Keywords: Printed Circuits Assembly

After a PCB has been created, mounting electrical components is the following stage in a typical PCB assembly procedure. For the finished product to perform as intended, a number of steps in the PCB assembly process must be carried out in the right order. The last stage of PCB production is PCB assembly, which results in a working Printed Circuits Assembly that is incorporated into a new product.

Printed circuit boards (PCBs) are the basic components of modern electronic gadgets. The PCB assembly process is where the genuine quality of a product will be established and proved, supposing the PCB creation process was carried out appropriately. PCB design revolves around inserting a bare board into any size of the standard assembly process, and professional designers make it their duty to comprehend the standard processing stages and restrictions.

Component Mounting Technologies

There are three possible processes in the PCB assembly process that include mounting and soldering components:

• Surface-mount technology
• Through-hole technology
• Hybrid assembly

The majority of contemporary circuit boards built for commercial items and cutting-edge technologies will have both surface-mount and through-hole components (hybrid assembly). The majority of designs will employ surface-mount components since they are often smaller; nevertheless, through-hole components are still used for bigger components like connectors or mechanical elements.

PCB assembly through-hole

Through-hole technology is not typically used for the main components in modern electronics. However, a lot of non-essential parts for digital or analogue processing are inserted as through-hole parts. The component leads are placed into the PCB's pre-drilled holes during this assembling phase. The leads are then soldered to the circuit board's other side. Transformers, pin headers, and bigger power transistors are a few examples of through-hole components that are frequently the bulkiest on a PCB.

Surface-mount devices are the tiniest components utilised in contemporary microelectronics. These parts don't need mounting holes since they are mounted directly by soldering them to the PCB. Be aware that certain connections will contain SMD pads for surface-mounted connectors in addition to drilled hole leads for mechanical stability. These components don't require drilled holes, which might lower the cost of PCB manufacture in simpler boards and speed up production. When compared to a traditional through-hole placement technique, component placement can be done approximately 10 times faster, and these components can have far greater component densities.

PCB assembly SMD

Surface-mount and through-hole components combined on one PCBA are a typical sight. The assembler has to know if the circuit board will employ both through-hole and surface mount components when sending it to a manufacturer for a quote. Because the two types of components demand various soldering techniques, the problem is that combining components might add extra soldering cycles.

Stenciling

PCB assembly is a step-by-step procedure that is guided. The procedure begins with stenciling, which involves applying solder paste to the areas of the board where components will be attached. Microscopic metal balls and a chemical called flux are both components of solder paste. Using a tiny stainless steel stencil, solder paste is applied to the bare PCB in a process similar to screen printing. While solder paste is applied to the stencil, the PCB and stencil are held in position by a mechanical device.

Automated Placement

A pick-and-place device automates component placement. With through-hole technology, some components that might be too big for some pick-and-place machines can be manually inserted, while at high volume the entire operation is usually automated. Precise numerical data obtained from the ECAD data for a design is needed for quick, accurate, error-free placement with automated component mounting. The pick-and-place file, which is produced by CAD software and used to programme a pick-and-place machine, comprises the X and Y coordinates of each component.

Soldering

There are three different soldering techniques that may be used to secure components on the PCB: wave soldering, reflow soldering, and selective soldering. Only wave soldering and reflow soldering are necessary for the majority of boards. In high-volume assembly procedures, hand soldering is often not done unless rework is being done.

Although this will often be utilised for through-hole soldering, wave soldering may be employed in both surface mount and through-hole assembly procedures. A hot wave of liquid solder is dragged over the PCB once it has been positioned into a solder line with all of its installed components. The metal components of the board that are not protected by solder mask have a solder bond to them. Component pins are firmly fastened to the PCB when the solder cools. This method is substantially more effective and can result in higher-quality assemblies when compared to manual soldering.

The typical method used for surface-mount assembly is reflow soldering. Boards with two sides will undergo two reflow soldering cycles. This results in a total of at least three solder cycles (1 wave, 2 reflow, and rework cycles) being needed to completely build the PCB when utilised with a hybrid assembly technique.

The board is then put into a reflow oven with a temperature setting of about 250 degrees Celsius to complete the reflow soldering. The exposed component pins and pads on the PCB are heated by the oven, which also causes the solder paste to solidify into a metal joint.

PCB reflow soldering

The constructed PCB has to be carefully cleaned once assembly and soldering is finished. Deionized water or other cleaning chemicals may be used in this process to remove any residues from the PCB's surface, particularly flux residues. PCBs are dried using compressed air after being cleaned, and assembled boards are then submitted for inspection.

Inspection and Testing

The constructed PCB will undergo certain automated testing to ensure that it is in good functioning condition. Additional quality checks will be necessary for high-volume manufacturing to make sure the system as a whole is generating faults at manageable rates. After inspection, certain boards could undergo in-circuit testing to further qualify them. This entails powering on the board and probing particular locations on the PCBA.

Automated optical inspection carries out visual examination (AOI). To visually analyze the PCBA's surface layers, this vision system employs high-resolution cameras that may be mounted at different angles. This inspection stage's objective is to find soldering and component positioning errors in the assembly. These include any lifting of components during cooling; solder connection quality, and component alignment. An AOI system can efficiently handle big quantities of PCBs.

Depending on the demands of the finished product and the level of dependability required, several testing procedures might be used for a finished Printed Circuits Assembly. In addition to in-circuit testing, PCBA qualification procedures may also involve mechanical testing, accelerated life testing, and stress tests including thermal and mechanical shocks. To ascertain the long-term dependability of the finished product, these tests go beyond the simple functioning checks and fault identification.