th en th rou gh th e h eat sin k attach m aterial (or th erm al in terface m aterial), an d fin ally
to th e h eat sin k wh ere it is rem oved by forced-air con vection . Sin ce th e silicon th erm al
resistan ce is qu ite sm all, for a first-order an alysis, th e tem peratu re drop in th e silicon
m ay be n eglected. Th u s, th e h eat sin k attach m aterial an d th e h eat sin k con du ction /
con vective th erm al resistan ces are th e dom in an t term s.
8.7.2 Adhesives and Thermal Interface Materials
A th erm al in terface m aterial is recom m en ded at th e package lid-to-h eat sin k in terface
to m in im ize th e th erm al con tact resistan ce. For th ose application s wh ere th e h eat sin k
is attach ed by a sprin g clip m ech an ism , Figu re 21 sh ows th e th erm al perform an ce of
th ree th in -sh eet th erm al-in terface m aterials (silicon , graph ite/ oil, floroeth er oil), a bare
join t, an d a join t with th erm al grease as a fu n ction of con tact pressu re. As sh own , th e
perform an ce of th ese th erm al in terface m aterials im proves with in creasin g con tact
pressu re. Th e u se of th erm al grease sign ifican tly redu ces th e in terface th erm al resis-
tan ce. Th at is, th e bare join t resu lts in a th erm al resistan ce approxim ately 7 tim es
greater th an th e th erm al grease join t.
Heat sin ks are attach ed to th e package by m ean s of a sprin g clip to h oles in th e
prin ted-circu it board (see Figu re 19 ). Th is sprin g force sh ou ld n ot exceed 5.5 pou n ds.
Th erefore th e syn th etic grease offers th e best th erm al perform an ce, con siderin g th e low
in terface pressu re. Of cou rse, th e selection of an y th erm al in terface m aterial depen ds
on m an y factors -- th erm al perform an ce requ irem en ts, m an u factu rability, service tem -
peratu re, dielectric properties, cost, etc.
39 of 44
PPC740 and PPC750 Hardware Specifications
Preliminary and subject to change without notice
IBM [ IBM ]
