More on the SECC-2

As you can see from the below pictures of an SECC-2 PII, the backside is exposed, allowing for cooling devices to be attached to it and directly contact the components that actually get hot. The previous packaging used a metal "thermal plate" that was much worse at allowing cooling. Take a look at the SECC-2 packaging, both front and back, below:

Celerons have their own heatsink and fan combos because they are "naked". There is no casing over the Celeron's circuit board, which was implemented as a cost cutting factor. Because of this, Celerons need different mounting for their heatsinks and fans. Also, because both sides are exposed and the Celeron package is rather thin, there are a couple of cooling solutions out there that utilize the bare backside of the Celeron.

Most of the current crop of heatsink and fan combos for Slot-1 CPUs have 2 or 3 fans. This is a better design because combos with a single fan mounted in the center lose airflow to the outer heatsink fins on the edges. By adding more fans, you are insured that air is collecting heat off all of the heatsink fins.