DFA demands upfront attention: failure to consider PCB design and production requirements leads to costly loss in time-to-market and time-to-revenue.
[FIGURE 1 OMITTED]
The prudent approach is to first examine the volume production considerations. There are different DFA considerations related to prototype assembly vs. production assembly. Each requires a different design skill set and equipment. If the right approach isn't used, costs can escalate and debugging the product will add additional costs as well. At prototype levels, validity of the circuitry and functionality of the board is far more important than the testability and manufacturing guidelines that are necessary to implement at the production level in order to make production smooth and flawless.
First article approval of the PCB is the most important step toward effective DFA. An OEM approved first article PCB proves valuable in answering all questions relating to manufacturing, assembly and test. It is the "proof of concept" before a PCB order goes into production, even if quantities are small. For the OEM, a smooth transition to production means key market demands such as time-to-market, quality, reliability, product delivery and critical time-to-revenue are met.
Multiple Team Interaction
DFA relies heavily on multiple team interaction. Design, fabrication, assembly and procurement teams assigned to a given PCB project must precisely tune into a project's specifications. Design engineers must be fully aware of certain assembly aspects, and likewise, assembly engineers must understand a project's design in detail. These teams need to know the particular areas that make assembly and testing go smoothly. They also need to anticipate problems that can arise at production levels and in the field, and should be able to resolve these issues quickly.
Working in tandem, these teams can address and resolve most of the issues and questions at root cause levels. For example, if the design team is specifying a component that has a much-extended lead time, let's say 12 weeks or more, the procurement team and designers can flag an issue and suggest a replacement component that is readily available in the marketplace.
To design a good product that has implemented DFA, the following considerations must be addressed:
Adequate test points should be deployed across the board to have complete access and coverage during flying probe or ICT test or for direct probing (FIGURE 2).
[FIGURE 2 OMITTED]
When fabricating a board, consider panelization vs. fabricating one board at a time. This is especially beneficial when the board size is relatively small, such as 30 in. (2) or less. Panelization increases the speed of fabrication and assembly of the product, thereby reducing costs for OEMs (FIGURE 3).
[FIGURE 3 OMITTED]
While placing the components at the layout stage, it will be prudent to use one side of the board vs. utilizing both sides, if possible. Placing components only on one side reduces NRE costs at assembly and test stages, plus it speeds up the component placement at the pick and place machines and reduces the testing time of the components. Placing components only on the top also reduces re-work, QC and debug times, all of which results in reducing assembly costs.
All the mechanical considerations should be kept in mind at the PCB design stages for DFA considerations such as designing at least two, if not three, mounting holes for the PCB and clearly defining the polarity on the polarized components. This practice eliminates any confusion that may arise at component placement, testing and debugging stages.
For specifying components at design stages, engineers should use the components, which are time tested and verified, as much as possible. Doing this will reduce debug and test times, as well as make the product more reliable and predictable. Effective DFA also calls for using commonly available components with specifications and features already proven in other system applications.
If you are using custom components, you are at the mercy of one or two suppliers whose production times are dependent on their capacities and production levels, and the delivery time can easily shift to a later date. Since there's a chance custom components may not have been used by other customers, they may not have gone through the test of time and application, causing potential latent defects in the field that the OEM may not be aware of.
Aside from using standard components, another major consideration involves careful monitoring of equipment-related features and physical tolerances. For example, take the drilling machines used during the fabrication process of PCBs. The drilling process is known to have some wander. Wander outside of the established tolerances will have a negative impact. DFA demands that tight tolerances are maintained in this regard, especially in smaller hole sizes; under 10 mils. Also, it is important to maintain proper alignment of multiple symmetrical components. If modular designs are involved, keep all modules alike so that when an IC is placed in one module, similar ICs are identically placed in the second, third, fourth and so on. In the same way, resistor and capacitor networks must be similarly placed to reduce SMT machine programming and placement time. In some cases, a PCB may be odd-sized or shaped. Hence, this particular feature is made clear at the outset in DFA so that when the required fixture is used in the assembly process its limitations are specified upfront.
For mixed signal designs, a clear cut strategy should be defined to reduce noise to signal ratios, power and ground signal should be properly defined according to the component placement, and shielding of high-speed digital signals should be incorporated. If these strategies are not implemented for DFA at the layout stage, product would create unacceptable noise levels and may need a redesign, costing more time, resources and money.
An experienced PCB layout team properly places components on the board at layout stages, keeping in mind electrical characteristics, heat dissipation and noise generating aspects of the components. For example, on a board that has analog and digital sections analog components are clustered together in a module and digital components are placed in a different segment of the board, with their respective power and ground planes properly placed underneath the component for a clean and quiet signal (FIGURE 4). Furthermore, DFA addresses such aspects as electrical characteristics, analog and digital PCB sections, and high vs. low frequency components.
[FIGURE 4 OMITTED]
Efficient product design provides the OEM with a competitive edge. It results in faster product delivery than a design using custom components and inadequate DFA, which can sometimes incur 8 to 14 weeks of delay, for procuring these components. Moreover, sound DFA speeds the product through assembly, as well as makes rework and field serviceability easy.
The bottom line is that properly executed DFA can save the OEM 10-30% of the pick and place, and testing and debugging time, which translates to major cost savings. A poorly designed product or one that uses nonstandard parts and processes will take more design and assembly time, incurring greater costs in manufacturing. Conversely, DFA techniques based on standard, proven components and practices will maintain lower costs and make the process smoother.
ZULKI KHAN is president and founder of NexLogic Technologies Inc. He can be reached at email@example.com.
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|Title Annotation:||DESIGN FOR ASSEMBLY|
|Publication:||Printed Circuit Design & Manufacture|
|Date:||Sep 1, 2006|
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