Inspection of fine pitch components’ solder joints
One of the fastest growing package types in the electronics industry today is the QFN (Quad Flat No leads) package from CSP (Chip Scale Package). This package type is very similar to the QFP package. It’s available in four-sided configurations with either sawed or punched leads. A common problem of QFN package is short circuit formation.
I was assigned in my thesis to make a proposal to eliminate the solder bridging of QFN packaging during the reflow. I will analyze with calculations and simulations, what the optimal stencil aperture and solder pad geometry are.
I acquainted an integrated system for prediction and analysis of solder interconnection shapes, the Surface Evolver uses this model. The Surface Evolver is an interactive program for the study of surfaces shaped by surface tension and other energies. The Evolver evolves the surface toward minimal energy by a gradient descent method. The energy in the Evolver can be a combination of surface tension, gravitational energy, squared mean curvature or user-deﬁned surface integrals. Evolver can handle volume constraints, boundary constraints and boundary contact angles. I used Surface Evolver to predict the solder interconnection shapes of the QFN-s. The simulation showed that, there is no solder bridging between the solder joints. In case of aperture E develops the greatest width of solder joint, 347 µm. The original distance of the pads is 350 µm, this distance decreases to 303 µm after the reflow, the simulation approaches this results with 1.64 % relative errors.
I imported this model to Comsol Multiphysics, and I inspected the mechanical strength of the solder joint. I did mechanical inspections on the solder joints. After that the solder joints can be distinguished on the basis of the mechanical properties and I’m able to decide, what the optimal stencil aperture and solder pad geometry are. Thus I can eliminate the solder bridging of QFN packaging. The least mechanical stress (39 MPa) is measured in case of aperture E with the prescribed displacement. I measured the greatest mechanical stress (55,4 MPa) in case of aperture A. In both cases the prescribed displacement was 6∙10-8 m. In case of aperture C the mechanical stress in the solder joint is 41,6 MPa. Based on the results, I recommend to use aperture E and C.