| The portable electronic products are easy to drop, slide or be impact in the course of using or transporting because of its small volume and low weight. The stress arising from falling is apt to cause the solder joint failure or even complete machine trouble, especially to the printed circuit board which mounted with TFBGA (Thin-profile fine-pitch ball grid array). As the requirements of microminiaturization and light of weight and the fast popularization of portable electronic products, TFBGA is used widely which causes the problem more severe. Meanwhile, with the implementation of lead free directive, compared with Sn/Pb solder joint, the higher rigidity and bigger modulus of lead free solder joint make the PCB easier to fall or impact which will cause failure.So the drop impact reliability of solder joint is the key content of unleaded portable electronic products.Whereas the drop test shows the limitation of high cost and long cycle, the theory analysis and dynamic simulation are investigated in this paper. Firstly, educing the drop and shock equation of the simplified PCB assembly and obtaining the analytic solution under the particular boundary condition according to the dynamics theory. Secondly, taking the typical lead-free TFBGA as the target and ANSYS software as the pre-processor, according to the drop test standard of JEDEC, two board level drop impact models of lead free TFBGA are developed. The drop simulation analysis is executed through the LS-DYNA explicit solver, the dynamic responses of PCB and solders joints during drop impact are obtained and their results are compared. Thirdly, the influence of the solder pad type, pad thickness, height of solder joint, the diameter of solder joint and the solder joint material to the drop impact performance is studied by combining the Input-G model with dynamic submodel technology. Meanwhile, the drop impact life-span of lead free solder joint is predicted based on the stress standard. Fourthly, the relationship between the number of screws which support the PCB and the bending of PCB under drop impact is studied through the theory and simulation modal analysis, and the influence of every stage mode to the PCB bending deformation is discussed. At last, a board level drop test for the components of portable electronic products is introduced.The study about board level drop impact shows that: (1) The mechanical impact and bending of PCB are the main reasons which induce the solder joint stress even the drop failure; (2) For the PCB which is supported by 4 screws, the solder joint at the outermost corner of the TFBGA mounted at the center of PCB is apt to failure most; (3) The solder joint drop impact life is decreased along with the increase of maximal normal stress; (4)Adopting the SMD pad design on components side is easier to cause the stress concentration which results in the solder joint with worse drop impact performance; (5) The maximal stress position of solder joint appeared on the components/solder joint side or PCB/solder joint side depends on the thickness of the pad; (6) Compared with leaded solder joint, lead free solder joint have greater drop impact stress under the same drop impact condition, According the stress failure standard, lead free solder joint own shorter drop impact life compared with leaded solder joint; (7)The results of modal analysis shows that the first bend mode is the main factor which influences the bend deformation of the PCB. Adopting more screws to support PCB as well as reducing the influence of the first mode is good for improving the drop impact performance of solder joint.For the first time, a method combining the use of dynamic submodel technology and Input-G model is proposed to study the drop impact reliability of solder joint, and the support optimization design method of PCB based on the modal superposition is developed,which provide a convenient and practical approach to study the drop reliability of solder joint further.Through the systematical study of the board drop reliability for lead free TFBGA and the influence analysis of the individual factors to the joint drop performance, it can provide a theory foundation to designers to improve the drop performance of solder joint during the conceiving of the layout of components as well as a guideline for board level mount structure design, solder process optimization and the connection optimization design of PCB to machine body. |
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