PCB DFM Checks – Fixing Problems Before They Get to Manufacturing

Before any PCB design goes to even the prototyping stage, the engineers at Electronic Interconnect (EI) subject that design to a number of reviews and checks to make sure that the PCB that the customer requests can actually be built. Quite often, designers will submit the artwork for a PCB that is superb in concept, but may not be compatible with the limits of the manufacturing process, explains Shehryar Abbasi of EI’s Engineering Department. Beyond that, a PCB design may also create issues for the assembly stage, such as a lack of solder mask between component leads that might cause bridging and shorts during the soldering process, when components are attached to the bare board. All of these things must be looked at, Abbasi says, in light of DFM: Design for Manufacturability and DRC: Design Rule Check principles and limitations. “We don’t run files ‘as is’; every board that we make initially goes through a comprehensive DFM check” Abbasi says.Design for Manufacturability, or DFM, is essentially a set of rules that facilitate the smooth manufacturing of PCBs, to ensure that the artwork, drilling/routing, and v-scoring data are compatible with the tolerances of fabrication. Normally, most of the Gerber data that EI receives has some feature that needs to be corrected or reported back to PCB Designer.
DFM checking is a service that EI provides to all customers ordering PCBs, and is intended to identify and solve problems before they end up creating headaches downstream. DFM at EI includes the following checks:
Soldermask clearances - distance to keep mask away from solderable areas;
Soldermask bridges - mask between SMDs to prevent solder bridging during later assembly;
Trace/Pad Widths & Spacing - minimum spacing and trace widths, trace width tolerances allowed;
Copper finish - to determine how much artwork compensation needs to be done for chemical processes.
These are the primary checks, but there is a lot more, all having to do with the fabricator’s ability to make the PCB in accordance with the customer’s requirements. These checks include Drill to Pad Ratio, Inner Layer Copper clearances, Silkscreen Line Widths, Silkscreen clearances from Solderable Pads, Drill Diameter tolerances, Aspect ratio (ratio of smallest drill to thickness of board), Board Outline's spacing to Copper/Drill features, and Board Outline's tolerances for Routing and V-Scoring.
“We go through their proposed board, layer by layer, to make sure that we can actually manufacture their PCB and to make sure that it is within our technology range” Abbasi says. “The checks that we do include minimum trace width and minimum trace spacing, as well as other tolerances. If there are issues that are not easily solvable, then we contact the customer’s designer, or design engineer, to let them know that there are issues that will impact our ability to manufacture their product. We also check the solder mask for clearances, etc., we will adjust as needed, or contact the customer once again to solve the problem.”
EI checks everything with regard to drilling, including drill sizes, and aspect ratio of drill size to board thickness. “For example, if you have a 10-mil via on a 125-mil thick board, we may not be able to drill such a small diameter via on such a thick board, that’s a reason for us to contact the customer. We also check for annular rings and pad to drill ratios, especially keeping in mind what classification the customer wants us to build to, for example a Class II, or a Class III product.”
In terms of assembly-related issues, EI has been noticing that more and more customers are requesting the presence of mask between finer pitch SMT pads. This is because IC footprints are constantly shrinking, and in many cases there is no mask between adjacent pads. This causes shorts when solder flows over to an adjacent pad during the assembly process because there is no mask there to inhibit that flow. In that instance, EI’s design engineers check to see if it is possible to apply mask within the space given during the fabrication of the PCB.
Many designers – indeed most – are not familiar with the many constraints of the PCB fabrication process, and as a result it is imperative that EI’s engineers work in close collaboration with the customer’s designer so that they can ultimately produce a working, robust PCB that will also lend itself to the board assembly process and yield a robust product. “It does not benefit anyone if we are able to fabricate a board that is full of problems for the assembly process, such as a high rate of soldering defects that cannot be reduced, excessive rework, and problems like that” Abbasi adds.
The customer’s designer needs to be aware of EI’s capabilities. “For example, if a designer asks for a 1-mil silkscreen line width, he isn’t aware that it is something for which we simply cannot manufacture a screen. So they need to be aware of the limitations of the PCB fabrication process.” Most of the time, EI engineers will be able to change or modify the design; otherwise, the designer has to make the necessary design modifications.
For most of these problems or issues, the ongoing miniaturization of electronic assemblies is the culprit. “The technology is shrinking; we’re doing as much as possible to catch things in the front end before they go to manufacturing” Abbasi says.
Design Rule Check (DRC) and DFM checks can be initiated at the CAD Design stage, during PCB Layout. However, from Abbasi’s experience, EI has seen that most PCB designers either do not know what manufacturing tolerances they should set, or they leave it for the PCB fabricator to fix and/or report back. DFM tolerances differ from manufacturer to manufacturer, but the engineers at EI know their shop's tolerances from Artwork to Drilling/Routing, and Scoring.
For more information: Electronic Interconnect (EI) is a professional printed circuit board manufacturer located in the Chicago area, and manufacturing printed circuit boards in the U.S. since 1985. EI serves design engineers and contract assemblers, providing all types of PCBs from single-sided to complex multilayer boards from prototype through production

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