This invention relates to the art of cooling integrated circuit packages, and more particularly to a new and improved aerodynamically-enhanced heat sink.
With the introduction of very large scale integrated (VLSI) devices, heat dissipation has become an ever-increasing problem. The idea of cooling fins for electrical apparatus is quite old, and there are legions of different types of heat sinks that have been used in the past. For the most part, these heat sinks have been employed in larger devices and typically include some form of cooling fins adapted to dissipate heat into the air.
It has been the practice to apply these older heat sink techniques to the newer VLSI circuit packages. However, there are numerous unique problems associated with heat dissipation for integrated circuits that make these older heat sinks unsatisfactory. One such problem is the reduced size of the newer integrated circuit packages vis-a-vis the older electronic apparatus. Another common problem associated with dissipating heat from integrated circuit packages has been growth compatibility with the material of the semiconductor package and that used to make the heat sink.
An integrated circuit is a fragile device fabricated on a silicon substrate. Thus, a suitable heat sink should not cause any warpage of the device or the package housing such device. The warpage problem occasioned by expansion and contraction of a heat sink becomes more acute when used with a brittle package material, such as ceramic which is typically used for housing integrated circuits.
As a general rule, the heat sink is made of metal, such as copper or aluminum. The heat sink is rigidly attached to the semiconductor package by an epoxy or a solder. During this attachment process, the epoxy or solder is heated to a fluid state to effect attachment of the heat sink to the semiconductor package. Thereafter, the epoxy or solder is allowed to cool and harden. This cooling and hardening step also induces stress in the package, and particularly in the material used to attach a lid to the semiconductor package. These stresses vary in magnitude with the overall shape of the particular heat sink that is being attached. And, depending upon the shape of the heat sink, the stresses can become so large as to cause cracks in the lid attach material. When this occurs, the hermetic seal of the integrated circuit is broken which makes the device inoperable.
So long as the heat sink attach material remains liquid, the stresses induced in the lid attach material remain relatively small. However, once the heat sink attach material soldifies, the stresses in the lid attach material rapidly increase. This rapid increase in stress is due to the fact that the heat sink contracts much more rapidly than the ceramic substrate. For example, the coefficients of thermal expansion for copper and aluminum respectively are about 2.6 and 3.6 times the expansion coefficient of ceramic.
For a more detailed analysis of VLSI device heat sinks, their problems and several prior art solutions, reference is made to a technical paper entitled "VLSI Thermal Management in Cost Driven Systems" by T. E. Lewis and D. L. Adams, which was published in the EIA Proceedings, 32nd Electronic Component Conference, 82CH1781-4, May 10 through 12, 1982, at page 166.