ATI R300: Thermal Problems Caused by the Shim

ATI R300 Package: Thermal Problems Caused by the Shim

The root of the problem is the shim, but is this gap thermally significant, or simply unimportant?

To put it bluntly, there is problem with the ATI R300 processor package that impacts just how well heatsinks can cool on the Radeon 9700 Pro.

With the area around the R300's silicon core populated by ultra small capacitors and resistors, the manufacturer has placed something there for protection. That protection comes in the form of a 4mm wide band of copper which runs around the perimeter of the 40mm square package. It helps to prevent the tiny components from getting crushed, or the core from being chipped.

This copper shim is glued to the surface of the PCB, and if it were installed flush to the height of the silicon core, it would actually be very useful.

Unfortunately, the problem is that the copper shim is actually slightly thicker than the silicon core by about 0.0023". This makes it impossible for the flat bottomed heatsink ATI use to actually come in contact with the R300 core for optimal cooling.

In the retail package, ATI have gotten around this by using a thick yellow thermal interface material (TIM) between the extruded aluminum heatsink and the silicon core. That yellow TIM bridges the gap.

However, most users looking to push their ATI Radeon 9700 card a little farther with the stock heatsink will tend to want to remove the thermal interface material, and replace it with a good quality thermal compound. TIM's are generally not known for possessing the best thermal conducting properties.

You will of course remember that for a thermal compound to work well, the smallest possible amount should be used. After all, the material is just meant to fill in the microscopic voids between the silicon and the relatively "rough" aluminum base of a heatsink. [Please see "Heatsink Fundamentals: Contact Resistance" for more explanation on this topic]

With the ATI R300 FC-BGA packaging, the shim necessitates the use of a rather thick layer of thermal compound just for the heatsink to make some sort of contact with the core, and that isn't good. Several web sites have even gone so far as to remove the shim with a sharp X-Acto knife, but I'm not too keen on taking a chance in scratching (cutting) a trace line on an expensive videocard, rendering it useless.

We can only guess that someone perhaps miscalculated the thickness of the copper shim used on the R300 FC-PGA processor package, or forgot to include the height of the adhesive in their calculations. If you look very closely, there is a very small gap between the copper and the package surface.

Since it is unlikely that ATI will make any changes in their manufacturing process to even out the heights between the shim and the R300 core, the real question that remains for us is what kind of temperature hit are we talking about here? Is the gap caused by the shim thermally significant, or simply unimportant? And what's more, if the stock heatsink were able to sit flush to the silicon core, instead separated by a thermal interface material, what would the gain be?

To answer this question we decided not to remove the shim (though some people have without any problems other than a voided warranty), but rather to mill out a small area from the base of the stock heatsink. Once we cut away some of the metal, the heatsink would be able to sit squarely on the core.