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Printed Circuit Board Assembly | SMT and THT Components

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Wed, Nov 13, 2019


Printed circuit board assembly or PCBA refers to the process of soldering or connecting electronic components to printed circuit boards. When the electronic components are not connected to the circuit board then it is known only as a printed circuit board. Once the electronic components are assembled and soldered on the circuit board, it is known as the Printed Circuit Board Assembly. Terms like Printed Circuit Assembly and PCB Assembly are also used to describe printed circuit board assembly.

Is Printed Circuit Board Assembly Same As Printed Circuit Board Manufacturing

It should be noted that when we talk about printed circuit board assembly, we do not mean the assembly of circuit boards during their manufacturing process. Printed circuit board assembly is actually what is done after the initial design has been done. Once the circuit board has been manufactured, electronic components have to be connected and soldered over the board. These electronic components can be both active and passive components.  After the electronic components have been soldered over the circuit board, only then does it become available to be used in electronic equipment. There are three basic things that have to be kept in a note during printed circuit board assembly. These things are mentioned below;

·  Type of electronic components

·  Type of PCB

·  Purpose of the printed circuit board

How Can You Carry Out Printed Circuit Board Assembly

When it comes to connecting the electronic components on the printed circuit board then there are two methods to choose from. The first one is the Thru-Hole Method and the second one is Surface Mount Method.

Thru-Hole Method

The thru-Hole assembly method is used when you need to connect electronic components with wires on the board. These wires can be plugged through the hole and soldered on the other side of the board. This method is usually used for large-sized components like coil and capacitors.

Surface Mount Technology

As Thru-Hole Methods is used for relatively larger components, the Surface mount technology is used for smaller items like diodes and resistors.

FPGA circuit board assembly

The PCB Assembly Process

1.  Apply Solder Paste To The Circuit Board

The first step in the printed circuit board assembly is to apply solder paste to the board. A thing stencil made up of stainless steel is placed on the board. The stainless steel stencil is used so that the solder paste can be applied to specific parts on the board. These are the same parts where the electronic components will be attached to the end product. i.e. PCB.

Type Of Solder Paste Used

The solder paste used in the assembly of printed circuit boards is made up of copper, silver, and tin. Tin is used in relatively higher proportions which is why the paste has a grayish color. It is very important to apply this solder paste to exactly the right place. If you apply large quantities of solder paste at the wrong place then that would waste your entire board and you will have to try again.

2.  Surface Mount Components

Once the solder paste has been applied to the board the next step is to add the surface mount components. Most of the components on a printed circuit board that doesn’t need to be connected to other components are surface mount components. Certain spots on the circuit board are fixed for these components. Once the solder paste has been applied, these components can be placed on the solder paste in their specified spots.

3.  Reflow Soldering

After you have placed the components on top of the solder paste, the next step is to make those components stick to the board. This can be done by solidifying the solder paste. This process is known as reflow soldering. This is carried out by heating the board and melting the solder. The melted solder then cools down and solidifies the components in their place.


4.  Quality Assurance And Control

Like all other products, you need to perform quality control and inspection for the circuit board.  Once the board passes through the soldering process and the reflow process, you have to check the board for any problems. The quality inspection performed at this stage is very strict and if there are any errors, then they usually get caught at this time. There are many different methods to inspect the item. Three of the major quality control methods are manual inspection, automated inspection, and X-Ray inspection. The inspection methods can vary with different manufacturers however these are three of the most common methods that most of the manufacturers use. Sometimes the manufacturers use a collection of these methods or to optimize their quality control, they use all methods to check the product for any errors.

5.  Thru-Hole Components

We have already talked about the surface mount components. At this stage, the Thru-Hole components are added to the board. Different circuit boards required different types of components. If your board requires thru-hole components then they will be added at this stage after the initial quality control.

6.  Final and Function Inspection

After the complete assembly of the printed circuit boards, the last step is the final inspection. This is called the functional test where the end result is tested. The PCB is tested in different ways to see if all aspects of the circuit board are working properly or not. If there is an issue with a product then it is sent to the back of the assembly line. The PCBs that pass the test are sent forward for commercial use.


In the old days, the  printed circuit board assembly  was done in a manual way.  From start to end every step was done by humans. However, with time the machines took over the assembly process. Humans could work in an efficient and effective manner but they used to get tired after a little while. Working with such small components made people tired and made their eyes hurt. The thing about using automated machines is that they are efficient and they don’t get tired. Automation has remarkably boosted the productivity of the printed circuit board assembly.