Printed Circuit Board (PCB)
A printed circuit board (PCB) is the basic structure of almost all electronic products. Various components such as microprocessors, other integrated circuits, resistors, capacitors, and other standard or specialized components are attached to the PCB. The purpose of the PCB is to serve as a connection and wiring platform for different components and modules. The PCB conducts signals and operating voltage to and from the components. Additionally, the PCB dissipates heat, acts as a mechanical support structure, and protects against electromagnetic interference.

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PCB Laminates
The board used for manufacturing PCBs is called a laminate. There are many different types of laminates depending on the application, and they come in different thicknesses and with various copper strengths. For more specialized applications, metal-based boards, flexible polyimide (flex), or a combination of rigid FR4 and flexible (flex-rigid) materials might be used.

The most common laminate types include FR4, CEM, and FR2.

The most common laminate thicknesses for standard FR4 are, for example, 0.5, 0.8, 1.0, 1.2, 1.6, 2.0, 2.4, and 3.2 mm. These apply to 1- and 2-layer boards.

Multilayer boards can be constructed using different "prepregs" so that the final thickness can deviate from the aforementioned. For example, a 1.5 mm or even a 2.1 mm thick multilayer board is possible, depending on the parts (core + prepreg + copper) used in its construction.

Most laminates are available with different initial copper thicknesses. The most common are 18 µm, 35 µm, 70 µm, 105 µm, and 140 µm. Although other options exist, PCB manufacturers typically use the most common variations to minimize the number of laminate variations. If every laminate thickness and copper option were available, laminate storage would grow to enormous proportions.

Laminate manufacturers produce large sheets approximately 2 x 2 meters in size. PCB factories cut these into smaller, production-suitable blanks. Typically, the factory places multiple customer panels into its production blank, aiming for the smallest possible waste percentage.

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PCB Finishes
The most common finishes are HAL, LF HAL, chemical tin, and chemical gold. In this context, the final finish refers to the coating of copper areas not covered by solder mask. If solderable surfaces were left as bare copper, they would quickly oxidize upon contact with air, rendering them unsolderable.

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HAL (Hot Air Leveling)
HAL stands for Hot Air Leveling. In this process, an almost complete PCB is dipped into flux and then quickly immersed in molten tin at 250-270°C. It is immediately lifted and excess tin is blown off with hot compressed air. The smoother the surface, the better. If too much tin remains, it may hinder the application of solder paste and component placement during assembly. The commonly used tin alloy is 60% tin and 40% lead. The RoHS directive, which took effect in 07/2006, practically bans this alloy in most products. The shelf life of solderability is fairly good, and boards can remain usable for a year in dry conditions.

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LF HAL (Lead-Free HAL)
The process is exactly the same as mentioned above, but the tin alloy is lead-free. A commonly used alloy is Sn100C, which contains 99.3% tin, with the remainder being copper and nickel. It meets the RoHS directive requirements. The shelf life is approximately the same as leaded HAL. It increases the cost slightly.

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Chemical Tin (Immersion Tin)
Chemical tin is a uniform, slightly matte tin surface created on the board's surface through various chemical baths. It meets RoHS requirements. The shelf life (solderability) is not very long and should not be stored for months before use. It is more expensive than LF HAL but cheaper than chemical gold.

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Chemical Gold (Immersion Gold)
Also known as ENIG (Electroless Nickel Immersion Gold) or AuNi.

This is a chemically produced surface consisting of a nickel layer grown on copper, followed by a gold layer. The process is old, proven, and widely used. However, it requires good expertise from the manufacturer because if the process is not perfectly controlled, soldering problems can arise. A smooth surface is a good feature that is beneficial in the soldering stage, especially with components requiring smoothness. The shelf life under normal conditions and proper handling is good. However, the gold can tarnish over time if the soldering surfaces are handled with greasy fingers. It is the most expensive option in terms of cost. It is suitable for product batches where surface smoothness is important, and where a long shelf life of solderability is also a plus.

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Chemical Silver
Chemical silver is a silver surface created through specific chemical baths.

The advantage is a smooth surface, but the disadvantage is the short solderability shelf life. It is approximately on par with chemical gold in terms of cost.

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OSP (Organic Solderability Preservative)
This is an organically produced, oxidation-preventive surface on the copper. It has been used for years and was common in consumer electronics for 1- and 2-layer boards. The advantage is a smooth surface. In wave soldering, unsoldered pads stand out well in inspection because the unsoldered surface is copper-colored. The disadvantages are weak solderability shelf life. If a board with mixed technology (reflow on the top side, wave soldering on the bottom) is used, the protective effect of OSP significantly deteriorates after the first heating cycle (reflow stage) on the unsoldered side. It may require more aggressive fluxes to ensure solderability. It is cost-effective.

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Glossary, Measurements, and Terms
- 1 µm (micrometer) is one-millionth of a meter or 0.000001 m, which equals 0.001 millimeters.
- 1 mil is one-thousandth of an inch. US-based programs use mils as a unit.
- ½ oz is 18 µm. 1 oz is 35 µm. 2 oz is 70 µm. 3 oz is 105 µm.

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Fiducial Marks
Fiducial marks are typically small copper dots at the corners of the panel, often on waste edges. They are used in the placement of SMD (Surface-Mount Device) components. The placement machine first checks these fiducials with a camera, enabling much more precise component placement. Without these marks, placement accuracy suffers. The size and placement of the marks vary, but a practical size is a 1 mm round copper dot at three corners of the panel, with the mask opened (i.e., green mask is not printed over the dot). The marks are placed at three corners so that if the panel is fed into the placement machine in the wrong orientation (180 degrees off), the machine stops because it does not find the fiducials where it expects to. Placement should not be right at the edge, as conveyors and guides in the placement machine require some space. If the fiducial is under a guide, the machine will not recognize it.

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Stencil
A stencil is a thin metal tool used in component assembly to apply solder paste to the copper dots on a PCB. Practically, it is a thin 0.1-0.25 mm thick metal sheet with holes etched or laser-cut at all the spots where paste is needed on the board. The stencil is precisely aligned over the PCB so that the holes match the intended spots, and then a squeegee is used to spread the paste over the board. When the stencil is lifted off, low "cakes" of paste remain at the hole locations. In the next stage, the placement machine places the components on top, where they stay loosely attached because the sticky paste holds them in place. After placement, the board goes into an oven, which melts the paste, soldering the components permanently to the PCB. When ordering a stencil, it is extremely important that the stencil file is paneled exactly the same as the PCB. If there is any discrepancy, neither the paste application nor the placement will be successful.

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EMS (Contract Manufacturing)
Electronics Manufacturing Services. The outsourcing of electronics manufacturing and assembly by original equipment manufacturers (OEMs) to companies specializing in these services.

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HDI Technology
High Density Interconnection (HDI) technology. A PCB with small holes, known as microvias (< 0.15 mm), as well as thin traces and insulating spaces (< 0.1 mm).

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Microvia Technology
A microvia is a small hole (< 0.15 mm) through a thin insulation layer (< 0.1 mm). After metallization, they connect the conductor layers to each other. Microvias are usually found in the outermost layers of a PCB in one or more layers. There are several methods for manufacturing microvias, including laser drilling, plasma etching, and photomechanical processing.

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Multilayer PCB
A PCB consisting of multiple conductor layers separated by insulating support structures.

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OEM
An Original Equipment Manufacturer (OEM) sells products to resellers and/or end customers. The term also refers to the reseller. OEM product buyers may add value by further developing the product before reselling it, branding it with their own name, and/or incorporating it into their own products.

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Thick-Film Hybrid
An electronic component where the conductive circuits are printed onto a ceramic substrate using conductive paste. Hybrids are used in industries such as pharmaceuticals, telecommunications, and automotive.

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Metal-CORE PCBs
A PCB can also be made on a metal base. This special solution can be used for example in applications that generate a lot of heat. The metal base (often aluminium) acts as a heat spread.

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