Gerber files are the life blood of PCB fabricating. For everything to go well, the Gerber files have to be perfect; they are referred to at every stage of PCB fabrication, from initial artwork generation to automatic optical inspection and final electrical inspection. They specify details in the finished product that are invisible to the human eye.

So, a small imperfection here, a wrong spec there and the trap is set. Sooner or later it will be sprung and the consequences are never pleasant: Manufacturing delays, product recalls, re-engineering or, a worst case scenario, equipment failure causes fatality.

View of a typical Gerber file

Honey we Shrunk the Universe

Getting specs exactly right has always been crucial, but it gets ever more important as the world of electronics vanishes before our eyes.

In 1965, George Moore, co-founder of Intel, came up with what has become known as Moore's Law. His original comment related specifically to the number of transistors that could be fitted onto an integrated circuit. It went something like this: "The number of transistors on an integrated circuit doubles every two years." He went on to say that this trend would probably continue for "at least the next ten years." Forty five years on and it seems George sold himself a bit short with his ten year prediction. His theory is more relevant now than it ever has been.

Example of a modern chip

In 1966, IBM had developed the first solid state memory, a device about the size of a large cell phone. It held one bite of information. Sometime in the remaining portion of this year, Intel will be launching chips with their 22nm technology and their website is gleefully talking about cloud computing.

Just to get this into perspective, the acronym "nm" stands for nanometre. One nm is one billionth of a metre. That's a 1 followed by 9 zeros if you live in the USA and 12 zeros if you live in the UK – but let's not split hairs.

In their pursuit of Moore's Law, the guys at Intel have made sure that things get smaller, more powerful and less energy hungry every two years. In 2007, they rolled out their 45nm processors, 2009 saw the 32nm technology and now in 2011 they are bringing us 22nm, 3D transistors.

Before we get back to Gerber files, here's a test to see if you've been paying attention: How many 45nm transistors can you fit onto the surface of a healthy red blood cell (assuming, in the first place, that you would want to?) The answer is 400.

One last interesting bit of speculation before we get back to Gerber files. If Moore hadn't come up with his Law and we were stuck with the materials and technology of 1965, a computer with the processing power of an average laptop would weigh 268 000 tons, or about the same as four QE2 cruise ships.

Back to Reality

Although Moore's original observation was about transistors, it actually applies to everything electronic: pixel density on flat screens and in digital cameras, the size of LEDs which are now so small that you can hardly see them is they are not on. Virtually all components designed to fit onto a PCB are shrinking before our eyes.

This means that electronics are being applied to more and more functions that weren't possible before; making our lives more efficient and us more and more dependant on electronics.

Microelectronics are now found in everything from singing greeting cards to drive-by-wire systems in high end motor vehicles.

Fewer and fewer decisions are left up to us to make; for a long time now, cars have been better judges of road surfaces and braking conditions than the drivers; they switch their own head lights on when they think it's getting dark and a generation of "lost people" is growing up in Europe, unable to read maps and unable to find its way without GPS assistance.

And so the onus to deliver safer, more accurate and more fail-safe electronically controlled components weighs more heavily on designers and manufacturers.

Back to Gerber Files, Finally!

Everything to do with PCBs starts with Gerber files. Once the electronics design engineer has designed the product, it's up to the PCB designer to craft the PCB in the most efficient and cost effective way possible.

A net list is created from the engineers schematics. This details every connection between every electronic component in the product. These components have to be laid out in the best possible way, having regard to what the components functions are and what their specific requirements are: Will some components heat up more than others and require cooling; will another component need to be in a specific position within the finished product to enable wireless connectivity for Bluetooth functions or satellite exposure for GPS functions; will a radio transmitter or receiver need to be positioned away from another component that will create interference and if so how far away?

The list grows with the sophistication of the device.

Once these things have been considered, the connections between them have to be designed and the decision made as to how many layers will be required to achieve the optimum result. In deciding on the layers, thought must be given to the space between the layers and the composition of the base material.

Budget also comes into the equation, particularly if the product is intended for the mass market.

Design for Manufacture

There's a myriad things the designer has to take into consideration, but one of the most fundamental aspects of PCB design is that boards must be designed for manufacture. This concept has two angles. First, the board has to be made from materials that will stand up to the rigours of PCB fabrication and the processes of being populated and built into the product that they were designed. Apart from mechanical and thermal stress, the boards must be compatible with the assembly machinery.

A Designer for all Reasons

As electronic components shrink in size, there is pressure on PCB designers to make correspondingly smaller boards. So, virtually every board designed today has a lot of microscopic real estate on it. Tracks have to be smaller which in turn means that tolerances are finer and as track width and line spacing shrink, the possibility of accidental connections between adjacent tracks rises.

The designer is quite literally in a microscopic world that is bordering on the atomic, designing product that will be entirely machine made and tested. The components are quite simply too small for humans to get their fingers on. So, if a spec details a measurement of 0.033 mil, it's not talking about 0.034 or .035 and divergence from the specified measurement can result in product failure.

The PCB designer must also have an understanding of what the final product is designed to achieve and what sort of conditions it is going to be used in; will it be used in high temperatures, high humidity, or be subject to mechanical shock? The list is endless.

Now, Gerber Files.

Now that we know what goes into PCB design, let's have a look at Gerber files and why they are central to the whole process.

Why Gerber? The Gerber Scientific Instruments Company pioneered photo-plotter manufacturing. Technology developed by the company revolutionised industries such as clothing and shoe manufacturing and the PCB manufacturing industry. Their name is synonymous with graphics conversion systems and digital data reduction systems which lie at the heart of PCB manufacturing.

Before any design can be taken to the manufacturing stage, the specifications of the design have to be reduced to Gerber file format. In other words, the specs are formatted for the photo-plotters, drilling machines, routers, automatic optical inspection machines and flying probes.

One Layer, One File

A complete set of Gerber files for a PCB means that there is one file for every layer of the board: Each conductive layer, the top and bottom component- bearing layers, solder masks, solder paste screens, and the boards final shape with routing and scoring specs.

The Gerber files are lists of X and Y coordinates with specs for everything that must go onto the board. They define track position and width, separation between tracks, or line width; the exact positions of vias that have to be drilled and the drill diameters, the outline of the board and the number and type of fiducials and their positions.

Because each layer has its own file, it stands to reason that the files must be carefully and clearly named. This seems to be stating the obvious, but in a surprising number of cases, the files are given hopelessly inadequate names and extensions. This wastes an inordinate amount of time as either the camming process person has to unravel the puzzle or the job is sent back to the client. Either way time and money are wasted.

The Humble Index – the Hallmark of Good Housekeeping.

Gerber files must be accompanied by a text file that identifies the PCB, indexes all the layers and clearly lists the corresponding Gerber file with its correct file name. The file names should identify the PCB and reflect the layer number and possibly a descriptive reference that will clearly identify the layer and its function.

When it comes to file extensions, stick to .gbr, .gbx or .art. Extensions like .top, .bot and so on have to be converted and refer to detail better contained in the file name.

The index file must also contain a list of technical details. Final PCB size, panel size and the number of PCBs per panel, inner core size etc.

A great Index file will look like this:

The Perfect Job Submission

When jobs come in to Cirtech, they go straight to Marcel Henry who heads up the national sales and technical support function of the Cirtech Group. He reviews the files preparatory to drawing up a preliminary quote and sending the job through to the camming department.

Once the job is on the camming departments workstations Marcel obtains a complete spec analysis and is able to assign the job to the most appropriate manufacturer. At this stage a detailed quote and an accurate turnaround time can be given.

However, if there are problems with the Gerber files and the job has to be delayed, it could take weeks to get back to the camming stage after it gets bumped from the queue.

For a Perfect Package – Zip It

A correctly labeled zip file, relating to only one PCB, always bodes well; it speaks of good housekeeping. If all the relevant information is in the zip file, the job is far more likely to go without a hitch, come in on schedule and produce the product that the client is expecting.

This is because the design and planning stages have been executed thoroughly and there are no surprises. Apart from the individual layer files, the camming department needs detail on the finished PCB. This could include information such as number of boards per panel, break-off strips, tooling hole and fiducial positions and whether the PCBs must be V-scored or routed or have a combination of both.

If these details are included in the zip file, it shows attention to detail and that the designer has designed for manufacture. If a job is to go smoothly from start to finish, then it is essential that the designer consult the companies that will be populating the board with components and assembling the PCBs into the end product. Only then will accurate panel details be possible as panel sizes must suit the equipment used in the populating and assembly stages.

Check List for a Smooth Order

Every client likes their order to be hassle free, arrive on time and come in under budget.

Remembering that Gerber files are the only interface between humans and machines is helpful. The days when we could pick up a component and see if it was likely to function or not are long gone. For the most part, we can't see some of the components. With PCBs, even if the tracks and line spacings were clearly visible to the naked eye, once they have been assembled, we can't see anything.

So, as miniturisation bites ever deeper and accuracy becomes increasingly critical, it's through Gerber files that we can tell the assembly machines, down to the micron, what to do and we can tell the AOI and flying probe machines what the perfect product is and what they should be testing for.

Gerber files need our respect if they are to hold and deliver the right information. So, here's a quick check list to help ensure that your Gerber files are the useful mines of information that they should be:

• Ensure that your PCB designer communicates closely with the Design Engineer and the populating and assembly companies. PCB dimensions must be compatible with their equipment.
• From the outset, ensure that each file is appropriately labeled and that the file name assists in identifying the part that it relates to.
• Each layer must have its own, uniquely labeled, Gerber file. Use standard extensions like .GBR, .GBX or .ART.
• Ensure that drill files and tool files contain complete sets of relevant information.
• Double check the Technical Spec details and ensure that all measurements are accurate to the smallest degree possible.
• Ensure that critical design elements are clear and specify details that are special requirements or not industry standard.
• Compile a detailed index file listing every single Gerber file and specification that the PCB fabricator must be aware of.
• Zip file the whole package.
• Ensure that each PCB has its own, unique Zip file, that the label clearly identifies the project and that the ZIP file extension is standard.

Communication is Key

Once all precautions have been taken and your project is perfectly packaged, remember that communication is vital. Companies like Cirtech will be in a better position to deliver your perfect product if they have all the necessary information plus peripheral information about the project.

Having a broad picture may influence decisions like which fabricator to use and could enable our staff to advise on peripheral issues like base materials, surface finishes and local and foreign standards compliance to name few.

To close off, Stephen's infallible advice: "Always remember, measure twice, cut once!"