Optimization And Reliability Evaluation Of Lead-free Wave Soldering Process By DOE For Inverter Air-conditioner Motherboard

“Green Manufacturing” has become the development requirements of theelectrical and electronic products industry in the21st century. It has become aninevitable trend that the lead-free solders took the place of SnPb solder to conductwave soldering production. Because the wave soldering process was complicatedand difficult to control in the actual production, coupled with the lead-free solders’characteristics, it resulted in high defect rates of welding. As a method ofmathematical statistics and process optimization, DOE (Design of Experiment) hasshows its unique advantages in the optimization of lead-free wave soldering process.Reducing the defect rates and improving the welding quality are the initialrequirements for the products. However, it also should meet certain reliabilityduring the service time.The wave soldering process was preliminarily optimized by fractional factorialdesign (FFD). In this experiment, flux flow, spray speed, spray height, preheattemperature, orbital inclination angle, tin immersion time were regarded as theexperimental factors, and the number of bridging was regarded as the responsesindicator. The results show that: the flux flow, orbital inclination angle, spray height,the interaction effect of flux flow with orbital inclination angle and the interactioneffect of flux flow with spray height are the significant factors affecting the numberof bridging. When the flux flow is at the low level, the number of bridging is more.It is reduced with the orbital inclination angle increasing and increased with sprayheight increasing. The initial optimized combination of parameters has been got:flux flow is40ml/min, orbital inclination angle is6.8°, spray height is50mm, tinimmersion time is5s, spray speed is150mm/s, preheating temperature is100℃.The linear mathematical model of the number of bridging corresponding with theprocess parameters has been gained by regression analysis.By central composite face-centered design (CCF) of response surfacemethodology (RSM), the notable factors were studied and the quadratic regressionmodel was gained. The results show that: when flux flow is at the range of36~40ml/min, orbital inclination angle is at the range of6.2~6.6°, and spray height is atthe range of58~60mm, the number of bridging is the least and the welding qualityis optimal. By solving extreme points of the quadratic regression equation and beingcombined with the results of FFD, the final optimized combination of processparameters is got: flux flow is40ml/min, spray speed is150mm/s, spray height is58mm, preheating temperature is100℃, orbital inclination is6.2°, tin immersiontime is5s. Repetitive experimental verification has been conducted by the final optimized combination of process parameters, it shows that the defect rate oflead-free wave soldering can be significantly reduced and controlled within1000PPM by the optimized combination of parameters.Temperature cycling test was used to make reliability evaluation for the solderjoints of the inverter air conditioner motherboard. Regulation and causes of thecrack initiation and propagation of Sn0.7Cu-0.05Ni through-hole solders wereanalyzed from two aspects of macro and micro levels. The results show that: withthe cycles increasing, the number of cracks on the surface of solder joint increases,and the number of cracks on the PCB after optimization is significantly fewer thanthat on the PCB before optimization in the same period, verifying the scientific andreasonableness of DOE. The cracks mainly initiate at the intersection between thesolders and the device, and propagate along the contact interface of solders withpads or pins. The cracks also propagate from the internal micro-cracks or holes.Cracks fracture is mainly in the form of intergranular fracture. The mechanism ofcracks initiation and propagation causing solder failure is a combined effect offatigue and creep. The cracks have a relationship with mismatch of coefficients ofthermal expansion for different materials, grain growth coarsening, and theformation of (Cu, Ni)6Sn5intermetallic compound and Kirkendall holes under theeffect of temperature cycling.If the program of DOE optimization and reliability evaluation are promoted, itwill have a far-reaching significance on guiding the actual production process fortypical household appliances.