| Sn-Pb alloy coating and its effectiveness in inhibiting growth of Sn whiskers has been widely applied in the electronic packaging industry, due to the sound corrosion resistance and solderability. However, because of the negative influences of Pb on the environment, Sn-Pb alloy coating has been forbidden to be used as the solderable coating. Therefore, the development of Pb-free solderable coating is highly desirable for future electroplating. Through the analysis of the problems of the current alloy coating and nano particle composite coating, this thesis tried to exploit Sn-Si C composite coating to improve the solderability of the coating and promote Pb-free solderable coating process.DC(direct current), single pulse and periodic reversal pulse electrodeposition modes are respectively used to prepare Sn-Si C composite coating. Effects of plating solution compositions and electrodeposition process on content of nano Si C in composite coating, coating’s morphology and residual stress in coating are investigated. Finally, the plating solution composition and process details of periodic reversal pulse electrodeposition are as follows, 50g·L-1Sn(CH3SO3)2, 90ml·L-1 MSA(70%), 30g·L-1 nano Si C, 50mg·L-1 SDS, 2g·L-1 hydroquinone, 4 ml·L-1 brightener, 3ms forward treating time, 0.3 forward working ratio, 2.5A·dm-2 mean forward pulse current, 3ms reverse working time, 0.2 reverse working ratio, 0.25A·dm-2 mean reverse pulse current, 45 ° tilt angle of electrode and 20℃. Through the characterization of the coating composition, morphology and structure, it is indicated that the addition of nano Si C in the composite coating is beneficial to refine deposited crystallites, flat the coating and change the coating structure, so that the application properties of the coating can be improved. Dipping experiments and Tafel curve method are utilized to test the coating corrosion resistance. Wetting and spreading experiments are used to test the solderability of composite coating.For three electrodeposition modes above, the quantity of the Si C particle in the coating increases with Si C particle concentration in the plating solution. Furthermore, inclusion of nano Si C particle in the composite coating is beneficial to the growth of new crystal nucleus and thus refining Sn crystallites. Besides, with increasing quantity of Si C particles, the corrosion resistance and high temperature oxidation resistance of Sn-Si C composite coating are improved and the coating residual compressive stress is increased.Through the mode of periodic reversal pulse electrodeposition, the cathodic reduction overpotential can be higher and deposited crystallites can be refined and the quantity of Si C particle in the coating can be improved while keeping the same plating solution composition. But the effect of dissolution owing to reverse pulse current on matrix metal causes smooth coating, enormously decreased residual stress of composite coating, and less agglomerated particles in the coating. With similar quantity of nano Si C, the periodic reversal pulse electrodeposition mode provides best corrosion resistance and oxidation resistance, which by contrast, is worst for the direct current electrodeposition mode.The mechanism of composite electrodeposition is studied. The electrodeposition of Sn-Si C composite coating follow the two-step adsorption model proposed by Guglielmi, i.e., particles in the plating solution is first weakly absorbed to the surface of electrode, then only a minority of particles are strongly absorbed, which are embedded into the coating. The acting force between Si C particle and electrode is weak, leading to low quantity of Si C particle in the coating. When the current is less than 3A·dm-2, volume percentage of Si C in the coating increases with raising current. The electrode and field intensity of solution interface have outstanding promotion effect on the the codeposition of nano particle. The electrochemical mechanism can give reasonable explanation to the nano Si C composite electrodeposition process.Meanwhile, the excellent linear relationship between the cathodic peak current and square root of potential scanning rate in cyclic voltammetry curve under different potential scanning rate shows that the electrodeposition process is diffussion controlled.Rare earth elements can be used as additive in composite electrodeposition in order to improve the performance of the coating. The results of the influence of La3+ concentration shows that, after the addition of La3+ into the plating solution, zeta potential of the Si C particles is significantly increased, resulting in higher contents of Si C particle in composite coating. At the same time, the surface of the coating becomes more smooth. When La3+ coexists with certain amount of sodium citrate in the plating solution, the electrodeposition potential of lanthanum is shifted positively, so that the codeposition of rare earth La and Sn on the copper electrode is realized. The mass ratio of La in composite coating is 2.35% ~2.48%.The wettability of the solder in the coating and the property of inhibiting growth of Sn whiskers are two important indicators to characterize the solderability of the coating. This thesis attaches great importance to the above two factors, and thus the relationship between the coating structure and solderability is obtained. The studying on the inhibition of Sn whisker growth shows that the existence of crystal face orientation of Sn crystallites(220),(321) in the coating with intermetallic compound Cu6Sn5 is beneficial for the growth of Sn whisker. However, the effect of preferred orientation of(420) crystal face is in sharp contrast. The coating’s residual stress is one of the main factors promoting Sn whisker growth. Under similar residual stress, crystallites in the composite coating is refined and compact. Therefore, it is harder for the Sn whiskers to break through the coating surface thus slowing down the growth of Sn whiskers.Solderability of composite coating obtained by three electrodeposition modes is similar, i.e., within a certain range, increasing composite quantity of Si C, smooth and compact composite coating is obtained, with refined crystallites. Therefore, the wettability of coating can be improved. However, excess quantity of Si C hinders the spreading of solder during soldering and deteriorates the wettability of the coating. It is found that the spreading area of solder as the coating prepared by the mode of periodic reversal pulse electrodeposition is largest. The codeposition of rare earth La and Sn on copper electrode is realised for preparing alloy composite coating. During solder spreading tests, it is the cooperative effect of both the affinity between La and solder metallic element and the crystal boundary segregation of rare earth elements that reduces the interaction between coating element and melting solder liquid, thus increasing the spreading area of solder. |
PDF Full Text Request