Page 353 - A Practical Guide from Design Planning to Manufacturing

Microprocessor Packaging 323

1°C/W, allowing a 40°C rise when 40 W is applied. The overall product
thermal resistance is the sum of the thermal resistance of all the layers
giving a total of 1.5°C/W, allowing a 60°C rise on the processor die from
the ambient air around the heat sink at 40 W of power.
Thermal resistance could be improved by thinning the die, but if it is
too thin, it may crack during the assembly process. Applying the IHS
with more pressure thins the first TIM layer, but this again risks crack-
ing the die. Applying the heat sink with more pressure thins the second
TIM layer, but this can crack the PCB itself. An important function of
the IHS is to protect the die from the mechanical stress of attaching the
heat sink. A larger heat sink reduces thermal resistance but adds volume
and area. Running the heat sink fan at higher speeds also helps but pro-
duces more noise. External airflow over the board components also
affects die temperature. Adding more case fans cools the ambient air in
the case, but this again adds noise. Some heat sinks use liquid forced
between the fins instead of air, but liquid cooling solutions are more
expensive and difficult to make sufficiently reliable.
Compounding the problem of cooling individual components is the
trend of smaller packages that allow more components to be placed
closer together on a single PCB. The overall power density of the board
is made higher, and a large heat sink that cools one component may actu-
ally impede airflow to other components on the board. For systems
where space constraints require the use of very small or no heat sinks,
thermal resistance will be dramatically higher. This can easily limit the
processor performance that is possible without the die reaching tem-
peratures beyond its reliability limit. Processors can operate with on-die
temperatures above 100°C, but at higher temperatures the transistors,
wires, and solder bumps degrade over time until eventually the proces-
sor fails. As future processors use transistors with smaller dimensions
and increased power density, improvements in thermal resistance will
be required to maintain the same power levels and temperatures.

Multichip modules
There is a constant drive to add more functionality to each die. Perform-
ance is improved and the required board space reduced by combining
multiple die into a single larger die that implements the same func-
tions. However, yield loss for very large chips limits how large a die
can be manufactured at a reasonable cost. An intermediate solution is
to combine multiple separate die into a single package, a multichip
module (MCM).
The performance of an MCM is improved over that of separate pack-
aged die by having short high-quality interconnects between the die
built into the MCM. Although the performance is not as good as a single