How Wedgelock Thermal Performance Is Measured
Wedgelock thermal resistance is listed on product datasheets in °C/W. This article provides a brief overview of WaveTherm's testing methodology to determine °C/W and characterize the thermal performance of SOLIDWEDGE™ wedgelocks using VITA-compliant fixtures and standardized conditions. For the complete methodology, including detailed fixture drawings and calculation examples, see the full thermal testing report.
What Is Thermal Resistance?
Thermal resistance measures how much a material or interface resists the flow of heat, and for wedgelocks, it quantifies how effectively heat transfers from the heatframe to the chassis cold wall. It is expressed in °C/W, which tells you how many degrees of temperature rise you get for every watt of power dissipated through that path.
A wedgelock with a thermal resistance of 0.1°C/W will produce a 1°C temperature drop across the interface when conducting 10 watts. Lower thermal resistance means better heat transfer. For conduction-cooled VPX modules dissipating 50 to 100 watts or more, small differences in thermal resistance translate directly into component temperature differences that affect reliability and performance.
How Does Heat Flow Through a Conduction-Cooled Module?
In a conduction-cooled VPX module, heat generated by components on the PCB flows through the heatframe and into the chassis cold wall. There are two parallel paths for this heat transfer:
- Frame-to-cold wall contact: The heatframe cover presses directly against one side of the cold wall slot, benefiting from a large contact area.
- Wedgelock-to-cold wall contact: The wedgelock expands against the opposite side of the cold wall slot. This path is essential for clamping force and also contributes to thermal performance.
The percentage of heat carried by each path is not fixed. It depends on the relative thermal resistance of each path, which changes with wedgelock mounting orientation. In practice, this typically works out to somewhere between a 70/30 and 80/20 split, as detailed below.
Convective and radiative losses from the module surfaces are typically negligible in sealed enclosures and are not considered in wedgelock thermal characterization.
Because heat flows through both paths simultaneously, the total thermal resistance of the system follows the parallel resistance formula, similar to resistors in an electrical circuit. The split between paths is a result of this parallel combination, not an input you assume beforehand. The path with lower thermal resistance always carries more of the heat.
How Wedgelock Orientation Affects Heat Flow
Wedgelock mounting orientation is one of the most consequential decisions for thermal performance. In a representative design comparison, Secondary Side orientation achieved a total thermal resistance roughly 5-6 times lower than Primary Side orientation for the same card, driven primarily by removing the PCB from the primary conduction path. The exact improvement will vary by design, but the PCB is consistently the largest single contributor to thermal resistance in a Primary Side configuration, so removing it from the path has an outsized effect.
The heat split between frame-to-cold wall and wedgelock-to-cold wall contact is not constant. It shifts depending on wedgelock mounting orientation, because orientation determines which side of the card frame makes direct contact with the cold wall, and how much resistance the heat encounters getting there.
The primary side cover (component side of the PCB) receives the majority of heat from the board. Where this cover sits relative to the cold wall, and how directly the heat can reach it, determines how efficiently that heat is extracted.
Secondary Side Orientation (SOLIDWEDGE™ Wedgelock Beside PCB)
The primary side cover makes direct contact with the cold wall, and the PCB is removed from the primary heat path entirely. With this direct metal-to-metal contact, the frame-to-cold wall path becomes the lowest-resistance route, carrying approximately 80% of the heat. The wedgelock handles the remaining 20% from the secondary side.
Primary Side Orientation (SOLIDWEDGE™ Wedgelock on Top of PCB)
The secondary side cover makes direct contact with the cold wall, but heat originates on the primary side (component side) and must conduct through the PCB and secondary side cover to reach that contact. PCB substrate is a poor thermal conductor, so this path carries significant resistance. Because of that, most of the heat actually takes the shorter route through the wedgelock sitting on top of the primary side cover instead, approximately 70% of it, while only about 30% makes the longer trip through the PCB to the secondary side cover contact. Here the roles are reversed, with the wedgelock side now being the lower resistance path. This configuration is typically used when convection is the primary cooling method and conduction serves as a supplement.
For high-power conduction-cooled modules, Secondary Side orientation is preferred because it provides the most direct thermal path for the majority of component heat.
Test Equipment and Setup
Accurate thermal testing requires equipment that replicates real-world chassis conditions while allowing precise temperature measurement.
Cold Wall Test Fixture
The cold wall is a heat sink with a card edge slot machined to the appropriate VITA specification (VITA 48, VITA 78, etc.). Key requirements include:
- Surface finish: 16 µin RMS or better on slot contact surfaces
- Plating: Clear chromate per MIL-C-5541 class 3 to represent typical chassis conditions
- Active cooling: Finned heat sinks with fans sized to dissipate the planned test wattage
- Thermocouples: Placed on both sides of the slot to measure cold wall temperature at each interface
A non-thermally conductive spacer (typically ABS plastic) is placed at the base of the slot to prevent the heatframe from contacting the bottom and creating a third heat path that would skew results.
Test Plate
The test plate simulates a heatframe with a wedgelock attached. It is made from 6061-T6 aluminum with a 16 µin RMS surface finish in contact areas. The plate thickness is calculated based on the cold wall slot height, wedgelock height, and nominal expansion:
Test Plate Thickness = Cold Wall Slot Height - Wedgelock Height - Wedgelock Nominal Expansion
For a standard VITA 48 configuration with a 0.225" tall wedgelock and 0.025" nominal expansion: 0.525" - 0.225" - 0.025" = 0.275"
Load resistors mounted on the test plate simulate component heat dissipation, with capacity for at least 100 watts. The resistors are covered with PTFE insulation to minimize convective losses and ensure heat flows through the intended conduction paths.
Temperature Measurement
Three temperature measurements are required to calculate thermal resistance:
- Test plate temperature (TP): Average of four thermocouples evenly spaced along the test plate length, located between the heat source and wedgelock, positioned approximately 0.100" to 0.200" from the wedgelock edge
- Cold wall frame side temperature (TCWF): Average of two thermocouples centered along the cold wall length, positioned approximately 0.100" to 0.200" from the frame-to-cold wall interface
- Cold wall wedge side temperature (TCWW): Average of two thermocouples centered along the cold wall length, positioned approximately 0.100" to 0.200" from the wedgelock-to-cold wall interface
Type-T thermocouples are used for their accuracy in the relevant temperature range. Thermocouple placement is critical. Sensors must be as close to the interface as possible without interfering with contact.
Thermal Resistance Calculations
Because heat flows through parallel paths, each path's thermal resistance is calculated separately, then combined using the parallel resistance formula. The heat split used in these calculations is specific to the mounting orientation under test, approximately 80/20 (frame-side/wedge-side) for Secondary Side orientation, and approximately 70/30 (wedge-side/frame-side) for Primary Side orientation.
Frame-Side Thermal Resistance (RF)
RF = (TP - TCWF) / (SFP)
Where P is total power and SF is the fraction of heat flowing through the frame-side path for the orientation under test, approximately 0.8 for Secondary Side orientation or approximately 0.3 for Primary Side orientation.
Wedge-Side Thermal Resistance (RW)
RW = (TP - TCWW) / (SWP)
Where SW is the fraction of heat flowing through the wedgelock path for the orientation under test, approximately 0.2 for Secondary Side orientation or approximately 0.7 for Primary Side orientation.
Total Thermal Resistance (RT)
RT = (RF × RW) / (RF + RW)
This is the standard parallel resistance formula. The result is expressed in °C/W.
Test Procedure
Data is acquired at multiple power levels to verify consistent performance across the operating range:
- Power increments: 20W, 40W, 60W, 80W, and 100W
- Stabilization criteria: No temperature change greater than 1°C over five minutes (per MIL-STD-202)
- Initial calibration: Cold wall and test plate averages within 0.2°C before applying power
Testing at multiple power levels confirms that thermal resistance remains consistent and reveals any nonlinear behavior that could affect high-power applications.
Why Surface Finish Matters
Heat transfer across a metal-to-metal interface depends on actual contact area. At a microscopic level, even machined surfaces have peaks and valleys. Only the peaks make contact, so the effective contact area is a fraction of the apparent area.
A 16 µin RMS surface finish specification ensures consistent, repeatable contact conditions. Rougher surfaces reduce contact area and increase thermal resistance. This is why WaveTherm specifies surface finish requirements for both test equipment and production hardware.
Test fixture surfaces are considered wear items and must be cleaned between specimens. If surface roughness degrades outside the acceptable range, surfaces must be refinished to maintain test accuracy.
Vacuum and Altitude Testing
The same methodology can be applied in a vacuum chamber to characterize high-altitude performance. At altitude, reduced air pressure eliminates convective cooling, making conduction through the wedgelock and heatframe even more critical. Vacuum testing validates that thermal performance holds under these conditions.
What This Means for Your Design
Understanding thermal test methodology helps you interpret vendor specifications and predict real-world performance. When evaluating wedgelock thermal resistance data, consider:
- Test conditions: Were tests performed at power levels relevant to your application?
- Surface conditions: What surface finish and plating were used?
- VITA compliance: Was the test fixture built to the same VITA specification as your chassis?
- Repeatability: Does the vendor use a standardized methodology that produces consistent results?
- Clamping force: Contact pressure at the wedgelock-to-cold wall interface directly affects how well heat transfers across that interface. Higher clamping force means better contact and lower thermal resistance. See how SOLIDWEDGE™ clamping force is calculated for a detailed breakdown.
WaveTherm's thermal testing methodology is designed to produce accurate, repeatable characterization of SOLIDWEDGE™ wedgelocks under conditions that match real VPX chassis environments. The published thermal resistance values reflect performance you can expect in properly designed systems.
