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Electronic PCB Assembly is the Process of Populating or Stuffing Printed Circuit Boards with Electronic Surface Mount and/or Through-hole Components to form a Functional Printed Circuit Board Assembly (PCBA). The PCB Main Function is to Mechanically Support and Electrically Connect the Electronic Components.

Printed circuit boards (PCBs) are an essential element of every electronic appliance; their reliability and durability are of supreme importance. Accordingly, the PCB material, circuit design, components, everything is carefully selected and used but printed circuit board assembling (PCBA) is also a vital factor that directly impacts the functioning and life of a PCB. A poor-quality assembling can cause malfunctioning even if a PCB is developed with top-quality components and material. Evidently, PCBs must be assembled as per the standard procedures.

After a PCB is manufactured, the components are assembled on it to form a circuit. As long as the components are not assembled on a PCB, the bare board does not provide the required functionality. There are two major techniques of PCB assembling: through-hole assembling and surface mount technology. Both techniques involve component placement and soldering on board.

PCB Assembly 1

                                                                                                                                     PCB Assembly Process

Printed Circuit Board Basics

To understand how a PCB is assembled and the steps involved in PCB assembling, you must overview the layers that constitute a PCB. A circuit board is made up of primarily two types of layers: a substrate and a conductive layer. But some other layers are also imprinted on a PCB for enhancing its protection and reliability.

Copper Layer

The copper layer is also called the conductive layer; it is carved to form circuit tracks that eventually connect electronic components assembled on the board.

Substrate Layer

It is an insulator and the foundation of the circuit board. It forms insulation between traces to avoid the unwanted flow of signals. The substrate used In rigid PCBs forms insulation, but it also forms a robust base to carry heavy circuits. Similarly, the substrate in flexible PCBs gives them their flexibility.  

Soldermask Layer

It is produced over the conductive copper layer for protection. It protects the copper layer against corrosion, and accidental short circuits (if any other conductive device accidentally comes in contact). The solder mask layer is usually green in colour.

Silkscreen Layer

After the circuit tracks have been created, the symbols, values and essential details are labelled on the PCB with a white silkscreen layer. It provides the details about each placement, where and which component or what value has to be mounted.

Now that you have learned about PCB basics, you should learn about pre-assembling procedures that are essential for a reliable PCB assembling.

PCB Pre-assembling Processes

Commercially PCBs are not assembled right after assembling; they must go through the pre-assembly process. After a PCB is manufactured, the assembly company or department runs design for manufacturability (DFM) check. It is so that the design of the PCB can be analyzed and cross-checked with the design files to ensure it is correct. DFM check is run primarily to spot missing details, unwanted connections and other technical flaws that can affect the circuit’s reliability and durability.

The DFM test is essential because some PCB designs require special care during assembling. For example, the high-frequency circuits are prone to malfunctioning even if the track’s width varies a bit or the track’s shape is incorrect or even if the space between tracks is too narrow; it affects the impedance of circuits and alters the functionality. Similarly, some electronic components are susceptible to electrostatic discharge (ESD) which means that they cannot be assembled using ordinary assembling equipment.

The purpose of the DFM check is to ensure that the PCB is ready for assembly and save time and cost. Apparently, if a PCB is flawed and still assembled, the PCB won’t function as wanted, and in such cases, the design has to be improved, and the PCB has to be developed again; leading to cost and turnaround time escalation. But with a simple DFM check, the possibility of developing a faulty PCB is eliminated. To further learn about the possible problems and to validate the design, the assembler study’s the design files, data sheets App notes, and design requirements (if there are any).

Printed Circuit Board Assembly Types

Simple PCBs are assembled manually, but on a commercial scale, especially complex PCBs, the assembling process is carried out on an automated printed circuit board assembling machine. There are two main types of PCBA; the primary difference between both types comes from the different types of electronic components used in the circuit.

The two main types of electronic components are surface-mount devices and through-hole devices. The major difference between these types is their physical architecture; through-hole devices are larger and have longer pins. The detailed differences are as follows.

Surface Mount Devices

Surface mount devices are smaller than their respective through-hole packages, which reduces board size and requires extreme care. A 01005 size SMD is barely 0.4×0.2mm in size whereas a human hair is 0.1mm thick.

PCB Assembly 2

                                                                                                                          Tiny Surface-mount Devices

SMDs are soldered directly on the top of the printed circuit board but given their small size; they are soldered using special equipment; regular solders can burn SMDs.

Through Hole Devices

Through Hole Devices are comparatively larger and cheaper, they are suitable for circuits that do not need to be compact. Moreover, through hole components are commonly available, whereas the SMD package of many components does not exist.  The through hole components are places on one side of the board, and their pins are soldered on the other side. As the pins have to be passed to the other side, holes are drilled in the board.

PCB Assembly 3

                                                                                                                          Through-hole Components

Now since, both SMD and THD have certain differences in their physical architecture, their assembling processes also differ.

Surface Mount Technology Printed Circuit Board Assembly Process

Step 1: Apply Solder Paste to the Board

Surface mount devices are soldered on the top of the board, and their pins are small, which is why their soldering demands extensive care. Unlike through hole components’ assembling, surface mount devices are placed on the board after applying solder.

The solder applied on board is in the form of paste; that is why it is called solder paste. To ensure the solder paste is applied only to the specific pads where component pins have to be soldered, a solder stencil is placed on the board before the solder paste is applied. As the solder stencil is designed according to the circuit layout; there are cut through holes in places that have to be soldered. These holes enable the solder paste to apply on board; specifically in regions that have to be soldered.

Like solder wire, the solder paste is a composition of metals, and it is in the form of tiny balls. Solder paste contains 96.5% tin, 3% silver, a very small fraction of copper and flux. The flux enables the solder paste eases the process of solder paste melting and bonding to the board. When applying solder paste, it is important to the precise quantity and only in regions that have to be soldered; improper quantities can cause solder spreading and formation of unwanted connections.

As the solder stencil and the board are not fixed, even a slight push can displace solder stencil and cause solder paste spreading on unwanted regions. To ensure the stencil does not move, a mechanical fixture is used for firmly holding the board and stencil together. Then an automated machine spreads the solder paste through the stencil in the right quantity.

Step 2: Component Placement

Once the solder paste has been applied to the board, the board is ready for component soldering. As already explained, surface mount devices are sensitive, and even an ESD from human hands can damage them, and SMDs can be tinnier than a pencil tip. This is why on commercial-scale the pick and place process of SMDs is also automated; a robotic machine picks the components and places on their right position on the board. The machine is preprogrammed; the program instructs the machine about the positions where the components have to be placed. It uses a vacuum grip to pick and place components.

Conventionally, this step was performed manually with tweezers’ help; the assembler picks the components with a tweezer and places on the board. But since the SMDs are too small and precise placement is difficult with hands, the manual placement was not entirely reliable. Whereas with automated machines, the pick and place process is more effective.

Step 3: Soldering

The solder paste and components have been placed on the board by this step, but the solder paste is still unsoldered. To permanently bond the components to the board, the solder paste needs to be solidified. The process of solidifying solder paste and soldering the components to the board is called reflow soldering.

PCB Assembly 4

                                                                                                                          Reflow Soldering Process

To solder the components, the board is passed into a reflow through a conveyor belt. The oven is equipped with heaters and coolers. The PCB first passes through the heating section where the temperature ranges around 250 degree Celsius which is high enough to melt the solder paste and cover the solder pads and component pins. As the PCB moves through the oven, it is passed through a series of coolers which solidify the molten solder paste and form a joint. Consequently, the components are permanently soldered to the PCB.

Reflow soldering for single-layer PCBs is simpler and easier as compared to double-layer PCBs. In the case of double-layer PCBs, each side has to be soldered separately. The soldering begins with the side that carries fewer components.

Step 4: Assembly Inspection

Technically the assembly process completes by the end of reflow soldering but to ensure the PCB functions correctly, it is inspected for potential flaws. This is essential because in some cases, the solder does not properly melt and connect the pin with the pad, or the unintentional misplacement of an item causes improper solder. Moreover, in some cases, unwanted connections are established that cause malfunctioning. This is why the assembling process does not end with reflow soldering, and the PCB is thoroughly checked for improper connections.

The inspection is performed using any of the following methods (depending on the design).

Manual Inspection

Even though the machines’ inspection is reliable, the manual inspection still remains the most commonly practised method. This is because human vision is more reliable in comparison to machines. Manual inspections are suitable for small circuits, but it is impractical for large and complex circuits.

Automatic Optical Inspection

Unlike manual inspection, the automatic optical inspection (AOI) is performed by a machine. The machine directs light on each solder from multiple directions and analyzes the solder through cameras. AOI is a suitable and effective method if the circuit design is complex or a large number of PCBs need to be inspected.

X-ray Inspection

This method is used only for inspecting solders that cannot be visualized by human eye or camera. The –ray inspection can detect hidden solder flaws as the x-ray can pass through layers.

Step 5: Assembly Functionality Testing

After a PCB successfully passes the inspection phase, it goes through the functionality testing stage. This is to ensure that the PCB performs the required functions. Certain functionality tests are performed on the PCB to test different aspects of functioning.

The functionality test is performed by powering the PCB and matching the outcomes with the same circuit’s simulation. If the PCB’s output matches the simulation, it passes the test; otherwise, it fails.

PCB Assembly 5

                                                                                                                                PCB Testing Process

This stage helps analyze whether the PCB has been assembled correctly or not. In case a PCB fails functionality test, the problems are highlighted and corrected in the later boards; leading to cost reduction and time-saving. If a PCB assembly fails inspection or testing phase, the assembly team reworks on it or scrap it.

Through Hole Technology Printed Circuit Board Assembly Process

The assembly of through hole components is slightly different from the assembly of surface mount components but some steps are common. As already explained, the through hole components are placed on one side of the board and soldered on the other side; they are soldered using solder wire instead of solder paste. This is because the solder paste (in the molten form) can run through the holes to the other side of the board. Follows are the assembly steps of through hole devices.

Step 1: Drilling

As the pins of the components have to be passed to the other side, holes are drilled inside the PCB. Traditionally, hand drills are used for drilling purpose, but commercially automated drilling machines perform the task. Moreover, the automated machines can effectively drill dozens of PCBs within an hour, whereas the manual drilling process is slow and less reliable.

Step 2: Component Placement

Like SMD assembly, the components are placed on the board with their pins passing through the holes. The component placement is can be either manual or automatic.

Step 3: Soldering

The soldering of through hole components can be either manual or automatic. Their difference is as follows.

Manual Soldering

The manual soldering of through hole components is simple and easy for small-scale PCBs; an assembler can single-handedly solder a complete board, but the process can be hectic if the design contains several components.

Usually, a mechanical fixture is used for holding the PCB from the edges as the soldering process involves the use of both hands.

Automatic Soldering

Automatic soldering, also known as wave soldering is somewhat like SMD soldering in an oven; it automatic soldering involves passing the PCB through an oven for soldering. But in this case, after putting the components on board, the board is passed into the oven through a conveyor belt and the pins are washed with molten solder and then cooled. But automatic soldering has some limitations; it is suitable only for single-sided PCBs.

Step 4: Through Hole PCB Inspection and Functionality Testing

Alike SMD assembly, the through hole PCB is also inspected to find potential assembly flaws and later tested to check its functionality.

Evidently, PCBA is an extensively technical process and requires extra-ordinary care.

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