Cold Plate Manufacturing Methods Compared: How Data Center Liquid Cooling Plates Are Actually Made
Summary
A direct-to-chip cold plate is made by combining one fin-forming method with one closure method. Fins come from skiving, CNC machining, extrusion or brazed fin stock. The plate is closed by vacuum brazing, friction stir welding, fusion welding or a gasketed joint. This article compares each method on achievable fin pitch, flatness, pressure rating, tooling cost, volume breakpoint and lead time, and explains why a 20 percent difference between two quotes on the same drawing often reflects a different construction rather than a different margin.
KEY TAKEAWAYS
ARTICLE
Published information on cold plate manufacturing tends to fall into two groups. Supplier literature usually describes a specific process without stating the tolerances it can hold. Academic literature usually models thermal performance without reference to production tolerances.
Neither group directly answers the question a thermal engineer usually has at the drawing stage: given a die footprint, heat flux, pressure drop budget and annual volume, which construction should the drawing specify, and what does each option cost in tooling, lead time and thermal performance?
This article works through that question one method at a time.
Fin Forming and Closure Are Separate Decisions
Many published comparisons list skiving, CNC machining, vacuum brazing, friction stir welding and extrusion as five competing options. In practice, they perform different functions and are used together.
Three of them create the heat transfer surface. Two of them close the plate into a pressure boundary. Every real cold plate uses one from each group.
| Group | Methods | What it determines |
|---|---|---|
| Fin forming | Skiving · CNC machining · Extrusion · Brazed fin stock | Surface area, fin-to-base thermal resistance, pressure drop |
| Closure and joining | Vacuum brazing · Friction stir welding · Fusion welding · Gasketed | Pressure rating, leak integrity, distortion, serviceability |
A skived plate still needs a closure method, typically vacuum brazing or friction stir welding. A vacuum brazed plate still needs a fin-forming method, typically folded fin stock or machined channels.
Comparing skiving directly against brazing therefore compares a fin method with a closure method, and that comparison can’t lead to a clear conclusion.
In production, a smaller set of combinations is commonly used.
| Construction | Fin method | Closure | Where it fits |
|---|---|---|---|
| Skived copper, FSW cover | Skiving from solid | Friction stir welding | High flux, high pressure, no fin joint |
| Skived copper, brazed cover | Skiving from solid | Vacuum brazing | High flux, moderate volume |
| Machined channel, FSW cover | CNC milled channels | Friction stir welding | Complex flow paths, aluminum, prototype to mid volume |
| Machined channel, brazed cover | CNC milled channels | Vacuum brazing | Complex flow paths, higher pressure |
| Folded fin, vacuum brazed | Brazed fin stock | Vacuum brazing, same cycle | Highest fin density, large area, high volume |
| Extruded profile, machined ends | Extrusion | Welded or gasketed ends | Lowest cost at volume, lower flux |
| Machined channel, gasketed cover | CNC milled channels | Bolted with O-ring | Serviceable, prototype, lower pressure |
Method Comparison
The first table compares fin-forming methods on the parameters an engineer specifies and the costs a buyer evaluates.
| Parameter | Skiving | CNC machining | Extrusion | Brazed fin stock |
|---|---|---|---|---|
| Fin thickness | 0.2-0.6 mm | 0.8 mm minimum | 0.5-0.8 mm minimum | 0.1-0.2 mm foil |
| Fin pitch | 0.3-1.2 mm | 1.5 mm and up | 1.5-3.0 mm | 0.5-2.0 mm |
| Fin height | up to ~25 mm | cutter dependent | profile dependent | up to ~50 mm |
| Aspect ratio | up to ~40:1 in copper | ~10:1 practical | ~15-20:1 | 50:1 and above |
| Fin-to-base joint | None. Fin is the base | None. Monolithic | None. Monolithic | Braze layer at every fin |
| Materials | Copper, aluminum | Copper, aluminum, stainless | Aluminum only in practice | Aluminum, copper |
| Geometry freedom | Parallel fins only | Any channel geometry | Constant cross-section only | Straight or wavy fin |
| Tooling cost | Low. Blade and fixture | Very low. Fixture only | Moderate. Die plus straightening | Moderate. Braze fixture |
| Volume breakpoint | 500 to 50,000 per year | Under 500 per year | 10,000 and up | 5,000 and up |
| Tooling lead time | 4-8 weeks | 2-4 weeks | 6-12 weeks | 8-14 weeks |
The second table compares closure methods on pressure rating, distortion, defect risk and cost.
| Parameter | Vacuum brazing | Friction stir welding | Fusion welding | Gasketed |
|---|---|---|---|---|
| Joint type | Metallurgical, filler alloy | Solid state, no filler | Metallurgical, melted | Mechanical, O-ring |
| Pressure rating | 10-20 bar typical | 20-40 bar typical | 10-25 bar typical | 5-10 bar typical |
| Distortion | High. Full furnace cycle | Low. Localized heat | Moderate to high | None |
| Machine face after? | Required | Recommended | Required | Not required |
| Void or defect risk | Braze voids, under 10% of area | Root defects, tool wear | Porosity, cracking | Compression set over time |
| Materials | Aluminum, copper, stainless | Aluminum readily, copper difficult | Most | Any |
| Serviceable | No | No | No | Yes |
| Tooling cost | Moderate. Fixture and cycle | Moderate to high. Tool and fixture | Low to moderate | Low |
| Volume breakpoint | 5,000 and up | 2,000 and up | Any | Under 2,000 |
| Tooling lead time | 8-14 weeks | 8-16 weeks | 2-6 weeks | 2-4 weeks |
Skiving: Fins Formed From the Base Material
A hardened blade takes a shallow cut into a solid block and lifts the material rather than removing it. The lifted layer stands up as a fin, still attached along its root, and the blade indexes across and takes the next one.
This is the main advantage of skiving. Because the fin is formed from the base material, there is no joint between fin and base. Heat moving from the base into the fin doesn’t cross an interface, a filler alloy or an adhesive layer.
At high heat flux, this can matter more than fin count. A brazed fin stack can reach higher fin density, but each fin is attached through a braze layer, which adds a series thermal resistance in the heat path the design is trying to minimize.
Limitations of skiving
Skiving has three limits, all set by geometry and material behavior.
Fins are parallel and straight. The blade travels in one direction, so serpentine channels, pin fin arrays and variable-density regions can’t be produced by skiving. If the thermal design requires flow to change direction, that has to happen in the manifold or cover rather than in the fin field.
Aspect ratio is limited by the material. As fins get thinner and taller, they’re more likely to bend or fold during forming. Copper skives better than aluminum because it’s more ductile at the strain the process applies, which is why fine-pitch skived plates are usually copper.
The starting block also has to be thick enough. Skiving consumes material from the top of the block, so the finished base thickness equals the starting thickness minus the material formed into fins.
What to put on the drawing
Fin thickness, fin pitch, fin height and the tolerance on each. Then the base thickness under the fin field, because that is what remains after the operation and it is what the flatness specification acts on.
Specify skiving explicitly rather than describing only the geometry and leaving the construction to the supplier. A brazed fin stack at the same pitch will usually quote lower and will perform differently. If the thermal model assumes no fin-to-base resistance, the drawing needs to state the construction that provides it.
Extrusion: Lowest Cost at Volume, Limited by the Die
An extruded profile has integral fins along its length. The profile is cut to length, the ends and mounting face are machined, and a manifold or cover is added. At high volume, it offers the lowest cost per unit of surface area of the metal constructions covered here.
It’s also the most constrained. The die fixes the cross-section, so the fin field is uniform along the full length, with no variation in density, no local enhancement and no change in flow direction.
Limitations of extrusion
Aspect ratio is limited by how well aluminum fills the die. A tall, thin fin requires metal to flow into a narrow, deep die cavity under pressure. Beyond roughly 15:1 to 20:1, the fin may not fill completely or die life becomes uneconomical. Minimum fin thickness is around 0.5 to 0.8 millimeters for the same reason.
In practice, the material is aluminum. Copper can be extruded, but not into the fine fin geometry a high-flux cold plate requires at a practical cost.
Extruded profiles also have bow. The Aluminum Association standard straightness tolerance allows about 1.3 millimeters per meter. That’s roughly 0.13 millimeters over a 100 millimeter plate and roughly 0.8 millimeters over a 600 millimeter plate, and both exceed a typical 0.025 millimeter contact-face flatness requirement. The profile is straightened, and the mounting face is machined after straightening rather than before, because straightening a machined part would reintroduce the deviation.
What to put on the drawing
The profile drawing with circumscribing circle diameter, minimum wall, fin thickness and pitch. Straightness after straightening, called out explicitly rather than left to the standard. The machining sequence as a note, because the order of operations determines whether the flatness specification can be met.
Brazed Fin Stock: Highest Fin Density, With a Braze Interface
Thin folded aluminum or copper foil is formed into a fin pack, placed on a base with braze filler, fixtured, and run through a vacuum furnace. The filler melts, wicks into the joints by capillary action, and solidifies into a metallurgical bond at every fin root and at the cover.
Fin density is the highest of the methods covered here. Foil at 0.1 to 0.2 millimeters produces more surface area than a subtractive process can, and because the full assembly brazes in one furnace cycle, the cover joint is made at the same time as the fin joints.
Limitations of brazing
The braze layer adds thermal resistance in series with each fin, in the heat path the design is trying to minimize. The added resistance is small but not zero, and on a high-flux die it offsets part of the surface area advantage that led to choosing this construction.
Braze voids are the main quality risk. Filler that doesn’t fully wet the joint leaves an unbonded region, which is both a thermal defect and a pressure boundary defect. A void limit of under 10 percent of joint area is a commonly used acceptance criterion, and it should be stated on the drawing along with the verification method.
The furnace cycle also distorts the part. Heating the full assembly relieves residual stress in the base, so the plate leaves the furnace with a different shape than it had going in. For that reason, the mounting face is machined after brazing.
What to put on the drawing
Fin geometry and material. Braze filler alloy specification. Void acceptance limit with an inspection method. Flatness after brazing, on the contact footprint. And the sequence note that the mounting face is machined post-braze.
Friction Stir Welding: Highest Pressure Rating Among Closures
A rotating shouldered tool plunges into the joint line and traverses it. Friction plasticizes the material without melting it, and the tool stirs the two sides together into a solid-state weld.
Because the material doesn’t melt, there is no solidification porosity, no filler alloy and none of the cracking mechanisms associated with weld metal. Friction stir welding gives the highest typical pressure rating of the closure methods covered here and the lowest distortion of the welded methods, because heat input is limited to the tool path rather than applied to the whole part.
Limitations of friction stir welding
Aluminum welds readily. Copper is considerably more difficult: its higher thermal conductivity draws heat away from the weld zone, and tool wear at the required temperatures is costly. Friction stir welded copper plates are produced, but vacuum brazing is more common on copper.
The tool needs run-on and run-off areas, and the joint line must be accessible to a machine with substantial rigidity and downforce. These fixturing and part-geometry constraints should be reviewed at the design stage.
The weld also leaves a nugget with its own geometry: a shoulder impression on the surface and a stir zone below it. On a cover plate, this is usually cosmetic. Where the weld runs close to a sealing face or the mounting face, it can affect sealing and flatness and has to be accounted for in the design.
What to put on the drawing
Weld path with run-on and run-off locations. Minimum distance from the weld to the mounting face and to any sealed feature. Pressure test criteria after welding. And whether the shoulder impression is acceptable on the visible surface or has to be machined off.
Gasketed Closure: The Serviceable Option
A gasketed plate uses a machined base, a cover, an O-ring in a groove and a bolt pattern. It’s the only construction covered here that can be disassembled, which makes it suitable for prototypes, for plates that need internal inspection and for applications where field service is required.
It also has the lowest typical pressure rating and is the only construction with a wear-out mechanism. Elastomers take a compression set over time and temperature, so sealing force decreases over the service life. Seal material and a replacement interval should be defined for the application.
What to put on the drawing
Groove dimensions that give 15 to 25 percent squeeze on the cord and 75 to 90 percent gland fill. Under-fill can leak under thermal cycling, and over-fill can extrude and damage the ring. Sealing face finish at 0.8 to 1.6 microns Ra, with 3.2 as a maximum, and lay direction. Machining marks running across the seal rather than around it can create a leak path even when the Ra value is within specification.
Choosing a Construction
Four inputs drive the decision. We recommend evaluating them in this order.
Heat flux at the die.
Above roughly 100 watts per square centimeter, fin-to-base interface resistance starts to matter more than fin count, which favors skived copper. Below roughly 60, surface area has the larger effect and a brazed or extruded aluminum construction usually offers better economics. Between those values, either approach can work, and flow path, volume and pressure requirements usually decide.
Flow path complexity.
If the thermal design requires coolant to change direction, accelerate or concentrate under a specific region, machining is the appropriate method. Skiving and extrusion both produce straight, parallel fins, and manifold design can only partly compensate for a fin field that can’t vary.
Annual volume.
Under 500 pieces a year, machining is usually the most economical choice, because the tooling costs of the other methods can’t be recovered over that volume. From 500 to about 10,000 pieces, skiving is usually the most cost-effective option for high-flux designs. Above 10,000 pieces at moderate flux, extrusion or brazed fin stock typically offers the lowest cost per part, while skiving remains competitive for high-flux designs up to roughly 50,000 pieces.
Pressure and serviceability.
Above about 20 bar, friction stir welding is the most suitable closure. If the plate has to be serviceable, a gasketed closure is required, along with its lower pressure rating. Between those cases, vacuum brazing or fusion welding are typical choices, depending on material and volume.
What a 20 Percent Difference Between Quotes Can Represent
Two quotes on the same drawing that differ by 20 percent are a common reason engineers look into this question. The difference is often not margin.
It’s often construction. For example, one supplier quotes skived copper with a friction stir welded cover and another quotes brazed aluminum fin stock. Both can meet a drawing that specifies geometry and performance but not construction, yet the two parts won’t perform the same on a high-flux die.
It can be the flatness specification. A plate with flatness called out across the contact footprint and verified on every lot carries a real inspection cost. A plate with a general profile tolerance and a first-article check doesn’t. The second quote will be lower, but flatness across the production lot isn’t controlled, and a contact face that’s out of flatness increases thermal interface resistance at the die.
It can be the test regime. Pressure decay testing at 1.5 times working pressure on 100 percent of assemblies costs more than a sampled test. Helium mass spectrometry to 1 × 10⁻⁶ mbar·L/s costs more again.
It can also be the machining sequence. A supplier who machines the mounting face after brazing has an extra operation that a supplier who machines before brazing doesn’t. Parts machined before brazing are at risk of losing flatness during the furnace cycle.
The practical takeaway: a drawing that specifies geometry and performance without specifying construction, sequence, inspection and test allows suppliers to quote different parts. Specifying those four items puts quotes on a comparable basis.
Frequently Asked Questions
How are data center cold plates made?
By combining a fin-forming method with a closure method. Fins come from skiving, CNC machining, extrusion or brazed fin stock. The plate is closed by vacuum brazing, friction stir welding, fusion welding or a gasketed joint. Common direct-to-chip constructions include skived copper closed by vacuum brazing or friction stir welding, and machined aluminum closed by friction stir welding or vacuum brazing.
What is skiving on a cold plate?
A hardened blade takes a shallow cut into a solid block and lifts the material into a standing fin rather than removing it. Because the fin is peeled from the base, there is no joint between fin and base and no interface for heat to cross. Fins run 0.2 to 0.6 millimeters thick at 0.3 to 1.2 millimeter pitch, to around 25 millimeters tall.
Skived or brazed fins: which is better for a cold plate?
Brazed fin stock reaches higher fin density using 0.1 to 0.2 millimeter foil. Skiving produces no fin-to-base joint. Above roughly 100 watts per square centimeter, the interface matters more than surface area, which favors skiving. Below roughly 60, the density advantage usually has the larger effect. Between those values, volume and flow path requirements usually decide.
What flatness can a cold plate hold?
A flatness of 0.025 millimeters across the die footprint is achievable in production with any of these constructions, provided the mounting face is machined as a final operation. Every method introduces distortion: brazing relieves stress in the furnace cycle, welding adds heat to the part and extrusion leaves bow from the press. Specify flatness on the contact footprint separately from the general profile tolerance, and verify it across the production lot.
What pressure rating can a cold plate achieve?
It depends on the closure. Friction stir welding gives the highest at roughly 20 to 40 bar, vacuum brazing 10 to 20, fusion welding 10 to 25, and a gasketed joint 5 to 10. Test at 1.5 times working pressure with a defined hold time, and state whether the test is on 100 percent of assemblies or a sample.
At what volume does machining stop making sense for cold plates?
Around 500 pieces a year. Below that, the tooling costs of the other methods can’t be recovered, so machining is usually the most economical choice. From 500 to about 10,000 pieces, skiving is usually the most cost-effective option for high-flux designs. Above 10,000 at moderate heat flux, extrusion or brazed fin stock typically offers the lowest cost per part.
Why is a machined cold plate channel limited to about 0.8 millimeters?
Because of cutter geometry. A channel narrower than about 0.8 millimeters requires a cutter small enough to deflect under cutting load, and a deflecting cutter produces a channel that is neither straight nor consistently wide. The practical aspect ratio for a milled slot is around 10:1, compared with up to 40:1 for skived fins in copper.
Why do two cold plate quotes differ by twenty percent?
Often because of construction rather than margin. One supplier may quote skived copper with a welded cover and another brazed aluminum fin stock, and both can satisfy a drawing that specifies geometry without specifying construction. Differences in flatness verification, test regime and machining sequence can account for the rest.
Can copper be friction stir welded?
Yes, but it’s considerably more difficult than aluminum. Copper’s high thermal conductivity draws heat away from the weld zone, and tool wear at the required temperatures is costly. It’s used in production, but vacuum brazing is more common on copper constructions.
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MES machines, skives, brazes and extrudes cold plate constructions in copper and aluminum across eight countries, with engineering and quality staff based at our supplier facilities. We review every drawing before tooling is cut. Send us your cold plate drawing and our engineers will recommend a construction suited to the geometry and identify any specifications that may be difficult to hold as drawn.
