5ed4b506_CB_vs__MCM_c_Comparison_Matrix__1
[0] Heat dissipation [2] External heat management add-ons, such as heat sinks, or forced convection or spreading devices out, which adds cost and makes the design more complex [3] Unpackaged ICs die mounted directly to a high thermal conductivity alumina substrate provide a less complex solution [1] 
With inadequate heat dissipation chips run hot, degrading lifetime. Nonuniform heat dissipation causes some parts of the MCM to run hotter than others, changing electrical properties (edge-rate degradation)
Appendix A: A PCB vs. MCM-c Comparison Matrix
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