SMTA International Conference Proceedings


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

Authors: Mudasir Ahmad, Ken Hubbard, and Sue Teng
Company: Cisco Systems, Inc.
Date Published: 9/24/2006   Conference: SMTA International


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.

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 solder joint shape is presented. Large deformation viscoplastic finite element analysis is used to simulate incompressible fluid flow.

The effects of package weight, solder volume and pad size on solder joint shape are analyzed. Different combinations of the design variables were simulated and functional relationships developed based on multivariate polynomial regression. The combination of design variables that can cause bridging in lead free solders are outlined. The amount of warpage over different lengths for 1mm, 0.8mm and 0.5mm pitch solder joints that can cause bridging is provided. The relationship between warpage and standoff height and maximum diameter is also provided for the different pitch values. This can be used to determine how much effective warpage would be acceptable to achieve a desired standoff height and maximum diameter.

Finally, a method to predict solder joint opens is outlined and compared with another known and validated shape prediction method (Surface Evolver [19]). The amount of displacement needed to cause a solder joint open is outlined.

KEYWORDS
ball grid array, coplanarity, eutectic solder, finite element analysis, high lead solder, hydrostatic pressure, leadless, microelectronics, micro lead frame, packaging, perzyna, rigid plastic, solder joints, shape, standoff height, surface evolver, viscoplastic, von Mises, warpage, pitch



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