Us folks at Seeed Fusion are always telling customers to review their PCB production files before handing them over for manufacture, but there isn’t a lot of information as to what exactly you should be looking for.
So, in this article, we’ve made a checklist of things to look out for as you sift through the production files based on the real-life problems we see during file review every day. The manufacturer’s DFM checks usually have you covered, but if you know what to look for, you can perform these easy checks in seconds which would otherwise cost you valuable time confirming and modifying the design when it may be less convenient. Leave the in-depth checks to the CAM engineers and you’ll thank yourself for it.
As our specialty is PCB
manufacturing in China, this guide is applicable to most quickturn PCB services
in the home of manufacturing. We’ve tried to make it as general as possible
based on a common-sense approach, but we cannot guarantee it applies to all of
them. If in doubt, ask your chosen fab house.
The Gerber Format
Gerber is the name you
will be most familiar with if you have ever had a PCB manufactured. This
contains almost all the information needed to make a physical layer cake of fiberglass,
copper and epoxy that make up your printed circuit board. All the manufacturing
instructions, whether it is for copper etching, solder mask application, silkscreen
printing, board shape, plated and non-plated holes is communicated to the
manufacturer in typically 10 simple txt files.
By understanding a
little more about the Gerber format and how the information is interpreted by
the factory, you can avoid disastrous errors and soul-crushing delays.
The Gerber files, in the most basic sense, consist of a list of fundamental shapes, 3 different commands and coordinates that “draw” out the CAD layers that make up the PCB. The good thing about that, is that 1) a multitude of PCB design software can easily translate the design into this universal format that can be interpreted by almost all PCB manufacturers and 2) you can easily visualise the PCB layers in a Gerber viewer and find errors.
Why not just check the PCB design in the design package?
Many PCB design software packages come with advanced tools and features to verify your design, whether they are DRC checks (also very important) and 3D previews. But the important thing to note is that they are often not based on the actual manufacturing data that they export. By relying only on the design package’s tools, your actual boards will be testament to the software’s ability to accurately translate the design into the Gerber language. From our experience, even the most popular and oldest PCB design software packages have their flaws.
Pick your Tool
There are a multitude
of different Gerber viewers out there ranging from view-only freeware to
professional CAM editors. Whichever your weapon of choice, we suggest that you
pick one that has at least these two features:
- Layer by layer view: The ability to inspect each layer, and to view any combination of layers superseded onto each other is key to understanding what they represent and how they are related to each other. For example, for a simple plated hole, you need to make sure that the hole is surrounded by copper in the circuit layer, has the corresponding opening on both sides and a drill hole of course. If you can only view each layer side by side then it is difficult to verify the alignment and of each feature. With the layers on top of each other, even the slightest shift can be identified.
- Real-life rendering: A simulation of what you should expect your boards to look like and the results of the different combinations of features in the different layers. This can help you verify things like whether through-hole pads are opened, vias are tented and more importantly, whether the board outline is what you expected. This is actually quite a difficult feat to achieve and few Gerber viewers pull it off well. But it is good for reference and a reminder of what exactly you are looking at, especially if you’re not familiar with the Gerber layers.
Other fun features to have are a transparency toggle, renderings for different solder mask colors, surface finishes, and tools to measure things like dimensions.
Seeed Fusion’s PCB manufacturing service has a convenient Gerber viewer built-in to the online ordering platform. Just upload your files and load it up.
1. Make sure you have all the necessary manufacturing files
The first check you should always do is check that you have all the necessary layers! This typically includes (with the common labels):
- Circuit layers (GTL/GBL/GL1/GL2 etc.)
- Solder mask layers (GTS/GBS)
- Silkscreen layers (GTO/GBO)
- Mechanical/Outline layer (GML/GKO/GM1, etc.)
- Drill layer (TXT/DRL)
A large percentage of issues arise from missing layers, particularly drill or outline layers.
2. Look for shorts and broken traces
A common thing we see in some files is traces not quite reaching the pad. With a flick of the mouse, the user may not have noticed that they did not fully connect the trace, or the export process may have “shrunk” the trace, causing a break in the circuit. Shorts are somewhat less common since DRCs are very good at picking these up or the software may have measures to prevent them altogether, but still occur. A poorly routed trace that looks like a short or a leftover snippet of a trace could confuse the engineer and cause them to raise a red flag. So, make sure it looks neat.
3. Ensure plated holes are plated
Ensure that every plated hole is enveloped in a full ring of copper on both sides. This clearly indicates to the fab house that this hole needs to be plated through. Don’t be doing copper on top but not on the bottom unless you really have to as this may prompt the engineers to ask for verification.
4. Find stuff where it shouldn’t be
Sometimes the Gerber export process messes up (looking at you Eagle eyes), or it is easy to select the wrong options (Altium, ahem), causing the other Gerber layers to merge into the copper layer and shorting out the entire layer. You can imagine how disastrous that can be. This is a prime example of something you would never find in the design but is very obvious in a Gerber viewer.
Solder mask layers
5. Check for missing solder mask openings
One of the most common issues with solder mask layers are the lack of openings where they should be. Factory CAM engineers are typically good at guessing which areas should be opened, but this relies on judgement. And when you rely only on judgement, that is when inconsistencies and mistakes arise. You can perform some simple checks to find missing solder mask openings (it is important to point out that the solder mask layer is a negative, i.e. filled shapes in Gerber = openings, areas of no solder mask oil):
- Missing through-hole openings: If you can, change the color of the solder mask layer to a vibrant bright color, turn on the copper and drill layer, and look for drill holes that look like outcasts.
- Missing SMT pad openings: Likewise, turn on the copper layer, vibrant solder mask layer and the silkscreen layer. With the help of the silkscreen markings, you can better visualise the component shape on the boards and it will be easier to notice any missing pad openings.
6. Check if the solder mask openings on the wrong side
A mistake we sometimes see particularly when customers make their own footprints or use unverified ones, is openings placed on one side of the boards when the pad is actually on the opposite side. This can be easily spotted using the tip above.
7. Check that vias are tented
Few customers actually want all the vias opened, but some design software often exports the Gerber files this way by default. You can easily notice this in a Gerber viewer then make the appropriate changes.
8. Solder mask openings smaller than the solder pad
That is just weird unless employed in some special BGA pad designs and will certainly raise the CAM engineers’ eyebrow.
9. Verify the text format and size
The way in which some PCB software renders text or graphics when inside the design software and then exported in Gerber format may differ, sometimes drastically. Say, you may spend a good amount of time perfecting the layout of the logo or cheeky comment on the boards, only to find that it’s been enlarged and falls off the board edge. Silkscreen features are a comparatively low priority layer in a DFM review so it can easily be missed. But the same problem can happen in any layer actually, and if in the copper layer, this can cause fatal shorts. The problem can usually be fixed by modifying some text rendering settings.
10. Check for text on solder pads
This is where the real PCB render can really come in handy. It is a common rule that no silkscreen is to be printed on opened contact pads and CAM engineers can easily remove overlapping areas using software. Good for them. But if you have some component designators or polarity indicators in the affected area, you could lose vital assembly information.
Bonus – Paste Layers (for stencils)
11. Check for missing/extra openings in the paste layer
The paste layer should look like the solder mask layer minus the through-hole pads openings and perhaps reduced in size a little. Anything else added or removed should be investigated and verified against the component datasheet.
So, these are just some of the problems we encounter during PCB file review on a daily basis. We hope that by sharing them with you, you can plan ahead and save time and effort when ordering your PCBs.
You may have noticed that we haven’t even begun to mention the mechanical aspects of the board such as the outline, v-cuts, or even drill holes. That is another topic we’ll cover another day, so stay tuned! We hope you found this article a valuable insight into design for manufacture,
Ever got caught out during engineer DFM review? Leave comments here to share with us your experience. If you have more detailed inquiries or questions about DFM, welcome to communicate directly with our manufacturing experts on our Forum.
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