Thin-Film Resistors Shrink Electronic Systems

High reliability resistor applications usually tend toward the use of thin-film rather than thick-film technology. Thin film is the technology of choice because of its ability to produce components with greater stability over frequency and time.

Fig. 1 shows the variation a thin- vs. thick-film resistor value can have over frequency, whereas the thin-film counterpart remains constant. Fig. 2, on page 57, illustrates the importance of stable resistor performance over time. The thick-film resistor value shifts significantly, whereas the thin-film counterpart exhibits only minimal shift in value. You can apply thin-film technology to both discrete and multichip arrays.

For discrete devices, many industries have considered 0805 and 0603 sizes as “pushing the envelope” for thin-film components. In many applications, the 0402 size is the standard. Now, the 0201 size ultra-small thin-film resistor is available; its footprint is 0.02 (L)×0.01-in. (W). The BLU0201 is the smallest thin-film resistor in the industry. The reduced component size offers about 300% space savings over the more conventional 0603 package. The Table lists the dimensions in mm of the 0201, 0402, 0603, and 0805 packages. Fig. 3, on page 57, shows the chip layout.

The BLU0201 has a standard rating of 0.025W with an optional 0.0375W power rating. The current published resistance range is 10Ω to 22kΩ; however, we've completed preliminary testing on values as low as 1Ω and as high as 330kΩ, expected to be available by February 2002. It's a single discrete component available in tolerances as tight as 0.1% with temperature coefficients as low as 25 ppm. We are now testing 0201 prototypes with tolerances as tight as 0.05% and temperature coefficients as low as 10 ppm.

All our resistors feature virtually the same thin-film technology, except for increased component density. In developing the smaller size, we investigated board design guidelines for 0201 passives. Design variations included pad size, pad geometry, pad-to-pad pitch, and chip-to-chip pitch. Some important design aspects were: defect minimization and increasing component density while shrinking overall p. c. board dimensions. Our research involved redefining pad designs and the printing, placement, and reflow process windows to achieve high first-pass yield and high throughput.

New designs also called for the high-speed placement of the 0201 resistor, which can be as challenging to place as they are to be seen by the naked eye. The primary obstacle is the ability to reliably feed the components (with tape-and-reel and bulk feeding systems) to ensure accurate pick-and-place performance. The small size of the components tend to move or “swim” in the tape pockets, potentially causing problems.

Medical Applications

Size is a critical factor in “new” portable medical equipment designs, including: handheld biorhythm monitors, wireless devices used in 24-hr patient monitoring, hearing aids, etc. Using the BLU0201 chip rather than a larger thin-film chip array reduces actual board space by more than 40%. Until now, the industry used the 0603 size discrete chips or much larger chip arrays. The multichip resistor array offers more efficient handling, but the discrete 0201offers better performance and smaller package size.

You can use this new product in a wide range of new applications, including automotive, telecom, and many other high tech requirements.

Multichip Arrays

Thin-film resistor chip arrays have been around for many years, primarily for use in high reliability and medical implant applications. Such devices offer no improvement over the performance of chip resistors, but they do offer a significant improvement in insertion costs for p. c. board OEMs. Two, three, four, or eight chips are housed in an array, enabling the placement of multiple components on a board at once.

Multichip arrays typically save the OEM money in insertion costs, which explains the market growth between 1995 and 2000. In the 1990s, the rise of three new technology concepts had major impacts on the traditional thick-film resistor and resistor/capacitor network markets.

The successful introduction of the low-cost, multichip resistor array demonstrated the possibility of placing multiple components on a p. c. board at one time, at a fraction of the cost of a thick-film resistor network. Multichip resistor arrays were successful in their global proliferation in applications where four or eight individual discrete resistors or a single thick-film resistor network had been placed. Buyers experienced double cost savings from more efficient picking and placing of individual discrete components, as well as from the low price of thick-films arrays.

Components in an array operate independently. You can find them in many applications today where the use of single discretes or traditional resistor networks is popular. Large Japanese individual discrete resistor manufacturers and a few U.S. multinational joint venture affiliates primarily produce multichip arrays.

Over the next 10 or 20 years, the concept of increasing an OEM's throughput of manufacturing will become more important, especially in applications where volumetric efficiency is key — such as handheld devices and most laptop computers. Here, the passive content represents the largest component count, as much as 80% to 90% in most cases. To increase the functionality of handheld devices to make them more palatable to the existing customer base, you must employ additional ICs, which require additional support components. The best modern example of this is the addition of Internet-related functions to many state-of-the art handheld devices with functions traditionally found in PDAs. The problem centers on the small size of the device, which you can achieve only through use of increasingly smaller components.

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