Effects Of Substrate Roughness And Barrier Layer On Wetting And Spreading Behavior Of Sn-35Bi-1Ag Solder

In the soldering process,the key to ensuring the quality of the connection depends on the wettability of the molten metal solder.The feasibility of the soldering process should be based on the good wetting and spreading of the soldering material spreading on the base material.Therefore,improving the quality of joints requires quantitative modeling and prediction of the wetting and spreading process of liquid solder.On the other hand,with the development of high performance and miniaturization in the field of microelectronic packaging soldering,and the enhancement of environmental protection awareness,lead-free is becoming more and more strict.However,due to the shortcomings of their properties in the use of binary Sn-based solder,the application of binary Sn-based solder is limited.It is necessary to add a certain amount of the third element to prepare the ternary alloy to make up for some defects of the binary alloy.In this context,Sn-35Bi-lAg(178?)solder is considered to have an important prospect in the industry because that its melting point is similar to that of the traditional eutectic Sn-Pb solder(183?),and it has excellent mechanical properties and wettability.In this paper,the wetting and spreading behavior of Sn-35Bi-lAg solder on the typical metal substrate(Cu,Ni,and electroless nickel plating of Cu)was studied;and the effect of temperature and surface roughness of substrate on the wetting and spreading process of Sn-35Bi-l Ag/Cu system was studied.A platform for real-time monitoring of dynamic wetting angle and the triple-line movement was built independently.In-situ wetting and spreading tests of Sn-35Bi-lAg solder on randomly polished Cu substrate,Ni substrate and electroless nickel plating of Cu substrate in a high-purity argon atmosphere were carried out,and real-time digital synchronous recording of the top and side movement of the triple-line was carried out.Results reveal that as the soldering temperature increases,the final spreading area of the solder on the substrate increases,the final contact angle decreases,and the total time for the wetting spreading process becomes shorter.The characteristics of the wetting and spreading process are similar at 473K,513K,and 553K.The entire spreading process is divided into four stages:i)an initial stage,ii)a rapid spreading stage,iii)a limited spreading stage and iv)an asymptotic stage.The wettability of Sn-35Bi-lAg solder on Cu substrate is better than that of Ni substrate and electroless nickel plating of Cu.Meanwhile,the wettability of Sn-35Bi-lAg solder on Ni substrate is similar to that on electroless nickel plating of Cu,the electroless Ni barrier layer has little effect on the wettability of Sn-35Bi-lAg solder.According to the analysis,the wetting kinetics of a rapid spreading stage may be determined by the chemical reaction rate of the Cu or Ni and Sn characterized by the Rn?t model with an value of 1.For a limited spreading stage,the wetting kinetics can also be characterized by the Rn?t model with an value of 4,in which the diffusion rate may determine the process.At the same time,the evolution of the dynamic contact angle with the spreading time t can be expressed with an equation:cos?F-cos?=(cos?F-cos?o)e(-kt).The value of k decreases with the increase of the temperature.The effect of substrate surface roughness and temperature on the wetting behavior of the Sn-35Bi-lAg/Cu system was studied.Results revealed that the wettability of Sn-35Bi-lAg solder was enhanced with increasing surface roughness of Cu substrates and temperature.However,the surface topography characteristics(i.e.,anisotropic or isotropic)had little effect on the wettability of Sn-35Bi-lAg solder.The liquid solder had a preferential spreading direction,which caused an elliptical triple-line pattern when Sn-35Bi-l Ag solder spread over an anisotropic surface,while the solder spread as almost a perfect circle when the surface of Cu substrate was isotropic.Besides,the surface roughness had little effect on the interfacial structure,while the interfacial IMCs increased with increasing the temperature.Moreover,the spreading process of Sn-35Bi-lAg solder spreading on Cu substrates with different surface roughness was similar at 483 K and 553 K.The whole spreading process was featured with four distinguishable stages:i)initial stage,ii)rapid spreading stage,iii)limited spreading stage and iv)steady stage.Wetting kinetics of the rapid spreading stage and limited spreading stage were described by the Rn?t model the values of n of the rapid spreading stage and limited spreading stage decreased with the increasing limited spreading stage.Meanwhile,the dynamic contact angle and time t could be characterized by an equation:cos?F-cos?=(cos?F-cosOo)e(-kt).The value of k decreased with the increasing surface roughness of Cu substrates and the temperature,while the surface topography characteristics had little effect on the value of k.