Why are we writing this?

When designing a printed circuit board through EAGLE or other CAD softwares it is essential to understand how subtle design choices will influence manufacturability. When a PCB is designed in CAD it must be converted to the industry standard file format known as a Gerber file for the manufacturing process. This blog post will provide a basic introduction to some best practices for defining circles and polygons such that they match design intent when translated to gerber format and are manufactured with confidence!

What is a gerber? Why should you care?

Firstly, what exactly is a Gerber file? A gerber file is the industry standard file format used to manufacture the PCB boards according to the CAD design. The gerber format can be thought of as a series of vectors which represent the features on different board layers. Put simply, a gerber file uses a series of “flashes” and “drawings” which are created using “apertures'' and the respective vector coordinates to represent the features of a PCB.

Gerber Format Explained

To better understand the concepts behind the gerber file format consider the following analogy of a gerber as a hand-drawn picture. An “aperture” can be thought of as the tip of a pen used to draw the image, and a gerber file contains different sized apertures to draw different sized features, much like an artist with different drawing utensils. A “drawing” therefore can be thought of as a line, arc, or other shape which is created by moving the pen, or “aperture”, across the page. A “flash” however is different from a “drawing”, and can be thought of as a printed image or stamp of a unique shape such as a square, circle, or other simple shape which is then applied to the paper instead of drawn with a pen.

How does Gerber Translation affect PCB design?

Now that the gerber file is better understood, what does this have to do with accurately and precisely defining circles and polygons? A staple of PCB design is the humble circle and it is commonly used for creating board features such as copper pads, vias, through holes, pin indicators, and so much more. While it may seem straightforward to define a circle (or any other polygon for that matter) of a specific size to match the intended design there are some features that must be considered.

When defining a circle in EAGLE the user must define “width” and “radius”. In order to create a precise circle, the user must set width=0, and then alter the radius value to create a perfect circle “flash” image. Doing the opposite process by defining a circle by width, with radius=0 will cause the circle to be shown as imperfectly round. This occurs because instead of creating a “flash” image, the gerber translation process will instead create a polygon. The “Circles & Polygons” board design shown in the image below demonstrates this effect. Notice in the image below how the circles on the right side of the design are represented as octagons instead of a true circle, and as a result will become represented as polygons in the gerber, rather than “flashes” which would be preferable.

This effect can also have significant influence when attempting to create a copper plane of very precise dimensions. In the image below, the bottom edge of the copper planes from the above image are magnified to demonstrate the additional size added to the rectangle’s edge by not defining thickness=0.

In some instances these slight dimensional inconsistencies that occur during gerber translation go undetected. But there is a very real possibility that unintended design effects such as these are the difference between an accurately translated gerber resulting in a well manufactured PCB that precisely follows the original intended design, and a PCB which has electrical shorts, unintended overlaps or collisions, and design tolerances that are much tighter than necessary.

Vias are plated holes that connect copper on different layers of a PCB. A via can either be tented with soldermask or uncovered like a traditional through hole pad. Uncovered vias may be desirable for early prototypes where they provide convenient places to probe a circuit or a convenient place to solder a jumper. Tented or covered vias are desirable because they prevent shorts if wire or other conductive material is in contact with the PCB and the soldermask provides an extra layer of protection from mechanical damage to the copper layer. You may also just prefer the look of a covered via.

We can easily see if the vias are covered on the FlashPCB’s “Board Review” page in the previews. These previews show not only what the PCB will look like but the individual layers. For instance here we can see the uncovered vias in the Bottom preview.

But we can also see them in the mask preview as well.

And we can see the holes in the soldermask in Eagle in the tStop and bStop layers.

If a via is covered by Soldermask is determined by “Limit” setting in Eagle’s DRC under the Masks tab. If a via drill is larger than the “Limit” setting then Eagle will automatically uncover the via by adding a hole in the Soldermask. By default Eagle has this set to “0mil” so all vias are uncovered. We can enter these settings in either “mil”, “mm”, “inch” or “mic”. Here we change the setting to 0.4mm and see that our vias are covered.

After we upload the updated board we see the covered vias in the Bottom preview.

And the holes are removed from the Soldermask preview as well.

Demo Board

To experiment with this setting you can use the FlashPCB Soldermask Limit Demo Board that has multiple via sizes.

PCBs, or printed circuit boards, are present in almost every electronic device from computers, cell phones, cars, medical devices to fridges, toasters and light bulbs. Single-layer PCBs were the first used, but today multi-layer PCBs allow for increased functionality and miniaturization of electronic devices.

Layer Demo

We have created a PCB that shows each layer. From left to right we have: no layers, just substrate, just copper on the substrate, copper with soldermask and finally soldermask with silkscreen on top. Below is the Eagle project for this board.

Substrate

PCBs are made up of multiple layers adhered to a substrate. There are a variety of materials that can be used as the substrate for a PCB, including fiberglass (FR-4), Aluminum, Ceramics (Rogers) and flexible PTFE or Polyimide. The most common substrate material is fiberglass referred to as “FR-4”. Fiberglass is a popular choice due to its thermal stability, mechanical strength, mechanical toughness, and high thermal resistance. Aluminum substrates are more common in high-power applications as they have good thermal conductivity and transfer heat away from hot components like LEDs.

Most PCBs are rigid, however, there are some applications requiring flexible circuits. In these cases flexible PTFE or Polyimide substrates are used. These are typically seen in tightly packaged devices where multiple electrical connections need to be made between boards. Special consideration needs to be paid to the design of flexible PCBS.

Copper

The next layer is the copper layer, which is where the actual circuits are printed. This layer is made up of thin copper foil that is laminated onto the substrate. The copper is etched in a specific pattern to create the circuit pathways that will connect the various components of the device.

The thickness of the copper foil is usually measured in ounces, which refers to the number of ounces of copper in one square foot. A typical board will have 1oz on the top and bottom layers of cooper and 1 or 0.1oz on the inner layers of copper.

Solder mask

On top of the copper layer is the solder mask. Solder mask is what gives the PCB its distinctive green color. The solder mask helps to prevent short circuits by providing an insulator over the copper layers, only exposing the areas that need to be soldered. The solder mask layer also helps in the manufacturing process by forming “solder mask dams” between fine pitch pads preventing bridges between leads of the components.

Solder mask is typically green, although other colors like black, red, blue, white and purple are available today. The green color is most common because it aids inspections, allows for the smallest solder mask dams of all the colors and is typically cheaper due to its frequent usage. FlashPCB currently only offers green but will add support for black PCBs soon.

Silkscreen

The next layer is the silkscreen, which is a layer of ink that is used to print labels and other identifying information onto the PCB. This can include the names of the various components and their locations on the PCB. The silkscreen can only be printed on top of the solder mask.

White and black are the most common colors of silk screen printing. Currently FlashPCB only offers white silkscreen printing.

Layer count

The number of layers in a PCB refers to the number of copper layers in the board. Today two and four layer layer boards are most typically used, very complex PCBs like computer motherboards may have up to 32 layers. We have even seen a 180 layer board at a trade show (it was half an inch thick). In very high layer counts like this, many of the layers are dedicated to ground and power planes. Typically in a four layer board, the inner layers are ground and power planes while signal traces are run on the top and bottom layers.

It's worth noting that PCBs with a higher layer count are generally more expensive to manufacture due to the additional material and fabrication process required. However, the additional cost can be offset by the improved performance and functionality that is possible with a multi-layer PCB. FlashPCB currently supports two and four layer boards.