Page 115 - System on Package_ Miniaturization of the Entire System

90 Cha pte r T h ree

(a) (b)
FIGURE 3.8 Different heat transfer paths (a) with and (b) without a heat sink.

that will impose several thermal challenges that include high electrical interconnection
resistance, poor thermal transfer from chip to chip through polymeric adhesives, and
less space for power dissipation.
The first step in the thermal design of SIP is to understand the potential heat transfer
paths. Figure 3.8a shows an example with a heat sink mounted on top of a SIP. In this
configuration, the majority of heat generated by the SIP will be conducted to the heat
sink, and then to the external ambient by either natural convection or forced-air
convection. In addition to that, a small portion of heat is dissipated through the package
substrate, vias, solder balls, and then the printed circuit board. Only a very little portion
of heat is dissipated through radiation. Figure 3.8b shows an example without a heat
sink on top of a SIP. Under this configuration, the majority of heat generated by the SIP
is dissipated through the printed circuit board. Natural convection as well as radiation
can account for some dissipation through the package surfaces. In this particular
configuration, radiation usually plays an important role to help dissipate heat.
Neglecting radiation effects under this configuration may result in significant errors.
Thus, heat dissipation paths strongly depend on thermal designs. Understanding the
potential heat transfer paths and fully utilizing them in SIP designs leads to thermo-
mechanical reliability of SIPs.
The second step in the thermal design of SIP is to place hot components close to the
main heat transfer paths. Figure 3.9 shows examples of hot component placement under
different system designs. If the majority of heat is dissipated through the board or by
natural convection, the hot component should be placed close to the package substrate.
On the other hand, if the major heat transfer path is from the top surface such as through
radiation, the hot component should be placed near the package top.

Heat transfer through board Natural convection Heat transfer through top

High power die should be High power die should
located on the bottom be located on the top
FIGURE 3.9 Examples of the hot component placement under different system designs.