Thermally efficient semiconductor device assemblies including interposers carrying a subset of the external contacts of the assembly, and methods of making the same

A semiconductor device assembly, comprising:

• a substrate including a first plurality of external contacts at a lowermost surface of the substrate;
• an interposer having a length and a width each less than both a length and a width of the substrate, the interposer having a plurality of inner contacts on a first side and having a second plurality of external contacts on a second side opposite the first side, the interposer coupled to the lowermost surface of the substrate at least in part by one or more of the plurality of inner contacts;
• a die disposed between the lowermost surface of the substrate and the first side of the interposer;
• a first plurality of solder balls disposed on corresponding ones of the first plurality of external contacts;
• and
• a second plurality of solder balls disposed on corresponding ones of the second plurality of external contacts,
• wherein the first plurality of solder balls and the second plurality of solder balls are configured to bond with co-planar package contacts.