By Ila Pal
Flash memory adaptor design provides a solution for probing high-speed, high-density flash memory IC packages.
 Figure 1. Flash memory adaptor design. |
Application service providers can offer a new kind of thin client (a network computer without a hard disk drive) because of enhancements in flash memory technology. The thin client delivers applications embedded on and executed from flash memory, making the applications completely manageable via software. Flash memory integrated circuit (IC) packages feature high clock speeds and fine pin densities (0.75 mm pitch). Packaging for such IC devices must feature finer interconnections as well as better electrical and thermal performance. Probing these high-speed, high-density flash memory IC packages requires an innovative solution to the challenges of designing a high-quality, high-speed, low-cost adaptor.
Current Adaptor Technologies
Adaptors have many classifications. One common adaptor solution is having a socket on the top and through-hole pins on the bottom that solder to the target board. This is a solution for low-density IC packages, but cannot be transferred to accommodate the tight tolerances and microscopic features of high-density chip scale packages (CSPs).
 Figure 2. Electrical resistance characteristics. |
Another option is using surface-mount technology (SMT) with a two-piece, pluggable solution. An emulator base is surface-mounted onto the target board and the probe board with test pins plugged onto the emulator base. This method uses a commercial socket pin technology, and is limited by the insertion and extraction force (insertion/extraction force is high for high-pin-count and high-density ICs). Also, this configuration provides a longer signal path, which makes it inefficient for high-speed testing.
Flash Memory Adaptor
Another solution is a compression technology that uses a conductive elastomer to achieve high density and high speed; it can achieve this because of the short length of the elastomer. An example of flash memory adaptor design consisting of a probe board and emulator base is shown in Figure 1.
 Figure 3. Current-carrying capacity. |
The probe board consists of a zero-insertion force (ZIF) socket on the top and pin interface on the bottom. The pin interface is compressed against the elastomer in the emulator base. The emulator base is a low-profile, high-temperature FR-4 substrate interconnected to eutectic solder balls. The SMT emulator base can be soldered to the target board by standard flux and reflow methods. Gold-plated terminal pins on the probe board (below the ZIF socket) allow for interconnection to the SMT emulator base. All signals are made available for probing on 0.025" square posts around the perimeter of the socket.
The socket on the probe board is designed such that force is evenly distributed on the top of the IC, pushing the solder balls into a high-speed, Z-axis elastomer connection medium. A heat-sink screw and the socket body provide heat dissipation for the IC in the socket. Precision guides (alignment plate) for the IC body and solder balls position the IC device for connection.
Conductive Elastomer
The Z-axis conductive elastomer used in the adaptor is a low-resistance (less than 0.01 Ω) connector. The elastomer consists of fine-pitch matrix of gold-plated wires, which are embedded at a 63° angle in a soft insulating sheet of silicone rubber. The insulation resistance between connections with 500 VDC is 1,000 MΩ. 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 150°C. Sample elastomers with three different thicknesses (0.5, 1.0 and 2.0 mm) were used to perform functional tests.
Electrical Characterization
The first test examined the relationship between continuity resistance of the elastomer and the amount of compression (see Figure 2). The continuity resistance remained well below 30 mΩ, and was even lower at more compression of elastomer.
 Figure 4. Compression load characteristics. |
The second test was performed to determine the current-carrying capacity of the elastomer (see Figure 3). The result was that the temperature rise was exponentially proportional for the first minute and remained constant for the supply of both 10 and 5 A current. This test was followed by a burn-in test. The adaptor was connected in series with a 20 Ω power resistor. A 5 VDC power supply was connected in parallel to the adaptor. A 250 mA current was supplied continuously for 24 hours and the changes in continuity resistance were recorded. The continuity resistance remained the same throughout the burn-in period of 24 hours.
Mechanical Characterization
The fourth test focused on stress analysis of the adaptor. The elastomer must be compressed uniformly to obtain a reliable connection. The compression load is directly proportional to the amount of compression (see Figure 4). Less compression force was required for the 2.0 mm-thick sample. This factor accounts for packages with more than 1,000 solder balls. Also, if the package has a wide variation in its coplanarity, a thicker elastomer is required.
 Figure 5. Endurance characteristics. |
The fifth test measured the endurance characteristics of the elastomer. A compression pressure of 275 g per mm2 was applied with 1 second ON/1 second OFF compression cycle. The continuity resistance remained constant up to 60,000 cycles (see Figure 5). If more cycles are required, users can change the elastomer in the adaptor; two adaptors are not necessary.
Configurations of Emulator Base
The surface-mountable emulator base (see Figure 6) requires no tooling holes to mount on an existing target board. The emulator base solders directly to the target board lands, requiring a clearance of 2.54 mm from the perimeter of the IC package. Standard SMT methods can be used to attach the emulator base, employing low-temperature eutectic solder balls, to the target board. The probe board assembly is then dropped into the emulator base, which contains a screw mechanism, and tightened.
The solderless emulator base possesses the same real-estate characteristics as the surface-mount version. However, the flux and reflow requirement (a semipermanent attachment process) is eliminated by adding tooling holes to the target board and mechanically fastening the emulator base to it. The Z-axis conductive elastomer is the only component between the pin bottom and the target board pads.
Conclusion
 Figure 6. Flash memory adaptor with surface-mountable emulator base. |
Primary concerns of anyone testing high-speed, high-density flash memory ICs are keeping development costs under control, optimizing existing manufacturing capability and minimizing time-to-market. Flash memory adaptors solve a myriad of high-speed, high-density application needs. The design of the adaptors allows assembly to the target board using existing SMT methods.
High-density socketing and testing is not a futuristic concept. IC devices as well as adaptors are currently being distributed and used in a variety of portable and hand-held products. As miniaturization furthers, the main stream of the industry will also embrace this adaptor technology — another step in the evolution (or revolution) of electronic package testing.
ILA PAL is a Research & Design Engineer, Ironwood Electronics Inc., P.O. Box 21151, St. Paul, MN 55121; (651) 905-7916; Fax: (651) 452-8400; E-mail: mannan@ironwoodelectronics. com; Web site: www.ironwoodelectronics.com.
SPEC SHEET
End Applications:
Flash memory IC packages
Related Products:
Elastomers, sockets, PCBs
Main Point:
Primary concerns of anyone testing high-speed, high-density flash memory IC packages are keeping development costs under control, optimizing existing manufacturing capability and minimizing time-to-market. Flash memory adaptors can solve a myriad of high-speed, high-density application needs. They use a compression technology that involves conductive elastomers.
