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Step 1: Design for Manufacture
December 31, 1969 |Estimated reading time: 7 minutes
Manufacturability is not just about printed circuit board (PCB) fabrication and assembly, but also cost-effective board testing and assembly, and fundamentally this comes down to the quality, performance and capabilities of the design tool.
By Paul Barrow
Time-to-market and cost are key drivers for design for manufacturability (DFM), and designing without a range of DFM capabilities in the tools is a recipe for low-quality, low-yield products. In a highly competitive market where a matter of weeks can mean the difference between success or failure, DFM is becoming an increasingly vital part of the design flow.
These days, almost any computer-aided design (CAD) system will produce a PCB design, but few will produce a manufacturable design that does not require fabricator interaction. DFM begins in the place and route stage of the design, in which interactive and batch checks to monitor the placement and routing processes are necessary to highlight, and even prevent, potential problems.
However, the parameters that control these checks must be handled within the database behind the CAD tool through the parts and technology libraries. There also must be a mechanism enabling these parameters to be updated regularly, accounting for new part footprints and improved/different manufacturing processes as they become available.
The more control parameters that can be specified up front in the design as part of the layout, routing and copper generation processes means less rework and better design profitability.
But there is always the risk of over-constraining a tool, rendering a design unbuildable or too expensive. The engineering trade-offs in the number of layers, manufacturing technology required for board fabrication and the parameters for assembly are complex and have to be agreed to by the designer in conjunction with the production team. When a consensus is reached, the CAD system has to be flexible enough to accommodate these parameters in the technology files that form part of the design database. This is where leading-edge development becomes important.
The key for successful, cost-effective design and quick time-to-market is to have the signal integrity and electromagnetic interference (EMI) tools tightly integrated into a constraints-driven design flow. Consistency and concurrency are key to the design tools, integrating international standard (SI) and EMI analysis into the design tools and adding fast simulation that gives an indication of whether the design will pass, fail or is borderline, all in a matter of minutes rather than hours or even days.
The concurrency, vital as placement and routing, can be an interactive process at best, especially with tighter space and cost constraints. To add another constraint of EMI and SI analysis at the end of the process can render such designs unrouteable. It is far better to be aware of the SI and EMI problems right at the beginning and design with those constraints in mind or in the tool. Such technology has to be tightly integrated as part of prototyping tools.
That is why third-party tools tacked onto the edge of the design flow just will not work for modern designs that have to get to market in a reasonable time frame, avoiding multiple cycles through the EMC test center (Figure 1).
Figure 1. More upfront PCB assembly simulation parameters specified in the design result in less rework and better profitability.
Today's tools should guide the designer in specifying where and how a part is placed, taking into account its orientation, pitch and roll of the components as they come off the tape reel and are being placed. The tool also must take into account the jaw size of the placement machine (where applicable), the angle at which they approach the PCB and the kick out direction of tool heads on radial insertion machines.
All this must be defined in the database so that concurrent monitoring can prevent components being placed where they will cause a manufacturing problem. The full range of batch checks on the complete design, or an area of the design, also is required.
There are numerous software packages developed (mainly by computer-aided manufacture software tool vendors) to simulate the assembly process, highlight clashes and enable editing of the component assembly sequence.
There is a need to integrate this type of tool in automatic/interactive placement routines, ensuring that optimum assembly placement is achieved, while also considering the design's electrical characteristics and allowing the designer to "trade" one against the other. This data must be fine-tuned easily to allow subtle differences between similar machines to be incorporated, as well as to allow for tolerances in physical component size between different manufacturers.
Initially, the components may be positioned with some fairly loose parameters, using a matrix of spacing approach and general placement rules so that all the components actually fit on the board. This has to include specifying the particular orientation for different components because this can make a difference in the assembly.
Front-end manufacturing systems will look at the components used on the design and the machines required, placing those components automatically and working out the placement sequence. If such a tool is available in the design flow, working in parallel with the electrical constraints, component assembly clashes can be resolved at an early stage, avoiding expensive design reworks, or "hand" assembly processes.
Component soldering is another key issue in the manufacturing process and, therefore, a key issue for DFM tools. The placement tool has to be aware of problems such as shadowing defects in wavesoldering systems. Wave soldering can cause solder shorts in compressed areas in which vias are tightly clustered, and the DFM tool must be aware that this is a likely problem area. One technique used to tackle this is covering via pads on the wave side with a resist pad that is slightly larger than the via holes. Additionally, all components with pads that are close together require such solder-thieving pads on the trailing edge of the wave side to reduce solder build up.
Before commencing, and during a design, the design engineer should discuss the requirements of test engineers and the production department. Is it a requirement to test every net or every pin? What kind of test equipment do they want to use? Should/can the board be designed to provide sufficient test coverage from a single-sided jig or is a double-sided test fixture needed? These test parameters must be built into the design, and the design engineer must understand the test requirements, even at the schematics stage (Figure 2).
Figure 2. An example of testability parameters built into a design. The design engineer should be aware of these from the schematics stage.
Some manufacturers may use via pads as acceptable test pad locations, and this has to be recognized by the design tool. Another key element of a testability analysis tool is the ability to place test pads on nets and locate nets that do not have valid, useable testable pads, all the time ensuring that the pads are not so close together as to cause the test jig access problems to the pads. These test pads also must not be too close to high components that may interfere with the path of the test probe.
New production techniques, such as build-up technology, are changing DFM tool requirements by changing the tradeoffs in the manufacturing process. Now that adding microvias using photolithography has a negligible additional cost, these extra vias can improve board manufacturability and yield; however, they also must be considered as potential problem sources.
There are now tools with the capability to take the netlist, component list and board size, and analyze the manufacturing cost using different scenarios, e.g., different layer numbers, buried via, microvia and the different manufacturing processes used in build-up board fabrication.
Other factors that are just as vital to board fabrication and assembly, and have to be considered during the design process may include sliver checking and thermal relief considerations. The ability to check for slivers of copper or resist is very important as these areas may become detached during the fabrication process and adhere themselves to another area of the design, causing an electrical short.
Thermal relief checks also are vital. If the design is using a thermal relief pad through the power planes, the tools have to ensure that there is sufficient thermal linkage from the pad to the plane. This can be solved using double power pads but checks need to be in place to ensure the thermal capabilities of the board design.
Finally, any design tool should be able to show what will be produced through a true interface. This is important for positive and negative planes where the ability to view and check thermal relief pads is missing from many CAD systems.
DFM is a combination of many different requirements, all of which should be integrated into the place and route tools that are a vital part of the design process. While boards can be designed without these checks, what is acceptable for the manufacture of a hundred boards often cannot be used for a thousand boards and certainly not for a million. This level of design tool is required to get the design to market quickly with minimum design iterations and rework, and to maintain a profitable, competitive manufacturing business.
Paul Barrow may be contacted at Zuken, 1500 Aztec West, Almondsbury, Bristol BS32 4RF, United Kingdom; 44 1454 207 800; E-mail: paul.barrow@zuken.co.uk.