With the increasing use of personal computers (PCs), digital video disk, Internet terminals, Internet-TV and information appliances, the demand for huge amounts of memory devices is high. And with this demand comes the need to test a large variety of memory types.
The test industry must provide testers to fill a wide range of device and test objectives. Whether it is at the manufacturing, the memory-module assembly, dealer or the engineering laboratory level, these testers must perform with specific objectives in minad.
The Memory Evolution
The first microprocessor board I owned had 2k of static random access memory (SRAM) and 2k of electrically programmable random memory (EPROM) on which the operating system resided. Back then, it only took 2 s for the microprocessor to test the SRAM on the board. Likewise, it only took 2 s for the EPROM programmer to verify the checksum of the operating system.
Well, things have changed. The PC on my desk now has 32 Mbytes of dynamic random access memory (DRAM) on a 168-pin dual in-line memory module (DIMM), 256 kbyte of cache memory on a 160-pin cache on a stick, and a 4-Mbyte operating system on an 80-pin flash SIMM module.
For flexibility and easy upgrading, memories now come in modules made up of several memory chips soldered onto a small PCB. As a result, testing these modules is a very complex task.
Software Memory Testing
Yes, many software-only memory check programs test memories on the PC, but generally they fail to fulfill the various needs of the industry. The memory counting software during the boot-up of your computer simply lets you know the memory size of your computer without checking its operational integrity.
Other memory diagnostic programs usually take more than 10 minutes to finish testing, and these programs only simulate the conditions on one computer. So when installed in another computer, the memory module fails. The memory-module industry urgently needs a new way to test.
Different Levels of Testing
The OEM computer manufacturers want memory modules tested for all the timing parameters listed on the semiconductor specification sheet. This type of testing requires expensive ATE that generates signals in nanoseconds and hundreds of megahertz and that caters to a vast variety of products. This approach costs millions of dollars for the tester, the personnel, the training and the maintenance.
The third-party memory-module manufacturers supply most of the memory modules. They buy memory chips from the semiconductor manufacturers, then assemble the chips onto memory modules using either assembly manufacturer subcontractors or in-house assembly lines.
To minimize being financially hurt by memory-price erosion, they operate very efficiently, usually receiving memory chips in the morning and shipping them out the same day as thousands of modules. To achieve this efficiency, they spend only a few seconds testing each memory module.
The memory tester they use is usually called a functional memory-module tester. As the name implies, this tester only tests and detects assembly errors on the module. The integrity of the memory chips depends upon the memory-chip suppliers. In this high-volume production environment where time is of the essence, the automatic memory-module handler (a mechanical transport system) saves labor and streamlines the manufacturing process.
There is another class of memory-module suppliers that specializes in supplying notebook and workstation computer modules. These memory modules are usually custom-designed to fit tight spaces in specific computers.
Due to the proprietary nature of a custom module, it usually fetches more money for the supplier. However, testing the hundreds of different shapes of modules requires many custom fixture setups to host the module. The tester must be infinitely flexible to accommodate every possible combination.
Then, there are the memory-module dealers and the aftermarket module retailers who buy modules from the manufacturer and sell them to the end users and computer integrators. These are mainly standard modules with some small exceptions such as notebook and workstation modules.
The biggest concern for memory-module dealers is the return-material authorization. Although they can return a module to the manufacturer through a warranty program, the transport time often risks price changes and devaluations. They need a general-purpose, low-cost memory SIMM tester that accommodates many different kinds of modules simply since their trading volume usually cannot justify an elaborate tester.
The Memory Price Drop
Following a profitable 1995, the memory industry suddenly awakened to a sharp price and profit drop in 1996. The standard dynamic RAM price fell 80% in 12 months. This drop has forced some semiconductor manufacturers to produce higher-performance memories to maintain their profit margin.
Instead of the standard DRAMs, today’s manufacturers go for the 3.3-V, 64-Mbit extended data output (EDO) and the synchronous DRAM. On the memory-module market, newer cache-module specifications are released every other month, including pipelined burst synchronous SRAM modules to streamline computer performance. The latest trend is also pushing to store the computer built-in operating system in flash memory modules.
New Memory Technology
Besides the new memories now being used in PCs, other new technologies and applications are being used. In the 1996 Olympics, electronic identity cards using flash memory were used in the athletes’ villages. Nintendo uses Rambus DRAMs in its new game machine.
Tsang Lab and STB use MDRAMs (technology licensed by Mo-Sys) in their 3-D graphic cards. Samsung announced Window RAM and the graphic-accelerating SGRAM, each aimed at specific storage applications, yielding improved system results. New digital signal processor chips make the digital answering machine a reality by using DRAMs containing a few bad bits at a limited number of locations that do not affect quality in digital audio applications. These are called audio RAMs.
At the same time, memory modules come in new forms to keep up with applications. With the 64-bit Pentium and Pentium Pro computers, memory modules are progressing from 72-pin, 32-bit SIMM to 168-pin, 64-bit DIMM modules. Likewise, the new notebook computers are falling into the standard, using the 64-bit, 144-pin small-outline DIMM module.
EDO and synchronous DRAM probably will be the mainstream memory during 1997. More than likely, memory modules will operate at 3.3 V instead of the conventional 5 V. And PCMCIA memory card usage will increase with portable computing.
New Challenges for the Memory Tester
With all these application challenges, the next generation of memory-module testers must be flexible to test all kinds of new as well as old memory technologies and architectures. They also must have easy-to-use software that allows user configuration of new module structures, and the tester manufacturer must provide service to adapt the tester to new memory modules and memory subsystems. But most of all, the tester must be affordable to low-cost, low-margin memory merchants and manufacturers.
About the Author
Cecil Ho is president and CEO of CST. Before founding CST, he spent more than 14 years as a design and program manager with Texas Instruments and General Instruments. Ho holds various international patents and awards in digital system design; and is a member of Eta Kappa Nu, an electrical engineering honor society. He has a B.S.E.E. degree from the University of Texas. CST, 2336 Lu Field Rd., Dallas, TX 75229, (214) 241-2662.
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Memory Module Tester Feature Requirements
Memory Module Assembly Manufacturers
Super high speed (1 s for 8-MB module).
Shorts and opens test algorithm.
Automatic handler facilities.
Testing of standard 72-pin and 168-pin modules.
Testing of 3.3-V (LVTTL) and 5-V (TTL) modules.
Test summary report.
Quick turnaround after service.
Specialty Memory-Module Suppliers
Highly flexible.
Coverage of all memory technologies (DRAM, SDRAM, SRAM, pipelined burst, flash).
Large number of standard adapters.
Advanced test features such as analog pin driver, waveform editor, graphical presentation and leakage test.
Readily accessible, experienced, continued engineering support.
User-configurable device library and tester parameters.
Engineering test summary report.
Memory-Module Dealers
Low cost (<$1,500).
Testing of all standard modules plus some popular notebook modules.
Verifying and pinpointing bad chips.
Automatic module identifier.
Ease of use.
Self-contained portable unit.
Off-the-shelf availability.
Direct report printout.
Copyright 1996 Nelson Publishing Inc.
November 1996
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