How Liquid Cold Plates Are Made: Constructions, Materials and Cost Drivers

A liquid cold plate takes heat from a device mounted on its face and hands it to a coolant, which carries it to a radiator or chiller. How the plate is built decides how much heat flux it can handle, how much pressure drop the pump must overcome, what each plate costs and where it could leak.

We machine cold plates to drawing for EV and power electronics programs; these are the trade-offs we discuss before an RFQ. For the parts themselves, see our custom liquid cold plate manufacturing.

What is a liquid cold plate?

A cold plate is a metal plate with internal coolant passages. A power module, cell stack or controller sits on its flat face; heat crosses the thermal interface material (TIM), conducts through the metal to the channel walls, and the flowing coolant takes it away by convection. “Cold” describes the plate’s job, not refrigeration.

An air-cooled heat sink does the same job with fins and airflow. Liquid takes over when the heat flux is high, the footprint is small or the enclosure is sealed: inverters, onboard chargers, DC-DC converters and battery modules.

The five main constructions

Press-in tube (tubed) cold plates

A copper or stainless steel tube is bent to the flow path and pressed or bonded into grooves in an aluminum base, then the face is machined flat: the lowest-cost route for moderate heat loads. The weak point is the contact between tube and base, which adds thermal resistance.

Gun-drilled (deep-drilled) channel plates

Straight passages are drilled through a solid plate, cross-drilled where they meet, and plugged at the ends. There is no lid joint on the face, so the plate is robust and simple to qualify. Channel geometry is limited to straight lines and the wetted area is small, so it suits low heat flux.

Milled channels with a sealed or welded lid

The channels are milled into the base as a pocket, then closed by a lid. The lid can be sealed with an O-ring and fasteners, or joined by friction stir welding (FSW): a solid-state process with no filler and low distortion, described in ISO 25239. The channel layout is free, so channels can follow the heat sources, and design changes only mean a new program.

Vacuum brazed cold plates

In this industry process, layers of clad brazing sheet (3003 core with 4004 or 4104 cladding), fins and turbulators are stacked and brazed in a vacuum furnace. All internal surfaces join in one cycle without flux, which allows the most complex internal structures and multi-layer plates. Fixtures and furnace qualification add cost that only pays off once the design is frozen.

Microchannel and fin-enhanced designs

Skived or folded fins, pin fins and microchannels multiply the wetted surface under concentrated heat sources such as IGBT or SiC power modules. The price is a higher pressure drop and more sensitivity to contamination in the coolant.

Vacuum brazed plates, as the industry builds them, give very high thermal performance with medium to high pressure drop, but high tooling cost (fixtures, furnace), so they suit frozen designs and larger batches. Design freedom runs from tube bends (press-in tube) and straight paths only (gun-drilled) to high for milled channels and fin designs, and very high, multi-layer for brazed plates. Press-in tubes fit any volume where cost drives the choice, gun-drilled plates low to medium volumes, milled channels engineering samples to repeat batches, and microchannel or fin designs power modules.

Materials: aluminum vs copper vs stainless

Aluminum 3003 brazing sheet (about 160 W/m·K, low density) is used in vacuum-brazed layers. Machinability is very good for aluminum 6061 / 6063, good for 3003 brazing sheet, slower for copper and hard for stainless tube.

Aluminum is the default for vehicle plates because weight matters and a good channel design makes up for its lower conductivity; see aluminum automotive components. Copper earns its place under small, very hot sources.

Keep the wetted metals consistent. An aluminum plate in a loop with copper tubing or a copper radiator forms a galvanic couple through the coolant, and the aluminum corrodes until it leaks. Most loops run water and ethylene glycol (WEG, often 50/50) or water and propylene glycol, with inhibitors; some power electronics use dielectric fluids. Check the O-ring and gasket materials against the fluid too.

Flow path design: serpentine vs parallel

A serpentine path sends all the coolant through one long channel: even velocity, but high pressure drop, and the coolant warms along the way, so the last heat source runs hotter. A parallel layout splits the flow through several channels fed by a manifold: pressure drop is low, but the manifold must be balanced or some branches starve.

Large plates such as battery cooling plates often use split-flow manifolds; local fin or pin zones go where heat is concentrated. The layout comes from your CFD and thermal simulation; the manufacturer checks that it can be machined and inspected.

Performance metrics to put on your drawing

• Thermal resistance (°C/W): temperature rise from coolant inlet to device case per watt, at a stated flow rate and coolant.
• Pressure drop (kPa) at flow rate (L/min): what the pump must supply; give the maximum allowed.
• Operating and proof pressure (bar): the pressure the plate sees in service and the higher pressure it must survive in test.
• Mounting-face flatness (mm) and roughness (Ra, µm): they set the TIM bond line. TIM resistance is R = t / (k × A): for a 25 cm² module and a TIM of 3 W/m·K, a 0.1 mm bond line gives about 0.013 °C/W, or 6.7 °C at 500 W; a bowed face that forces 0.2 mm doubles that. Flatness of 0.05 to 0.1 mm across the device footprint is a common drawing requirement when a TIM is used.
• Leak rate (mbar·l/s), test method and cleanliness: the acceptance limits and how they are measured.

Testing a cold plate before it ships

• Pressure decay leak test: the plate is pressurized with air and the pressure loss over a hold time is measured; common for moderate leak-rate limits.
• Burst or proof testing: on samples, to confirm the margin above operating pressure.
• Flatness by CMM: mounting face and port positions measured against the drawing datums.
• Internal cleanliness: washing and particle analysis to ISO 16232 or VDA 19.1 when the customer specifies it.
• Documentation: material traceability for each batch, with the inspection report on request.

What drives cold plate cost

• Construction and tooling vs volume: machined channels need only a program and a fixture; brazing and FSW add fixtures and qualification that pay off only over larger, frozen batches.
• Material and plate size: copper costs more per kilogram, is denser and machines slower than aluminum; larger plates mean more material and machine time.
• Channel complexity and layers: deep, narrow channels, fins and multi-layer designs add cycle time and scrap risk; thin floors over channels distort like thin-walled EV battery housings.
• Mounting face and inserts: tight flatness and roughness on the device face and many threaded inserts add finishing and inspection time.
• Ports, fittings and surface treatment: custom port bosses, quick-connects and anodizing or plating each add operations.
• Testing scope: testing every plate costs more than sampling, and tighter leak rates need slower methods.
• Documentation: inspection reports, FAI and material certificates, supplied on request, take time; match the level to the program stage.
• Volume break-even: machined-channel plates fit engineering samples and low volumes; brazed or formed constructions become cheaper per plate only at higher, stable volumes.

Choosing a construction: a quick decision table

In industry practice, a frozen design where a sealing joint is preferred moves to milled channels with an FSW lid; very high heat flux with complex internal fins and stable volume points to a vacuum brazed plate with fins or microchannels, and local fin or pin zones can also sit in a brazed body.

A cheaper construction is often enough: if a drilled or tubed plate meets the thermal target with margin, brazing is not worth paying for.