By Ila Pal, Ironwood Electronics, St. Paul, Minn. [ironwoodelectronics.com]
Today's packages boast high clock speeds (often in the GHz range), fine pin densities (below 0.5mm pitch) and high pincounts (>1000). When these packages are assembled onto a PC board, they perform predetermined functions at certain speeds. Socketing is one way to test the functionality of IC packages without damaging them. Socketing high-speed, high-density IC packages requires an innovative solution to the challenges of designing a shorter signal path (less resistance), good electrical insulation (to prevent signal loss) and proper thermal management. Socket design is dictated not only by the functions mentioned above, but also by other parameters, including durability, power consumption, assembly methods and the environment in which the system will operate. A GHz socket (Figure 1) provides a solution that is fast, dense and durable. Socket Function A socket is an electromechanical device which provides a removable interface between IC package and system circuit board with minimal effect on signal integrity. Removable interface is the major reason for using a socket, and it is required for a variety of reasons, including ease of assembly, reworking, upgrading and cost savings. The cost advantage is a result of saving the IC by not attaching it permanently to the PC board. The socket is permanently attached (soldered) or semi-permanently attached (solderless methods) to the board, and the packaged IC can be inserted into or removed from the socket without disturbing the connections to the board. Socketing helps to test, evaluate and inspect the complete system. Also, sockets allow field maintenance, testing, replacement or upgrades. This has become a critical factor because of technology evolution.
A socket that can test IC packages with 0.5mm pitch and below has been developed. This GHz socket meets a range of high-speed, high-density socket requirements suitable for very compact production sockets to robust test and prototype applications. GHz Socket A cross-section photograph in Figure 2 illustrates the socket design. The sockets are designed such that force is evenly distributed on the top of the IC pushing the solder balls into a very high speed, Z-axis elastomer connection medium. Elastomer is the only medium between IC package and the circuit board. A heat sink screw and the socket body provide heat dissipation for the IC in the socket. Precision guides for the IC body and solder balls position the device for perfect connection. Wire on Elastomer The Z-axis conductive elastomer used in the socket is a low-resistance (<0.01Ω) connector. The elastomer consists of fine pitch matrix (0.05mm x 0.05mm) of gold plated wires (20µm diameter) that are embedded at a 63° angle in a soft insulating sheet of silicone rubber. The insulation resistance between connections with 500 VDC is 1000 MΩ and is ideal for high-current (50mA per filament) applications where a thin, high-density anisotropic connector is required. The gold-plated brass filaments protrude several microns from the top and bottom surfaces of the silicone sheet. The operating temperature range for the elastomer is -35° to 100° C. Sample (0.5mm and 1mm) elastomers with two different thicknesses were used to perform the functional tests, as noted.
Electrical Characterization The test shows the relationship between continuity resistance of the elastomer and the amount of compression (Figure 3). A 0.5mm diameter gold plated pin electrode was compressed on the elastomer at 0.05mm increment. The change in resistance was read using a multimeter. The graph shows that the continuity resistance remains well below 20 mΩ at proper compression of elastomer. Bode diagram (Figure 4) shows the low insertion loss at high frequency for the 0.5mm thick elastomer connection medium. Mechanical Characterization Removable interface requirements are generally stated in terms of the insertion/ extraction force and number of insertion/ extraction cycles a socket can support without degradation. These forces become increasingly important as the number of pin counts in the socket increase. Furthermore, the elastomer must be compressed uniformly to obtain a reliable connection. A relationship between the compression-load versus the amount of compression is shown in Figure 5. Samples with 0.5mm and 1.0mm thickness were evaluated, and the graph indicates that the compression load is directly proportional to the amount of compression.
If the package displays a wide variation in its co-planarity, a thicker elastomer is required. The next test measures the endurance characteristics of the elastomer. For this test, a 188-pin µBGA package (0.5mm pitch, 0.3mm diameter solder ball) was used. The 0.5mm thick elastomer was compressed to 0.2mm and 1mm thick elastomer was compressed to 0.3mm. The results are shown in Figure 6. The graph illustrates that the continuity resistance remains constant up to 50,000 cycles. In concluding the mechanical test results, the elastomer is an excellent medium for applications requiring 50,000 mating cycles with minimal mating force. If more cycles are required, elastomer replacement is effortless. Conclusion A primary concern to anyone utilizing the high-density µBGA package is that the socket must provide a low and stable value of resistance while meeting mating requirements, particularly those involving mating force and the number of mating cycles that must be withstood without degradation. GHz sockets solve such concerns and provide unmatched solution for high-speed, high-density, high pincount applications. The simple socket design makes it cost efficient and allows assembly to the target board using existing hardware. A unique feature of this socket is its separable components that can be easily replaced and reused.
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