When working on a complex PCB layout, designers tend to rely on the DRC engine in their CAD software to spot errors that can reduce manufacturing yield. Some electrically correct design choices will create DFM problems that need to be found and fixed before sending a PCB out for manufacturing. PCB designers normally do these checks in their PCB layout file before it’s exported to Gerber files, but the Gerber files should always be reviewed before a board is sent into manufacturing.
Design Problems You Can Find in Your Gerber Files
The list of design and DFM problems below is not an exhaustive list, but these are some areas that any designer should check before sending their Gerber files to a manufacturer. There are 6 common DFM problems that can be found in many PCB designs:
1.Incorrect solder mask opening on SMD pads: If there is no opening or too large of an opening, a solder ball may flow and bridge two leads on an SMD component.
2.Vias in SMD pads: This is often a mistake in the footprint, but you might not see this in your CAD tools if the view configuration is not set correctly.
3.Acid traps: Manufacturers will have their own corner angle requirements that are meant to prevent acid traps from occurring during etching.
4.Incorrectly sized vias: In multilayer boards, the vias might be too small for mechanical drilling, or their aspect ratio might be too large (greater than 10:1).
5.Clearances: Common clearance mistakes are conductors too close together (such as in high voltage boards), 1 mil spacing between grounded vias and polygons (clearance conflict), and overlapping drill hits.
6.Trace widths for controlled impedance: Some CAD tools do not enforce an impedance profile during routing, and it’s up to the designer to set the correct width on all impedance controlled nets.
Thin copper can be identified in the top layer, and there may be a clearance problem between traces and vias on the inner layer.
It’s important to note that these design problems might not trigger an electrical DRC error unless you’ve specifically created design rules for these points in your PCB design software. Therefore, designers need to check their Gerber files for these DFM problems either manually or with automated Gerber file checker software.
Problems with Manual Gerber Checks
Designers need to review these potential problems before starting their fab run, but not all designers have access to the same Gerber file checker software as professional manufacturers. This means designers have to rely on the DRC engine in their CAD tools, or they have to manually look through the Gerber files to make sure they exported correctly. Here are some problems with doing a manual Gerber file check:
1.Shorts and opens are not always obvious: Since manual Gerber file checks can only happen on each layer individually, it’s difficult to spot opens and shorts that involve a layer transition. These problems may not be caught with a DRC in your CAD tools, and not all Gerber viewer utilities will pinpoint shorts or opens.
2.Easy to miss DFM mistakes without measuring: Simple design mistakes like small vias, small solder mask clearance, excessive gaps, length mismatch, and much more require precise measurements. Unless every portion of the PCB layout is measured manually, it’s difficult to inspect every aspect of the layout.
3.Checking complex layouts take too long: Gerbers for simpler PCB designs can usually be done manually without missing any details, but complex multilayer PCB layouts simply take too long to review manually.
With so many potential problems to be inspected before fabrication, PCB designers need an automated solution to help them quickly get through
Use a Gerber File Checker Software
The fastest way to spot manufacturability problems in your Gerber files is to use Gerber file checker software. Most of these applications will only accept Gerber files in RS-274X and X2 formats, although other applications are available that support IPC 2158, ODB++, or native CAD files. Any PCB designer can find these tools useful as they can support multiple fabrication file formats and CAD formats.
All Gerber file checker applications are designed to spot any problems in your PCB layout that will reduce fabrication or assembly yield. Most of these applications are used for viewing, editing, and quickly inspecting some basic DFM requirements in a PCB layout. However, the best Gerber checking programs provide other important benefits:
1.Optimized analysis: High-quality Gerber file checkers have optimized the process of loading, rendering, and analyzing Gerber files. Some programs take up to an hour to process Gerber files for complex designs, while the best programs only take a few minutes.
2.Cost analysis: When reviewing a design and working up a budget, a designer can see how changes to the design will affect production price, which helps save time and money.
3.Automated calculations: As long as nets can be identified in the layout, Gerber file checker tools can use the geometry of interconnects to calculate things like impedance, routing length, utilization rate, pad/silkscreen overlap.
4.Gerber compare feature: The best Gerber file applications will let you compare two sets of Gerber files. When updating designs, helps speed up a design review before production.
CAD tools often apply 1 mil clearances by mistake when grounding a net to a plane or polygon. These vias are supposed to be connected to the surrounding plane, and this clearance mistake can be found with an automated Gerber file checker.
With these capabilities, a designer can quickly identify design problems before the PCB gets sent in for fabrication and assembly. Unless you pay for a turnkey service or enhanced inspection level, the fabricator might not inform you of these problems, leading to a failed fabrication run. You can take the risk out of manufacturing with the best PCB design review software.
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