Another cost adder in cold plate manufacturing is the addition of holes. One hole may add as much as $3 to the cost of a cold plate. One of the main reasons that holes add costs is that holes cannot be made in the fluid path. Therefore, for a tubed cold plate, a bend in the tube needs to be made to accommodate the hole, and each bend adds cost. For a vacuum-brazed cold plate, an island must be created in the fluid path, which also means electrical discharge machining (EDM) the internal fin. This can add quite a bit of machining time and therefore cost.
There are several types of holes. One type is a through hole, which passes from one side of the cold plate to the other. A second type is a tapped hole, which has screw threads. Since aluminum is relatively soft, tapped holes have a limited life if the components or boards are frequently changed. With tapped holes, helicoils are often used. A helicoil is a sturdy steel insert that adds strength to the threads for applications in which there is likely to be frequent component change out. Through holes are produced by a single drilling process, while tapped holes require an additional tool on the same machine set up. Helicoils require a tapped hole in order to be installed, and the helicoil installation itself is completed outside of the machining center. In summary, through holes are the least expensive and helicoils are the most expensive.
Tight tolerance of the location and spacing of holes can also drive up costs. A reasonable tolerance specification is ±0.005 inch (±0.013 cm). As with flatness, specifying local tolerances when possible will reduce cost. With big cold plates where holes can be relatively far away from each other, the tolerance becomes harder to maintain. One reason is that machine tool tolerances increase as the head has a farther distance to travel. Another reason is that there may be thermal gradients of as much as 18°F (10°C) in the machine shop, which can expand or contract the cold plate by as much as 0.005 inches (±0.013 cm). Through holes are the easiest to specify a tighter tolerance for because creation of a through hole is accomplished with a single tool operation, while tapped holes are not as easy to tolerance because making them involves two tools. Helicoils are the hardest to tolerance because the process requires a tapped hole and the helicoil itself has a tolerance. All the tolerances add up, making it harder and more expensive to manufacture. Avoiding small tapped holes will also help to reduce cost. Hole sizes of 4-40 or smaller become difficult to tap as the taps can break while drilling. In order to minimize this problem, the machine must run much slower. One way to counter tight tolerances requirements on a cold plate is to increase the size of the mounting holes in the component or board.