November - December 1999
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Putting New Technology to Work in a Growing Market
The rapid emergence of Rambus¬ technology has sent vendors scrambling to provide sockets that will test and burn-in Rambus-enabled memory ICs cost-effectively and to spec.
- By Ron Iscoff, Editor
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| Figure 1. Texas Instruments has expanded its Rambus burn-in sockets with an open-top, auto load/unload design for ease-of-use. The new sockets handle smaller RDRAM pitches, down to 0.75 mm, and a greater range of I/Os. |
A key difference in both test and burn-in sockets for Rambus-based DRAMs (RDRAM) devices compared to standard memory ICs, is the tighter electrical requirements, including inductance and capacitance, according to Melissa Frank, Rambus' manager of infrastructure and validation engineering, Mountain View, Calif.
The majority of socket vendors are now addressing the Rambus market, and are finding that the cost of entry presents some problems, both in dollars and in technical challenges, though not insurmountable ones.
"Rambus technology will drive the first high-volume application of CSP burn-in sockets."Rambus technology will drive the first high-volume application of CSP burn-in sockets, says Ed Craig, sales and marketing manager for Texas Instruments Interconnection Business (Figure 1), Mansfield, Mass. "This will test burn-in socket suppliers' ability to ramp the manufacturing and assembly processes, as well as their ability to accommodate the different size packages associated with each semiconductor maker's final package outline," Craig adds.
Socket design involves the functional requirements of mechanical, electrical, thermal, reliability and re-usability, as well as the commercial issue of cost," Craig adds. "The contact to the solder ball is the socket, which is the most significant and difficult issue due to the nature of the soft eutectic solder balls."
Major Tradeoff
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| Figure 2. The LibertyLine™ RDRAM burn-in sockets from Liberty Research are gold-plated, open-top units, claimed to be priced lower than any product with competitive specs now on the market. |
Randy Knudsen, product manager for Rambus applications at Johnstech International, Minneapolis, says the availability of a customer's package samples results in designers having only preliminary tool drawings and unknown tolerances to which they design. Then, too, the difference in RDRAM packages, such as length, width, thickness and ball sizes (both diameter and exposure) "are different from various partners, requiring device-specfic hardware for good alignment and contact."
Nick Langston Sr., president of Liberty Research Inc., Santa Clara, Calif. (Figure 2), observes that "The signi-ficant challenge for the RDRAM test socket is to provide low inductance (<0.8nH) to eliminate the noise in the ground loop while providing a long service life of 100,000 test cycles.
Langston says Liberty was "primarily employing Pogo Pins with an inductance of about 1.2nH, but a 0.8 Gb/second per pin (the Rambus' data channel rate), and the inductance was excessive." The pins offer the advantage, however, of low cost and excellent reliability beyond 100,000 insertions, according to Langston. "To reduce the inductance of the pins, we have been making them shorter and shorter."
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| Figure 3. Aehr Test Systems offers DiePak™ reusable carriers and sockets. |
Significant Challenge
"Due to the fine pitch of the RDRAM, creating burn-in and test sockets that offer reliable electrical contact throughout the temperature range" represents a significant challenge, according to Aehr Test System's Rick Pendergrass, Mountain View (Figure 3). "There is also the challenge of creating a standardized burn-in board and socket that accommodates the various die sizes and CSPs from different manufacturers," he adds.
While technical challenges tend to be foremost in the socket makers' thoughts, Ariane Loranger of Loranger Inter-national, Warren, Pa., reports "a continuous uncertainty as to whether or not Rambus will proceed" in high volume, in light of competitive approaches that have been gaining favor.
On the technical side, Loranger says planning for outrigger solder balls and designing sockets for multiple pitches are additional issues. "The number and locations of outrigger solder balls have changed over time. To minimize the customer's cost, we try to populate the socket for the maximum I/O count, which becomes an issue when addi-tional outrigger balls are added."
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| Figure 4. OzTek's HP 95000 RDRAM auto test contactor with 64 I/O is part of the OzTek i2™ test socket system which combines interchangeable inserts and spring-loaded probe contacts to produce an accurate and reliable high-speed signal path. |
Jim Brandes, product manager for PrimeYield Systems, St. Paul, Minn. (Figure 5), notes that Rambus devices are "very sensitive to parasitic inductance, which must be minimized in the socket design."
Parasitic Inductance
Brandes says that high Rambus data rates maximize contactor bandwidth requirements. Data rate and bandwidth needs can be satisfied "by keeping the signal path through the contactor as short as possible with maximized contact path cross-sectional area."
Many Rambus sockets today are arranged in 8 and 16 site configurations with some handler makers gearing up for 32-site testing.
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| Figure 5. PrimeYield employs "Plunge-to-board technology" and a minimized path length. |
In addition to a short signal path, cited as key by most providers of Rambus sockets, Bill Richline of Tecknit Interconnection Products, Cranford, N.J., calls for good coplanarity and a small footprint.
Another development goal, according to Frank Lessani, project manager for Yamaichi Electronics USA, San Jose, is meeting the design needs of the customer's time frame, considering the rapid changes occuring with Rambus.
Dimensional Variations
John Hartstein, memory marketing manager for Wells-CTI, South Bend, Ind., notes that developing a contact design that can accommodate the dimensional variations in solder balls, package bow and warp, while providing a high-quality electrical interconnection is at the forefront.
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| Figure 6. Synergetix has developed several spring-loaded contactors for Rambus memory testing the Micro Pitch Probe used for Rambus sockets tested at or near 800 MHz is pictured here. |
This is a technology, OzTek's Rogers adds, that will "be subject to the same price pressures as all DRAM products."