By Mark Shortt
Editorial Director, Design-2-Part Magazine

Miniaturization in printed circuit boards continues to point toward faster transmission of information

The impact of microelectronics on the lives of consumers today is immense. With such commonly used items as wristwatches, calculators, home appliances, and personal computers all making use of integrated circuit technology, it's hard to imagine going through a day without them. It's even more amazing when you consider their applications in telecommunications, information systems, and robotics—and, as well as internet phones, wireless communication products, and new specialty instruments that link biomedical technology with microelectronics.

"Technology is changing very fast, with a big trend toward miniaturization and advanced packaging technology," said Walter Chavez, President of AmTECH Microelectronics, Inc., San Jose, California. As a result of this trend, much of the work being performed by electronics contract manufacturers today involves placement of the latest chip-packaging technologies on printed circuit boards (PCBs).

Printed circuit boards are the hearts of electronic systems ranging in size from tiny wristwatches, personal digital assistants (PDAs), and wireless phones to large computers. They are the foundation for the final interconnections among all the completed integrated circuits (ICs), and serve as a communication link between the outside world and the microelectronic circuitry within each packaged IC, according to Serope Kalpakjian, author of Manufacturing Engineering and Technology (Addison-Wesley Publishing Co., 1995).

Smaller Means Faster

Advances in printed circuit board (PCB) technology today continue to be geared toward increasing the number of leads and connections into the circuit board. Miniaturization is a key feature of the latest chip packaging technologies, such as micro ball-grid-arrays (BGA), chip-on-board, and flip chip semiconductor devices, which increase the number of connections into the circuit board without increasing the size of the board. Ever-increasing amounts of information can be fired off more quickly as a result of the greater densities made possible by miniaturization.

"Smaller continues to be perceived as better," said Dennis Gleason, vice president of sales and marketing, Wolf Electronix, Orem, Utah. "The price of technology is leading to lower-cost automation," he continued, adding that the newer technologies have earned the confidence of manufacturers by providing increased reliability.

Evolving Surface Mount Technology

On an evolutionary scale, the path to smaller circuitry has led from pin-through-hole insertion, to standard surface mount, to increasingly finer-pitch surface mount technologies. These include BGA, chip-on-board, flip chip, and the most recent chip-scale packaging, representing extremely small devices with the finest pitches (center-to-center lead distances) available.

In through-hole assembly, PCB components have pins, or leads, that are inserted through holes in the printed circuit board. After being put in place on the PCB, the element is then soldered to the board with the use of a wavesolder machine.

In surface mount assembly, considered a fine-pitch technology, the pins are mounted on the surface of the PCB rather than inserted through holes. The much smaller size of the pins permits greater densities, but another important advantage is that the process more easily lends itself to full automation. Benefits include faster assembly at reduced cost, Mr. Chavez said.

But as pins became smaller, they and other fine-pitch components became increasingly difficult to place on the PCB. A surface mount technology known as ball grid array (BGA) packaging was developed a few years ago to solve the problem. Instead of using leads, BGA makes use of a grid of tiny balls on the bottom of the package. Besides ease of use, the balls permit more connections because of their reduced size.

Other advanced packaging technologies currently in use include flip chip, chip-on-board, chip-on-flex, and chip-scale packages. Flip chip technology, Mr. Chavez said, uses the silicon semiconductor in "bare die form." The silicon die, which has no plastic packaging or leads, is flipped upside down to make the connection.

Chip-on-board (COB) also uses a bare die, but instead of being flipped, the die faces up. Wire bonding technology is used to make the connection from the surface of the die to the surface of the PCB. Chip-on-flex is used in applications, such as disk drives, that require a flexible circuit board rather than a standard PCB, Mr. Chavez said.

Chip-scale packages (CSPs), literally the same size as the chip, are also being used to achieve greater density. These are extremely small, fine-pitch devices.

As PCB technology continues to mature, a number of factors are motivating OEMs to outsource services ranging from assembly to complete box builds. At the same time that high costs of capital equipment discourage purchases of new machinery, OEMs are pressured to bring products to market more quickly. Many of them are downsizing their manufacturing capabilities and materials inventories, and instead shifting more of their resources to product development and sales and marketing.

"Technology is changing so fast, it's very difficult for OEMs to keep up," explained Mr. Chavez. "It's also very difficult to control inventory and to manage material logistics. OEMs want a contract manufacturer to be good at material logistics," he said.

The range of services offered by job shops includes materials procurement, custom manufacturing and assembly, and both in-circuit and functional testing of printed circuit boards. While some companies provide full-service, turnkey electronic manufacturing, with design services and engineering support, others concentrate on contract assembly.

Options for Outsourcing

AmTECH Microelectronics, Inc. provides electronic and mechanical contract manufacturing services—both consignment and turnkey—at its 32,000-square-foot plant in San José, California. The company, which employs 125, has capabilities in chip-on-board surface-mount, through-hole, and electro-mechanical assembly. Its customers include manufacturers of computers, telecommunications devices, smart cards, and fiber optic transceivers. Typically, these customers outsource turnkey electronic manufacturing, including material kitting, assembly, and test services.

In addition to chip-on-board automated surface mount assembly, the company offers automated lead forming and cutting for all through-hole components. The firm also provides epoxy die bond, aluminum wire bond, eutectic die bond, and gold wire bond processes.

According to Mr. Chavez, AmTECH strives to provide leadership in new manufacturing technologies while continuing to refine its use of the more traditional methods, such as surface mount, through-hole, and electro-mechanical assembly. Advanced technologies employed by the company include ball grid array, chip-on-board, and chip-on flex.

For automated surface-mount assembly, the firm uses Philips and Quad Pick and Place Systems. Its chip-on-board manufacturing equipment includes K & S automatic wire bonding equipment.

The company recognizes its obligation to protect the environment and has taken the initiative to research and adopt environmentally friendly procedures, according to Mr. Chavez. It is part of a growing majority of companies, he said, that use no chemicals in its PCB assemblies. Instead, only de-ionized water or no-clean processes are used.

Wolf Electronix, a job shop located in Orem, Utah, recently earned ISO 9002 registration for its contract services in surface mount and through-hole PCB assembly. The company, which employs 60 at its 14,000-square-foot facility, provides contract engineering, turnkey and consignment manufacturing, and prototyping. Wolf counts ball grid array among its specialty capabilities and also provides testing, conformal coating, cable assembly, and final box builds.

Applications for the firm's services include various medical products, automated meter readers, computer peripherals such as IR printer interfaces, and PCBs for automated blenders and other consumer items.

For bottom-side SMT, the firm uses a screen-printing method that is said to be more effective and have higher quality than the dispensing adhesives traditionally used. According to Mr. Gleason, dispensing presents many opportunities for errors and defects, but screen printing ensures that the quality of the "glue dots" is consistent time after time.

KimchuK, Inc., Danbury, Connecticut, is a low-to-medium-volume contract manufacturer that supports a wide range of industries. James Marquis, vice president, said that the company is currently seeing significant growth for printed circuit boards in the telephony and specialty instrument markets. Examples of applications in these markets include internet phones and specialty medical instruments.

"We're working on six projects right now that tie the fields of biochemistry and biomedicine to electronics," he said.

"The majority of our customers outsource to us the layout of the printed circuit board and packaging of the assembly to facilitate the manufacturing process," said Mr. Marquis. "Many of our customers will design their products in a breadboard stage, and then forward the design to us for re-packaging in order to minimize the manufacturing costs," he explained.

Very few customer-supplied printed circuit board designs are optimized for manufacturing, according to Mr. Marquis. "We see significant activity in the area of re-spinning a design in order to reduce manufacturing costs," he noted. "We are also seeing significant outsourcing in design, purchasing, manufacturing, testing, and warranty repair services," he added.

Mr. Marquis said that many of the technological advances first employed by high-volume manufacturers several years ago have started to trickle down to the medium and low volume manufacturers. Ball grid array (BGA), flip chip, high-density flex circuits, and boundary scan testing are examples of these technologies, he said.

KimchuK employs fully automated surface mount assembly, BGA, and chip-on flex technology, but has avoided using chip-on-board technology because of the labor intensity that would be required to incorporate the technology into its operations. However, the company does employ flip-chip technology, which has not yet had widespread use among small-to-medium-volume manufacturers.

Another advanced technology currently being used by the firm is boundary scan testing, which the company is employing in higher-density surface mount assemblies. Although the technology has been available for approximately a decade, Mr. Marquis said, many of the company's customers are unaware of its existence and the design methods that enhance its use.

According to Mr. Marquis, the technology is effective in densely populated surface mount boards, lack of access to many of the nodes has a limiting effect on in-circuit testing. In the case of ball grid arrays, in-circuit testing is often not usable, he said.

"The alternative to in-circuit testing is the Flying Probe Scanner," said Mr. Marquis. "Again, if you do not have access to the nodes, flying probe scanners do not provide the necessary results. Boundary scan testing can and does," he stated.

References

1. Amstead, B.H., Ostwald, P.F., and Begeman, M.L. 1987. Manufacturing Processes, p. 658. 8th ed. New York: John Wiley & Sons.

2. Kalpakjian, Serope. 1995. Manufacturing Engineering and Technology, p. 1028. 3d edition. Reading, Massachusetts: Addison-Wesley Publishing Co.

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