If you have two planes of symmetry, XZ and YZ you can pick any vertex in the model and apply a Displacement (not a Fixed Support) of Z = 0 leaving the X and Y Free which will constrain the model and allow thermal stress to be correctly calculated. You will get the same stress no matter which vertex you choose. Only the Deformation plot will be different since the Z value will be zero at the vertex you chose.
If you really want the Deformation plot to show the zero Z value at some point in space between the two PCB packages, you can insert a Remote Displacement, type = Deformable and choose an edge on each package. Edit the global coordinates of the Remote Displacement to be on the Z axis.
Please reply with an image of the geometry.
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