[Cathar]

Waterblock design is all about balance, but ultimately, for a high performing waterblock design, it is about maximising convective thermal transfer (or CHTC as 8-ball points out above) from the metal surface to the coolant. Actually I believe it's better to refer to it by its density, being the convective heat transfer density (CTHD), expressed as W/m^2K.

Maximising surface area where it's needed most is implicitly accepting that it is not possible to maximise the CHTD to a high enough level whereby the heat may be dissipated using just the surface area immediately above the copper die.

This gets into base-plate thickness issues as well. The thicker the base-plate, the wider the thermal spread, and therefore the less of a need for CHTD to be maximised to deal with the wider heat distribution.

The issue with thicker base-plates as that they create a larger temperature differential from the heat source to the convective surface. Thick base-plates are essentially bad if we want to move forwards with block design. For a 10x10mm heat die of 80W, there's about a 2.5C difference to move through the first millimeter of pure copper. By then the heat has spread out a bit further laterally and the next millimeter "costs" less. To move through the next millmeter to a flat convective surface "costs" about another 1.8C. The 3rd millimeter costs about another 1.4C. The 4th millimeter costs about 1.1C extra. The 5th millimeter costs about an extra 0.8C. By the 5th millmeter though the bulk of the heat has spread out to roughly 4x the surface area of the 10x10mm die, consequently requiring a lower CHTD to deal with the heat.

So we want a thin base-plate, but this is not so easy, because this hinges solely on the CHTD. If CHTD isn't high enough to deal with a too thin base-plate, then the rise in temperature of the metal surface above the coolant may very well be greater than the cost of conducting heat through the copper for a slightly longer distance.

Let's assume we have a CHTD of around 25000 W/m^2K, which is fairly typical of a single inlet above the die design on a flat base-plate like the Swiftech MCW462 blocks. If the copper base-plate is 1mm thick, we incur a 2.5C cost for the copper conduction, and have about 144mm^2 of surface area with which to cool the bulk of the heat (I say bulk of the heat because some heat does conduct laterally further, but for the ease of simplicity we'll assume that 100% of the heat is concentrated in an inverted pyramid shape projecting upwards from the edges of the heat source). This gives a net C/W of the convective surface of 0.277, or for an 80W heat source a rise of (0.277 x 80 =) 22.2C. Add on the 2.5C of the copper as we get a total of around 24.7C.

At 4mm thick, we have 324mm^2 of effectual convective surface area, and a 6.8C copper conduction cost. Net convective C/W is 0.123, for a total temperature differential of 6.8 + 0.123 * 80 = 16.7C.

Make the same base-plate 5mm thick, and our copper conduction costs us 7.6C. Over the 400mm^2 area though, the net C/W of the convective surface is 0.10. Total temperature differential therefore is now 7.6 + 0.10 x 80 = 15.6C. We could keep on going but this expresses the point well enough.

Please note - these are just all rough calculations to express the point, rather than intending to be factual statements of reality. They also don't take into account the cost of a thermal interface material layer.

Clearly the CHTD of around 25000 is not high enough to counteract the increased heat density of a 1mm thick copper base, and a 5mm thick copper base gives lower temperatures, despite the extra 5.1C of copper conduction cost.

So we have three choices available to us at this stage if we want to see an improvement.

1) Keep the base-plate thin and raise the effective surface area (ESA)
2) Raise the CHTD
3) Attempt to do both

What jaydee is suggesting is 1). By attempting to raise the surface area say 4x after the first millimeter of base-plate we're attempting to lower the cost of copper conduction while maximising surface area to the same levels of travelling through 5mm of copper base-plate. of course the heat still has to conduct up the pins and "furniture" so it's not going to be a 1:1 tradeoff. We're attempting to claw back some of that 5.1C temperature differential of the 5mm base-plate.

By adding jets we also attempting to do 2). If we increase the CHTD as well, then we win on two fronts.

CHTD may be improved through various jet impingement methods.

So a good block design will therefore minimise base-plate thickness, maximise CHTD and maximise ESA. What becomes the real trick though is deciding how to manufacture the design such that these goals are achieved. Another more subtle point for those that have followed this far is that the need for more ESA can become diminished if CHTD is raised far enough (ie. effective direct die cooling).