Printed circuit boards (PCBs) have been around for quite a while. They are an essential part of modern electronics, and nearly every device has at least one. But there are so many things you will need to know if you plan on designing one by yourself.
One of the exciting aspects of PCBs is the use of copper and the role it plays in the production. Here, we will go through everything you need to know about copper layers, dimensions, and why it is so important for the PCB manufacturing process.
Basic Anatomy of PCB
Before we go in-depth about copper thicknesses, we should cover the basic anatomy of printed circuit boards. There are several layers on the regular PCB. Firstly, we have the base material or substrate, which is usually fiberglass. Most commonly, the base is made of FR-4 or FR4.
Now, on top of the FR4, the manufacturer will laminate a layer of copper to the board, using heat and adhesives. These are known as one-layer PCBs. The most common type of PCBs has two layers of copper, one on each side. The number of copper layers can be any from one to sixteen or even more.
On top of the copper layer, there is a solder mask. That is what gives that recognizable green color to the printed circuit board. Of course, a solder mask can be any color, but it is usually green or red.
Finally, silkscreening is a printing method that adds letters and marks on the top of the solder mask. It is often white. As with a solder mask, the color of silkscreen can be anything you choose, but the price might be higher if you opt for anything other than black, yellow, or white.
Now that we’ve covered the basics of PCB design, you might ask — Why should we use copper? The primary reason why manufacturers use copper for the layers is due to its high conductivity.
What that means is that it can easily transmit a signal without much loss along the way. It also means that the company designing the PCB won’t need to use a lot of copper during the process. Even the smallest amount will do the job.
As a result, PCB manufacture requires thin layers of copper, which can be quite inexpensive. Needless to say, this seems like a dream come true since copper can do a better job compared to other materials while being a lot cheaper.
While other materials can carry electric charges, another advantage of copper is the fact that it is highly conductive.
Copper Weight and Thickness
Calculating the thickness of the copper is quite easy, and the whole process is standardized. Naturally, the thickness of the copper is determined by the manufacturer. They will also decide which type of copper to use.
The weight of the copper is calculated in ounces (oz). The more ounces of copper a manufacturer uses, the thicker the copper will be. That leads us to the second part of the equation — thickness.
You can easily calculate the thickness of copper. It is a thickness of one oz of copper rolled out across the area of one square foot. We already mentioned that the thickness is standardized, and you won’t need to measure anything by yourself.
One ounce of copper is equal to 1.4 mils in thickness. For everyone unaware, a mil is a unit of measurement, and it is equal to one-thousandth of an inch, which is approximately 35.56 micrometers (μm).
Similarly, two ounces of copper is equal to 2.8 mils or 71.12 μm, and so on. Let’s take a look at some of the common values you might encounter on the market.
- 0.5 oz = 0.7 mils (17.78 μm)
- 1 oz = 1.4 mils (35.56 μm)
- 2 oz = 2.8 mils (71.12 μm)
- 3 oz = 4.2 mils (106.68 μm)
- 4 oz = 5.6 mils (142.24 μm)
- 5 oz = 7 mils (177.8 μm)
- 6 oz = 8.4 mils (213.36 μm)
Most manufacturers design PCBs with a copper weight between 0.25 oz to 6 oz, and it is not common to find values (or weights) out of this category.
We mentioned before that PCBs can have more than one or two layers. If we take a look at a 4-layer PCB, we can notice that there are top and bottom layers, as well as two layers inside.
The top and bottom layers are known as outer layers, while the two in the middle are the inner ones. Of course, if we talk about a board with sixteen layers, fourteen of them will be inner, while the top and bottom will count as outer layers.
The reason why that is important is due to the fact that the inner and outer layers have different dimensions. More specifically, different thicknesses.
Needless to say, even if the inner and outer layers have different thicknesses, the table we mentioned before will still apply to both. For example, the outer layers (top and bottom) can be half an ounce in weight, and the inner ones can weigh one ounce.
That means that the thickness of the outer layers will be 0.7 mils or 17.78 μm, while the inner ones will have 1.4 mils or 35.56 μm. The thickness of the foil or layer is always specified in ounces per square foot (oz/ft²) or g/m². One ounce per square foot is equal to 300 grams per square meter.
Types of Copper Based on the Weight
Now that we have covered the basics of thickness and weight for copper, we should mention different types of this material used in printed circuit boards. There are three main categories of copper you can see on PCBs.
The first one is known as standard copper. Any copper that weighs between a quarter of an ounce and four ounces per square foot falls into this category. One of the main reasons why this type of copper is called standard is due to the fact that it was a standard copper for PCBs for many years. Many companies will use it even today, and if the client does not specify it, the manufacturer will use 1.4 ounces per square foot.
The second category is heavy copper. Here, we have any copper that’s heavier than 4 ounces, meaning between 5 and 19 ounces per square foot. Initially, designers started using heavy copper as an experiment. Not only is it possible to design a PCB with heavy copper, but it is superior in every way to the standard one.
Finally, we have extreme copper, which is between 20 and 200 ounces per square foot. For some designers, heavy copper wasn’t enough. They decided to push the boundaries of PCB manufacturing. Extreme copper is not nearly as popular as the other two versions, and many people believe that there is no need for anything heavier than heavy copper.
Dimensions of Traces
As we could see, calculating the thickness of the copper layer is quite easy. But the main problem is calculating the dimensions of traces in the PCB. Naturally, the thickness of the layer will determine the thickness of the trace, and it is calculated in mils. That means that one ounce of copper will also lead to 1.4 mils trace thickness.
Since traces act as a highway of the PCB, the main part for calculating the trace width is the current that will flow through it.
To avoid having to go through difficult calculations for copper trace width, you can always use either a universal chart or a trace width calculator. Naturally, the width of the trace on your printed circuit board can be any value you like. That is, as long as you stay inside the boundaries for spacing between the traces.
If you want to learn more about traces and dimensions, as well as ways to calculate it by yourself, check out the article about this subject.
Copper is an essential part of nearly every printed circuit board. But to ensure that everything works properly and according to your design, you will need to calculate the weight and thickness of the copper layer.
Fortunately, the process is quite easy, and it is standardized. If you are looking to learn more about printed circuit boards, please contact us with any questions you might have. MKTPCB has over a decade of experience designing high-end PCBs for clients all over the world.