Wave soldering dross formation analysis and reduction measures
As the Sn ingredients containing is more than 95% in the SnAgCu lead-free solder , so in comparison with the traditional solder , the increasing of ingredients of Sn and temperature of Lead-free soldering process will lead to oxidation of the solder increasing .Before reducing the oxidation of solder slag, dross, we must first understand the types and molding processes.
The following three shall be considered:
(1) The static surface of the oxide film, this is a natural phenomenon of Sn oxide, as long as the oxide film is not broken, since it would prevent further production of the amount of oxidation. As shown below:
   
(2) Black powder, as a result of the friction of high-speed rotating impeller shaft and Sn oxide film, was the spheroidizing product production, and the particles are larger. As shown in the figure below:
    
（3）Bean curd residue, was mainly in the nozzle periphery of turbulent waves and peace wave , accounts for the vast majority of the entire weight of oxide slag.
    
Bean curd residue was caused by the negative pressure oxygen to shear slag, and the waterfall effect in the combination result of various factors , the specific dynamic process is as follows：

 
   The black area is the air interface, the liquid temperature tumbling white Sn. t = t3 figure we can see a small portion of the air is swallowed at the solder solution, a small portion of the air due to the rapid oxidation of oxygen inside the tin will surface but cannot eliminate N2 gas, and therefore form a hollow balls, Since the density of the hollow ball is much smaller than that of tin, tin surface is bound to emerge when these hollow ball once stacked to form floating in the Bean curd residue tin surface.
Knowing the causes and tin forming species, we believe that reducing the formation of Bean curd residue is to reduce wave soldering tin slag most effective measures. From the above it can be seen dynamic process: hollow of solder balls are two necessary conditions：
The first prerequisite is the boundary effect, the tin surface with a dramatic roll, forming phagocytosis
The second requirement is a hollow ball inside to form a dense oxide film, the nitrogen gas is formed within the package. Otherwise it floats on the surface of at the solder when hollow ball will be broken, unable to form a "Bean curd residue."
These two necessary conditions are indispensable.
The measures of reducing dross in wave soldering show as following :
1.        Reducing the gap generated when switching wave, which will be reduced reflow solder bump efforts to reduce the roll, thereby reducing the generation of phagocytosis.
So we changed the cross section of solder pot into a trapezoid, and make the first wave as far as possible close to the edge of the solder pot

 
2.        In the both of the first wave and second wave we add the unfiltered barrier device to the tumbling-flow solder.

3.        Take the N2 protection to avoid the generation of the dense oxide membranes in solder ball.