By guest poster Phillip E. Tuma of the 3M Company
Use of passive 2-phase immersion for computing equipment is largely limited to IBM's exploration of the technology for cooling bipolar chips in the 1970s. The liquid encapsulation module or LEM, for example, was a 10×10 array of 4.6×4.6mm chips immersed in C6F14 liquid that boiled on the bare silicon. A vulnerability of this technique is the phenomena of incipience overshoot, a large temperature excursion before the inception of boiling that can allow a chip to overheat or stress it mechanically during the sudden temperature drop that follows. This issue was overcome by modifying the silicon surface with sandblasting followed by an aqueous KOH treatment. It was also observed that fluid-borne contaminants could distill out of the fluid onto or under the chip. Under-filling with beeswax kept contaminants away from the C-4 solder bump connections.