With the proper sockets, you can anticipate and overcome a number of potentially troublesome and costly design issues early in the development cycle.
Electronic designers have long been familiar with using sockets as a fast and efficient method of reliably mounting (or removing in some cases) packaged electronic devices on pc boards. However, current state-of-the-art microelectronic device technology merits a fresh look at socketing.
Other than speeding the production process, there are some major reasons for using sockets today. These include protecting sensitive components from physical damage and thermal shock while the motherboard is being assembled; protecting the motherboard itself when key components are installed or removed; facilitating device testing, emulation, and programming; and simplifying the replacement of failed devices on the assembly line, in the test lab, or in the field. Moreover, as the dollar value of vital electronic devices, such as microprocessors, continues to escalate with each succeeding generation, the technical and financial justifications for using sockets increase correspondingly.
Employing sockets like the one in Figure 1 gives designers novel opportunities to anticipate and overcome numerous potentially troublesome and costly design issues early in the development cycle, well before releasing a new circuit board or system to manufacturing. In many cases, resolving potential problems early means better-performing and more reliable products with fewer field failures and, consequently, fewer redesign and rework requirements.
SOCKETS BOOST PRODUCTION EFFICIENCY
As electronic devices—especially high-reliability types—move from design through prototyping into production, there are critical points at which testing, emulation, and programming activities must occur to validate performance and quality and permit the device to function operationally. Sockets provide a fast and dependable means of temporarily mounting packaged devices onto test fixtures while minimizing or eliminating thermal or mechanical stresses (i.e., soldering, desoldering, package deformation, excessive handling, potential droppage, etc.) that could damage the devices (Fig. 2). This benefit then carries over to subsequent assembly steps on the production line and ultimately to any field test/repair/upgrade activities that might be required during the service life of the device and its motherboard.
Using sockets can also have a positive impact on new-product-development timetables. Typically, electronic design and production activities involve various processes that must occur in parallel to maintain tight production schedules and maximize the efficient use of available resources. Often it's impractical to synchronize these processes so that they all reach "critical mass" at the same time. Thus, a ready-for-production motherboard design for a state-of-the-art computer system may be completed months before a new microprocessor chip, destined for the same computer, successfully exits the design-validation process. Under such circumstances, advance production of socketed motherboards will enable packaged chips to be inserted at a later date as soon as they become available.
Designing boards with sockets also affords other manufacturability benefits. For instance, many production lines use solder reflow to attach sockets to boards (see "Build A Custom Board-To-Board Connector System Using Sockets," below). Sockets typically have a smaller mass than the device package to be mounted. This often significant reduction in mass helps simplify the process of profiling the reflow oven for optimal heat distribution, facilitates socket attachment, and reduces time in the oven. These factors yield payoffs in reduced energy costs and faster processing times.
Another critical solder-related factor during initial board design is the anticipated strength of solder bonds retaining the sockets on the motherboard and the resulting influence on reliability for the service life of the board. Socket pins are frequently butt-soldered to pads on the motherboard. However, variations in the board's relative flatness (coplanarity) and the package to be mounted may cause the solder joints to vary in bond strength, as well as in resistance to shock and temperature cycling. One way to alleviate this problem is by specifying sockets that incorporate low-temperature eutectic solder-ball terminals, which yield stronger solder joints than typical pin-to-pad connections.
DESIGN CRITERIA FOR BOARDS WITH SOCKETS
In an ideal world, the socket would be "transparent" in dimensional, electrical, mechanical, and thermal terms. In other words, the perception would be the same as if the packaged device were being mounted directly to the board. In the real world, however, the socket needs to be viewed in the same light as any other electronic component, with discrete dimensional, electrical, thermal, mechanical, and cost properties.
Understanding Dimensional Issues:
Packaged electronic devices vary widely in configuration, size, and modes of attachment, as do the sockets associated with each package type. For illustrative purposes, Figure 3 details the various components of a soldered socketing system for mounting BGA devices to a circuit board (for a closer look at BGA-packaged chip testing, see "Build A Socketed Chip Test-Fixture Board," right).
When designing a new board configured from the outset to accommodate socket-mounted devices, the engineer must consider—jointly—a number of factors. These factors include the package and socket dimensions (i.e., combined footprint and height), the clearances required around and above the socket-mounted package for proper airflow and heatsinking, and general accessibility to allow for problem-free mounting or removal of the device.
But if the objective is to rework an existing motherboard design to retrofit sockets so as to facilitate the mounting of packaged devices, then other considerations bear attention. For instance, suppose the devices were previously soldered directly to pads on the motherboard. The footprint and height of the package, as formerly mounted, might be significantly less than the footprint and height of the same package when mounted in a socket.
This disparity will reduce the clearances around and above the socketed device to the point where it may be technically impractical to accomplish the retrofit unless the motherboard is completely redesigned. The redesign must ensure an adequate "keep away" zone around the device to permit proper airflow, heatsinking, and unobstructed mounting of the motherboard to a chassis.
Under such circumstances, the use of low-profile sockets with footprints identical to that of the packaged device may help minimize or eliminate any board-redesign requirements. In fact, even in the case of a new board design (where the installation of sockets is planned from the outset), low-profile sockets can save valuable board "real estate."
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 (for a birds-eye look at how socketing averted thermal problems with power converters, see "Mounting Power Converters On Printed-Circuit Boards With Sockets," below).
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.