DFM Guide: Designing Liquid Cold Plates for CNC Machining
01Pick the Construction First
How the plate is closed decides who can make it, what it costs and which material grade it needs. There are five common constructions.
| Construction | How it is sealed | What the machine shop delivers | Best for |
|---|---|---|---|
| Bolted, o-ring or gasket sealed | Elastomer seal in a machined groove, cover bolted down | The finished plate: body, cover, groove, ports. No joining step. | Prototypes, test hardware, serviceable plates, low to medium volume |
| Brazed (vacuum or furnace) | Filler metal joins cover to body across every fin top | Machined body and cover, ready for the brazing vendor. Often a finish-machining pass after brazing. | High pressure, thin plates, production volume |
| Friction stir welded | Solid-state weld around the cover perimeter | Machined body and cover with the weld step; finish machining after welding | Aluminum and copper plates in volume, no filler metal in the loop |
| Tube in plate | Copper or stainless tube pressed into a milled groove | The grooved plate. Tube forming and pressing are separate operations. | Low heat flux, lowest leak risk, coolant never touches the plate |
| Gun drilled | Deep holes cross-connected and plugged | The finished plate, if the hole depth is within reach | Simple straight flow paths in thick plates |
The bolted design is the one to reach for while the thermal design is still moving. It can be opened, inspected and re-machined, it needs no outside joining vendor, and the first article can be on your bench without a brazing queue in the schedule. Its costs are thickness, since the cover needs bolts and the groove needs land, and an elastomer that has to be compatible with the coolant for the life of the product.
02Material: Copper or Aluminum
Copper moves heat better; aluminum is lighter, cheaper and much faster to machine. The numbers:[1][2]
| Property | C110 copper | 6061-T6 aluminum |
|---|---|---|
| Thermal conductivity | 391 W/m·K | 167 W/m·K |
| Density | 0.322 lb/in³ | 0.098 lb/in³ |
| Machinability | Rated 20 (free-cutting brass = 100): soft, gummy, burrs at every edge | Excellent: high speeds, clean chips, small burrs |
| Typical finish | Bare, or nickel plated for corrosion resistance | Bare, chem film or anodize outside the wetted area |
Copper earns its cost where heat flux is high and the plate is small, which describes a processor cold plate. Aluminum suits large plates with spread-out loads, such as power electronics and battery modules. Two cautions apply to both. First, if the design will be brazed or welded, the copper grade matters; C110 vs C101 explains why oxygen-free copper is required there and wasted everywhere else. Second, do not put aluminum and copper in the same coolant loop without an inhibited coolant rated for mixed metals; the aluminum becomes the sacrificial half of a galvanic cell.
Order plate in a stress-relieved or half-hard condition where you can. Cold plates are wide and thin and have most of one side milled away, which is the recipe for a part that bows when it is unclamped. For aluminum, 6061-T651 (stress relieved by stretching) stays flatter than plain T6, and cast tooling plate is flatter still where its lower strength and conductivity are acceptable. For copper, H02 half-hard cuts more cleanly than annealed stock.
03Channels and Fins a Cutter Can Reach
Every channel is a slot cut by an end mill, so the slot inherits the tool's limits. Three ratios keep a channel design inside standard tooling; all three come from our DFM guide for CNC milling.
The same logic applies to the plan view. Channel ends and serpentine turns take the radius of the cutter, so draw them as full radii. Inside corners in header pockets need a radius too, and a larger one is always cheaper; the corner radius rule gives the minimums by depth. Keep a uniform channel depth across the plate where the thermal design allows it: one depth means one tool and one Z level.
If the thermal model wants fins much finer than these ratios allow, that is a signal the design has moved from a milled plate to a skived or bonded fin insert. Those are different processes with their own suppliers; the machined body that holds the insert is still a milling job.
04Flatness and Finish on the Thermal Face
The thermal interface face is where the tightest tolerances on the part belong, and usually the only place they belong.
Apply flatness to the contact patch, not the whole plate. A flatness tolerance on the full face of a long plate is expensive and rarely what the design needs. What matters is the area under the device. A per-unit-area control ("flat within X per inch") or a flatness callout on a defined zone gives the thermal interface material a consistent bond line at a fraction of the cost. Overall bow can take a looser number because mounting screws pull it out.
Expect stress relief to move the part. Milling the channels releases stress in the plate and the part bows toward the machined side. The standard answer is to rough both sides, let the part relax unclamped, then finish the thermal face last with light cuts and low clamping force. This sequence is routine, but it is why a cold plate costs more than a bracket of the same size.
Do not over-specify finish. A well-milled face lands around 32 to 63 µin Ra, and thermal interface materials are designed to fill that. Finer finishes need fly cutting, grinding or lapping. The surface finish chart shows what each step costs. Decide whether machining lay matters, too: if the TIM is a thin grease, say so and the shop can control the direction.
When mounting holes, dowel pins and the contact patch must line up with a board or a socket, position them from the same datums as the device. The true position calculator and the stack-up calculator will tell you whether the tolerances you chose actually assemble.
05Seal Grooves
On a bolted plate the o-ring groove is the pressure boundary. Seal manufacturers publish the gland dimensions for every standard cross-section; use their tables for depth and width rather than deriving your own. The points below are the ones that get missed on drawings.[3]
Put the flatness of the two sealing faces on the drawing as well. An o-ring tolerates a small, uniform gap; a cover that rocks on a bowed body does not seal no matter what the groove looks like.
06Ports and Fittings
Tapered pipe threads (NPT) seal by wedging metal into metal with sealant in the gaps. In a thin aluminum or copper plate that wedge can crack the boss, the orientation of an elbow fitting is a matter of luck, and the sealant ends up in the coolant. Straight-thread o-ring ports avoid all three: the thread only clamps, an o-ring does the sealing, and the fitting can be removed and reinstalled without damage.
The common choices are SAE straight-thread o-ring boss ports (SAE J1926, the industrial counterpart of the aerospace AS5202 port, which uses the same o-ring-in-a-counterbore principle with J-form threads) and ISO 1179 (BSPP) ports with a bonded or elastomer seal. Our AS5202 port chart gives the spotface, thread and seal-cavity dimensions of the aerospace port by dash size. Whichever standard you pick, name it on the drawing; "1/4 port" is not a specification. Give the port boss enough thickness for full thread depth plus the tap drill point, and leave room for a wrench or a quick-disconnect sleeve beside it. The coolant manifold design guide compares the port standards side by side.[4]
07Burrs, Chips and Cleanliness
A cold plate has more hidden edges than almost any other machined part, and whatever is left inside ends up in a pump or a quick disconnect. Three design habits make a clean part achievable:
Make every edge reachable. A burr can only be removed where a tool can touch it. Cross-drilled intersections deep inside a header are the classic trap. Where passages must intersect, do it where a deburring tool can reach through a port, or make the intersection part of an open pocket that gets closed by the cover.
Avoid blind pockets that trap chips. Dead-end channels and deep blind holes hold chips and coolant residue through washing. A flow path that is open from inlet to outlet can be flushed; a dead leg cannot.
State the requirement. "Clean and free of burrs" means something different to every reader. If the system needs a particulate limit, a specific cleaning process, or bagging and capping of ports for shipment, put it in the notes so it is quoted and inspected.
Pressure and leak testing happen after the plate is closed. For a brazed or welded design that is at the joining vendor or your own facility; for a bolted design it is wherever final assembly takes place. Decide early who owns that test and what the acceptance pressure is, because it sets the bolt pattern, the cover thickness and the seal.
08Drawing Checklist
The rest of the quote package is covered in How to Write an RFQ.
Have a cold plate to quote?
Send the model and drawing. We will quote the machined body and cover, flag anything a cutter cannot reach, and respond within one business day.