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what is printed circuit board (PCB) ?

PCB Basics

Electronic circuits are normally manufactured through the use of PCBs (Printed Circuit Boards).

The boards are made from glass reinforced plastic with copper tracks in the place of wires. Components are fixed in position by drilling holes through the board, locating the components and then soldering them in place.

The copper tracks link the components together forming a circuit. The animation shows the components arranged on the component side’ of the PCB and as it rotates, the copper tracks are also shown.

There are traces that electrically connect the various connectors and components to each other. A PCB allows signals and power to be routed between physical devices. Solder is the metal that makes the electrical connections between the surface of the PCB and the electronic components. Being metal, solder also serves as a strong mechanical adhesive.

http://www.technologystudent.com/images5/pcbcir1.gif

Composition

A PCB is sort of like a layer cake there are alternating layers of different materials which are laminated together with heat and adhesive such that the result is a single object.

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Let’s start in the middle and work our way out.

FR4

The base material, or substrate, is usually fiberglass. Historically, the most common designator for this fiberglass is “FR4”. This solid core gives the PCB its rigidity and thickness. There are also flexible PCBs built on flexible high-temperature plastic (Kapton or the equivalent).

You will find many different thickness PCBs

Perf board

Cheaper PCBs and perf boards (shown above) will be made with other materials such as epoxies or phenolics which lack the durability of FR4 but are much less expensive. You will know you are working with this type of PCB when you solder to it – they have a very distictive bad smell. These types of substrates are also typically found in low-end consumer electronics. Phenolics have a low thermal decomposition temperature which causes them to delaminate, smoke and char when the soldering iron is held too long on the board.

Copper

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PCB with copper exposed, no solder mask or silkscreen.

The next layer is a thin copper foil, which is laminated to the board with heat and adhesive. On common, double sided PCBs, copper is applied to both sides of the substrate. In lower cost electronic gadgets the PCB may have copper on only one side. When we refer to a double sided or 2-layer board we are referring to the number of copper layers (2) in our lasagna. This can be as few as 1 layer or as many as 16 layers or more.

The copper thickness can vary and is specified by weight, in ounces per square foot. The vast majority of PCBs have 1 ounce of copper per square foot but some PCBs that handle very high power may use 2 or 3 ounce copper. Each ounce per square translates to about 35 micrometers or 1.4 thousandths of an inch of thickness of copper.

Soldermask

The layer on top of the copper foil is called the soldermask layer. This layer gives the PCB its green. It is overlaid onto the copper layer to insulate the copper traces from accidental contact with other metal, solder, or conductive bits. This layer helps the user to solder to the correct places and prevent solder jumpers.

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Silkscreen

PCB with silkscreen

The white silkscreen layer is applied on top of the soldermask layer. The silkscreen adds letters, numbers, and symbols to the PCB that allow for easier assembly and indicators for humans to better understand the board. We often use silkscreen labels to indicate what the function of each pin or LED.

Silkscreen is most commonly white but any ink color can be used. Black, gray, red, and even yellow silkscreen colors are widely available; it is, however, uncommon to see more than one color on a single board.

Terminology

Now that you’ve got an idea of what a PCB structure is, let’s define some terms that you may hear when dealing with PCBs:

  • Annular ring – the ring of copper around a plated through hole in a PCB.

Annular ring on resistor

Examples of annular rings.

  • DRC – design rule check. A software check of your design to make sure the design does not contain errors such as traces that incorrectly touch, traces too skinny, or drill holes that are too small.

  • Drill hit – places on a design where a hole should be drilled, or where they actually were drilled on the board. Inaccurate drill hits caused by dull bits are a common manufacturing issue.

Bad drill hits

Not so accurate, but functional drill hits.

  • Plated through hole – a hole on a board which has an annular ring and which is plated all the way through the board. May be a connection point for a through hole component, a via to pass a signal through, or a mounting hole.

Plated through hole resistor

2 comments

  1. Jeremy

    PCBs can be single-layer for simple electronic devices. Printed circuit boards for complex hardware, such as computer graphics cards and motherboards , may have up to twelve layers. PCBs are most often green but they can come in any color.

  2. Deutschland

    A legend is often printed on one or both sides of the PCB. It contains the component designators , switch settings, test points and other indications helpful in assembling, testing and servicing the circuit board.

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