A Detailed Example "The Agilent V4400 Flash Memory Test System is a good example of a modern tester with a large number of interface connections. Pogo pins are used, but of a special design. Romi Mayder, a project manager with the semiconductor test section of Agilent Technologies’ Automated Test Group, said, “A pogo pin basically is a barrel that houses a spring and a plunger that move in and out. We ran a life test on some pins and found that, after a time, the plating flaked off, debris would get inside, the springs started to anneal, and the contact between the plunger and barrel deteriorated. Our custom pins have a significant bias applied,” he explained, “so the plungers actually are cocked slightly to one side and always in contact with the barrel and rubbing.”  Figure 1 (at right) shows an assembly of V4400 connections for three sites being tested for both DC and RF characteristics. The blue wires are equal-length coaxial cables that terminate in blind-mate connector blocks at the tester’s pin-electronics boards. A close-up view of the pogo-pin connection areas is presented in Figure 2 (see below).
The 64 signal connections are surrounded by smaller ground pogos. The outer ring on the signal pogos is a coaxial structure that carries the 50-W environment right out to the pogo tip, even though the actual ground connection is made separately by the ground pogos. The third type of pogo pin carries power, sense, and other auxiliary signals. To maintain the controlled impedance of the 64 signal lines through the interface connection requires 94 grounding pogo pins. In addition, there are 40 auxiliary pins. Each of the 36 segments comprising the V4400 interface assembly contains 198 pogo pins, making 7,124 in total. And, each of the four quadrants comprising nine sets of site electronics has its own connections, all together making 7,290 pins requiring an overall force of 640 lb to compress. In the V4400, vacuum is used to force the probe or load board into contact with the pogo assembly. A 14"-dia probe board has an area of 154 in.2 so atmospheric pressure of 15 lb/in.2 could provide a force of 2,310 lb, assuming a perfect vacuum on the other side of the board. Obviously, only a reasonably good vacuum is needed to obtain the required 640 lb. When the pogo pins are sufficiently compressed, the board seats against a large-diameter flat hard stop, so it is clear that the assembly is correctly and completely mated at that point. The use of a hard stop removes the mechanical stress that high forces otherwise could impart to the probe board material. This means that vertical positioning is very accurate and repeatable. Accuracy is maintained even under large probe-to-wafer loads encountered with a high degree of parallel test because the probe board doesn’t flex. The lack of an integral mechanical stiffening frame, by Mr. Mayder’s estimate, made available 20% to 30% additional routing area, which helped ensure controlled impedance tracking and uniform lengths. Because the V4400 is designed to test flash memory devices, edge speeds and clock frequencies are only moderate. Timing tolerances of 5 ns are common, affording the tester’s designers with greater opportunities than, for example, the sub-100-ps tolerances of fast RDRAM testers. On the other hand, the nature of the flash devices themselves creates challenges. Most DRAM testers are strictly 50-W terminated systems. This means that the actual length of the cabling has little effect on DUT signals as long as all cables are equal in length. Similarly, RDRAM devices are designed to operate in a 28-W environment. Conversely, the output current available from flash memory devices is much less than from DRAM or RDRAM parts. Also, the output impedance is from 100 W to 300 W, and it varies dynamically with output level. Consequently, terminating the DUT with a 50-W load isn’t practical. Instead, the V4400 operates with unterminated lines when the DUT is driving output signals. Simulation and actual measurements have shown that an acceptable pulse edge results from the combination of a 1-ns rise time and a 2-ft long interconnect path from the DUT to the pin electronics." "...Ben Morris, the Agilent product manager for the V4400, said, “Customers are testing SRAMs, RFID tags, USB controllers, and some really diverse applications. Our systems have the right price point and a large enough pin count and are sufficiently flexible that people use them to test many other devices in addition to flash memory.” Reprinted with permission
Published by EE-Evaluation Engineering
All contents © 2001 Nelson Publishing Inc.
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