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Today, we’re going to take a brief look at the fundamental item that keeps the electronic world turning: the PCB (meaning “Printed Circuit Board”).

In our modern society, PCBs are essential in everyday life, from improving convenience at home to powering business operations to offering endless entertainment. They allow you to control your microwave, your car’s dashboard, your X-Box, your lawnmower. . . The list is endless.

But what is a PCB? How does it work? What is it made of?

 

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Invented to replace the bulky wiring of the early 20th century, PCB assemblies started becoming commonly used in the late 1940’s, particularly by the U.S. military. At that time, circuit boards consisted of a sheet of material such as Bakelite or even wood, a wiring layout on one side and components connected through from the other side.

Throughout the following decades, PCB designs allowed for more and more complexity within smaller areas. New board printing methods improved efficiency and functionality. Single-layer boards became multi-layer boards. Components could eventually be placed on both the top and bottom of the PCB.

Today, PCBs can be mass-produced faster than the eye can follow, and sport components almost smaller than the eye can see. Yet for nearly 100 years, the concept of the circuit board has remained the same since the beginning: connect electronic components through a wired surface.

 

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If you’ve looked closely at a PCB before, you may have noticed the visual similarity it has to a sky view of a city. In fact, there are quite a few parallels between a populated PCB and a bustling metropolis. Roadways are mapped out for electricity. Traffic instructions are posted at every other corner. Buildings control the major functions of the city from power plants to IT departments. And just like a city, the PCB fabrication process begins with a foundation.

Starting from that foundation, we’ll take you through a basic overview of the PCB fabrication and assembly process, including the following topics:

  • Core Material
  • Copper Traces
  • Layers
  • Vias
  • Plated Through-Holes
  • Pads
  • Solder Mask
  • Surface Plating
  • Legend
  • PCB Assembly

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pcb core substrate copper foil diagramThe first step puts the literal “board” in “PCB” and forms the groundwork for everything to come. This core consists of two parts. The internal base material or “substrate” is generally made up of woven fiberglass and resin. While there are numerous available compounds that can be used to make up this base, the most common is called FR-4. Copper foil then covers both sides of the substrate surface.

In addition to physically supporting all the traces and components that will be attached, the FR-4 base insulates the two sides of copper from electrically interfering with each other during operations. Furthermore, FR-4 has a level of heat-resistance that protects it during the rigorous high-temperature assembly processes.

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circuit copper trace plane diagramRows of copper, otherwise known as “traces”, form the electrical pathways between components. In order to create the small, elaborate trace patterns needed to function, the copper foil must be chemically etched. A light-sensitive photoresist coating is applied and cured to protect the desired copper pattern. Then, any unwanted copper is chemically dissolved and washed off leaving only the protected traces.

In addition to traces, PCBs will often contain massive copper planes. These vast areas of metal can serve a number of purposes. They are frequently used as ground or power planes that run through the extent of a board, providing easy connection access from any location in the circuit. Sometimes they are used to dissipate heat. In spite of their size, the primary function remains the same as any other trace.

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pcb layers diagramOnce the traces of the core have been formed, new layers can be added. A layer of insulating and bonding agent called prepreg (meaning “pre-impregnation”) is placed over the copper layer. The prepreg usually consists of a similar material to the substrate, such as a softer form of FR-4. With the prepreg in place, another layer of etched copper foil or core is added. This process repeats for every new layer.

How many layers can a PCB have? Technically, as many as the designer wishes. Multilayer boards with over 100 layers have been known to exist. But in reality, more layers mean more complications during production. Just like trying to bake a deep-dish pizza with too many layers, overly complex PCBs have a high risk of coming out of the reflow oven overcooked, undercooked, or just outright broken. That said, 2-16 layers is a common range for modern PCB production.

More layers also mean more preparations ahead of time. Any items that will be inaccessible to work on from the outside, such as blind or buried vias (more on those in a second), must be inserted at this point. Once the stack of layers is ready and riveted together, it undergoes an intense lamination process involving 375°F heat and massive pressure.

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via through buried blind circuit boardNow that the 2-dimensional patterns have been stacked and laminated, the traces need to be connected into one large 3-dimensional network. To link a trace on one layer to a trace on another, holes are drilled and vias are placed vertically through the layers. These small copper barrels may travel through only a couple of the PCB layers or all the way from top to bottom depending on the need.

There are three major types of vias:

  • Through Vias, which link from the top to the bottom layer
  • Blind Vias, which link from an external layer to an internal layer
  • Buried Vias, which link from an internal layer to another internal layer

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Similar to vias, plated through holes (PTH) connect from the top to the bottom side of the PCB. However, these barrels are typically larger and serve another major purpose. To attach a component to the PCB, the legs (or “leads”) of the components are inserted into the PTH and soldered in place. This creates a secure connection both mechanically and electrically between the component and the PCB trace.

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Pads carry a similar function to through holes in that they allow for secure connections between components and the PCB traces. Unlike plated through-holes, pads are found only on the surface of the circuit board. Surface-mount components are designed specifically to adhere to pads.

blank circuit board pads through hole masking legend

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The substance that grants a PCB its color is the masking. Historically, green has been the overwhelmingly popular color choice, but PCBs can sport a multitude of color options these days. The main duty of the masking is to protect the copper from contamination and physical damage. It can also help protect traces from undesired connections which could short out the PCB’s connections and damage components.

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After the masking has been added, the remaining exposed copper is plated. This process focuses mostly on plating vias, pads and PTHs. The plating both protects the copper and assists the formation of strong metallic bonds between solder and copper.

Commonly used plating metals include tin, gold on nickel, or solder. The metal can be deposited a number of ways, including but not limited to:

  • Electroplating: This process works with tin, nickel and gold. Using a DC current, the metal plate is bonded chemically to the copper.
  • Hot Air Level Soldering: The PCB is dipped in molten solder, then leveled out using high-pressure air.
  • Immersion: The PCB is submerged in a chemical solution that deposits the metal to the copper.

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Often the final step in the PCB fabrication process, information called a “legend” or “silkscreen” is printed on the surface of the masking. This print shows vital assembly information on the surface of the PCB, meaning assemblers can easily spot component locations and directions. It can also show information such as a company logo, date code and anything else the designer wishes to add.

 

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pcb components smt surface mount through holeThe term “PCBA” stands for “Printed Circuit Board Assembly”. Unlike simply “PCB”, a PCBA implies a circuit board that is fully populated with components. This population process is usually considered a completely separate process from PCB fabrication. Oftentimes, circuit boards are fabricated by one company and assembled by another. At Meyer Electronics Manufacturing Services, our focus is on the assembly side of the equation, allowing us to maximize quality on each assembly process.

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The entire basis of a circuit board revolves around its components. All of those chips, switches and connectors are what give life to the PCB. Some components create power flow or control its amount and direction; Others handle complex calculations and memory storage, or even produce light and sound. The applications appear to be limitless, from growing crops to making movie magic to exploring the far corners of the earth and beyond.

There are two main processes by which components are attached to the PCB. Both surface-mount and through-hole components have one major thing in common…

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Solder is the metal that connects components to the PCB. It has a low melting temperature and good conductivity, making it the common choice for adhering electronic components to circuit boards. Though solder has historically used lead as a major ingredient, recent decades have seen the rise of RoHS-compliant lead-free solder. Many countries now require all PCB sales to adhere to strict lead-free guidelines.

Solder comes in different forms such as wire spools for hand soldering, paste for stenciling, and large bars for automated equipment.

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The SMT process connects surface-mount components to the PCB pads. First, solder paste is stenciled onto the blank PCB’s plated pads. Then high-speed high-accuracy component placement equipment loads the components from reels and trays onto the PCB. During the jarring process of placement and transport, components are temporarily held in position by the solder paste. Finally, the PCB is run through a reflow oven, liquidizing and then solidifying the solder paste into firm solder joints.

Unlike through-hole components, surface-mount components are made to withstand the extreme temperatures of the reflow process. Therefore, if both SMT and through-hole processes are involved in a PCB’s assembly, SMT comes first.

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The through-hole process is a bit more manual than SMT. Components must be prepared, which typically includes removing them from packaging, then cutting or bending their leads to fit the PCB. Once prepared, the components are placed or “stuffed” into the PCB’s through-holes either by hand or auto-insertion equipment. Finally, the components are soldered in place either by automated machinery or by hand soldering.

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Once the component assembly has been completed, the PCBA may undergo various additions such as conformal coating, wire modification, programming… Far too many to discuss here. After all of the additional processes have been completed, the PCBA is hooked up to its housing, ready to change the world.

 

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New innovations constantly move the craft of PCB design, fabrication and assembly towards amazing feats. Even so, the concept behind what a circuit board is remains as concrete as it ever has. Through a combination of inventive design, basic materials, metal manipulation and precision use of natural elements, we shape our modern way of life. Through small “cities” of components, we control the world around us.

With any luck, you’ve discovered a little something new today on how the wonders of electronics are made possible. Or at the very least, you found a new tidbit that’ll impress your coworkers. Either way, head to the comments below and let us know!

Chris Meyer has been in electronics manufacturing for over 15 years. He writes from his circuit-board-littered cubicle to share understanding and quality improvement tips in the PCB industry.
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