Traditional drive mechanism wedge locks, or conduction-cooled embedded module card retainers, have typically been selected by physical size: i.e. height, width, length, and mounting locations, then by thermal performance as stated in physical test results that are typically provided by the manufacturer. The data sheet specifications will also provide other data such as clamping force and additional options such as lock patches, torque-limiting features, plating options, etc.
What is not stated in any manufacturer’s technical datasheets for traditional wedge lock retainers, is a description of failure modes, or design guidance on how to mitigate against the risk of potential mechanical failures of these devices in operation.
A conduction-cooled embedded computing module utilizing wedgelocks with traditional drive mechanisms can become jammed inside the deployed environment in a few ways. Once a mechanical malfunction occurs, the module cannot be easily extracted from the system, and could jeopardize or completely incapacitate critical embedded systems during mission sets for the end-user.
Several years of real world feedback from customers has led WaveTherm to highlight this fundamental design implication, and the impact it could have on customers’ ability to deploy reliable embedded systems. Our intent is to help OEM Module Suppliers, and System Integrators, identify this key risk to their systems during the early stages of development, so that the risks can be properly factored into the board-level design decisions before those decisions hinder the reliability of the deployed system.
There are three main ways that traditional drive mechanism, U-channel shaped extruded wedge segmented wedgelocks can become inoperable:
Disengaging a wedgelock with an extraction tool(typically a hex key torque beam), can lead to the front piece/wedge segment of the wedgelock becoming disassembled, and falls wayward of the still mounted wedgelock. See animation –
Mis-aligned wedgelock assembly -
Many individuals have had the experience of being unable to release a circuit board module when wedge segments on traditional-drive mechanism wedgelocks have jammed in the raised position. Often, the only solution is to
disassemble the host chassis with the potential of having to destroy the chassis by drilling through the side to gain access to the seized wedge segment(s).
The best solution is, of course, prevention.
The use of a wedge lock with positive retraction will prevent the disaster that is caused by stuck wedge segments. The SolidWedge™ design, where each wedge segment is connected to adjacent wedge segments, inherently provides positive retraction.
When the drive screw is turned counterclockwise, the threaded drive wedge is retracted which in turn pulls each of the connected segments down to its released position.
Other designs have attempted to assist in the retraction of each wedge segment, but the frictional forces involved that hold a wedge segment after it has been inserted using hundreds of pounds of force can not be expected to be retracted by spring tension.
In today’s critical applications, where easy access to embedded computing modules and rapid field replacements are crucial to successful system operations, it makes sense to eliminate the threat of stuck wedge segments and the resulting non-extractable circuit boards.