Package Tilt Prediction Using Surface Evolver

In board level package mounting process, many parameters will influence the final joint shape including the package stand-off height. If the stand-off height of solder joints at opposite ends does not equal, package tilting will occur. As a proactive measure to ascertain no tilting issues for newly develop packages, a methodology of predicting package tilting for BGA is developed using an energybased simulation model with surface evolver. The solder joint prediction model developed by Brakke proved to be very useful in predicting solder joint geometry after reflow and it will also be used to predict package tilting for BGA during board level mounting. This paper will discuss the capability of Surface Evolver in predicting solder joint geometry like solder ball height and diameter, and solder joint height. Then it moves on to discuss the methodology in using the same tool in predicting package tilting in board mounting process. It will be shown that the result of the surface evolver is well within the experimental data. Surface evolver program requires a command line input programming which is not very user-friendly, so a user interface was created using Visual Basic^® 6 so that the engineer will only need to input relevant parameters into the program and command encoding is done automatically.