Liquid Bridge Shape Analysis Of QFN Solder Joint And Structure Design Of Self-assembly Welding

With the rapid development of portable intelligent terminal and electronic device, the elec-tronic components have been impelled to develop rapidly toward miniaturization, thinning, cost of cheapness and high performance. Therefore, it is urgent to improve the packing density, reduce the package size, increase the pins numbers of the package, reduce the dis-tance between the pins and strengthen the electronic and thermal properties of the packaging body. With the electronic packaging rapidly developing toward miniaturization and high per-formance, electronic packaging is researched more and more at home and abroad. With the merits of structure and performance, such as, small, thinning, low-cost, high heat dissipa-tion, low impedance, low self-inductance and so on, therefore, the QFN package (Quad Flat Non-leaded Package) well corresponded to the development trend of electronic packaging has been widely employed in electronic packaging industry.However, the characteristics of QFN package determine there are many technical problems in the practical application. For instance, due to the low welding height of QFN packaging device, there is prone to a solder bridge, solder bulge and other defects. With the decreasing of the pad size and pitch, the welding yield decrease. Because the package style of the QFN is leadless, the stress on the pin solder joints cannot be adjusted by lead deformation, causing an adverse impact on the service life of the small solder joints. There is great difficulty and high cost repairing printed circuit board (PCB) on which the QFN devices were mounted, that put forward higher requirements for the QFN device welding yield for the first time.In order to solve the QFN devices are faced with the problem in the practical application, the following researches are carried out and based on interface science and capillary mechanics theory.(1) A pseudo-three-dimensional surface tension equivalent model of the liquid bridge was proposed. Starting from Young-Laplace equation, the reasonable geometric approximation was put forward, the differential equations defining the shape of the liquid bridge between the rectangular structured surfaces were developed. The boundary conditions were given based on the wettability region. Using optimization method of minimizing the function of boundary value error, the method of transforming the initial problem of nonlinear ordi- nary differential equations into boundary value problem has been proposed. By this way, the morphological characteristic parameters and the force variation law of the liquid bridge were obtained, and the stiffness characteristic curve was given to reflect the relationship be-tween the force and the height of liquid bridge. Comparing the results of the finite element simulations performed by Surface Evolver with those of the method employed in this paper, the practicality of the method was demonstrated. The research results could be used as a CAE method for the analysis of QFN solder joints.(2) According to the specified QFN package devices, three types of common solder liquid bridge form were summarized. Based on the preliminary simulation analysis results of SE, the geometric characteristics of the solder joints were summarized. The cylindrical approx-imation was used for liquid thermal solder joint, the liquid gas interface at the two ends of the solder joint on the inner pin and the outer pin was approximated as ellipse surface, then the differential equations of the liquid bridge corresponding to each solder joint was established. According to the wettability change of the molten solder on the pad surface, the reflow soldering process was divided into "slip" and "hinge rotation" stage, then the bound-ary conditions were given and the corresponding objective functions of the boundary value error was established, which could provide theoretical basis for the morphology and force analysis of the liquid solder joints of QFN.(3) For purposes of improving the self-assembly adaptive ability of the solder joints, the design idea of the pad structure was proposed. The minimization method of the boundary value error function was used to transform the boundary value problem into the initial value problem, and the stiffness characteristic curve of liquid bridge is obtained. By analyzing the stiffness characteristic curve, it can be find that when the self-assembly equilibrium position locates in the middle of the stiffness characteristic curves of the inner and outer ring small solder joints, QFN has the maximum welding height tolerance and high capacity to adapt the uneven distribution of the welding gap caused by chip and PCB board’s flatness error and thermal warp distortion.(4) The object of the pad structure design is to improve the ability of self-assembly of the solder joint group. Based on the stiffness characteristic curves of liquid solder joints, the design criterion of PCB pad which makes the QFN have the maximum height tolerance for welding is proposed. By changing the PCB pad size to meet the design criteria, can signif- icantly improve the welding yield and reduce production costs. As shown in the examples, the results and SE simulation results were in good agreement, which could not only greatly improve the computational efficiency, but also avoid the shortcomings of SE, such as grid surface intersection with each other or unable to determine whether the energy has converged to minimum level.