The Interface Is a Thermal Resistor in Series
A cold plate is only as good as the path between the die and the coolant. That path has four resistors in series: die to case, case to thermal interface material, interface material to plate, plate to coolant. Machining only controls the third one directly, but it sets the thickness of the second, and on high power devices that is often the largest single term.
What Flatness Error Actually Does
A plate that is concave by 0.05 mm across a 40 mm footprint creates a gap that the thermal interface material must fill. Grease fills it at the cost of a thicker bond line, and every extra 0.01 mm of bond line adds roughly 0.1 to 0.2 degrees Celsius per square centimetre per watt depending on the material. On a 200 W device that is not a rounding error, it is the difference between a design that meets its junction temperature and one that throttles.
| Plate condition | Typical bond line | Effect |
|---|---|---|
| 0.01 mm flatness, Ra 0.8 | Thin and uniform | Lowest interface resistance |
| 0.05 mm flatness | Thicker at the centre or edges | Higher resistance, uneven die temperature |
| 0.1 mm bow after brazing | Highly variable | Hot spot, possible pump out over cycles |
Why Brazing Moves the Face
Vacuum brazing runs at a temperature that relaxes the copper and redistributes residual stress from machining. A plate that measured flat before brazing will not necessarily measure flat after it. This is why the finishing pass and the flatness inspection belong after the furnace cycle, not before, and why a supplier who quotes flatness on the pre-braze part is quoting the wrong number.
Specifying the Interface Correctly
- State flatness over the die footprint, not over the whole plate. A plate can be flat overall and still be dished where the die sits.
- State surface roughness with the flatness value. Flatness without roughness is incomplete for a grease interface.
- State whether the plate will be screwed down under torque, and provide the screw pattern, because torque bows thin plates.
- State the interface material. A pad tolerates more gap than grease and changes the required flatness.
Where the Money Should Go
Buyers often ask for a thicker copper base when the real fix is a flatter face. Copper thickness helps spread heat laterally, but it cannot repair a bad interface. If a design is marginal, the cheapest improvement is usually the contact face: better flatness, better roughness, and a spot faced screw pattern so the fasteners do not distort the plate.
Machining tolerances and test records for the copper vapor chamber cold plate are listed with the flatness measured after brazing.
