly grew into solder at a rate of 0.04 μm/s to form high aspect ratio grains at the cooling stage; the interfacial Ag3Sn plates dissolved into the solder at a rate of 0.2μm/s at the heating stage and randomly precipitated at the interface at a rate of more than 5μm/s at the cooling stage with an undercooling of 31-55 ℃; Ag3Sn plates tended to precipitate on the cold end under temperature gradient, and those precipitated on the hot end tended to dissolve into the solder. The functional mechanisms of liquid clusters on random precipitation of Ag3Sn plates as well as cold end preciptiation of Ag3Sn plates under temperature gradient were clarified; interactions between thermomigration flux and the chemical potential flux of Ag atoms leading to Ag3Sn phase dissolution on the hot end and growth on the cold end were revealed.3. For the effect of crystal orientation and cross-solder interaction of substrates on interfacial reaction:in micro solder joints with polycrystalline substrates, scallop interfacial Cu6Sn5 grains formed in Cu/Sn/Cu micro joints, and grew ripened and combined larger until the full IMC joints formed after long reaction time; in Cu/Sn/Ni joints, cross-solder interaction induced that Ni atoms participated in the interfacial reaction on Cu side forming fine (Cu,Ni)6Sn5 grains, and Cu atoms diffused to the Ni side and promoted the grow of needle type (Cu,Ni)6Sns, finally, full IMC joints formed with holes inner the joint. In micro solder joints with single crystal substrates, the orthogonal and regularly arranged prism Cu6Sn5 grains formed at the interface of (001) Cu at 300℃, but transformed to the randomly orientated scallop ones with longer reaction time; parallel needle type (Cu,Ni)6Sns grains formed at the interface of (001) Ni at 250℃. The driven force of the accelerated growth of Cu6Sn5 grains from the opposite two interfaces after growing closed was clarified to be the cross-solder interaction of Cu ripening fluxes; grain boundary migration was revealed to be the mechanism of single grain formation from the opposite two grains; based on atomic diffusion flux controled kinetics and the cluster nucleation theory, the growth and evolution of morphology and orientation of IMCs under Cu-Ni cross interaction and the crystal orientation of substrates were clarified.4. For the tensile properties of micro solder joints:the tensile strength and brittleness of micro solder joints increased and the fracture position transformed from inner solder to IMC interface with the increasing of IMC thickness. The full IMC joint had the highest tensile strength, where Cu/IMC/Cu joint reached 93 MPa and Cu/IMC/Ni joint reached 110 MPa.The fracture position of Cu/IMC/Cu solder joint was at Cu3Sn/Cu6Sns interface, while that of Cu/IMC/Ni joint was at the contacting interface of (Cu,Ni)6Sns from the opposite two sides. The failure processes induced by the interfacial defects and the stress concentrated positions were in situ characterized. The mechanism that the constraining effect of IMC interface induced the increase of solder joint strength was clarified. |
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