Printed circuit boards are the driving force behind a wide range of electronics. You can find them in simple calculators and advanced military and satellite systems. However, do you know how to assemble a PCB? Do you know the steps involved in the process? There’s no need to fret if you don’t! We have created this in-depth guide on the Bill Of Materials (BOM) of a PCB to get fitted onto aboard. PCB Assembly to get to know everything you need about the process. Then you can assemble your PCB and give life to your project.
The Printed Circuit Boards Assembly
The printed circuit board assembly is mounting or placing electronic components that give the board its functionality. Manual and automatic techniques can mount the electronic components, and then they are soldered in place.
A PCBA or printed circuit board assembly. It would help if you did not confuse it with printed circuit board (PCB) manufacturing, which produces and creates prototypes. Covers the installation of electronic components during the assembly process. The Printed Circuit Board Assembly) is the board presented after the entire parts and pieces get soldered and rightfully installed.
Different technologies can be used to assemble a PCB, including SMT and THT processes. We will explore them in detail in chapter 4 but for now, let’s take a look at a PCB design.
Printed Circuit Design and Fab
Using schematics, you can use a Computer-Aided Design (CAD) solution to design PCBs. According to the design, the design files or Gerber files are then handed over to the manufacturer who produces or assembles the PCB.
The basic design of PCBs include-
Substrate: The base material of a PCB is known as the substrate. It is what makes the board rigid and durable.
Copper: Each functional side of the PCB is applied with a thin conductive copper layer. The number of sides will require a copper layer depending on whether the board is single-sided or double-sided.
Solder mask: The green-colored substance on PCBs is due to the application of the solder mask. It provides insulation to the copper traces so that they don’t contact conductive components.
Silkscreen: Use a white screen as the last layer of the PCB. It contains the labels of different components in symbols and characters.
PCBs can also be of three types-
- Rigid PCB made of solid material such as fiberglass
- Flexible PCB made of bendable materials such as Kapton
- Metalcore PCB made of metalcore
The next chapter will discover the printed circuit board assembly process.
Printed Circuit Boards Assembly Process
3.1 Tools Required
You will need a minimum number of tools if you plan to solder your PCB manually. The things to arrange include-
- Soldering iron or soldering station
- Solder flux
- Wire Cutters
- Volt/OHM meter
3.2 PCB Assembly—Soldering Equipment
You can choose from a wide range of soldering equipment for manual printed circuit board assembly. The most simple ones plug right into the power outlet and don’t have many options for temperature control. You should select a 15 to 30-watt soldering iron for your PCB assembly.
The thermostatically controlled soldering irons can be suitable as they come with options to control the temperature. Some have dials to control the temperature, while others use a magnetized tip with a specific temperature.
It would help if you chose a soldering iron with replaceable tips of different sizes. The bonuses begin to magnetize as you increase the current flow causing the temperature to rise. When magnetism comes down, the heat also reduces.
You may also need a hot air soldering station that uses hot air to melt the solder.
3.3 Types of Solder
You will find different types of solder in the market and choose according to the purpose and application of your project. There are three types of solder used for electronics-
- Lead alloy solder
- Lead-free solder
- Silver alloy solder
Lead Alloy Solder
Solders are made from a combination of lead and tin and may also contain traces of other metals. It is responsible for giving the solder a lower melting temperature which is significant as most electronics are heat sensitive.
Lead alloy solders are defined by the ratio of the weight of tin followed by the weight of lead. For example, it could have a balance of 60:40 or 63:37- the first number represents the amount of container while the second refers to the amount of lead.
You can use both solder types for typical electronic applications. The 63:37 alloy can effectively transform into a liquid state and helps prevent cold solder joints.
Lead-based alloys are used as a standard in the electronics industry but can have health consequences.
You may be able to come across lead-free alloys such as the 96.5:3:0.5, which has 96.5% tin, 3% silver, and 0.5% copper. Lead-free alloys are more expensive than lead-based alloys and have a higher melting temperature.
Lead-free alloys create stronger solder joints though they may be brittle.
Silver Alloy Solder
Solders may or may not contain lead. Silver was first used in solders to create more robust and durable solder joints. Silver alloy solders tend to be more expensive than lead-based and lead-free alloys.
3.4 PCB Assembly—Proper Soldering Technique
PCB assembly process. You have to take the help of soldering methods multiple times during printed circuit board assembly. It would help if you used proper soldering techniques to get the highest quality end products possible. Here we will tell you how you can use appropriate soldering techniques during the Bill Of Materials (BOM) of a PCB to get fitted onto aboard.
The proper way to solder is to heat the surfaces that will solder beyond the melting point of the solder. It enables the solder to flow over the surfaces freely. It would help if you also kept a check on the amount of the solder, making sure not to use too much.
You also must ensure that the surface is heated enough to prevent cold solder joints. It happens when you use too little heat to the surface, and the solder cannot move freely.
The rest of the soldering process is carried out automatically by machines. The reflow soldering uses a series of heaters and cold heaters to melt and solidify the solder and make it firm.
You should ensure that the temperatures are correctly set in the reflow soldering machine. It needs to heat up to 250 degrees Celsius to melt the solder.
The manual soldering technique may be needed when dealing with THT components. You have to place the parts by hand and then solder the extra lead or wire on the other side of the board. It has to be done carefully so that the solder or flux does not touch the different components and only in the right place.
It may be complex or challenging to solder PCBs if you are entirely new on the job. It would help if you practice first on some small projects and then try your hands on PCB soldering after you are skilled. It would help if you also were careful not to inhale the fume or smoke coming from the flux in the solder.
Now we will check out the differences between the processes used to assemble PCBs.
Differences in Printed Circuit Boards Assembly Process
You can use different technologies to assemble the electronic components on a PCB. The main methods include Thru-Hole Technology (THT), Surface Mount Technology (SMT), and Mixed technology.
We will discuss the main differences and the process of SMT). This will make it easy for a pick and place machine to easily pick and place the components on the PCB (PCB assembly for each method.
Through-Hole Technology (THT)
THT method of PCB assembly is used for electronic components that come with a wire or lead. The PCB comes with holes drilled into it to fit the pieces. The extra charge that goes through the holes is soldered on the opposite side of the board.
THT is used for significant components such as coils and capacitors. It is also used for other plated through-hole or PTH parts that go through the plated through-hole of the PCB. Various PCB components use the holes on the board to transfer signals from one side to the other side of the PCB. For this reason, you cannot rely on soldering paste which will pass right through the holes.
PCB Assembly–Assembly Process
THT assembly uses both manual and automatic processes to place the components on the PCB. Proceed as follows –
1. PCB Assembly—Placing Components
Electrical engineers manually place the components on the PCB according to specifications. It has to be done quickly and accurately with full compliance to operation standards or regulations of the THT assembly process for proper functioning.
For instance, it is essential to define the orientation and polarity of electronic components so that operating elements don’t affect them.
2. PCB Assembly—Examining and Correction
You need to check if all the electronic components on the PCB have been placed accurately. It can be done automatically with the use of a transport frame. The engineers can quickly rectify any errors or mistakes if you find them.
These electronic components have to be soldered to the board in this step. You can do it manually, but a far more efficient and automated process called Wave soldering can be used.
The PCB is placed on a conveyor belt that carries it inside a special oven which contains molten solder at high temperatures. The solder is applied to the bottom of the board, which covers all the pins at once.
The electronic components are attached with all lead or wire connections to the board.
Surface Mount Technology (SMT)
WellPCB, we have professionals and experts who have the right skill sets to make your PCB needs come to life. We offer PCB manufacturing PCB assembly (SMT enables you to speed up the production process, but there are increased chances of defects. SMT is the automatic placing or mounting of electronic components on a PCB. For this reason, the method also employs failure detection for creating functional products.
The electrical components mounted with SMT are smaller than PTH components and may or may not have leads. They sometimes come with pins, flat contacts, or terminations on their body.
1. PCB Assembly—Applying Solder
You have to use a solder paste printer to apply solder to the PCB. A solder screen or stencil is used to ensure the proper application of solder at valid points where electronic components will be placed.
It is crucial to have an efficient solder paste printing process as it directly impacts soldering quality. A solder inspector is used to find any defects in the quality of solder paste printing. If any errors are on the spot, the solder is washed off for a second printing.
For minor defects, a rework can be sufficient.
2. Placing Components
A pick-and-place machine is used to mount the electronic components after solder printing. The engine automatically mounts the IC or components through component reels. The components reels are responsible for feeding the details to the machine, which are then fixed onto the PCB.
3. PCB Assembly—Reflow Soldering
This step uses a specialized oven to harden the solder paste to fix the board’s components firmly. The PCB is carried inside a series of heaters that raise the board’s temperature to 250 degrees Celsius. The high temperature melts the solder on the board.
Next, the PCB moves through a series of more excellent heaters, which bring down the temperature and help the solder to harden. That adheres all the electronic components firmly to the PCB.
PCB Assembly–Mixed Technology
Electronic products have increased in complexity, requiring different electronic components on PCBs in the modern age. You will find the use of both THT and SMT technologies in a single PCB that contains both surface-mounted and through-hole details.
Mixed technology is used in the following cases-
Single Side Mixed Assembly
In single side mixed assembly, the usual solder paste printing process is carried out first. Then the surface-mounted components are placed, followed by reflow soldering.
Then you have to place the THT components and carry out wave soldering. You can also go for manual soldering if you use a small number of THT components.
One Side THT and One Side SMT
First, you need to apply the surface mount adhesives and then mount the SMT components. Then you have to place them in the oven for solidification, followed by flipping.
Then you have to mount the THT components and perform wave soldering. This assembly process requires high cost because of the use of an increased amount of adhesives.
Double Side Mixed Assembly
It is a complicated and lengthy process where SMT components are placed after solder paste printing. Then you have to use adhesives for placing the SMT components on the other side, followed by solidification.
Next, the THT components are mounted, ending with wave soldering. You can also carry out the process without using adhesives but need to use heating three times, resulting in decreased efficiency.
The following chapter will determine how a PCB is tested to ensure its functionality and quality.
Printed Circuit Board Assembly Testing
You will need to run various tests to determine the functionality of your PC assembly. It starts with the DFM check, which involves checking the design specifications of a PCB for missing or problematic features.
The next test for functionality is carried out after the surface-mounted components are placed and soldered. The board is tested for its connections and shorts, which can occur if inappropriate links have occurred.
Manually inspect the PCB after the reflow process. It can be helpful in a small batch of PCBs but is not feasible for large-scale production. An automatic optical inspection machine is also used to test the PCBs using advanced cameras.
The machine can detect low-quality solder by analyzing the way they reflect light.
One of the less conventional methods used for highly complex or multi-layered PCB involves X-ray inspection. The X-ray provides visuals of all the layers and can help find hidden problems from plain eyesight.
After the assembly process is over, a final inspection and functionality test are carried out. You can find out different aspects through the trial, such as signal output, current, and voltage. You can determine the electrical characteristics of the PCB through simulated signals.
This will make it easy for a pick and place machine to easily pick and place the components on the PCB (PCB assembly can be a complicated process or a simple one depending on the number of electronic components you are using and their types. The best way to assemble a PCB is to specialize in maintaining all specifications and standards.
You can contact us for printed circuit board assembly and high-quality end products quickly passing the function tests. We will ensure that the PCBs are assembled as per your design and instructions to deliver the best results.