Tombstone Example tomb-D for the Surface Evolver

[Click for the tomb-D.fe datafile in a second window.]

This model permits the liquid solder to wet the sides of the chip.

Starting geometry.	Evolved with gogo.	Evolved further, showing problems.

Notable features:

• The solder is still completely enclosed with facets, so the contact angles are all implemented by setting the surface tensions of facets.
• The problems due to the explosive nature of negative tension facets are getting serious here. Evolution in gogo had to be stopped before the surface got close to equilibrium.
• The solder contact facets on the sides of the chip do not have all possible one-sided constraints on them, to speed calculations. This means special steps are needed in deleting edges, since the Evolver will only delete edges when the constraints on the edge match the constraints on one of its endpoints. So a special edge deleting command deledge is defined that removes one-sided constraints from the edge endpoints before deleting the edge. The command is used by setting the variable deledge_id to the id number of the edge to be deleted, and calling deledge.
• Note how the contact line dips to a cusp along the vertical front edges of the chip. Since the sum of the contact angles on either side of such an edge is less than 90 degrees, the contact lines will ideally become tangent to the edge, or else reach all the way down to the chip bottom.
• Also note how the contact line cusps along the bottom edge of the chip on the side. Such cusping requires careful evolution, particularly when the contact surfaces are explicitly represented with facets.