| With the development of modern electronic information industry,modern electronic assembly has developed rapidly towards high-density,fine-spacing packaging and high reliability.Fine-pitch QFP devices face new challenges in traditional welding processes such as hot air reflow welding.As a new brazing technology,laser spray bead microjoint has the characteristics of small heat input range,fast welding and high flexibility.Laser welding can effectively control the flow of molten filler metal during welding,so as to avoid welding spot bridging and short circuit caused by QFP device in reflow welding.At the same time,it can avoid the thermal deformation of the plastic sealing body and the popcorn phenomenon caused by damp during the whole heating welding process of the traditional reflow welding.Therefore,laser spray bead micro-connection technology has been paid more and more attention in the board level assembly of QFP devices with fine spacing.However,there are still many outstanding problems in the welding of QFP devices.For example,it is difficult to accurately control the heat transfer in the welding process,and it is difficult to adjust the process parameters of obtaining high quality QFP solder joints.The result can be reliability problems with solder joints.It is very necessary to develop the laser spray bead micro-connection technology for practical QFP device welding.Based on this,this paper studies the laser spray ball micro-connection technology for QFP device welding.The specific research contents and conclusions are as follows:（1）Mechanical properties of solder joints of QFP devices under different laser peeling-ball micro-connection parameters.The welding test of QFP device was carried out by laser petting-ball micro-connection technology,and the laser process parameters were adjusted to establish the process window of QFP device laser petting-ball micro-connection.According to the process parameter window,the corresponding welding experiment and welding joint tensile experiment were designed to explore the influence of laser power,heating time and nitrogen pressure on the mechanical properties of welding joint.The results show that the influence of laser power and heating time on the mechanical properties of solder joints is greater than that of nitrogen pressure.With the increase of laser power,heating time and nitrogen pressure,the tensile force of solder joints increases first and then decreases.（2）Microstructure and fracture analysis of laser petting-ball micro-connection solder joints of QFP devices.The microstructure of solder joints and tensile fracture of solder joints of QFP device were observed by SEM and EDS,and the effect of laser ball spraying micro-connection process parameters on the microstructure of solder joints was investigated.The macroscopic mechanical characteristics of tensile fracture were analyzed,and the fracture mode and fracture structure composition of QFP device were confirmed under laser petting-ball micro-connection.The results show that:1)The intermetallic compound composition at the upper interface of the solder joint is（Cu,Ni）6Sn5,and the intermetallic compound composition at the lower interface is Cu6Sn5.The changes of laser power and heating time have great influence on the microstructure of solder joint.Under high laser power and heating time,the intermetallic compound layer between solder joint and pad is thicker and exhibits dendrite shape.With the increase of laser power,heating time and nitrogen pressure,the thickness of intermetallic compound layer between solder joint and pad increases.2)The fracture modes of solder joints are different under different laser parameters.It is mainly reflected as follows:under the condition of short heating time,ductile fracture appears,and the fracture surface presents dimple morphology.Under the condition of longer heating time,brittle fracture appears,and the fracture surface presents smooth transgranular fracture morphology.（3）Finite element simulation analysis of temperature field of QFP laser nozzle micro-connection.The temperature field simulation model of QFP laser pellet microconnection was established based on ANSYS software and verified by infrared temperature measurement experiment.Combined with the experimental process parameters,the weld temperature field under different process parameters was calculated and obtained.The results show that the relationship between laser power and weld temperature is almost linear,and the weld temperature increases about 6℃for every 5%increase of laser power.The heating time is proportional to the weld temperature,and the tangent slope increases gradually.（4）Parameter optimization of laser pellet micro-connection technology for QFP device.The response surface model of mapping relationship between welding joint tensile force and laser process parameters was established,and the significant degree of laser process parameters’influence on welding joint tensile force was analyzed.And based on the response surface model,the laser process parameters were optimized by genetic algorithm,and the combination of laser process parameters under the maximum tensile force of solder joints was obtained.The results show that the order of influence of laser process parameters on tensile force of QFP solder joints is laser power>heating time>nitrogen pressure.After optimization,the optimal laser process parameters are as follows:laser power 81%,heating time 87ms,nitrogen pressure 860Pa,and the maximum tensile force is 2218gf.In this paper,the relationship between QFP device solder joint quality and main process parameters（laser power,heating time and nitrogen pressure）was obtained through the study of laser peak-ball welding technology of QFP device.Aiming at the mechanical properties of QFP solder joints,the process parameters of laser peeling-ball welding are optimized,which has certain engineering significance for the process development of laser peeling-ball welding in practical electronic device welding application. |
