With the usage of PCBs becoming ubiquitous with increased usage of electronic products, it is important to understand the PCB Assembly process in detail.
Essentially, a Printed Circuit Board with components mounted on it is known as an assembled PCB and this process of mounting components on the board is referred to as the process of PCB Assembly.
The PCB Assembly process, in turn, has a number of steps- some of these are automated and some are manual. Here are the basic steps involved in Printed Circuit Board Assembly:
The basic unit of the PCB has a number of layers. These layers include:
- The base material of the PCB that gives the PCB its rigidity is known as the substrate
- On each side of the Printed Circuit Board is a copper layer.
- Above the copper layer is the solder mask. Essentially, what it does is to insulate the copper traces. The solder mask is therefore responsible for smooth working of the PCBA as it ensures that the soldering does not happen on unwanted parts.
- The final layer on the PCB board is the silkscreen which adds labels and therefore indicates the function of the various components.
Following are the basic types of Printed Circuit Boards:
- Rigid PCBs: As the name indicates, these are inflexible PCBs. For such PCBs the base is made of materials such as FR4, epoxy or phenolics.
- Metal core PCBs: As the name suggests they are made of a metal core that spreads heat efficiently and therefore components that are heat sensitive, are protected.
- Flexible PCB: Flexible PCBs offer more pliability than their rigid counterparts and therefore are of use in several applications.
- Rigid-Flex PCBs: This is a hybrid design that essentially has the qualities of both rigid and flexible PCBs. Typically these are multi layered with flexible circuit substrates joined with rigid boards.
When it comes to the component mounting technology for printed circuit board assembly, two types of technologies:
- Surface Mount Technology: This is particularly useful for small, sensitive components such as resistors or diodes.
- Thru-hole technology: This is useful when components are to be mounted on the board through a process of plugging them through holes on the board. When there are large components, this technology is preferable.
Prior to the PCB Assembly (PCBA) process, however, there need to be some preparatory steps. This largely includes assuring that the design of the PCB is right for which a Design for Manufacturability test is undertaken. This is to ensure that no operational issues are faced once the PCB is ready. A DFM check looks at the various design specifications, any missing features and more, which can impact the functionality of the PCB. For example, is the spacing between components too less, if so, if it not caught at the DFM stage, it could lead to short circuits at a later date. Design for Manufacturing can therefore go a long way in ensuring that costly errors are later avoided.
Printed Circuit Board Assembly Process Flow:
Post the Design for Manufacturing (DFM) test comes the actual Circuit Board Assembly process.
Solder Paste Stenciling
The first stage begins with the Solder Paste Stenciling. Essentially this means applying a solder paste on the board. A thin stencil is placed over the PCB, this allows the solder paste to be applied on certain parts of the PCB. Essentially, the solder paste is a mix of solder with the flux, which needs to be applied to the board at the correct places. Once the stencil is removed, the solder paste remains in the exact location in which it was intended.
Pick and Place
The next step is what is known as Pick and Place. A robotic device now places the surface mount components on the PCB. These are then soldered to the board. While earlier this process was manual, it is now automated that adds to precision and accuracy.
This process ensures that the solder paste solidifies and that components are affixed to the board. Essentially, after pick and place the PCB board is moved to a conveyor belt. It is here that the board is heated to about 250 degrees Celsius, which melts the solder. Now it moves to cooler heaters where the solder cools and solidifies. This now connects the components to the PCB.
After this process, comes the very important process of inspection. It is important to check to see that the connection isn’t poor and that there are no shorts as a result of misplaced components. There are a number of inspection methods that are in use. These include:
- Manual Inspection- This works well for small batches. However, when the number of boards increase, visual inspection becomes impractical
- AOI or Automatic Optical Inspection- This is suited for large batches where an AOI machine is used that has cameras at different angles. With different quality solders reflecting light in different ways, it is easy to identify the low quality solder.
- X-Ray inspection- For layered PCBs, this method works well
Through Hole Components
The board also may contain a number of through-hole components, in which case these components require a special kind of soldering method.
This could be manual soldering where components will be inserted one at a time. The process can be lengthy depending on how many components there are.
There is also wave soldering which is an automated version. However, this isn’t possible for double-sided PCBs
Post this comes the final inspection to test for functionality. With simulated signals running through the PCB at this stage, its electrical characteristics are tested.
This completes the Printed Circuit Board Assembly (PCBA) process. However, soldering can make the process messy. What is therefore extremely important, is to wash the product after the soldering process. For this, a high pressure washing apparatus is used, where the PCB is washed in deionized water. Washing is then followed by a drying process followed by packaging and shipment.
Through Hole Technology Assembly Process
In this assembly process the first step is that of component placement where components are placed manually conforming to a number of regulations regarding polarity and orientation of components. This is, then followed by a process of inspection. The last step is to do with wave soldering which involves the THT components to be accurately soldered onto the circuit board.
SMT PCB Assembly Process
This process involves the steps of Solder Paste Printing where the solder paste is applied through a solder paste printer. This is then followed by mounting of components through an auto pick-and-place machine that sticks parts to the board. The final step is to do with reflow soldering which binds the components firmly to the board.
With electronic products becoming increasingly complex and miniaturization becoming the order of the day, most boards have both thru-hole and SMD components. In such cases it is important to have a proper sequence in order. Hand soldering works well where there are many components on both sides and where SMD components are more. Where there are a small number of THT components, wave soldering is recommended.
It is important to choose the right Printed Circuit Board Assembly (PCBA), USA who is technically sound and aware of all these nuances to do justice to your project.
Technotronix offers professional PCB Assembly services with 4o years of expertise. Our accurate PCB assembly process meets the quality standards, including ISO-9001:2015, RoHS, and more. You can rely on our PCB design tools that, in turn, lead to that perfect PCB Assembly. Our superior engineering capabilities mean that each board matches your exact requirements and technical specifications. In case if you have any questions, please feel free to contact us via email at firstname.lastname@example.org