[Pros] Applies a preload to the thermal interface material, improving thermal performance. Requires no holes or anchors. Easy removal/rework. [Cons] Must have "keep out" zone around the BGA for the clip. Extra assembly steps. [Cost] Low [Method] 
Clip-on
T h e r m a l t a p e
E p o x y
W i r e f o r m Z - c l i p s
C l i p - o n
P u s h p i n w i t h c o m p r e s s i o n s p r i n g s
S t a n d - o f f s w i t h c o m p r e s s i o n s p r i n g s