A Medical Electronics Manufacturing Fall 1996 Feature
By combining discrete passive components into multielement packages, designers can save more board space than by simply reducing the size of the components themselves.
Miniaturization has been a key contributor to advances in electronic technology. Certainly, miniaturization has been made possible mostly through remarkable breakthroughs in reducing the size of active components. But as integrated circuits (ICs) get smaller and more complex, there is an increasing need to also reduce the space required for the supporting passive components.
Integrated passive components (RC circuits) and passive component arrays (MLC capacitors, MLV transient suppressors, and thick-film resistors) used in medical electronics. Photo courtesy of AVX Corp., Myrtle Beach, SC.
Many electronics applications have serious space considerations that are pressuring manufacturers to reduce component size. Much of the motivation for this has come from military and aerospace needs, but today's miniaturization demands are more likely to come from other market segments including telecommunications (cellular phones), computers (laptops), instrumentation (handheld devices), and medical electronics (pacemakers). Such applications continue to drive size reduction in components for commercial uses as well as for applications with very high reliability requirements, such as lifesaving medical equipment.
However, simply reducing the case size of a part is not always the most effective way to miniaturize. Consequently, passive component manufacturers have begun to combine discrete components into volumetric-efficient multielement packages.
Multielement Leaded Packages
Before the popularization of surface-mount technology (SMT), the most effective use of space was provided by multielement leaded packages such as single and dual in-line packages (SIPs and DIPs). Originally no more than discrete chip components mounted on small boards with leads for insertion into mounting holes, these products quickly advanced to thick- and thin-film circuits composed of components such as resistors and capacitors that were deposited directly onto boards.
The chief benefit of these circuits was that they saved board surface area by using the advantage of height; they could be set on end. Also, the printing or depositing processes allowed much closer spacing between discrete components than board mounting did.
From about 1980 to 1987, a large number of these combinations and networks were developed, but as board technology improved and multilayered boards became the norm, these devices were less desirable because of the space that was required for their mounting holes and corresponding margin areas. However, these through-hole leaded packages are still used in sizable numbers in the general marketplace and can be found in many external medical products. Improvements have greatly increased the range of values available for these types of circuits.
The advance that brought this concept to the next developmental level was providing networks in molded packages that had L- or J-shaped leads designed for surface mounting. Using thin-film technology, several manufacturers have introduced a wide variety of these circuit designs, such as networks of capacitors, resistors, inductors, diodes, and combinations of these elements, that offer significant space savings over discrete components. The use of thin film also provides greater accuracy in component values, lower power consumption, and the use of higher frequencies.
These packages can be found in many telecommunication, computer, and medical applications. Although they save space and are convenient and easy to use, they are also still rather large compared to leadless designs.
There are other standard leaded parts that are being combined to form sim-
ple arrays. For example, the tantalum chip capacitor, because of its inherent design, requires leads, but even so, medical-grade tantalums have long been used in miniaturized medical applications from implantable pacemakers and defibrillators to hearing aids. These components can be grouped into arrays with two to six capacitors joined either side-by-side or stacked to eliminate the space between components on the board. Though simple, these configurations represent a significant space savings for those applications in which multiple components of the same value are needed.
Figure I. Four discrete components require a 0.027-sq. in. board area, but require only 0.012 sq. in. when grouped into an array.2
Although size reductions have occurred across the board in discrete passive components as technology and packaging have improved, the leadless surface-mount products are responsible for the most dramatic advances in passive electronic miniaturization. As materials and manufacturing processes keep improving, component manufacturers continue to produce more complex devices in smaller packages. However, there are limits to this type of technological improvement.
For example, the multilayer ceramic (MLC) capacitor has been reduced several times since it was first developed, and each reduction has caused some loss of capacity. Improvements in dielectric materials and in the manufacturing of thinner dielectrics as well as a lowering of the voltages used for electronic devices have allowed OEMs to produce surface-mount chips in the Electronic Industries Association (EIA) standard case dimensions of 1210, 1206, 0805, 0603, and 0402. However, because of internal design issues and because the EIA specifies that chips cannot be thicker than they are wide, there also has to be a reduction in the actual capacitance-generating portion of the chip with each size decrease.
As shown in Table I, 1206 and 0805 MLC capacitors use only a little more than 40% of their total volumes for capacitance. This percentage drops to 32% for a 0603, and to less than 20% for a 0402 chip. The total volume is based on the outside measurements of the chip, and the useful volume is this total volume minus the space used for the margins and covers. As the chips become smaller, the margins and covers take up a larger percentage of the total volume.
These very small chip sizes can also challenge user ability to process them or require major upgrades in pick-and-place handling equipment that was built before their invention. Nevertheless, several medical device companies regularly use 0603, 0504, and 0402 chips in applications for which saving space is an important consideration.
The real goal in miniaturization is not just to make smaller components, but to achieve board-area savings and overall volumetric reduction. Even the smallest components still require placement clearances and board-mounting pads. Combining the passive elements into arrays before mounting them addresses these limitations.
Figure 1 shows four 0603 components. When the components are combined into an array, they have the same total component area as the discrete elements did, but the mounting pad areas are reduced and the clearance areas around each component are eliminated. Therefore, the array configuration uses only 0.012 sq in., which is less than half of the 0.027-sq in. board area that the independent components require.
Also, the 1206 array, which is 0.06 in. wide, can have twice the height of the four discrete components, which are only 0.03 in. wide. Thus, if this diagram represented an MLC capacitor, for example, its capacitance could be more than doubled. Using an array configuration and taking advantage of its additional height allowance would result in more than twice the capacitance and less than half the board area that using discrete components would.
Capacitor and Resistor Arrays
Even though they are considered to be recent innovations, some leadless multielement packages have been in use since the 1980s. By 1984, multicapacitor MLC discoidal arrays were already being manufactured for EMI filtering in military and aerospace connectors.
At the same time, smaller versions were being introduced to the medical market for filtering in implantable pacemakers. However, these were and have remained custom components and have not become available as commercial products. Custom connector arrays with up to 150 capacitors have been manufactured since the mid 1980s. Many implantable pacemakers, defibrillators, and nerve stimulators use high-reliability versions of these planar arrays that can have up to six capacitors in a part with a diameter that is less than 0.150 in.
The first commercially available SMT products were the thick-film resistor arrays. These arrays, which were available in 2-, 3-, 4-, and 8-resistor packages, used the same thick-film-on-aluminum technology that is used in manufacturing discrete resistors. Most of these arrays were just discrete parts assembled in larger packages. This very basic construction was the initial method used in manufacturing multielement SMT components.
Later versions of multielement SMT arrays have used space even more efficiently. For example, an eight-resistor array takes up only 0.157 x 0.083 in. when additional terminations are placed on each end as common grounds so that there can be four separate resistors on each side.3 This version provides twice the number of resistors with the same pitch, or spacing between mounting pads, as one four-resistor array did with only a 60% increase in area.
More recently, commercial MLC capacitor arrays in surface-mount chips have become available. First developed was an eight-capacitor feed-through array that consisted of an SMT version of the discoidal arrays used for EMI filtering. In 1995, manufacturers announced 2-, 3-, and 4-capacitor arrays for use in general surface-mount applications. The EIA is planning to standardize dimensions for these components soon, which will accelerate their acceptance by OEMs. In addition to the obvious board-real-estate savings, these capacitor arrays also reduce pick-and-place operations and are much easier to handle than the 0603 and 0402 parts that they replace.
Custom MLC capacitor arrays have been in use for miniaturized medical applications since almost three years before the introduction of the standard arrays described above. One custom nine-capacitor array used for high-end hearing aids has terminals on all four sides and can serve as the platform for a wire-bonded die. High-reliability arrays that have from 3 to 10 capacitors each are being used in implantable pacemakers and defibrillators. One of these arrays has two similar parts that can accommodate flip-chip or wire-bond attachment by simply changing the termination material. Custom arrays allow unique configurations, widely differing capacitor values in the same array, and higher voltages.
Resistor and capacitor arrays are only the beginning of small, leadless passive combinations. For example, one manufacturer has just released a multilayer varistor array, which provides voltage- dependent resistance for ac circuit transient suppression.
A step beyond combining similar components is to integrate different passive elements into a single package. This development is evolving in two different technological directions.
The simpler direction is the use of standard MLC capacitor technology with innovative internal designs and added thick-film surface elements. The eight-capacitor feed-through array was the first and simplest example of this.4 At the time this product was released, medical applications were already driving developments in miniaturizing resistance-capacitance (RC) circuits, or circuits that contain both resistors and capacitors. Because size is such a critical factor in implantable pacemakers and defibrillators, customized miniature RCs were pursued to reduce the size of the electronics. Today there are RCs in production that have as many as 12 capacitors and 6 resistors in a single package.
The more difficult direction is the use of the latest MLC technology. To the previously mentioned capabilities of complex internal designs, surface-screened elements, and ability to serve as a platform for ICs and other SMT components, the latest developments add the capabilities for fine-line screening, cofired copper electrodes, blind vias internal resistors, and inductors. Some simple inductance-capacitance chips, circuits, and filters are being offered, but the more complex possibilities are available only as high-volume custom devices.
Although severely lagging behind developments in active components, passive component integration is allowing the development of an assortment of new product offerings. Some of these items have been possible for several years, but lack of widespread customer acceptance and high costs have slowed their introduction into the general marketplace. Some items are yet to be developed. For example, because several manufacturers can perform both thick- and thin-film manufacturing, hybrid components combining both technologies may be forthcoming. Passive component integration is and will continue to be an important contribution in the development of increasingly smaller medical electronics.
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