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By Ila Pal, Ironwood Electronics Inc., St Paul, MInn. Large ICs are moving toward very high clock speeds (in the GHz range for some applications), while pin densities below 1 mm pitch and pincounts over 500 are becoming common. Accordingly, packaging for such devices must feature finer interconnections and better electrical and thermal performance. Socketing these high-speed, high-density IC packages requires an innovative solution to obtain high-quality, low-cost and high-speed sockets for test, burn-in, development and production. GHz* BGA sockets, which employ conductive elastomers, provide a solution unmatched in pin density, electrical performance, and socket size. Current Technologies Commercially available socket technologies offer pros as well as cons. One common socket type employs stamped leads in a variety of configurations, such as "Y," "pinch" and "fork" contacts. While these are suitable for low-density packages, they cannot accommodate the tight tolerances and microscopic features of high-density BGA packages. The spring and probe contact has been successfully employed in sockets for fine- pitch packages, but is prohibitively expensive. Additionally, this contact can leave a permanent "witness" mark on solder balls, which in turn causes contamination and poor solder joint quality when packages are reflowed for attachment to the production board. The various configurations of stamped-lead contacts also provide a longer signal path, making it inefficient for high-speed operation.
Another technology, and one that lacks most of the negative aspects of the spring and probe contact, employs conductive elastomers to achieve high density and high speed in an affordable BGA socket. The results of our study have confirmed that this is a BGA socket that provides a range of high-speed, high-density BGA sockets from very compact production models to robust units for test and burn-in applications. A typical BGA socket design is shown in Figure 1. The sockets are designed so 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. A heat sink screw and the socket body provide heat dissipation for the device in the socket. Precision guides for the IC body and solder balls position the device for perfect connection.
Conductive Elastometer The Z-axis conductive elastomer used in the socket is a low-resistance (<0.01Ω) connector. The elastomer consists of multiple rows of metal filaments arranged symmetrically in a sheet of soft silicone rubber. The insulation resistance between connections with 500 VDC is 1000 MΩ, making it ideal for high-current (50 mA 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 -30° to 100°C. A sample (P=0.1mm, Ps=0.2mm and T=0.3mm) elastomer (shown in Figure 2) was used to perform the following functional tests. Electrical Characterization This test examined the relationship between continuity resistance of the elastomer and the amount of compression (shown in Figure 3).
The graph shows that the continuity resistance remained the same between a 16%-67% compression of elastomer. The second test was to determine the elastomer's current carrying capacity. Figure 4 illustrates the result. The graph shows that the temperature rise was exponentially proportional to time for the supply of a 3-amp current.
For a 1 amp current, the temperature rise is almost flat over a 15-minute period. This was superseded by a burn-in test in which the socket was connected in series with a 20Ω-power resistor. A 5 VDC power supply was connected in parallel to the socket. A 250 mA current was supplied continuously for 24 hours and the changes in continuity resistance were recorded. Figure 5 shows that the continuity resistance remained the same throughout the 24-hour burn-in period.
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