A NEW METHODOLOGY TO PREDICT AND MINIMIZE MOLTEN SOLDER JOINT OPENS AND SHORTS DURING REFLOW

Abstract Ball Grid Array technology has become the default standard for high performance packaging. With package sizes exceeding 50mm, warpage, coplanarity, field reliability and reworkability are becoming ever more challenging. As reliability margins drop, there is a greater need to control more of the parameters that could impact field life. It has been shown that the post reflow shape of solder joints can significantly impact the reliability of a package [1]. Warpage of large packages places several constraints on the minimum standoff height that can be obtained without causing shorts (bridging) and opens during reflow. This constraint will become even more of a concern as these large BGA packages migrate to finer pitches, i.e. below 1 mm pitch. In addition, for small packages with pitches as low as 0.5mm, the optimal solder paste volume needed to avoid solder bridging becomes a challenge.

To ensure good solder joint formation and prevent solder bridging, it is critical to understand the amount of paste volume needed during assembly and reflow. The final shape of the solder joint is a function of surface tension, wetting area, gravity and applied forces.

In this study, a new methodology to simulate eutectic and lead free solder joint shape is presented. Large deformation viscoplastic finite element analysis is used to simulate incompressible fluid flow. A numerical model for surface tension is outlined and validated with closed form solutions. The results of the numerical model are compared with other known solder joint shape prediction methods.

The effects of package weight, warpage and pad misregistration on solder joint shape are then analyzed. Recommendations to maximize standoff height and to avoid bridging and opens for tin-lead and lead free solders at 1mm and 0.8mm pitch are provided. Finally, the formation of leadless solder joints is studied and compared with experimental data.