Jim Murphy - President, Advanced Interconnections Corp.

ED Online ID #7840

April 26, 2004

Understanding Electrical Issues:

When planning to mount an electronic device with a socket, it's important that the socket interfere as little as possible with device signals. In the case of a low-speed device, the socket's electrical properties may make minimal impact on signal processing, so using a socket may prove simple. If the device operates at very high speed, the socket might alter the signal path, impeding signals and adversely affecting the device's functionality. This situation may call for the incorporation of additional electronic components into the motherboard design to preserve signal integrity. For board designers, this is key as it's generally easier and more cost-effective to resolve component specification and placement issues early in the design cycle.

Understanding Mechanical Issues:

Mechanically, sockets and socket-adapter systems must exert adequate force to retain the packaged device securely and ensure proper electrical contact with the motherboard. On the other hand, the physical force needed to mate the adapter to the socket (or to remove it) must be low enough to prevent distortion or damage to the device or the motherboard. Hence, it's beneficial for designers to know the precise retentive qualities of a socketing system before specifying it. This will ensure that when the device is inserted, it won't loosen when subjected to vibration or fit so snugly that attempts to remove it will possibly cause damage.

Equally important is the material and mode of construction of the socket contacts. For example, a screw-machined beryllium-copper contact with heavy gold plating will typically deliver more predictable and consistent mechanical and electrical performance than a stamped contact of the same material with thinner gold plating. Plated thickness is particularly important when dealing with test fixtures. Repeated insertion and extraction of adapters into a socket will eventually wear away the gold plating, changing the contact resistance and potentially skewing test results. So if the plating thickness is inadequate, the number of permissible insertion/extraction cycles may be reduced, which will necessitate replacing the socket itself more frequently.

Understanding Thermal Issues:

To maintain optimal mechanical integrity and electrical performance throughout all temperature cycles, the designer should match the socket's temperature performance characteristics (i.e., thermal resistance, temperature stability, coefficient of expansion, melting point, etc.) to those of the package to be mounted. This will alleviate potential problems that may adversely affect heat dissipation, mean time before failure (MTBF), frequency of field repair, and consistency of lab simulations with actual field experiences.

If, for example, the maximum operating temperature of a device package exceeds the maximum service temperature of the plastic used in the socket and adapter, either of the latter might deform or melt. Moreover, if the package has a different thermal coefficient of expansion than the socket and adapter, localized deformation problems, such as warpage, may result in the board or the package.

Understanding Cost Issues:

Any time additional components are required in an electronic system, cost tradeoffs must occur. For example, if a motherboard or packaged device carries a low price tag, it's intuitive to treat them as disposable items. In such cases, specifying sockets would drive up production (and selling) costs unnecessarily without improving the product. However, sockets will more than justify the added cost of incorporating them into the design if the board or device carries a high value or if the development/test/emulation/assembly process must be accelerated. This is true too if the device must be field-programmed before mounting to the motherboard or the board and device is to undergo field-testing, repairs, and upgrades.