PCB assembly is a final step in taking a newly manufactured circuit board and making it functional. This process is about more than just placing components — it’s also about quality assurance and meeting design specifications.
The first step in the PCB assembly process is to apply solder paste. A stencil is used to ensure that the paste lands on only the areas that require it.
Substrate
Printed circuit board (PCB) is the core of electronic devices. The conductive pathways engraved in the laminated copper sheets of PCBs help to dissipate heat and connect all the electronic components and accessories within a device. When you purchase a product that has a PCB attached to it, it’s called a PCBA or PWB.
The PCB substrate is made from fiberglass, composite epoxy or other insulating material with conducting material on one or both sides. These layers are etched or plated with copper and coated with tin-lead to prevent corrosion. The circuit patterns etched or plated onto the copper circuits are insulated with solder mask and held together with an epoxy resin.
In PCB assembly, the copper traces and pads are used to carry electronic signals between different parts of the circuit. These signals can be high-speed digital or discrete analog. The traces can be thin or thick printed circuit board assembly depending on the circuit design. The PCB can be assembled with either SMD or thru-hole components. SMD components are small and have short pins that are placed on the PCB surface using a SMT mounting technology. The pins are then soldered into place using a reflow soldering process.
Both SMD and thru-hole technologies can be automated. However, SMT is the most popular technique because it allows for a higher production capacity and is more cost-effective than through-hole techniques. A good PCB manufacturer should have expertise in the various soldering methods for each project and the proper equipment to execute those techniques.
Circuit Pattern
Historically, a PCB consisted of wires connected point-to-point to a chassis—typically a sheet metal frame or pan. This assembly method was time-consuming, cumbersome, and expensive.
PCB patterns are essential for establishing reliable connectivity, maintaining signal integrity, managing electromagnetic interference (EMI), distributing power, and placing components optimally. Strategic component placement is akin to choreographing a precise dance for electronic elements. This intricate process minimizes alignment lengths, reduces signal propagation delays, and augments overall circuit functionality.
For boards with more than two layers, a pattern is used to mark the locations where components will be placed in the finished product. A stainless-steel stencil affixed to the surface of the PCB is coated with solder paste, which is applied only to the locations where components will be soldered. This allows assemblers to quickly and accurately place component leads in their proper positions.
A legend, or silkscreen, is also 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. The legend is typically printed using a liquid photo imaging technique that provides superior accuracy compared to screen printing epoxy ink.
Maintaining high-quality standards is a top priority for manufacturers engaged in the business of printed circuit board assembly. This ethos translates into heightened operational efficiencies, product reliability, and environmental sustainability.
Contact Fingers
The tiny gold-plated connectors on the edges of a PCB printed circuit board custom are commonly called “fingers”. These contact points make electrical connections with the edge sockets of add-on cards. Their superior strength and conductivity ensure reliable connectivity and can withstand thousands of insertion and ejection cycles. They can be found in computers, graphics cards and other products that require an external power source.
They are designed as protruding tabs, strips or leads on the edge of the board and align precisely with the corresponding contacts in the socket. Achieving the right spacing, plating thickness, manufacturing specifications, inspection methods and maintenance procedures can prevent many of the problems associated with these small, but crucial contacts.
Ideally, a distance of 0.5 mm must be maintained between the gold fingers and the circuit board outline. Solder masks and screen printings must also be placed far away from the finger surfaces. A tape test should also be performed to verify the plating quality of these edge connectors.
Gold is a popular material for the connecting fingers because it has excellent corrosion and abrasion resistance. This prevents the fingers from being tangled and worn out, while its low contact resistance allows for good signal transmission. Additionally, the high density of the gold surface helps maintain a stable contact force and minimizes deflection. This is especially important in a scenario where the fingers are exposed to frequent insertion and ejection.
Components
A bare printed circuit board (PCB) is a substrate that serves as a platform for mounting and connecting electronic components. It features a design with conductive copper planes and lines (similar to wires on a flat surface) that are etched from sheet layers of copper laminated onto or between insulating sheets. It then facilitates electrical connections through the traces with soldering, which both electrically and mechanically fastens the component leads to the copper PCB traces. The resulting finished product is called a functional printed circuit assembly (PCA).
In order to maintain the highest standards of quality during PCB assembly, manufacturers must adhere to rigorous quality control procedures. This includes meticulous visual inspections as well as thorough testing procedures to identify any errors and irregularities before the finished board is integrated into electronic devices or systems.
The first step in PCB assembly is to ensure that all required tools and equipment are available and ready for use. This is particularly important for manual processes that involve human hands, which can introduce variability and inconsistencies into the final assembly. For example, hand soldering may result in sloppy joints that are difficult to inspect and repair. To overcome this issue, automated soldering technologies have become increasingly prevalent in the industry, enabling manufacturers to produce high-quality PCBs with increased efficiency and accuracy.