Why is my laptop overheating after (trying to) switch processors?

A lot of unknowns, such as which IC? Intels, for example, run cooler than AMD's. Heat dispersion is critical...the more contact between the heat source and metal coolers, the better. The closer the metal heat sink is to the source, the better. The larger the heat sink is, the better. Heat compounds act as electric insulators, while allowing some heat transfer. The tighter they are between the source and sink, the better.