Liquid cold plates and electronics cooling

A liquid cold plate is a machined channel closed by a leak-tight lid, and the closure joint decides the part. StirLight manufactures and develops friction stir welded cold plates in the UK for electronics and power cooling: a machined pocket or channel in aluminium, closed by a solid-state weld made below the melting point, with no filler, flux or furnace cycle. Because heat is applied only along the weld line, the plate keeps its bulk temper and moves far less than a furnace-brazed equivalent, and the coolant is separated from the electronics by parent metal rather than by braze filler, an adhesive bond line, or a gasket.

Why is the closure joint the hard part of a cold plate?

Everything else about a cold plate is machining. By the time the lid goes on, the part carries its full material and machining value, and the joining step decides whether that value survives. The closure joint has to do four things at once:

• Contain coolant at working pressure with no leak path.
• Conduct heat, because the wall above the channel is part of the thermal path.
• Not distort the plate, because flatness at the device interface sets contact resistance.
• Not degrade the base material, because the plate is also a structural and mounting element.

Furnace brazing satisfies the first two and fights the second two: a cycle above roughly 570 C anneals 6061 and relaxes the stress left by machining the pocket, so flatness has to be bought back by post-braze skimming. Adhesive bonding satisfies the flatness requirement and inserts a polymer into the thermal path. A friction stir welded closure addresses all four, at the cost of requiring a reachable joint line.

What geometries suit friction stir welded cold plates?

Cold plate architectures and their suitability for friction stir welding

Architecture	Closure joint	Suitability
Machined pocket or serpentine channel with a flat cover plate	Perimeter and internal rib weld lines, externally accessible	The natural fit. Weld lines are long, straight or gently curved, and reachable.
Channel closed by a lid with internal pin-fin or rib structures	Perimeter plus weld lines over ribs	Suitable where rib positions can be followed and supported.
Two-piece manifold or header assemblies	Perimeter closure and port welds	Suitable; ports usually handled as separate features.
Embedded tube in a machined groove	Weld over or beside the tube	Case by case, driven by tube wall and support.
Dimpled plate exchanger, hundreds of internal joints	Internal, not tool-reachable	Not a friction stir welding candidate. Brazing is the correct process.
Aluminium to copper assemblies, for example a copper base with aluminium body	Dissimilar joint line	Feasible as a dissimilar friction stir weld; requires development.

What does StirLight actually run?

Two platforms cover the range from coupon trials to full plates, both at the StirLight facility in Chesterfield:

• Greta — an ESAB SuperStir gantry platform for large and thick-section components, with force control for repeatable penetration. Panel builds beyond 10 m in length are supported by modular handling and repositioning.
• Agatha — an ABB IRB 8700-800 robot with an ESAB EWH 50-FSW head, for smaller parts, curved weld paths and complex geometries, with straight welds to around 5 m.

Traverse speed across the equipment envelope spans roughly 30 to 5000 mm/min depending on material, thickness and tool; for aluminium cold plate closure welds the useful range is typically 750 to 1500 mm/min, against roughly 130 mm/min for TIG. Tooling, including stationary-shoulder tools where distortion must be pushed lower still, is designed and made in-house. See capabilities for the full equipment envelope.

Materials

• Aluminium alloys across the 1xxx, 2xxx, 3xxx, 5xxx, 6xxx and 7xxx series, in plate, cast and extruded form. 6061 and 6082 dominate cold plate work.
• Copper in thick section, including for high-conductivity bases and busbar work.
• Dissimilar combinations, notably aluminium to copper.

Thickness envelope StirLight will publish for cold-plate work:

• Thin sheet, including cooling ribbons: micro friction stir welding down to 0.3 mm.
• Typical cold plates: 2-4 mm, feasible on the robotic system (Agatha), typically around 0.3 m by 1 m.
• Battery-tray panels: typically around 1.5 m by 3 m.
• Very high pressure applications: 5-12 mm, feasible on the gantry system (Greta).

How is quality demonstrated?

• Weld procedure development referencing ISO 25239, the standard for friction stir welding of aluminium.
• Because there is no melting, the weld contains no solidification porosity and no hot cracking, which removes the two defect families that normally drive leak failures and NDT burden in fusion-welded aluminium.
• In-process force, torque and position data captured during welding, with StirSense in-process monitoring available for production traceability.
• Leak and pressure testing to the customer specification. Typical automotive inverter coolant-loop figures, as an example of what that specification often looks like rather than as a StirLight guarantee: normal operating roughly 1-2.5 bar(g) on 50/50 water-glycol; design pressure 2.5-3.5 bar(g) at 65-85 C; proof typically 3.5-5 bar(g) held 5-15 minutes with no leak and no permanent deformation; burst typically 6-10 bar(g) minimum. Measure lid deflection on proof, not only leakage.
• Sectioning and mechanical testing during development to confirm penetration and joint properties. A related StirLight measurement — on an automotive battery-tray panel built from extrusions, not on a cold plate — achieved 0.5 mm flatness per square metre over a panel containing 4 metres of weld, with no post-weld straightening and no post-weld heat treatment. Cold-plate flatness is measured on the actual geometry in the trial.

An anonymised example

A UK electronics-cooling fabricator was closing machined aluminium cold plates by a furnace route and losing parts at final leak test, with additional loss from flatness recovered by post-braze skimming. Because the failures appeared after all machining was complete, each loss was the full value of a finished part rather than the value of a joint. Moving the closure joint to a local, solid-state weld removed the whole-part thermal cycle, so temper and shape were no longer being reset by the joining step, and welds could be inspected individually instead of the assembly being condemned as a unit.

How an engagement runs

1. Reachability and design review. From the drawing: is the closure joint line reachable, and is there enough wall above the channel. This is quick and it is the gate that matters.
2. Feasibility trial. Representative material and joint geometry, welded and tested against your leak and flatness requirements, with a structured report and a go or no-go recommendation.
3. Process and parameter development. Parameters, tooling and fixturing, taken to a qualifiable procedure.
4. Production welding. Prototype through low and medium-volume production in the UK, with traceability.

Details on the services page. Indicative commercial shapes for trial work belong there rather than here.

Frequently asked questions

Can friction stir welded cold plates hold pressure and pass a leak test?

Yes, that is the normal requirement. Typical automotive inverter coolant-loop figures, as an example of what the specification often looks like: normal operating 1-2.5 bar(g) on 50/50 water-glycol; design 2.5-3.5 bar(g); proof 3.5-5 bar(g) held 5-15 minutes; burst commonly 6-10 bar(g) minimum. Acceptance is always demonstrated on the customer test specification.

Is a friction stir welded joint better thermally than a brazed or bonded one?

The heat path across the joint is parent metal rather than braze filler or a polymer bond line, so it avoids the bond-line thermal resistance that adhesive designs carry. The dominant thermal term in most designs remains the wall thickness above the channel and interface flatness at the device.

What plate sizes and thicknesses can StirLight weld?

Typical cold plates of 2-4 mm and about 0.3 m by 1 m run on the robotic system; 5-12 mm high-pressure plates on the gantry. Battery-tray panels are typically around 1.5 m by 3 m. Micro friction stir welding takes cooling ribbons down to 0.3 mm. The gantry supports panel builds beyond 10 m; the robotic system covers complex paths and straight welds to around 5 m.

Do you make aluminium to copper cold plates?

Dissimilar aluminium to copper joining is within the process capability and is treated as a development activity rather than an off-the-shelf procedure.

What volumes make sense for friction stir welded cold plates?

Prototype through low and medium volume, which is also where furnace fixturing costs are hardest to amortise. High-joint-count, high-volume furnace parts are usually better left brazed.