Quality Control of Vacuum Brazed Copper Cold Plates for IGBT Cooling
In the previous articles, we discussed the internal design of this vacuum brazed copper cold plate, including the folded fin structure, offset fin structure, and the reasons for choosing pure copper and silver-copper brazing alloy.
However, having an efficient internal flow path is only part of the solution.
For IGBT modules and other high-power electronic devices, the final thermal performance also depends heavily on two factors that are often overlooked:
* The flatness of the mounting surface
* The sealing reliability of the cooling channel
Even a well-designed cold plate can lose performance if the contact surface is not flat enough or if the brazed structure cannot withstand long-term pressure and thermal cycling.
This is why every cold plate goes through final machining and pressure testing before shipment.
Why We Perform Fly Cutting After Vacuum Brazing
During the vacuum brazing process, the copper assembly is heated to a high temperature and then cooled back to room temperature.
Although the process is carefully controlled, small amounts of distortion can still occur. In some cases, minor brazing residue may also remain on the mounting surface.
For many mechanical parts, this may not be a major issue.
For an IGBT cold plate, however, surface flatness directly affects thermal resistance.
To restore the mounting surface, we perform fly cutting after brazing.
Fly cutting is a machining process that uses a single-point cutting tool to create a very flat surface across a large area.
After fly cutting, we control the mounting surface flatness to within 0.05 mm across the entire contact area. We also limit the height difference between the edge and the center of the surface to less than 0.01 mm to ensure uniform contact pressure when the IGBT module is installed.
These values are based on practical thermal interface material performance rather than arbitrary manufacturing targets.
Most thermal grease and phase-change materials can compensate for small surface gaps, but once the gap becomes too large, air pockets remain between the module and the cold plate. Because air is a very poor thermal conductor, contact thermal resistance increases significantly. The result is higher junction temperatures and reduced cooling performance.
Why Edge Flatness Is Also Important
When discussing flatness, many people focus only on the center of the mounting surface.
In reality, edge conditions are equally important.
If the edges are rounded or slightly lower than the center after machining, the IGBT module may only contact part of the cold plate surface.
This can create uneven clamping forces and uneven heat transfer across the module baseplate.
Over time, localized hot spots may develop, especially in high-power applications.
For this reason, we inspect both overall flatness and edge quality during production.
The goal is to ensure that the entire mounting surface can achieve uniform contact pressure after assembly.
Every Cold Plate Is Pressure Tested
After machining is completed, each finished cold plate undergoes a pressure test before shipment. The test is performed using dry nitrogen at a pressure of 6 Bar, and the pressure is maintained for 10 minutes. To pass inspection, the pressure drop must remain below 0.05 Bar throughout the test, with no visible leakage and no permanent deformation of the structure.
This test verifies both the integrity of the brazed joints and the strength of the overall structure.
Why We Test at 6 Bar
Most EV cooling systems operate at pressures significantly below 6 Bar.
Typical coolant loop pressure is often around 1.5 to 2.5 Bar during normal operation.
Testing at a higher pressure provides additional safety margin for real-world conditions such as:
* Pressure fluctuations during operation
* Thermal expansion and contraction
* Vehicle vibration
* Long-term aging
In other words, the cold plate is tested beyond normal working conditions to ensure reliable performance throughout its service life.
For vacuum brazed assemblies, the pressure test also confirms that the multi-layer brazed joints are fully sealed.
Why This Design Works Well for EV Inverters
Traction inverters generate substantial heat within a relatively small area.
To maintain performance and reliability, the cooling system must remove heat efficiently while keeping temperature distribution as uniform as possible.
Several features of this cold plate contribute to that goal.
High Heat Flux Capability
Pure copper offers thermal conductivity of approximately 400 W/(m·K), allowing heat to spread quickly from the IGBT baseplate into the cooling structure.
The folded fin section increases internal surface area, while the offset fin section promotes stronger coolant mixing and turbulence.
Together, they help reduce overall thermal resistance.
More Uniform Temperature Distribution
The center-fed inlet and symmetrical flow path help distribute coolant more evenly across the cooling surface.
This can reduce temperature variation between different areas of the module and improve overall thermal balance.
Reliable Mechanical Structure
The cold plate uses vacuum brazing with a 72Ag28Cu silver-copper filler alloy rather than adhesives or soft solder.
The result is a fully metallic joint with high structural strength and good long-term stability.
Combined with pressure testing, this provides additional confidence for demanding automotive and industrial applications.
Typical Quality Checks
As part of our standard production process, we perform:
*Surface flatness inspection
*Edge quality inspection
*6 Bar pressure testing
*Pressure drop recording
For customers who require additional validation, we can also arrange thermal cycling tests, random vibration testing, pressure pulse testing, and other application-specific reliability evaluations based on the operating environment.
While internal fin structures often receive the most attention, the final machining and testing processes are equally important.
In many high-power cooling applications, long-term reliability depends not only on how effectively heat is removed, but also on how consistently the cold plate performs after thousands of operating hours.
