This article will look at bench testing methods and equipment useful in ascertaining the performance capability of waterblocks used as part of a CPU cooling system.

There are several different approaches to such testing; a unit’s in-place performance can be measured under a single set of (sometimes unquantified) conditions, or it can be “bench tested” under a range of conditions such that the unit’s performance capability can be predicted in any likely installation (assuming of course that the other components' performance capability data are available).

A previous article (HERE) described the relative merits of bench vs. system testing. Here we will consider a somewhat complex bench testing equipment setup and a testing methodology designed to characterize a waterblock’s capabilities.

It is hoped that the test method outlined here might be a 'talking point' for some future collaboration between interested testing parties. After a general agreement on test methods, the real issue of cross-calibration could be addressed to enable the comparison of test results from different sources. And farther yet down the road, standardized test procedures could be drafted and validated with conventional round-robin testing.

Readers are advised that this is an article about testing - using waterblocks for sure - but about testing; the equipment and the procedures. Many technical terms are used without definition (in the interest of brevity); when an unknown phrase is encountered, please use Google as an understanding of much of what follows will likely depend on an accurate understanding of that particular word or phrase.

An understanding of how a waterblock functions is helpful in visualizing the temperature reactions of the various elements in the heat dissipation path from the CPU to the coolant.

Waterblock (wb) is the generic name of the small heat exchanger placed on the CPU die face to reduce its temperature. The wb consists of a baseplate (bp) - which may be flat or finned for increased area, whose face is pressed in contact with the die and which removes the die’s heat by conduction; and a water box through which the coolant flows in contact with the backside of the bp. The heat is transferred from the backside of the bp (and fins if any) to the coolant by convection.

Note that at equilibrium all the heat from the CPU (less the secondary heat path losses) will be transferred to the wb - at any flow rate. The heat in must equal the heat out or it is not at equilibrium; and the temperatures will continue to rise (or fall) until equilibrium is established. This is true of all wbs:

It is quite incorrect to say that a good wb transfers ‘more’ heat than a poor wb, it is the temperature gradient across the wb that is different.

The wb’s bp temperature (gradient) will be materially affected by changes in the coolant flow rate.

A higher coolant flow rate (implying higher velocity) will improve the heat convection effectiveness within the wb.

This will enable equilibrium to be reached at a lower temperature differential between the coolant and the bp, which in turn will reduce the temperature of the bp, and hence the temperature of the die itself.

Differences in wb efficacy will manifest themselves in this way as well, the more efficient wb (with better convection rates) will have a decreased temperature differential between the coolant and the bp. Differences in conduction 'resistance' through the bp will also differentiate between wbs' performance.

Note that the coolant temperature rise is a consequence of the heat input, divided by the flow rate. The coolant temperature rise is therefore the same for all wbs at the same heat load and flow rate. (This is a greatly and excessively simplified description of the interaction between Fluid Mechanics and Thermodynamics in a 2 inch box. Many more words are needed to adequately describe the functions and relationships within a wb - but this article is not about wb design, just their testing.)

In between the wb’s bp and the die’s face is a thermal joint, which typically contains some form of Thermal Interface Material (TIM) to improve the heat transfer from one surface to another.

This joint will have the effect of a series resistance to the “flow” of heat and will always result in a temperature offset of some magnitude. As no wb can be tested without also including a TIM joint, the characterization, and more importantly CONTROL, of this variable becomes essential. A method of so doing, and the one utilized here, was described in this article (HERE).

To recapitulate then; under consideration is the CPU die face (the source of the heat), the thermal joint (filled with a TIM), the waterblock’s baseplate (between the TIM joint and the coolant), and the coolant.

Bill Adams