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DFM: Is the Industry There Yet?
December 31, 1969 |Estimated reading time: 6 minutes
Because DFM is still a relatively new concept, assemblers, designers and fabricators are often working on a different page when it comes to putting DFM to use.
Erik Nilsson
Designers have finally recognized the importance of DFM. What is DFM?
Everyone generally agrees that the acronym DFM stands for design for manufacturing/manufacturability. That is where the agreement on the subject ends. Some people think of DFM as part-spacing rules and fiducial requirements. Others think of trace widths and annular rings. Still others think of accessibility for in-circuit test (ICT) probes. In an attempt for greater clarity, people break down DFM to design for assembly (DFA), design for fabrication (DFF), design for test (DFT) and so forth. But this explanation lacks clarity and simplicity. For example, pad-spacing rules relate to solder bridging in assembly, feature resolution in fabrication and probe clearances in ICT.
Has the meaning of DFM been lost? Has DFM become meaningless at the precise moment it has become essential? Not yet. People who need DFM are seeing that it gets done - even the people who disagree about what DFM actually is. As boards continue to become denser and more complex, the definition of DFM must become uniform. This article discusses one particular kind of DFM, analysis of the fabricatability of the board, and discusses its importance in the assembly environment.
Everybody`s Doing It
Leading assemblers agree on the importance of DFM. Assemblers analyze boards for their compatibility with assembly processes. Many assemblers also analyze boards for compatibility with fabrication processes. This seems strange at first glance, but there is motivation to perform this analysis.
Assemblers use fabrication DFM for three main purposes:
1. Predicting the cost of the bare board.
2. Controlling the cost of the bare board.
3. Choosing a fabricator for the board.
As with so many other aspects of assembly, the transformation of the assembly industry from predominantly captive factories to predominantly merchant contract manufacturers (CM) is driving the use of fabrication DFM in assembly. CMs deliver assembled boards to their customers. The bare board is one part among many that the CM buys from its suppliers. Time pressures dictate that CMs must often bid on a board before suppliers are chosen. To avoid expensive surprises later, CMs analyze the printed circuit board (PCB) to estimate how much it will cost. In the past, there may have been time to ask the fabricator to estimate the cost, but today that is not often the case.
Beyond merely predicting the cost of the bare PCB, advanced CMs use fabrication analysis to show their customers why bare board costs are high, and what the designer can do to bring the cost down.
It is not a secret that CMs make most of their money on the parts that go into the PCB. Thus, successful CMs control parts` costs scrupulously - the cost of bare boards is no exception. A CM will typically work with several fabricators. A fabricator is selected based on available capacity, quality and price. It is too time-consuming and expensive to send a board out for bid to more than a few fabricators. It is important to pick fabricators that are appropriate for a particular board. Assemblers perform fabrication DFM analysis to determine the best fabricators for a particular design.
Is This the Designer`s Problem?
Designers do perform design rule checks, but they lack the tools to perform fabrication DFM. Also, most designers may not know it, but many lack the manufacturing background to perform DFM analysis.
In a PCB design system, design rule check typically means checking a layout to make sure components have the pins they are supposed to, no nets are broken or shorted, and other similar checks. Design systems typically give the designer only a vague idea of manufacturability for two reasons:
1. The typical design system does not perform many important manufacturability checks.
2. Many design systems do not check against an accurate model of circuit patterns.
For whatever reason, most PCB design systems creators have not included manufacturability checks. Design systems concentrate on the electrical behavior of the board, admittedly a tough job. PCB design software makers have generally been unable to devote resources to understanding the changing requirements of different manufacturing processes. Useful fabrication checks are described below. Some designers perform such checks and derive great benefits from them, but most depend on their manufacturing counterparts.
Many PCB design systems, including some market leaders, cannot accurately represent the metal patterns they are ostensibly designing. The whole point of design is to produce a design for manufacture. The biggest problem areas are in plane layers. Many design systems do not model plane layers in detail. The result is that an inexperienced designer may foolishly trust the design system to produce plane layers that maintain the signal integrity of nets. It is left to fabrication DFM to find these problems.
Why is This the Assembler`s Problem?
The short answer is because it is not anyone else`s problem. It is not the designer`s problem because design is too early in the process to solve these problems. It is often too late to solve these problems in fabrication. Consequently, assembly is often the only place left where these problems can be addressed.
A more positive way to look at it is that CMs increasingly play the role of comprehensive manufacturing expert for their design customers. CMs often have special departments for new product introduction, prototype development, etc. Some CMs are even able to charge extra for this manufacturing expertise. Consequently, CMs are being asked to be expert in the entire manufacturing process. Demands for fabrication expertise are a reflection of the trust placed in CMs by their customers.
What to Look For
Start by looking for the basic things that a fabricator will look for: trace width and air gap. Designers will often reveal what design rules they intended to follow. "This is a five and five board," translates into a 0.005" trace width and 0.005" air gap between traces. Sometimes, the designer is correct. Most of the time, the designer is correct for about 99 percent of the board, but the remaining 1 percent violates the rules the designers thought they were using. That 1 percent has a drastic effect on board cost.
The typical computer-integrated manufacturing (CIM) system cannot perform an accurate check for trace width and air gap, let alone some of the more elaborate tests that are becoming increasingly necessary. To perform even this simple fabrication check, either a computer-aided manufacturing (CAM) station or a bare-board analysis system (a CAM station, except with fewer features and a higher price) is needed.
A CAM station can check for other things that will cause problems with the fabricators if they are not caught. Take a look at the process capability indexes from the fabricators. Items such as a line annular ring and soldermask clearance will be found. The CAM station should be able to check for these, as well. The results reveal which fabricators can make a given board or a design that is next to impossible for anyone to make.
Also, when choosing CAM stations, look for a system that will work well with the CIM system or manufacturing execution system (MES). Evaluate the tool for ease of use and make sure it is easy to learn. Some DFM systems can be very complex, with obscure resolution and speed controls to manipulate complex setup required before starting. However, some DFM systems are comparatively straightforward to use.
Conclusion
Now that DFM has arrived, its results have surprised many within the industry. But it has arrived and designers now expect advice on how to make their boards manufacturable. To prosper, an assembler must be able to provide this information. SMT
ERIK NILSSON is president of GraphiCode, a Mania Technologies Co., 6608 216th St. S.W., Suite 100, Mountlake Terrace, WA 98403; (425) 672-1980; Fax: (425) 672-2705; Web site: www.graphicode.com.