How a Copper Cold Plate Gets Leak Tested, and What to Write

September 26, 2026

A liquid cooled copper cold plate is a machined part, a joining operation and a pressure vessel. The joining is where it fails, and the test is where the failure is found. Specifying the test in the RFQ decides who pays when it fails.

Three tests

Pressure decay. Pressurise with air or nitrogen, hold, watch the gauge. Cheap, common, done at most shops. Good for water cooled electronics at modest pressure. Sensitivity depends on pressure, volume and duration, so all three are specified.

Bubble test. Pressurise and submerge, or apply solution. Finds the location of a leak rather than measuring it. A screening and diagnostic test, not a specification on its own.

Helium leak test. A mass spectrometer detects helium passing through the joint. Sensitive enough for vacuum and refrigerant service. Needs equipment and expertise not every machine shop has. If you specify a leak rate in mbar litre per second, this is the test you are asking for.

What a leak rate means

A leak rate is a flow, stated at a pressure difference. Specifying a rate without the pressure, the fluid and the duration is not a specification. The number itself comes from the system: what leaks, into what, with what consequence. That is a question for the system engineer, and a machine shop cannot answer it.

Where the joint is

Copper cold plates are joined by brazing, by soldering, or by drilling and plugging channels. Each has its own failure mode and its own test timing.

Brazed two piece: test after brazing, before final machining of critical faces
Drilled and plugged: test after plugging, then again after any machining near the plugs
Soldered: test at a pressure and temperature the solder will actually see in service

Joining machined copper parts covers the joining choice.

What to write in the RFQ

Leak test: pressure decay, [fluid], [pressure] for [duration],
pass if decay less than [value].
Test after brazing, before finish machining.
Test performed by [shop / buyer].
Failed parts: [scrapped at shop's cost / returned for review].
Test report required with shipment.

Or for a helium test:

Helium leak test to [rate] mbar l/s at [pressure differential].
Performed by [shop / third party / buyer].

The line about who owns a failed part matters more than any other. Joining consumes finished machining, and a cold plate that fails after brazing has already cost most of its price.

Design decisions that reduce leaks

Fewer joints. Two pieces braze more reliably than five.
Generous braze land width so the filler has somewhere to flow.
Channel routing that avoids thin walls between passage and surface.
Plugs designed for the test pressure, not just for sealing.
A test port or a plan for how the part is pressurised.

The design side is in how to design a machined copper heat sink and the service page is copper heat sink and cold plate machining.

Frequently asked questions

What leak rate should I specify for a cold plate?

It depends on the coolant, the pressure and the consequence of a leak. Water cooled electronics commonly use a pressure decay test at a multiple of working pressure. Vacuum or refrigerant systems need a helium leak rate. Ask the system engineer, not the machine shop, for the number.

Who does the leak test, the shop or me?

State it. A shop can pressure test after brazing. A helium test needs equipment not every shop has. If the shop tests, say the method, the pressure, the duration and the pass criterion. If you test, say so, and agree who owns a failed part.

Should the test happen before or after final machining?

After brazing and before final machining of critical faces is common, so a failed joint does not waste the finishing work. Final test after all operations is the safe answer where the finishing could disturb a joint.

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