Anatomy of a Card Retainer

The simple card retainer may not have the glamour associated with the latest embedded processors, which tout faster data throughput, low power consumption and a host of other performance-related attributes, but this often-overlooked structural element is an integral part of an embedded system. Mounted directly to the printed circuit board (PCB) itself, a card retainer, also called a wedge lock, securely holds the board in place, reducing the risk of damage to the electronics within an embedded system.

Most applications requiring card retainers are found in avionics. One of the critical aspects of a quality card retainer is the ability to maintain uniform clamping force across the length of the card, especially when subjected to the extreme shock and vibration levels found in these types of application environments. This force is needed to maintain solid mechanical rigidity as well as maintain the surface-to-surface contact that provides heat management. The number of wedges used, a customizable feature on a card retainer, has direct bearing on the performance of the retainer. A higher number of wedges, the more clamping force and thermal transfer.

Complexity in a Simple Component

There are many other features hiding behind the scenes of the simple card retainer. A close examination will uncover elements like coined edges facilitating a smooth insertion and extraction from a system. (Figure 1) The use of extruded aluminum gives the card retainers a lightweight, yet exceptionally strong, construction—a critical component in mobile and airborne applications.

Flexcom Cutout Surelocks

And these versatile components offer a surprising number of options to enable customized solutions that match the operating environment. So, what exactly makes a card retainer right for a given application?

Popular design elements include multi-segment retainers that can be manufactured to a specific length to meet the needs of an application as well as the choice of the size and style for mounting holes. Both of these are important attributes in the growing small form factor (SFF) space.

There are other elements that allow an engineering to “custom design” a card retainer to his or her application. Drive type is typically user or project dependent. For example, if used in a metric environment, the drive will usually be metric. Striking the right balance between a lightweight design and the proper finish is also a large part of selecting a card retainer.

Different locking mechanisms, typically either screwed-in or lever-based, are used to ensure that the card retainer is secured, with some offering visual indicators that confirm whether the retainer is closed or open.

Most card retainers for high G force applications require the use of a screwdriver to ensure the board is securely in-place. However, there are a few ‘tool-less’ retainers that can adequately secure a board, typically using a lever built into the card retainer itself. While this design may work for certain, less rugged applications, care must be taken to avoid overtightening the lever. This can result in a costly repair if the walls are damaged, since most avionics enclosures are milled and will therefore require the entire unit be removed for repair. Similarly, any imperfections in the spray-coated finish will impact thermal management, typically requiring an entirely new finish be applied to the enclosure.

Your best bet is to keep the rugged, mobile and harsh nature of your environment in the equation, as it may necessitate a screwed-in card retainer.

Built-in Heat Transfer Properties

Although typically used throughout mobile, mission-critical and rugged applications, card retainers have found a home in stationary industrial environments as well, thanks to their inherent thermal management properties that exist due to the strong contact the card rails have to the enclosure.

For example, with a simple turn of a screw, Elma’s SureLock retainers expand to both securely hold the PCB in place as well as facilitate conduction cooling by pulling heat from the card and transferring it to a cold plate or extruded wall of the enclosure. They also offer a visual indication that the card has been properly secured in the form of a red screwhead that is visible (or not) when the proper tightening has been achieved.

Test performed on these types of card retainers have shown impressive thermal transfer capabilities in two different types of finishes, Yellow Alodine and Black Anodized (Figure 2). Another popular finish is electroless Nickel, which typically rates even higher than an anodized finish.

Surelock 290 Series Thermal Resistance

It’s important to note the difference between the types of finishes and the associated clamping force.

Yellow Alodine and Black Anodize are popular options, both offering good clamping force and thermal properties. The most commonly-specified finish is anodized since it typically provides the optimum cost-to-performance ratio. If you were to review this finish under a microscope, you’d see an almost peak-and-valley type architecture that provides increased stability to hold two surfaces in place. If frequent removal of the cards is needed, this may not be the best option, since retainers can get ‘stuck’ from this friction when removing.

Electroless Nickel may be a better option for these types of applications, although it comes with a heftier price tag, sometimes in the neighborhood of a 25% increase. However, it’s also the most efficient as it provides superior clamping force as well as exceptional thermal properties, while the smoother surface does not produce the binding effect of the anodized finish.

Specify Early to Avoid Headaches

Some of the simplest engineering oversights have caused catastrophic system failures and card retainers are not immune. If your application will require a card retainer, it’s best to evaluate your needs early in the design process or you run the risk of being stuck with some unforeseen design challenges. Some common pitfalls when specifying card retainers in at the end of the process include:

• No room on the board: if you don’t make provisions for the placement of the card retainers, certain components may be in the way of where the rails need to go on the board itself.
• Ground planes on the outside: to ensure proper thermal transfer, the ground planes should be positioned to conduct as much heat as possible to the outside of the enclosure. Waiting until the end may limit your options.
• Rails are too short: Nothing is worse than discovering your rails don’t extend across the entire length of your board. Make sure the retainers are properly matched to your system’s dimensions.

Improve System Stability

Card retainers are an extremely integral component in mobile, rugged and harsh environments as well as in dynamic, commercial environments exposed to high shock and vibration. They ensure that the card stays seated and properly cooled, allowing for a consistent, reliable interface between the board and the backplane of an embedded computing system. And with a secured board, that won’t succumb to thermal stresses, system reliability is strengthened, meaning better operation and overall functionality...all with the help of the ‘simple’ card retainer.