Ultim ately, th e fin al selection of an appropriate h eat sin k for th e PPC740 an d PPC750
depen ds on m an y factors, su ch as th erm al perform an ce at a given air velocity, spatial
volu m e, m ass, attach m en t m eth od, assem bly, an d cost.
7.7.1 Internal Package Conduction Resistance
For th e exposed-die packagin g tech n ology, sh own in Table 3 , th e in trin sic con du ction
th erm al resistan ce path s are as follows:
•Th e die ju n ction -to-case th erm al resistan ce
•Th e die ju n ction -to-lead th erm al resistan ce
Figu re 20 depicts th e prim ary h eat tran sfer path for a package with an attach ed h eat
sin k m ou n ted to a prin ted-circu it board.
External Resistance
Radiation
Convection
Heat Sink
Thermal Interface Material
Die/Package
Chip Junction
Internal Resistance
Package/Leads
Printed-Circuit Board
Radiation
Convection
External Resistance
(Note th e in tern al versu s extern al package
Figure 20. C4 Package with Heat Sink Mounted to a Printed-Circuit Board
Heat gen erated on th e active side (ball) of th e ch ip is con du cted th rou gh th e silicon ,
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.
7.7.2 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
37 of 43
PPC740 and PPC750 Hardware Specifications
Preliminary and subject to change without notice
IBM [ IBM ]
