Integrating Design for Fabrication analysis tools with PCB/CAD tools is now a necessity for you and your customers. Here’s why.

The need for Design for Manufacturing (DfM) and, more specifically, Design for Fabrication (DfF) checks as a part of the overall PCB design process are well documented. Articles in many of the industry trade magazines over the past few years have detailed both the wide range of DfF rules, which should be analyzed, along with the benefits of doing so. All PCB fabricators use sophisticated (and not-so-sophisticated) tools to prepare the data from their CAD system prior to ramping up their manufacturing lines. So what is the problem? You send the data, they fix it and you get a board prototype back that functions as expected first time, right?

But what does it mean to “prepare the data from your CAD system”? Think about that for a minute. The PCB designer works diligently with the engineer to ensure that all of the signal integrity requirements are met and then the fabricator manipulates the data to adjust for specific processes and tolerances. But that works because you used an appropriate set of design rules that allowed for different process tolerances--and you always use the same fabricator. Good!

My intent is not to demean the fabricators here. They are the experts in their field and the goal should be to form a close relationship with them so that they can help you to help themselves. That way everyone benefits and saves time and money through reducing the number of prototypes and design re-spins and improved yield and reliability.

How it Works Today

When designing a PCB, the electrical and signal integrity characteristics are the most important factors considered. Why? Because more often than not, the logic designer is heavily influencing the PCB design; his primary concern is to get a prototype in his hands that functions electrically correct. So many hours are spent designing the PCB to a set of electrical rules. Quite often we pay lip service to DFF requirements. Hopefully, after only minimal design changes and re-spins, the PCB is “finished” and made ready for production, and possibly volume fabrication and assembly.

So what happens next? The logic designer loses all interest in this PCB because he has already moved on to the next project. At this point in the design process, the PCB may be outsourced completely for fabrication and/or assembly or it may be prepared for fabrication by internal manufacturing engineers. They must deliver a data set that is both manufacturable, while maximizing yields, but at the same time consistent, so that potentially multiple manufacturing sites that fabricate the PCB receive exactly the same data.

In the case where multiple manufacturing sites are used for PCB fabrication, without the internal manufacturing engineers preparing the data first, there are opportunities for inconsistencies to occur in the finished product that may cause reliability issues later in the field. This may occur because each of the manufacturing sites may and probably will have their own third party Computer Aided Manufacturing (CAM) tool suite that front ends their fabrication process. These CAM tools are normally used to perform a variety of DfM related checks, including DfF, Design for Assembly (DfA) and Design for Test (DfT), to ensure that the PCB is manufacturable with regard to their processes and tolerances.

The DFF checks are used to determine if the PCB is manufacturable based on a known set of process rules, which may vary from site to site. In fact, there are probably no two manufacturing sites used by a single company that have identical rule sets. That’s where the inconsistencies can occur: each site uses its own set of rules to perform the DfF checks on the “completed” PCB. Remember that each site may also use a different CAM tool suite that supports different DRC rules, may use different algorithms, etc. to perform the DRC.

Other fabrication and manufacturing related operations may also be performed at this stage, such as silkscreen clipping and generation, unused inner pad removal, copper balancing, etc. Again, unless all manufacturing sites are using the same CAM tools with the same rule sets, it should not be surprising that there may well be reliability issues later and how are field engineers supposed to debug three different but “same” versions of the PCB?

The third party CAM tools used today, as mentioned previously, are used after the fact when the manufacturing data sets have already been generated for the PCB from the PCB CAD tool. After each of the manufacturers have performed their own DFF analysis and other fabrication related DRCs, there is inevitably a cycle of feedback to the OEM, which results in design changes, which in turn spawns the next generation of a new manufacturing data set, which is then sent back to the manufacturer which in turn. This all precipitates additional time and cost. As most readers of this publication know, PCB manufacturers make their livelihood by fabricating boards, not identifying DfM related violations. They want their production lines running at capacity.

One additional point to consider here is how often do the manufacturers make the decision to modify the manufacturing data to suit their process, time constraints and profit margins without telling you? Although it may have no visible or actual effect on the design integrity it may introduce consistency errors again, later when subsequent and additional fabrication runs are outsourced to alternative suppliers. The OEM simply supplies the design manufacturing data set that they thought was used, untouched for the initial fabrication run.

How Would You Like It to Work Tomorrow?

It has become apparent that making DfF analysis tools very tightly integrated with PCB/CAD tools is now a necessity. It is no longer sufficient to rely on those expensive CAM tools to perform the DFF checks on the manufacturing data. More control over design data quality is required and expected.

The ‘Left Shift’

Enlarge this picture Figure 1. Integrated and autmated verification steps are needed earlier in the design process.

A colleague of mine, David Wiens, who has claimed technical expertise, was in the realm of high-speed design a few years back. He has long been preaching the virtues of “left shift,” of high-speed analysis tools in the PCB design flow, i.e., making those analysis tools available throughout the design cycle, not just after placement and routing are complete. Of course, it is common practice now and we all expect that our CAD tools are very tightly integrated with our analysis tools.

The more I thought about this principle, the more it became apparent that the left shift philosophy can and should be applied to other areas of the PCB design, especially DfF (Figure 1).

How can we ensure that a consistent data set is provided to the manufacturers so that they can all fabricate the PCB to the same rules and within acceptable tolerances? A couple of options are available.

Enlarge this picture Figure 2. Easy rule setup and definition enables non-experts to understand the manufacturing requirements.

One-way to partially achieve this is to poll the manufacturers and determine a common set of rules that they can all comfortably accommodate in their fabrication process. That way you own the rules and they alone have the authority to change them. A second, more appropriate solution is to move the

DfF analysis tools and other areas of fabrication and manufacturing checking closer to the PCB design--a la “left shift.”

Remembering that the PCB designer may not also be a DfM/DfF expert, these tools should be easy to learn and use. They should be tightly integrated and in fact, should function seamlessly with the PCB/CAD tools. DfF rule set up (Figure 2) and hazard/violation review (Figure 3) should function just like the rest of the PCB/CAD tools.

Enlarge this picture Figure 3. Clear and concise review capabilities readily identifies individual hazards.

Ideally, they should be able to reuse rule sets or schemes based on multiple manufacturing process and tolerances. Fabricators should be willing to share those rules sets with you. After all, the more you do to prepare the data prior to generating manufacturing outputs, the less they have to prepare the data from your CAD system.

Of course relatively low cost, readily available licenses should be accessible from within the PCB/CAD tool and not a stand-alone application. Moving DfF analysis into the design space makes sense; more PCB designers recognize this need as companies take a more proactive role in helping to control their manufacturing costs beyond just bargaining for the best board price.

Conclusion

The most obvious benefit of moving the DfF rule checking from a post design, place and route operation to become an integral part of the PCB design process is to alleviate multiple cycles between OEM and manufacturing. Defining target manufacturers early in the design process and understanding the related costs helps increase a company’s profitability by increasing yields and reducing manufacturing times.

Forming a close relationship with

fabricators and manufacturers enables the OEM to retain control over the design data intent at the manufacturing stage while also ensuring that the manufacturers are free to maximize their productivity and therefore profit by focusing on what they do best: fabricating boards. This gives you the confidence that PCBs can be manufactured anywhere, assuming the design rules are process tolerant, while providing a single, consistent data set that you and you alone own and control. The time is now to “left shift” your DfF analysis. CT