| In electronic packaging and manufacturing fields,Sn-Bi solder alloys with low melting point and processing temperature can be used for assembling temperature-sensitive electronic components and step soldering.Meanwhile,the feature of low processing temperature meets the concept and trend in energy-saving and environment-benign,thus Sn-Bi based low temperature solder pastes have attracted increasing attention worldwide.However,the low reflow soldering temperature requires the flux to have low activity temperature,which may in turn cause high reactivity of the flux at ambient temperature,consequently lead to deterioration of shelf-life and printability of the solder paste.Therefore,the conventional flux systems,such as those used for Sn-Pb solder paste and Sn-Ag-Cu(SAC)lead-free solder pastes,which have high reflow temperature,can not be used for Sn-Bi solder pastes,and the new flux system must be designed and prepared to suit Sn-Bi solder powder.Furthermore,compared with conventional Sn-Pb solder pastes and SAC solder pastes,the reflow processing defects,such as solder ball spattering and solder joint blacking defects,are more likely to occur when using high Bi-content(?58%)Sn-Bi solder pastes,which may bring about serious problems to the product reliability and thus should be solved before the large-scale commercial application of Sn-Bi solder pastes.Along with development of low temperature surface mount technology in electronic packaging,there is an increasing demand for high-performance Sn-Bi low temperature solder pastes.In this thesis study,firstly,based on the function and requirement for each component added in the flux of Sn-Bi solder pastes,a series of single-factor experiments or multi-factor orthogonal experiments were designed and optimized for selection of different components.Then,the percentage of Sn-Bi alloy powder was determined by viscosity index,and a basic formula of Sn-Bi solder paste was determined.Based on the basic formula,the components of the flux were adjusted to reduce the reflow defects through an exploratory study of defect formation mechanism,and finally an optimal formula of high-performance Sn-Bi low temperature solder paste was obtained,which shows clear innovation and practicality.Further,the self-developed Sn-Bi solder paste was evaluated in comparison with two commercial Sn-Bi solder pastes,and the factors influencing reflow process performance of Sn-Bi solder paste and the underpinning mechanism were discussed.Results show that the optimal components of ingredients in solvent,rosin and activator exhibit better performance than a single ingredient.The addition of the active salt can increase soldering ability of the flux effectively but will result in an increased electrochemical corrosion of Sn-Bi solder powder,and the inhibitor can decrease the degree of corrosion effectively.The viscosity of the solder paste can be affected by the content of thixotropic agent,tackifier and solder powder,and the high content of solder powder leads to high sensitivity of viscosity.Solder ball spattering and dewetting defect can be reduced by properly regulating the content of Sn-Bi solder powder or flux composition.The results obtained from the comparison tests between the self-developed Sn-Bi solder paste and two popular commercial Sn-Bi solder pastes indicate that the self-developed solder paste is comparable to two commercial ones with some properties superior over them.The results of evaluation of the processing performance of selfdeveloped Sn-Bi solder paste under different process parameters show that higher ramp-up rate and peak reflow temperature can increase the wettability of the solder paste.Further,a higher ramp-up rate significantly increases the severity of solder ball spattering,and a longer dwelling time can also increase the severity of solder ball spattering for the high volatility of the solder paste.Solder ball spattering mainly takes place during the ramp-up stage of the reflow stage,in which the high Bi content liquid solder ball on the surface of molten solder pool is pushed to the edge of the solder pool when liquid flux film breaks up,this finally results in the formation of solder ball spattering. |
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