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STEP 1: Left Shift the Bottom Line
December 31, 1969 |Estimated reading time: 6 minutes
It’s documented that using DfF techniques for PCB design produces higher yields and lower cost. The real question is how. After taking a brief look at the problem, the “how” steps are enumerated in the left-shift methodology.
A major trend in design for fabrication (DfF) analysis is a concept called left shift. This concept moves the analysis tool out of the hands of the manufacturer and into the hands of the PCB designer, integrating analysis tools with design tools. This is a major improvement in PCB design because it eliminates the multiple design iterations necessary for DfF analysis, making the entire design process more efficient.
Contemporary Processes
Most PCB designs are influenced heavily by logic and analog circuit designers. Their chief concern is ensuring electrical and signal integrity across the board. The electrical rules are intricately determined, taking a good deal of time.
In contrast, DfF considerations often are more or less an afterthought in the process. The DfF requirements are left up to the manufacturing engineer, in hopes that there will be few design changes and re-spins.
Once prototypes have been verified, the manufacturing engineer must deliver a data set that is producible - while maximizing yields - and consistent to allow the board to be fabricated at multiple sites. Each site is provided with the exact same data. Without the OEM’s manufacturing engineers preparing the data first, too many opportunities exist for inconsistencies to occur in the finished product, a situation that may cause reliability issues later in the field. This situation is caused partially by each manufacturing site having its own third-party computer-aided manufacturing (CAM) tool suite used in their fabrication process. The manufacturer’s DfF-related checks are performed by these CAM tools. The CAM suite also ensures rules compliance for design for assembly (DfA), design for test (DfT), and manufacturability of a PCB within the limits of set processes and tolerances.
Figure 1. A traditional production flow for PCB design.
The manufacturer’s DfF checks determine if the PCB is producible based on that manufacturer’s set of process rules, which can vary from site to site. It is unlikely that any two PCB fabricators used by an OEM have identical rule sets. These slight variations in rule sets generate inconsistencies in the boards produced by multiple manufacturers.
Several other production operations occur once the board is at the manufacturer. Silk screens are added, unused inner pads are removed, and copper balancing is done. As manufacturers use different CAM tools and identical rules, the same board can have a different appearance from one manufacturer to another.
Usually, these inconsistencies are caught by the OEM, which results in a design change and cycling back to the manufacturers with a new data set. This causes additional time and cost expenditures. Figure 1 illustrates the contemporary design methodology.
A better system to ensure the quickest design-to-production time as well as the fewest re-spins requires that DfF tools be tightly integrated with PCB computer-aided design (CAD) tools. Running DfF checks solely at the PCB fabricator/manufacturer is no longer sufficient. The left shift is required to make those analysis tools available throughout the design cycle, not just after placement and routing are complete.
Left Shift in DfF
There are four basic steps to ensure the complete left shift of the DfF process: defining target PCB manufacturers early; forming close working relationships; moving verification up the line; and providing clear, user-friendly, easy-to-follow tools. Incorporating these into the design and manufacturing cycle can reduce overall PCB cycle time and cut down on the number of cycles. Plus, follow-on boards are much easier to design.
Define the Target Manufacturers Early. Select a small set of PCB manufacturers early in the product design cycle. This provides the longest lead time for all DfF activities by establishing the rules for each manufacturer at the front end. Not only can the rules be incorporated early, but the process and development of a common set of rules is in the hands of the OEM.
Form Close Relationships with the PCB Manufacturers. The first step is coupled tightly with this second step. When the target manufacturers are identified early in the design process, there is time to form a close relationship with fabricators and manufacturers. The goal of the relationship is for the OEM to retain control over the design data content at the manufacturing stage, while also ensuring that the manufacturers are free to focus on what they do best: fabricate boards.
Figure 2. Flow of left-shift PCB development.
When the manufacturers are selected early and relationships developed, they should be willing to share the design rules with the OEM. Then, the rules taken from each of the manufacturers can be analyzed and a design-rule set can be developed that is process-tolerant among the chosen fabricators. The more the OEM does to prepare data prior to generating manufacturing outputs, the less often fabricators’ time is required to modify the data from the OEM’s CAD system to a form usable with their manufacturing system.
Move Integrated and Automated Verification Steps Forward. The goal of DfF is to have multiple manufacturers provide identically functioning and appearing PCBs. How can an OEM ensure that a consistent data set is provided to the manufacturers so that they all can fabricate the PCB to the same rules and within acceptable tolerances?
It may be possible to ask all the manufacturers to create a common set of rules that they all can accommodate comfortably in their fabrication processes. That way, the OEM controls the design and fabrication rules and any modifications.
The other option is to left shift the DfF analysis tools to an earlier part of the design and manufacturing timeline: closer to the PCB. Moving the tools earlier in the process generally lets users identify potential problems at a stage when relatively minor effort is needed to remedy the problem.
This is the point in the left shift concept - push problem solutions to locations in the design and manufacturing timeline where they are easiest and least costly to fix.
Use Tools That Non-DfF-experts Can Use. Moving the analysis tools to the left means that the people carrying out those tasks will have to learn to use a new analysis tool. Though they are all cooperative, it still is important not only to demonstrate the advantages of the new procedure, but also to consider the designer’s background and choose tools that can be most easily learned.
These tools should be tightly integrated and function seamlessly with the existing PCB and CAD tools. DfF rule setup and hazard or violation review should function just like the rest of the PCB and CAD tools in use at the company. Figure 2 shows PCB design and manufacturing processes after the left shift, blending the design and manufacturing sides.
Conclusion
Performing the left shift by relocating DfF from place-and-route and manufacturing operations to an integral part of the PCB design process can eliminate design cycles and add to the product’s bottom line, reducing time and costs. Identifying potential PCB fabricators early and developing relationships are the first steps to implementing left shift. Defining a common set of rules can increase yields, while at the same time reducing manufacturing times.
Equally as important, the left shift gives the OEM control of the design data content. This single set of consistent manufacturing data, which is process tolerant, enables a potentially larger universe of fabricators to be used, lowering costs and realizing the goal of DfF.
Steve Hughes, marketing manager, Systems Design Division, Mentor Graphics Corporation, may be contacted at steve_hughes@mentor.com.