By Byoung Sung Kim

Almost all of the telecommunications products designed and manufactured by Lucent Technologies are based on printed wiring boards (PWB). Therefore, the company has placed a strong emphasis on how to design PWBs for ease of assembly and testing. But the equipment that houses and makes use of these boards can also benefit from traditional Design for Assembly (DFA) techniques.

In the design of the 1B24 processor for Lucent's long-distance switching system, several cables were combined into one thick cable with multiple connectors, and the fan-out of wires was designed to take a natural shape.

Lucent is the former systems and technology company of AT&T. The DFA method used is called Design for Simplicity (DFS). It was originally developed by Hitachi in Japan, where it was known as the Assembly Evaluation Method (AEM). General Electric licensed AEM, modified it, and renamed it DFS. Lucent has licensed the methodology from GE and modified it again to make it more suitable for Lucent products. Many Lucent engineers are trained in DFS, and the company has also developed a software tool.

The well-known Design for Manufacturing and Assembly program from Boothroyd/Dewhurst Inc. of Wakefield, R.I., is a time-based method that tells how many seconds an assembly operator needs to pick a part and assemble it. The Lucent DFS method is based on penalty points that convey the relative ease or difficulty of the assembly operation. DFS first gives each part 100 points for its existence. More penalty points are given if the part is assembled in any way less than optimum. For example, if the part is assembled along any direction other than straight down, it gets penalty points (for example, 20 points if it must be assembled sideways, 30 if it must be turned like a screw). If it has to be bonded or soldered, it gets an additional 20 or 30 points. Then, the total penalty points are converted to the assembly time.

Lucent designers quickly learned that the choice of which tool to use was far less important than when to use the tool and how to implement design improvements. The best time to use the tool is as early in the design cycle as possible. Otherwise, design changes are too costly or delay the schedule.

The first Lucent product that was designed with a thorough and structured application of the DFS technique from the very beginning was a three-line, voice-only business telephone (ISDN 8503T) for small office applications. One of the designers and the author examined an existing similar ISDN product. They compared this product with the requirements for the new telephone set and eliminated parts that the new one did not need. They came up with the most likely design concept and the assembly scenario. With that design in hand as an original concept, they gathered the entire development team, consisting of two design engineers, a manufacturing engineer, and a product manager, and the author taught a Design for Simplicity class.

The group analyzed the original design concept as a team exercise and came up with many ideas for improving the design. For instance, one large rubber keypad replaced three small ones under the buttons. Eleven parts were eliminated or combined from the original 55 parts, and the original assembly time of 4.5 minutes was reduced by 30 percent.

The design concept was improved even further. Instead of assembling the individual buttons and light pipes in-house, a vendor was commissioned to pre-assemble them on a rubber pad. Then, there were only 11 parts and subassemblies for the final assembly, and the assembly time was reduced to less than a third of that for the original concept. Many of the design improvements were later copied for other products within the family and for other product families.

Lucent's No. 4ESS, a digital electronic switching system for toll calls, is the backbone of the AT&T long-distance network. Every long-distance call made through AT&T goes through the 4ESS system. When the long-distance market grew in the early 1990s, a new processor with more capacity and speed was urgently needed. The 1B24 processor was the answer and became the new processor for the 4ESS.

The 1B24 consists of two identical cabinets, about as wide and deep as a large refrigerator but 8 feet tall. Each cabinet consists of several electronics units, a fan unit, and a fuse/filter unit. An electronics unit, in turn, has a metal cage with a backplane where all printed wiring boards are terminated and where all inter-PWB connections are made. The front of the cabinet is open for the service and repair of PWBs. But the backside is almost totally covered by several hundred cables that make connections between units within each cabinet and between the two cabinets.

The cables and the fuse/filter unit posed the two major hardware design problems. First, there were just too many cables in the cabinets and, second, the fuse/filter unit was too congested and difficult to assemble. The problem was so serious that service seemed nearly impossible in the case of failure. Once all the issues were identified, the development team got together and went through a DFS training and brainstorming session.

Numerous improvements were made in cable design and documentation. For ease of attaching cables behind cabinets, many cables that originated and terminated close together were combined into one thicker cable with multiple connectors, thus reducing buildup. All cable labels were color-coded, depending on the gross vertical position to save time in the reading of coordinates and to make inspection easier. Finally, a detailed document was crucial; it clearly indicated the order in which to attach the cables, and showed how to route them and where to tie them down.

The fuse/filter unit is usually about 6 inches high, 2 feet wide and 1.5 feet deep. It receives bulk dc power, which it distributes in smaller, individually fused increments to the rest of the cabinet. The unit has as many as 150 fuses and 10 large feed-through diodes and wires among all those parts; the unit is very congested. In addition, the fuse has to be field-replaceable while the system is running. This condition was met by mounting fuse holders on a PWB and making all PWBs accessible from the front. Next, all wires were bundled into a few connectorized cables. At one end, all wires were terminated on a connector. At the other end, wires were formed so that the fan-out of wires would take the same shape that the cable would have after it was attached.

All of these design changes made the assembly of the individual cables somewhat more difficult, and clear documentation took more time. But the changes made the final assembly much easier and reduced the total assembly time drastically. To install all the cables behind the two cabinets took more than 20 days for the original model, but less than 10 for the new model. In addition, cabinets became serviceable in the case of cable failure. To assemble a fuse/filter unit took more than 20 hours for the original model but only about six for the new model.

The No. 5ESS digital electronic switching system for local calls is used in the United States by all regional telephone service companies and in many foreign countries, such as China, Japan, and Korea. The power distribution architecture of a 5ESS is similar to that of a 4ESS. Rectifiers convert the commercial ac to dc and provide bulk dc power to several power distribution cabinets located at various positions in the system. Next, power distribution cabinets distribute the dc power in smaller and individually fused increments to each cabinet. Then, a fuse/filter unit redistributes the power to the rest of the cabinet as described earlier.

To permit automatic restart without human intervention after an extended power failure and to reduce the cost, the cabinet had to be redesigned. First of all, a simple change of architecture made all other improvements more effective. The old cabinet had three large 48-feed units, one smaller 32-feed unit, and a charge control unit. In the new architecture, the charge control function is simplified and now a part of the cabinet infrastructure. Thus, a new cabinet houses four identical 48-feed units, and the manufacturing process is much simpler. We now need to manufacture only one kind of unit instead of three different kinds. DFS was rigorously applied at all levels of the product. The total number of parts was reduced by 68 percent, from 3,208 parts to 1,015, and the total number of different parts by 46 percent, from 136 kinds to 73 kinds. Finally, total assembly time was reduced by 87 percent, from 77 hours to 9.9.

One improvement was that the old fuse block consisted of 33 parts (of 11 different kinds) and one subassembly, and required more than six minutes to assemble. The new fuse block is now a single Lucent-designed, outsourced item. This change alone has reduced the parts count by over 1,500 parts and saved almost five hours of assembly time. Another example is the stand-offs used to mount a bus bar on another metal part and to insulate from it. The old design used several different types of stand-offs, each having its own fastener set. The new design now has only one stand-off and one set of fasteners.

The frame's improved architecture also reduced its assembly time. In the old design, all 48-feed units had an identical set of wires with maximum required lengths. When the unit was installed at some positions, these wires were shortened and reterminated during assembly of the frame. In the new design, all wiring is now a part of the frame infrastructure and is met by a formed, connectorized cable installed before the units are. Thus, installing a unit now takes only a few minutes instead of the hours that were spent in modifying wires. The unit is now mounted, secured with four screws, and connected with a few connectorized wires that are already in place.

Lucent's Type-80 cabinets receive voice signals and data from subscribers, multiplex them, and transmit them to a central office. They also receive signals from the central office and transmit them to subscribers.

There used to be four different cabinets in the Type-80 family for different capacities and applications. Although all four looked very similar, a quick study clearly indicated that all critical dimensions, subsystems, and interfaces in particular were very different in their details. These differences had led to a disastrous proliferation of parts and subsystems.

The development team set a clear goal: All cabinets would have to be designed on common platforms and optimized for the whole family with identical overall dimensions, an identical assembly sequence with the same fasteners, and an identical interface to limit the variation caused by optional features. And DFS would be applied rigorously at all levels.

A good example of a common platform can be seen in the heat exchanger, a high-cost subsystem. In the old designs, each cabinet had its own heat exchanger; in the new designs, there is only one type. A smaller cabinet now uses one of these heat exchangers, and larger cabinets use two or three. Although DFA traditionally tries to reduce the number of parts, using two or three small, identical heat exchangers is much more economical than a new larger one.

The sheet metal shop has benefited most from the redesigning. To assemble one each of all four cabinets, it required 1,067 sheet metal parts of 329 different kinds. Now, only 567 sheet metal parts of 132 different kinds are needed. This represents a savings of 47 percent fewer parts and 60 percent fewer kinds of parts. The old cabinets used 126 different kinds of fasteners, including about a dozen different types of rivets. Now, there are only 14 different fasteners and only one type of rivet.

In the past, each cabinet was designed individually and took about a year from the concept to a first model ready to test. By developing a family of products on a common platform, engineers took about the same time—one year—to design the first new cabinet as they had for one of the old cabinets. But the second and third cabinets took only a few months each. For the last cabinet, all platform subsystems were already designed; the team designed fewer than 20 unique sheet metal parts and needed only a few weeks to design, manufacture, and assemble a working prototype for testing.

The cases reported here are some of the documented examples of successful DFA at Lucent. There are many more undocumented but equally successful cases. They include many cases of consumer telephone products, the cable joint of submarine fiber cables, the pump laser and repeater for submarine fiber cables, many shelf-level products for the 5ESS and 4ESS, and the product packaging of an early mobile telephone. Many of these products broke new ground in their design and in the application of DFA.

The concept of DFA is now extended far beyond product assembly to many different aspects of a product. They are generally referred as DFX where "X" is a certain aspect. The best known of these extensions is Design for Environment (DFE), a design practice that aims to minimize the environmental impact of a product and process during the entire life cycle. Lucent has a strong program aimed at minimizing the use of water and power in the semiconductor manufacturing process. Another strong program at Lucent is Design for Installation (DFI). The goal of this program is to design telecommunication equipment for ease of installation.


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