Page 352 - A Practical Guide from Design Planning to Manufacturing

322 Chapter Ten

Transistors on the die surface release heat. In a flip-chip package, the
transistors will be on the side of the die facing the package substrate.
Some heat will flow out through the solder bumps and package leads.
The rest will diffuse through the silicon chip to the backside of the die.
Packages for high-power processors may include an integrated heat
spreader (IHS). This is a metal plate that touches the backside of the die.
It helps cool the die by allowing the heat of the die to rapidly diffuse over
a much larger area.
To make sure there are no gaps between the die and the heat spreader,
a polymer paste is spread across the backside of the die. This type of
layer, added specifically to allow heat to carry between separate pieces
of a package, is called thermal interface material (TIM). Heat from the
die passes through the TIM into the IHS. The heat then moves through
the metal of the IHS before reaching a second layer of TIM. This layer
fills the space between the IHS and the heat sink.
Heat sinks are fabricated of aluminum or copper fins to have as much
surface area as possible. They allow heat to be dissipated into the air
around the processor. Many heat sinks use a built-in fan to blow air
through the fins and remove heat more rapidly. Although a processor in
3
a package might occupy less than 1 cm and weigh only a few grams,
3
the heat sink required to use the processor might be 150 cm and weigh
500 g. Some heat sinks are attached to the processor package, but many
are so large and heavy that they must be attached instead to the PCB
or directly to the external case. Being bolted directly into the product
case gives a heat sink more support but prevents it from being attached
until the final assembly of the product. The motherboard and processor
must be installed in the case before the heat sink, and if the processor
is to be replaced, the heat sink must be unbolted from the case first. The
size, weight, and noise generated by a computer system are all signifi-
cantly impacted by the processor heat sink.
The thermal resistance measures the effectiveness of the package and
heat sink in cooling the die. It is the ratio temperature rise versus power.
In the example in Fig. 10-10, the ambient air around the heat sink inside
the product case is assumed to be 40°C. When the processor is using 40 W
of power the hottest spot on the IHS is measured as being 60°C. This
means that the thermal resistance of the heat sink and its TIM is
0.5°C/W, allowing a 20°C rise in temperature when 40 W is applied.

C 40
°
Ψ = (T IHS − T ambient ) = (60 °− ° C) = 05 CW
/
.
heat sink
power 4 40 W
At the same time, the die temperature is measured as 100°C (near its
reliability limit). The thermal resistance of the package is therefore