Socket-adapter Systems: A Practical Test Alternative
09/01/2006
An Innovative, Space-Saving Approach
BY GLENN GOODMAN, Advanced Interconnections Corp.
Traditional one-piece, clamshell-design test sockets are useful diagnostic tools for establishing or evaluating device functionality. However, they tend to be less satisfactory for validating fine-pitch packages such as ball grid array (BGA) and land grid array (LGA), which are gaining popularity for field-programmable gate arrays (FPGAs) and programmable logic devices (PLDs) as the trend toward convergence in cell phones and other portable devices continues. Innovative two-piece socket-adapter systems offer design engineers significant size, speed, and cost advantages over traditional sockets.
At least 10 mm larger than BGA devices, one-piece sockets are bulky and require extra heat for the solder attach process. This size difference can lead to problems when many components must fit in a small area. The answer has been a time-consuming, costly redesign to properly fit the socket onto the board.
Two-piece socket-adapters can be made only 2 mm larger than a BGA device. Once in place, a socket is soldered to the PCB and a device (or devices) is soldered to an adapter, which can be plugged in as needed (Figure 1). Test, validation, or field programming can be done on the production-level board without redesign for testing.
 Figure 1. A socket-adapter system converts an SMT package to a thru-hole assembly that can easily be plugged into a board-mounted socket. |
As pitch decreases because of the proximity of the socket shells, older socket systems’ limitations reduce operational bandwidth and leave circuitry open to crosstalk. Next-generation socket-adapter systems must keep pace - both mechanically and electrically - with fine-pitch CSP, BGA, and LGA packages. A novel socket-adapter design uses a hybrid concept in which half of the male terminals are arranged in an interstitial pattern and applied to the female socket. The mating adapter contains the other half of the male terminals, which alternate with female terminals between the rows (Figure 2), allowing for a previously unattainable pitch (down to 0.50 mm). Additionally, increased distance between adjacent terminals improves the electrical characteristics of the entire connector system. By alternating male and female terminals, screw-machined terminals with multi-finger contact redundancy can be used, and are preferable to stamped and formed terminals for their durability for fine-pitch.
 Figure 2. Innovative designs provide a practical alternative to traditional test sockets at only 2-mm larger than a typical BGA or LGA package. |
Electrical and mechanical benefits accrue from socket-adapters. Shorter signal path permits greater socket signal integrity. Fewer working parts mean less likelihood of component failure. Lightweight construction makes this type of socket almost invisible to the PCB, and requires minimal thermal profiling.
Socket-adapters Cut Costs
A typical 0.50-mm clamshell socket, with its die-cast and molded mechanical parts, is expensive because it requires that tooling holes be drilled into the board so the socket can be bolted down. A fine-pitch socket-adapter system does not contain multiple parts and is far less expensive to manufacture. Soldering a device to an adapter not only protects the device from damage during board processing, but also facilitates future device upgrade or replacement. Eliminating board redesign reduces test time and expense.
In certain applications where space is not a concern, traditional clamshell-type test sockets are preferred because devices can be quickly and easily socketed with a simple turn of a screw; no soldering is necessary. Socket-adapter systems require device soldering and must be plugged in - introducing an insertion force - and separated through some type of mechanical procedure, often through the use of an extraction tool. Either step carries with it some potential for damaging a component or a board.
Applications for Socket-adapter Systems
Innovative socket-adapter systems are being used in any number of fine-pitch applications. For example, perhaps an OEM discovers an unstable chip after boards are already designed and in production, but can’t fit anything more into that space, because the components are already up against the side of the device. Installing a socket-adapter system that matches circuit parameters eliminates the need to redesign the board and go back into test mode (Figure 3).
 Figure 3. Space-saving socket-adapter systems offer a viable test and validation solution for fine-pitch packages. |
Cell-phone mobility groups build large diagnostic boards to perform diagnostic and life-cycle testing on various iterations of next-generation chips, miniaturizing what may have been a 12"-square board down to 4" square. Fine-pitch socket-adapter systems simplify this process.
Companies that make flash memory are using 0.5-mm socket-adapter devices in the development process. Once the board has been finalized, they can easily move several flash memories in and out to determine which one will work best in a particular application.
In the automotive industry, manufacturers use test heads to perform diagnostics on vehicle engines and rear ends. Fine-pitch technology is essential for the tiny satellite function cards that can be swapped in and out to replicate other functions.
Socket-adapter technology is gradually gaining acceptance in medical applications. Manufacturers of complex medical equipment are sometimes reluctant to abandon reliable approaches. Miniaturization is advancing, however, and fine-pitch devices eventually will come into their own in that industry as internal circuitry continues to become smaller.
Lead-free Compliance
Compliance with the European Union’s Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS) directives means that sockets must be not only RoHS-compliant, but also RoHS-compatible. The lead-free solder balls on a BGA device, for example, require higher processing temperatures - as high as 260˚C - that can damage older components.
Interconnect manufacturers are developing a steady stream of innovative sockets and adapters to ensure a smooth transition to these new material and processing requirements. Rather than reconfiguring a PCB processing profile to accommodate a lead-free BGA device, a design engineer can use a high-temperature molded or FR-4 socket in place of the device. Soldered to a lead-free BGA adapter, the device is converted from SMT to thru-hole; the device/adapter assembly can then be plugged into the board-mounted socket.
If a socket is substituted for a semiconductor, the semiconductor is protected from damage during high-temperature board processing; a path for future replacement or upgrade is created. A BGA device with tin-lead solder balls, or any device package such as DIP or PGA, can be used on a lead-free board, allowing the manufacturer to reduce costs by stocking only one lead-free board design for both RoHS-compliant and RoHS-exempt applications.
Summary
Innovative new two-piece socket-adapter systems offer a number of advantages over traditional clamshell test sockets in miniaturized circuitry applications. In particular, the large size of older sockets can present major problems when dozens of components have to be crammed into a small space on a PCB.
Interconnect manufacturers are developing a steady stream of next-generation sockets and adapters not only to meet the mechanical and electrical demands of new IC packages, but also to ensure a smooth transition to WEEE- and RoHS-compliance.
GLENN GOODMAN, design engineer, may be contacted at Advanced Interconnections Corp., 5 Energy Way, West Warwick, RI 02893 USA; 401/823.5200; E-mail: [email protected]