WEDGELOCK THERMAL & CLAMPING FORCE LIFECYCLE TEST METHODOLOGY

Each SolidWedge is tested under a 100 Cycle Test consisting of four thermal tests and 96 stress analysis (clamping force) tests. Below consists of how WaveTherm goes about conducting these test. 

Thermal Test Methodology

Thermal testing is done to determine the total thermal resistance across the contact surfaces of a test plate and wedge lock to a cold wall. 

There is a measurable temperature drop when thermal energy flows across the contact area of two surfaces. By measuring the temperature differential between the two surfaces and knowing the thermal energy being inserted into the test plate, a calculation of the thermal resistance of the joint can be calculated.


Parallel Resistance Theory
There are three ways in which the heat from the test plate can be dissipated. Of the three there are two parallel paths to consider in calculating the total thermal resistance of the test plate and wedge lock system:

• Conduction to the cold wall is assumed to account for 70% of the heat dissipation from the test plate.
• Conduction through the wedge lock to the cold wall is assumed to account for 30% of the heat dissipation from the test plate.
• Convective and radiative losses from the test plate are assumed to be negligible due to the insulation described in the setup procedure and air flow shielding that can be placed around the unit under test during the testing process.

Terminology

The following terms are used throughout this test procedure:

• Wedgelock – The mechanical device which locks a heat frame or PCB into a cold wall and provides the clamping force necessary to facilitate heat transfer and resist displacement due to shock or vibration
• Heat frame – A metal plate or frame which attaches to a PCB and is designed to conduct thermal energy from the active components on the printed circuit board (hereafter PCB).
• Test Plate – A representative heat frame with a wedge lock on one edge, thermocouples adjacent to the wedge lock, and resistors mounted on it to simulate the effect of active components on a PCB/heat frame combination.
• Cold Wall – A heat sink with one or more card edge channels formed in it to support the heat frame, and which uses active cooling to dissipate the thermal load generated by the PCB/heat frame.
• Test Fixture – A representative cold wall with active cooling, typically finned heat sinks and fans.
• Thermocouple – A pair of dissimilar wires used to determine a temperature at its point of contact.
• Data Reader – An analog-to-digital converter which converts the micro-voltages generated by a thermocouple into temperature values.

Equipment Requirements

Cold Wall
The cold wall is of sufficient size to allow the cooling fins and fan combination to effectively cool the wattage that is planned to be dissipated during testing. There is a slot machined in the cold wall that is manufactured to an appropriate specification corresponding to the wedgelock specimen (VITA 48, VITA 78, etc.) to be tested. The cold wall slot contact surfaces has a surface roughness of 16 µin (RMS) or better. There are thermocouples placed on each side of the slot to determine the temperature of the cold wall adjacent to the slot. The cold wall is plated Clear Chromate per MIL-C-5541m class 3 to represent a typical chassis.

In order to prevent the heat frame under test from contacting the “bottom” of the channel and thus providing a third surface to transfer thermal energy, there is a spacer approximately 0.5 inches wide and approximately 0.040 inches thick made of a non-thermally conductive plastic such as ABS, or similar, and placed at the base or non-contact side of the cold wall slot. This spacer is removed after the test plate is locked in place.

Figure 1: Cold Wall for Thermal Analysis
Test Plate
The test plate is long enough to accommodate the wedge lock to be tested, and approximately 2.50” wide to accommodate the resistive elements to be used. The thickness of the wedge lock mounting edge is determined by the wedgelock height and the cold wall slot:

𝑇𝑒𝑠𝑡 𝑃𝑙𝑎𝑡𝑒 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 = 𝐶𝑜𝑙𝑑 𝑊𝑎𝑙𝑙 𝑆𝑙𝑜𝑡 𝐻𝑒𝑖𝑔ℎ𝑡 − 𝑊𝑒𝑑𝑔𝑒𝑙𝑜𝑐𝑘 𝐻𝑒𝑖𝑔ℎ𝑡 − 𝑊𝑒𝑑𝑔𝑒𝑙𝑜𝑐𝑘 𝑁𝑜𝑚𝑖𝑛𝑎𝑙 𝐸𝑥𝑝𝑎𝑛𝑠𝑖𝑜𝑛

Here is an example calculation using a standard VITA 48 cold wall with a 0.225” tall wedgelock which has a nominal expansion of 0.025”:

𝑇𝑒𝑠𝑡 𝑃𝑙𝑎𝑡𝑒 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 = 0.525 − 0.225 − 0.025 = 0.275𝑖𝑛

Material for both the cold wall and the test plates is 6061-T6 aluminum with a surface finish specified as 16 µin (RMS) in the areas of contact to the wedge lock and between the test plate and the cold wall.
Figure 2: Test Plate for Thermal Analysis
Temperature Measurement
There are three temperatures required to calculate total thermal resistance:
  • Test Plate (TP) - The test plate temperature is determined by averaging four thermocouple readings. The thermocouple locations is evenly spaced along the length of the test plate, at least an inch (1.0”) from either end. The thermocouples is located between the heat source and the wedge lock, as close to the wedge lock as possible (typically 0.100” – 0.200” from the center of the thermocouple hole to the wedge lock edge).

    • Cold Wall Frame Side (TCWF) - The cold wall frame side temperature is determined by averaging two thermocouple readings. The thermocouple locations is centered along the length of the cold wall, at least an inch (1.0”) from either end. The thermocouples is as close to the frame-cold wall interface as possible (typically 0.100” – 0.200” from the center of the thermocouple hole to the specified interface).

    • Cold Wall Wedge Side (TCWW) - The cold wall wedge side temperature is determined by averaging two thermocouple readings. The thermocouple locations is centered along the length of the cold wall, at least an inch (1.0”) from either end. The thermocouples is as close to the wedge lock-cold wall interface as possible (typically 0.100” – 0.200” from the center of the thermocouple hole to the specified interface).
    Calculation

    To calculate the total resistance of the system, each path’s thermal resistance is first calculated separately. The total resistance of the system can then be calculated using the same method as parallel resistors in an electrical circuit:

    • Frame-side Thermal Resistance (RF) - The thermal resistance from the test plate to the frame side of the cold wall can be calculated as the temperature difference between the test plate and the cold wall frame side, divided by the amount of power dissipated through that path.

    •  Wedge-side Thermal Resistance (RW) -The thermal resistance from the test plate to the wedge side of the cold wall can be calculated as the temperature difference between the test plate and the cold wall wedge side, divided by the amount of power dissipated through that path. 


    • Total Thermal Resistance (RT) -The total thermal resistance of the two parallel paths can be calculated the same way as two parallel resistors in a circuit.
              This calculation results in units of °C/W. 

    Initial Temperatures
    The system power energizes the heat sink cooling fan or fans, or other cooling apparatus, and the data acquisition system. All channels stabilize to room temperature before initializing testing. The channels are adjusted so that the numerical average of the four readings on the cold wall is within 0.2°C of the numerical average of the four readings on the test plate. Note: an accurate absolute temperature is not required since all readings will be taken from the same known starting value and only the differential temperature will be used in calculating test results. 

    Power Levels
    Test data is acquired at intervals of 20 watts, up to 100 watts by intervals of 20 watts. For each power level, the power supply is adjust to provide the desired wattage on the test plate. All thermocouple readings stabilize, then the test plate and cold wall thermocouple readings are recorded in a formatted excel file. Stability is defined as no temperature change of greater than 1°C over the span of five minutes.

    Clamping Force Test

    Abstract

    Clamping force testing is performed on the WaveTherm SolidWedge as well as other various wedgelocks to compare the output force generated by a given input torque. The wedgelocks are secured onto their corresponding mounting plates and the mounting plate will be inserted into the clamping force test assembly. Load cells in the assembly are used to read the clamping force of the device at a specified torque. There is a specific software that pairs with the data reader utilizes (OM-DAQ-USB-2401, Omega Data Acquisition Module) that graphs the pounds applied to the load cells of the test fixture as the test commences.

     Test Setup

    The test setup requires a test plate with the correct dimensions correlated to the size of the test specimen and the test fixture as seen below. When combined the test specimen and test plate need to be .525”. The width of the testing fixture with the standard load transfer block is .600”. The clamp force test fixture is meant to simulate a real chassis. The torque wrench is used to mimic real world application of a SolidWedge applied within an embedded system. Each specimen should have a cycle of 2-24, 26-49, and 51-99 clamp force cycles for a total of 96 cycles. The 1st, 25th, 50th and 100th cycles are the thermal tests. All data for each test is collected within separate excel files and then consolidated into one overall spreadsheet. 

    Figure 3: Stress Analysis Fixture