Effect Of Process Parameters On Composition And Microstructure Of Copper Phosphorus Solder

Copper phosphorus solder has excellent welding performance,which is widely used in aviation,aerospace,automobile,chemical industry,machinery,electronics and household appliances.The manufacturing process of copper phosphorus solder is hot working process.After hot extrusion and pickling,the surface of copper phosphorus wire is prone to color difference.In view of this phenomenon,this paper takes BCu93P copper phosphorus wire as the research object.Firstly,the key process parameters of casting and extrusion before extrusion were analyzed statistically,and the microstructure of the original ingot and extruded wire was analyzed by SEM and EDS.The high temperature oxidation experiment with temperature gradient of 50℃（400℃,450℃,500℃,550℃,600℃）and time of 20s was designed.The oxidized solder was pickled and passivated.SEM,EDS and XPS were used to analyze the composition and microstructure of the surface after high temperature oxidation and pickling.The wet thermal oxidation experiments with temperature gradient of 50℃and humidity gradient of 20%（0,20%,40%,60%,80%）were designed.The oxidation behavior of solder surface under different temperature and humidity was analyzed through the thermal weight gain curve of solder.According to the experimental results,the improvement scheme of the existing process is put forward,and the effect is good.The results are as follows:（1）The internal structure of ingot and wire has no effect on the color difference of solder surface.The original ingot microstructure of BCu93P solder is gradually refined from the center to the edge,and there is dendrite segregation.The ingot is mainly composed of dendritic Cu phase and eutectic（Cu+Cu₃P）phase.The cooling time of the ingot in the mold has no obvious effect on the composition and microstructure of the ingot.In addition,the internal phase of the extruded wire is the same as that of the original ingot,and the microstructure of the edge region of the extruded wire is more uniform and finer than that of the center.The reasons include:first,the ingot is preheated before hot extrusion,and the internal structure of the solder recrystallizes to form equiaxed grains;The other is that during hot extrusion,Cu₃P phase is broken by external pressure in the edge area of wire,which recombines with Cu phase and makes the microstructure of solder relatively fine.（2）Temperature is the key process parameter that affects the color difference of solder surface.When the temperature is higher than 600℃,the surface of pickled wire is red;When the temperature is lower than 600℃,steel gray appears on the surface of the wire after pickling.The valence state and content of Cu element are the direct cause of color difference on the surface of wire.The content of Cu and Cu²⁺on the surface of red wire is 68%and 11%,respectively.The content of Cu and Cu²⁺on the surface of steel grey wire is 37%and 33%,respectively.The reason is that when the temperature is higher than 600℃,Cu₂O is mainly formed on the surface of the solder,and after pickling,Cu is attached to the solder substrate,which is red;When the temperature is lower than 600℃,CuO is mainly formed on the surface of solder,and less Cu is attached on the surface after pickling.Due to the oxidation consumption of Cu,the relative content of P increases,forming a new phase CuP₂,which is steel gray.（3）Humidity reduces the temperature which the surface of solder begins to oxidize.When the humidity is 0,the solder begins to oxidize at about 420℃;When the humidity is higher than 20%,the temperature decreases to about 300℃;When the humidity increases from 20%to 80%,the temperature at which the solder gains weight by oxidation does not decrease significantly.This is because the water in the environment is adsorbed on the solder surface,the acid gas in the air dissolves in the water to form electrolyte,and the potential difference at each point on the solder surface is generated due to the uneven composition,which forms the micro battery,which provides the driving force for the chemical reaction on the solder surface,thus reducing the oxidation temperature on the solder surface.（4）Through the above experimental analysis,the process improvement scheme is proposed and verified.By increasing the extrusion pressure and the ventilation of die holder,the discoloration of solder surface can be improved obviously.With the increase of extrusion pressure,the extrusion temperature can exceed 600℃;More Cu₂O can be produced on the surface of solder by increasing the ventilation rate of die base.After the improvement of the process,the discoloration frequency of the silk is reduced from 42% to 8%.