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What is PCB

In our day to day life we are encountered with many electronic products. From smart phones to home entertainment systems to appliances to our cars, everything needs electronics in its core & the core of the electronics is the printed circuit board, also known as a PCB.

Printed circuit boards are the small green or blue chips covered in lines which work as the main connector between the components mounted on it. The main ingredients are fiberglass, copper lines and other metal parts. PCBs are insulated with a solder mask. This solder mask is actually responsible that characteristic green color. Though in many instances now, we are seeing it blue too but irrespective of the color the overall functionality of the PCB remains same.  

What is PCBA (PCB Assembly)

A circuit board prior to assembly of electronic components is known as PCB.

A PCB with components mounted on it is called an assembled or populated PCB and the process is called PCB assembly or PCBA Process. You must have seen fine copper lines on bare boards, these are called traces. These traces electrically link connectors and components to each other which allow the circuit board to function in a specific way.

One should not get confused that PCB assembly is different from PCB manufacturing

. PCB Manufacturing involves PCB designing and creating PCB prototype. However PCB assembly starts only after PCB is ready. The assembly process further depends on type of PCB & Components.

The PCB assembly process consists of several automated and manual steps. These processes must be integrated and compatible to each other in order to maintain the highest quality output. 

Let’s understand the PCB & its designing first before moving further. 

PCB Designing:

There are several layers in a PCB Base, all are designated to perform a significant role in the functioning of it.

It all starts with a BASE which comprises several layers, designed and developed to play a significant role in the functioning of a PCB. These layers include:
• Substrate: This is the base material of a PCB. Main work of it is to provide the rigidity to the PCB sheet.
• Copper:  Next layer is of copper. The copper layer determines the nature of the PCB i.e if the copper layer is present on one side, the PCB is known as single side PCB & if the copper layer is present on both sides then it is known as double side PCB.

• Solder mask: Above the copper layer is solder mask, which is responsible for PCBs characteristic green/blue color. The main work of solder mask is to insulate copper traces from contacting other conductive materials, which in return protect it from short circuit. One can also see many holes in the solder mask, These holes are to fix components on the board with solder. The work of solder is to keep every component in place on the PCB board. Masking is a very important process in PCB manufacturing & assembly as it insulates the copper traces one side & on the other, it precisely defines the places where soldering needs to be done thus keeps it error free.

• Silkscreen: One must have seen different symbols and characters printed on the top of PCBs.   A white/black silkscreen is identified with the characters, numbers, symbols or specs mentioned on the top of PCB board. This is the final layer on a PCB board. This is basically to indicate role of components on the board.

There are two types of mounting technologies prevailing in the modern PCBA industry:

Surface Mount Technology & Thru-Hole Technology which will be described briefly in the article as the PCBA process progresses. Due to distinctions between THT and SMT, the assembly processes are different as well.  

PCB Assembly Process:

PCB Assembly process includes several steps such as solder paste stenciling, pick & place, Soldering, Inspection & Quality Control, Through hole insertion and functional testing etc. Each step is equally important and monitored with precision in order to get highest degree of output.               

Step 1: Solder Paste Stenciling

Here in this step, a solder paste is applied on the bare PCB board. The solder paste consists of tiny metal balls consisting metal, silver and copper in different proportions. This is mixed with flux which allows solder to melt and bond with the PCB surface. Basically the work of solder paste is to prepare PCB for adding components on the board. A solder paste is a grayish substance which is spread equally on the bare PCB board, on the areas where solder needs to be done or components need to be placed or added. These areas are called component pads.

solder pasting is done using a solder screen that is correctly placed directly on to the bare PCB board. Then a runner is moved across the screen pouring solder paste from the holes of the solder screen on to the board. Holes in the solder screen are positioned right on the solder pads on the board so that the solder paste gets poured exactly on the solder pads where components need to be added.  

Step 2: Pick and Place 

In this step of the assembly process, the solder paste added PCB then proceeds for pick and place process. This process starts with a robotic machine loaded with reels of components picks the components from the reels and other dispensers and places them on to the component pads where solder pasting was done in the previous step. Solder paste helps components to stick on the board at the desired position.

Surface mount technology is for small size components and integrated circuits (ICs). Very small & sensitive components, such as ICs, resistors or diodes are placed automatically onto the surface of board. This is also known as SMD assembly.

Pick and place process is all preprogrammed and the machine programming is done as per the layout and design of actual printed circuit board. This is how the whole process seems so simplified and precise.

This process is generally different if in case the components need to be wave soldered, here the machine adds glue dots to keep the components in place on the board. The glue attached with the board may make any repair difficult however some glues are available now which degrades during the next stage of soldering thus making the PCB more convenient to use. 

Step 3: Reflow Soldering

After pick & place process of PCB assembly, The next process is reflow soldering. Reflow soldering is done to firmly joint SMDs to the PCB board. The surface mounted PCB is now moved to a large reflow oven which has a series of heaters and coolers inside to maintain the desired temperature. The heaters gradually take the temperature of the board to 250 degree Celsius. At this temperature the solder present in the solder paste on the boards melts down, this allows SMD components to sit down properly.

During this whole process, the PCB continues to pass through the oven and then it is encountered with series of coolers , which solidify the solder and thus a firm joint is established between the SMD components and the PCB board.

Reflow soldering process is different in case of double side PCBs. Stenciling, Components placing and Reflowing are done separately in case of double side PCBs. The side with fewer and smaller parts is stenciled, placed and reflowed first, followed by the other side. 

Step 4: Inspection & Control

Reflow process ensures soldering of components on the components pads. Now the next step of PCBA is inspection & quality control. Here the surface mounted PCB is properly inspected to ensure that the product is free from all the errors such as components misplacing, poor connections, short circuit or connection missing as these are common errors, occur during subsequent processes. These inspections are done in order to produce highest quality of product.

The inspection is done through following ways:
• Manual Checks: Manual check is not an option for SMT mounted PCBs as these are loaded with hundreds or more components. This method becomes unrealistic and inaccurate as the number and the complexity of the boards increases.

• Automatic Optical Inspection: This is the most common inspection process used now a days. Considering the components on the boards and complexities of the PCBs, Automatic optical inspection is a more viable solution to ensure the quality of the PCBs. PCBs are inspected with high powered cameras installed in an automatic optical inspection machine. Automatic optical inspection machine is also known as an AOI machine. The AOI machine is capable of recognizing poor joints, connections & misplaced components more precisely and that too in a very short time & thus provides error free PCBs as a result.

• X-ray Inspection: X-ray inspection process is used in case of complex or layered PCBs. it is not very common. With the help of X-ray, one can see through different layers of a multilayered PCB and thus does quality control.  .

Step 5: Through-Hole Component Insertion 

The next process of PCBA is through-hole components insertion. In this step, all the components which have leads/legs or wires are mounted manually by plugging them through the holes present on the board. This technology is for large components such as capacitors, coils etc to be assembled. Mounting of these components are not possible with SMT hence manual insertion is required. These components are known as PTH i.e plated through-hole components.

The process starts with PCB going through different hands for manual insertion of components. Every person in the line is given a specific component. One person inserts a specific component on the board and passes it to other for another component insertion and the process goes on continuously till the last component is added to the board. 

Soldering in case of PTH components is bit different as it requires a more specialized kind of soldering. The extra lead part or wire has to be soldered on the other side of the board.

Manual soldering is outdated now and not used instead, wave soldering is most common practice now days.

Wave Soldering: After Through-Hole component insertion, the PCB is moved for wave soldering on a conveyer belt. The conveyer belt is run through an oven with ‘solder melt’ inside it. As the PCB moves on the belt inside the oven, the ‘solder melt’ just below the PCB board solders all the pins of the through hole components at once.  Wave soldering is impractical for double side PCBs as soldering the entire PCB side may defect the PCB and components both. Once the soldering process is finished, PCB moves on to the final inspection & functional testing. 

Step 6: Final Inspection and Functional Test

This is the final step of the PCB assembly. Final inspection is to check the functionality of the PCB. This is known as "functional test" as well. This step is very important as it determines the functionality of the PCB and if the PCB fails to perform as desired during the test, it is then recycled or scrapped. Once final testing is done, PCBs are now ready to be used.  

The PCB assembly process is very important and should be well integrated with the other production processes in order to produce the highest quality of product. Considering the technicalities and complexities in products that are being manufactured today & also the expectations on quality, this whole process is very critical to the success of any project.