Reading time ( words)
This article originally appeared in the January 2013 issue of The PCB Design Magazine.
What is a circuit board? Doesn’t that seem like a silly question to ask in a publication like this? Our readers are involved in all aspects of circuit board development, and many of us have devoted our entire careers to them. But what if you’re new to this industry? Where can you go to learn the basics?
I have a degree in electronics (BSEET), and although we studied electronic theory and spent a lot of time in the lab during those four years, we didn’t discuss circuit boards at all. We learned how to test our ideas by building little circuits, and about simulating and emulating them, but nothing about how to transform them into real products. Nothing about what our customers really need in a production environment. Sure, there’s a lot of information available on the Internet, not to mention on-the-job training, and private training, technical conferences, user groups, and publications like this. But many of these resources assume you already know something about the subject. Sometimes it seems as if there is simply too much material to wade through.
Where do you start?
Connecting the Dots
Fortunately, this column is reserved for beginners, and I’m grateful to The PCB Design Magazine for providing it. We now have a blank slate. We can learn step by step. The benefit I have over many other technical writers is that I don’t have to decide how much to explain, or what to exclude. I’ll try to explain everything. I don’t have to make assumptions about how much you already know; I’ll assume you know nothing. In fact, if you think I missed something significant or didn’t explain something well enough, you can e-mail me.
But let’s get back to the question: What is a circuit board?
TerminologyHere’s the official definition from the IPC-T-50 publication “Terms and Definitions for Interconnecting and Packaging Electronic Circuits” Revision J:
Printed Board (PB)The general term is for completely processed printed circuit and printed wiring configurations. (This includes single-sided, double-sided and multilayer boards with rigid, flexible, and rigid-flex base materials.)
OK, that didn’t teach us very much about what a circuit board is, but it used the term “printed circuit,” so let’s get that one, too:
Printed Circuit 60.0912A conductive pattern that is composed of printed components, printed wiring, discrete wiring, or a combination thereof, that is formed in a predetermined arrangement on a common base. (This is also a generic term that is used to describe a printed board that is produced by any of a number of techniques.)
I often turn to IPC when I want to understand something better, and while IPC definitions are technically correct, they just don’t create a very good mental picture of what a circuit board is. I’ll try to explain more and add a picture or two, but first I need to mention something about terminology. IPC has traditionally used the term “printed circuit boards” or the abbreviation PCB, but the organization has recently been replacing those references with the term “printed boards,” or PB.
The reason I prefer “circuit boards” is that I’m not sure boards should be thought of as printed (they are usually etched), and the term “printed board” may soon become confused with the newer “printed electronics.” Printed electronic circuits are truly printed, and that industry is now maturing rapidly. And the term PCB can also be confused with the other kind of PCBs that are environmentally harmful, and PB can be confused with the atomic symbol for lead or even peanut butter.
Seriously, I am a strong proponent of clear communication and the use of unambiguous terms, and I applaud IPC for its efforts to standardize, but in this case I prefer the term “circuit board.” Regardless, I predict that the widely used acronym PCB will be used forever, so you should know that “PCB layout” means the same thing as “circuit board layout.” We’ll learn more about IPC later, but right now we have to get back to discovering what a circuit board is.
The Simplest Circuit Board
From the definitions above we learned that a circuit board is a conductive pattern formed in a predetermined arrangement on a common base. The conductive pattern can be as simple as a single layer of copper, with portions of it etched away to leave the desired connectivity. This is called a “subtractive process” because the material starts out as a full sheet of copper, which is relatively inexpensive and is a good conductor of electricity, and then the unwanted areas are etched away to leave the conductive pattern. The copper used in this process is usually very thin, so it needs to be supported on some type of insulating material, which is the “common base” mentioned in the IPC definition.
In addition to providing support for the flexible copper pattern, the base also provides mechanical support for the electronic components that will be mounted to it. This insulating material is most commonly a thermally cured flame-retardant fiberglass, which is an organic resin system reinforced by one or more layers of glass fibers woven together like cloth.
The most common form of this base material is called FR-4 (flame-retardant), but many other materials are available and each has its own specific properties. For example, some materials are more stable at higher temperatures, while others are better for high-speed circuits and other can flex continuously, for example. We‘ll learn more about material properties later, but for now we only need to understand that the simplest circuit board is made from a single-sided laminate material, which is a single layer of copper bonded to a base material of the desired thickness. Copper-clad laminates are typically 36 x 48 inches, which are then cut down to 18 x 24 inches for the bare board fabrication process.
Single-sided boards are used for simple circuits in very inexpensive, high-volume products like toys or smoke detectors. Holes can be drilled in the board for leaded components to be inserted from the other side and soldered to the copper conductive pattern, without having to add the expense of plating the holes. Surface-mount components can be soldered directly to the conductive pattern. It is difficult to design a circuit of any complexity on a single layer without using a lot of jumpers, so instead of using a single-sided laminate, it is more common to start with a laminate that has been coated with copper on both sides.
By using a laminate that has copper bonded to both sides, the bare board fabricator can etch a different conductive pattern on each side and connect them together with plated-though holes, which are formed by drilling holes through the laminate and then using a plating process to deposit copper on the hole walls. This technology allows copper-clad laminate material to be processed in “layer pairs,” with the ability to place conductive paths connecting the layers together wherever they are needed. It should be obvious that more complicated circuit designs can be accomplished by using double-sided boards with plated through-holes, but let’s take this idea one step further. Why not process several layer pairs with different conductive patterns, insert more insulating sheets of material between the layer pairs, laminate them all together, and then process the outside layers just like a double-sided board?
The hole drilling and plating process could connect all the layers together, and in this way we could make boards that are 4-layer, 6-layer, 8-layer, 10-layer, etc. The advent of multilayer boards has opened the door to designing extremely complex electronic circuitry, adding layer pairs as needed to make all of the necessary connections. OK, that was a fast-paced and all-too-brief introduction to multi-layer technology, but we need to move on. We can dive into the details of board fabrication later.
The Rest of the Story
A casual glance at almost any modern circuit board will show more than just copper conductive patterns on insulating material. What’s the rest of the story here? To continue our general overview of the common circuit board, it might be helpful to have a picture:
Figure 1: A typical circuit board.
Figure 1 shows one small area of a circuit board I designed many years ago. You can see that the conductive pattern has been formed so that three components can be soldered to it. You can see the connections between the component land patterns, some plated holes called vias, and a few square areas for test points. There are three other important things to notice in this picture:
- Green soldermask has been applied to the surface of the board, leaving openings in areas that will be soldered and for test point accessibility. This protects the outside layers and also makes the soldering process more reliable.
- White ink is commonly called silkscreen, and provides identification of parts by reference designator, and sometimes includes component outlines and other information.
- Bare copper will oxidize quickly (like a new copper penny turning brown), which makes it difficult if not impossible to solder. A final finish has been applied as a protective coating over the exposed copper, keeping it solderable. In this example, it is an alloy of tin and lead (eutectic solder coat applied in a process called hot air solder levelling, or HASL), but it could also have been silver, gold, tin, an organic solder preservative (OSP), or some other coating.
Here’s another area of the same board to show some variation. This part of the design has large copper areas on the surface of the board and uses thicker connections between components.
Figure 2: Another view of the same board.
So that’s what a circuit board is. Conductive lines that connect land patterns together are called traces. Larger conductive areas are called planes, which are also commonly referred to as copper pours or area fills.
Notice that the silkscreen can be used to provide extra nomenclature (like the "+12V" label in the photo above), which may help people in the lab or during field service. It’s a good design practice to consider the needs of others who may be using your product long after you have moved on. Try to include information on the board that will be helpful to them, if possible.
See you next month!
Jack Olson, CID+, has been designing circuit boards full-time for over 20 years. Jack would like to thank the Orange County PCAD User Group of 1987 for freely sharing so much knowledge and experience, and especially to Jack Miller of RoyoCAD for his sponsorship and support. He can be reached at PCBjack@gmail.com.
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