What is the structure of a PCB and how it's made??
Like you I'm also interested to know how a PCB is made, not just because I'm a student of EEE but because every electrical components relies on it! Let's see what I can find out researching over the internet and present to you the valuable information on this article and most importantly understanding the engineering and mechanism behind it.
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| ©Arturo A |
[This article is focused mostly on the building mechanism of PCB]
[PCB stands for Printed Circuit Board]
Brief about PCBs
PCB is the heart of an electronic device. The design of PCBs is said to have been invented by Albert Hanson in 1903, resembling today's plated through-hole PCBs. Since then PCBs have undergone many changes. Currently there are two main types of PCB manufacturing: THT(Through Hole Technology) and SMT(Surface Mount Technology).
THT based manufacturing process and technology itself is old and almost out of date. These type of technology we've seen inside of old TV's and often being used in medical devices and gadgets that operate at a high temperature. Another convenient use of this technology is in educational sectors.
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| Through Hole Technology© |
SMT based technology is latest and can fit multiple components in a compact size what we can see in modern technologies like smart phones, computers, tablets etc.
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| Surface Mount Technology ©Skitterphoto |
However, in this article let's focus on SMT based PCBs since this technology is latest, compact and convenient.
Structure of a PCB
If you take an X-ray of PCB you'll see dark areas of traces which are conductive areas and rest of it is a non-conductive insulator made of a woven fiberglass with an epoxy resin binder which is flame resistant called FR4.
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| X-ray of a smartphone's PCB ©Branch Education |
If we look at the PCB to see how it's organized, we'll see components like microchips, resistors, capacitors, connectors are directly connects to the PCB. And other components like Displays and Cameras are not mounted directly with the PCB, they're connected through a set of mating connectors and a flat cable.
The SoC or the main micro-chip which is mounted differently on the PCB than other components. The SoC is connected in grid connections points or pads which is called a ball grid array that connects it to the wires or traces underneath that runs through the PCB.
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| Ball Grid Array |
The PCB allows each of the component to communicate with the SoC(System on a Chip) and other micro-chips through hundreds of wires through it. Each of these traces of wire are electrically separate and cannot touch any other trace.
The higher number of traces allows for more data to be sent through components like camera to micro-chips or SoC(System on a Chip).
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| Traces of copper wire ©Antony Nettle & Alamy |
And also if you take the x-ray image again you'll see multiple layers of wires on top of each other but not actually touching. This is because they're in different layers.
For better understanding let's look at an average smartphone's PCB. It's made of 10 conductive layers; the top and bottom layers are used for mounting components in order to make the design compact and use less space. In other application a PCB could contain 2-50 or more layers.
Since the components of the circuit board needs power and ground and PCB has entire layers that are dedicated for power and ground and also named as the power and ground planes. Additional ground plates are used for electromagnetic shielding(to lower electromagnetic field) and heat dissipation.
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| Layers of PCB ©PCBWay & Photo Express |
Remaining middle layers are used to carry all the communication traces/signal wires. Each of conductive layers is made of copper. And between these layers is an insulating fiberglass and epoxy resin that prevents the flow of electricity.
Most PCB stay around 2-10 layers. Additionally the top and bottom have a colored solder mask that insulates electrically while keeping the component mounting pads accessible. And the silkscreen is on the top of that which is basically ink used for marking to tell where components are placed.
If you think are these layers interconnected? The answer is "Yes". They're connected through VIAs(Vertical Interconnected Access).
VIAs are fabricated through mechanical drilling or laser technology which are metal(copper) plated holes that connect two or more layers. VIAs make separate conductive layers and insulating material interconnected.
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| VIAs ©PCBWay & Lesics |
Through hole VIAs are the holes that go from top to bottom layer. It is a hole through the circuit. It may or not connect all the layers depends on how it's designed. When hole or other VIA pass through a layer that they don't want to connect to, the copper is removed from around the VIA on that layer.
Blind VIAs connected either with the top or bottom layers to a middle layer. And you cannot see through this that's why they're Blind VIAs. Buried VIAs connect internal layers inside to one another. There are other VIAs on the top of the PCB which act as a heat sink for heat dissipation.
Regarding how a PCB is going to be designed each of them are different. Some smartphone stack PCBs on top of one another in order to save space for other components like large battery or cameras. Some separate PCB in top and bottom parts; with the bigger one containing SoC called the motherboard and the smaller one called the daughter board.
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| ©cadence |
Manufacturing Process of PCB
[Manufacturing process of Surface Mounted(SMT) PCBs only]
First the PCB is designed in a software(KiCad, OrCad, CircuitMaker, EasyEDA are popular PCB designing software).Then the design is being printed out in photo negative in black ink in a plastic sheet with the help of a plotted printer.
On a copper foil a photo resistive mask is placed first and the printout is placed on the top of it. Then this setup is placed(exposed) under UV light. When the setup is placed under UV light, the light passes through the clear/transparent parts of the printout which hardens the photo resist.
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| Fabrication Process ©Lesics |
The other parts where black ink of the printout prevents the light to pass through and those areas underneath the ink remains unharden. Then the copper is being removed from the unharden area with the help of few different chemical process.
At this point the PCB is fabricated. The copper areas are then being protected by solder mask. Then the circuit board goes through optical inspection process where the circuit board is being inspected with a machine scanner that uses light imaging.
If the PCB comes out ready for mounting components then the components are being mounted on the surface of the PCB using reflow soldering to their contact pads where the assembly of components are subjected to controlled heat. And that's how SMT PCBs are made.
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| Through the process of Reflow Soldering ©chipcodelab |
PCBs come in multiple layers to prevent occurring the issue of interference. When current is flowing in one trace a magnetic field is generated that affects another trace which then both trace starts to act as a capacitor. This results change in voltage which becomes serious with high frequency signals.
To solve the issue of interference multilayer technology is used by separating ground layers and power layers with insulating layer. The number of layer varies and depends on requirements.
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| Interference Issue ©Lesics |
The layers are then pressed together alternately (The press operator loads them into bonding press which then uses heated press plates and pressure together to merge them entirely).
These layers are connected internally through VIAs about which we've learned previously in this article.
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| Photo by Pixabay |
More about PCBs
PCBs are lightweight. Average weight of a smartphone PCB is 19.5± 3.4g, which is 18% of the weight of the smartphone containing all the important parts. Some of the components of modern PCBs are smaller than 1mm.
Engineers are continuously innovating to make components such as microchips, resistors, capacitors, inductors and connectors as small as possible. Thanks to their hard work, we can now carry a first-generation room-sized computer in our pockets.
Older design of PCBs use to take much more space because of their large components and mounting mechanism.
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| Old THT PCBs | Photo by Pixabay |
PCBs are integral to all electronic components. It provides a nice neat organized home for all the components. PCB sizes are now smaller than 0.4 inches. In the future, they may become even smaller which seems impossible today.
As they continue to evolve and we get compact and efficient electronic device, we can thank all the engineers who worked countless hours. Their dedication has revolutionized this technology, making our life easier and more connected.
Components or Terms in details
SMT: Surface Mounted Technology; All the components are being mounted on the surface of the PCB not holing through the PCB like THT(Through Hole Technology).
Epoxy resin binder : It is functional thermoset polymer generally made of epoxy, polyester or phenolics. It is used in patching and grounding applications as a seal coat for non-skid surfaces outside and also as a primer to ensure high strength bond for coating and patches.
Photo Resistive Mask: Also known as photomask is a transparent plate usually made of glass or fused silica, with an opaque pattern etched on its surface. The opaque pattern represents the layout or design of the circuit that needs to be transferred onto a substrate e.g. a PCB.
References
What are PCBs? || How do PCBs work?
This ⬆ video explains it very well. I would recommend you to watch this video.
PCB | At the heart of every electronics device
Watch these videos for more precise understanding.
So, That's it. The PCB, looked like a complex thing to me at first but it's not. If you take a deep look it'll come out as clear as a burger to you. Take the piece out separately you'll understand everything and how it works.
If you've found this article informative then you can visit my other blogs in this website. There are more amazing contents like this one. And comment down what do you think about the structure, mechanism and engineering of SMT PCBs"? Let me know and I hope I would see you again.
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