PCB stands for Printed Circuit Board. It is a board made of insulating material and conductive pathways (usually copper) that mechanically supports and electrically connects electronic components such as resistors, capacitors, integrated circuits (ICs), transistors, and connectors. A PCB replaces large numbers of wires by using printed copper tracks, making electronic circuits compact, reliable, and easier to manufacture.
THE DESIGN PROCESS
You usually start the design in the schematic view. It lets you lay out the components logically and make the necessary connections without having to worry about exactly where they’ll sit in physical board or whether the tracks will cross each other.
Components are represented by their standardised symbols, and they are usually arranged according to their function or purpose in the circuit rather than their actual physical positions on the PCB. Because of this, the grouping of components in the schematic may be different from how they are placed in the real board layout.
The ideal software for PCB schematic design depends on your experience level and budget. KiCad or Autodesk Eagle is highly recommended as the best free, open-source option for beginners and professionals. For industry-standard, professional workflows, Altium Designer or OrCAD PCB Designer is the top choice.
Your software package includes most of the common items you need in its library: common resistors, diodes, capacitors, transistors, integrated circuits (ICs), and more. Adding these components is usually simple — you select the required component and sometimes specify the exact part number. For example, with a resistor, you specify its resistance and possibly its tolerance value.
You should also make sure you choose the correct package for the component; this is the physical format for it. Many parts are available in a range of different formats, depending on the target application. The manufacturer’s datasheet lists the available packages for that part and how they differ. You usually have a choice of packages depending on the soldering style (through-hole or surface-mount) and several other criteria. Over time you will come to recognise some of the package types that you commonly use, so you’ll know without checking that a SOT-23 is a compact three-pin surface-mount package typically used for transistors and small signal devices, while TSSOP-16 is a 16-pin surface-mount IC package with reduced pin spacing for compact designs, and QFN-16 is a highly compact 16-pin surface-mount package that offers improved thermal performance and higher PCB density.
Now that you’ve designed your PCB, the next step is to make one or lots of them.
ETCHING BOARDS
The most common PCB-making technique for home use is to etch the board. Many easily available kits include the materials and tools needed to perform the PCB etching process.
The first step is to get the PCB design onto the board that will be etched. This process generally involves printing out the design from your PCB design software onto a stencil. This printed pattern tells which copper should remain and which copper should later be removed. If you are using a photo-resist board, the stencil is used to block selected areas during UV light exposure. If you are using the toner-transfer method, the design is printed using a laser printer onto glossy paper so it can later be transferred onto the PCB surface.
Photo-resist board
A photo-resist board has a light-sensitive coating. The printed stencil is placed over the board. The board is exposed to UV light (usually you will expose it under a bright lamp for a few minutes). The stencil blocks light in selected areas and allows light in others. After development, only the required circuit pattern remains protected.
Etching board using photo-resist board
In toner-transfer method the design is printed with a laser printer onto glossy paper. The printed paper is placed on the copper board. Heat (usually from an iron or laminator) transfers the toner onto the PCB. The transferred toner acts as a protective layer for the required copper tracks.
Toner-transfer method
Before etching, the PCB already has a protective pattern (photo-resist or toner) on the copper surface. The protected areas represent the required tracks and pads. Now, you can immerse it into a chemical etching solution (such as ferric chloride or another etchant). The chemical reacts with the unprotected (exposed) copper and dissolves it and copper covered by toner or photo-resist remains protected. After all unwanted copper is removed, only the protected copper stays on the board. These remaining copper paths become the PCB tracks (circuit connections).
Etching board
Unwanted copper is removed
After all the unnecessary copper has been etched away, and you’ve removed the board from the etching bath and cleaned off any remaining etchant, your board is almost ready for use.
The last step is to drill the holes for any mounting points or through-hole components (resistors, capacitors, connectors, etc.). Mounting points used to fix the PCB to a case or structure. You can do this by hand, or, you can drill automatically using a CNC (Computer Numerical Control) drilling machine, if you have access. PCB design software generates drill data files (commonly Excellon format) containing hole locations, hole sizes, and hole types. You can export the drill file along with Gerber manufacturing files from your PCB design package to provide the drill locations for your mill and drills holes at the exact positions specified. Modern PCB fabrication generally uses automated CNC drilling machines.
MILLING BOARDS
PCB milling is an alternative process to etching a PCB. In addition to using a CNC machine to drill the holes in your PCB, you can also use it to remove copper around the tracks and create the circuit pattern, instead of using chemical etching. To do this, you need to export the copper layers from your PCB software as Gerber files, which are the industry-standard format used to describe copper layers, pads, solder masks, silkscreen, and board outlines. These were first defined by Gerber Systems Corp., hence the name, and are now the industry standard format used to describe PCBs in manufacture. Gerber files are the standard format used in PCB manufacturing and contain details about copper tracks, pads, and board layout.
To use a CNC mill for PCB manufacturing, the Gerber files must first be converted into G-code, which is the machine control language understood by CNC machines. Drill information is commonly exported separately as Excellon drill files. For CNC-based PCB prototyping, these files are converted into G-code, the machine control language used by CNC machines, by CAM (Computer-Aided Manufacturing) software such as FlatCAM, Autodesk Fusion, Vectric Cut2D. Some CNC milling systems include this conversion software as part of their package.
In standard industrial PCB manufacturing, the process usually does not involve converting Gerber files into G-code for a desktop CNC mill. Instead, manufacturers use dedicated PCB fabrication systems that directly read manufacturing files. Gerber + Excellon files processed by dedicated PCB fabrication equipment, rather than G-code-controlled PCB milling.
The CNC mill works by cutting a path around each track (trace) so that it becomes electrically separated from the surrounding copper. Because of this process, milled PCBs often look different from etched PCBs. Large copper areas that are not connected to anything are usually left on the board instead of being removed, because removing all of that copper would require more milling time.
PCB milling using CNC machine
ASSEMBLY
After your PCBs have been manufactured, you still need to get the components soldered onto them.
For assembling surface-mount PCBs, you need an additional file from the PCB design called the solder paste layer. This layer is used to create a stencil, which helps apply solder paste accurately onto the PCB pads. The stencil has openings only at the component pad locations. The stencil can be made by laser-cutting it from a thin Mylar plastic sheet or having it manufactured from thin steel. Obviously, the steel one will last much longer and let you solder up lots more boards before you need to replace it.
Apply solder paste on PCB
The solder for surface-mount work comes as a paste, supplied in containers or tubes. A squeegee is used to spread solder paste across the stencil surface. The paste passes through the stencil openings onto the PCB pads and then carefully lift the stencil off the board, leaving solder paste only on the required component pads.
Now comes the tricky bit. Using tweezers and ideally a loupe or magnifying glass, place each component onto the relevant spot on the PCB. but you should avoid moving or shaking the board because at this point in case some of the parts get displaced.
Hand soldering
After placing all the components on the PCB, you need to melt the solder paste so that the components become permanently attached to the board. This can be done with a soldering iron, but for surface-mount assembly, the process is usually easier with a hot-air rework station. A hot-air rework station is similar to a combination of a soldering iron and a small heat gun because it uses heated air to provide the temperature needed to melt the solder.
Placing through-hole components on PCB with a soldering iron
Placing surface-mount components on PCB with a hot air station
You can solder all the connections at once if you use a reflow oven. A reflow oven works by heating the entire PCB and its components evenly until the solder paste melts and creates the electrical connections. Professional reflow ovens also allow you to set different temperature profiles, which helps match the heating requirements specified in component manufacturers’ datasheets, especially for sensitive or complex components.
Reflow oven
Source: Designing the Internet of Things by Adrian McEwen & Hakim Cassimally (Wiley)
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