Guide to PCB Assembly
Electronics technology is an integral part of our daily lives. Almost all electronics technology from our smartphones to our cars requires electronics components to function properly. And at the heart of these electronics are printed circuit boards (PCBs). As PCBs are becoming more advanced, you will find PCBs with various active and passive components mounted on them. Such PCBs are known as assembled PCB and the manufacturing process is called PCB assembly or PCBA for short. Besides just electronics components, you may also find other parts and accessories such as wires, connectors, shields, and heat sinks. As the demand for innovative technologies continues to grow, manufacturers are constantly trying to fit more into technology, while keeping their sizes small.
To achieve this goal of creating thinner and smaller equipment, manufacturers of electronics components look to create smaller components and high-definition inter-connect PCBs. Because of the challenge of manually assembling such tiny components, new technologies have been introduced to solve this problem. The solution is to use different PCB assembly methods based on the application and design of the equipment.
These methods are:
- Through-Hole Components Assembly
- Surface Mount Components Assembly
- Mixed Assembly
- Single-Sided PCB Assembly
- Double-Sided PCB Assembly
Through-hole components (THC) are those that involve the use of wire leads for connecting. The lead on the components is inserted through the holes drilled into the PCB, which is then soldered to pads on the opposite side through the use of a wave or a selective soldering machine. Any excess leads that are leftover during this process will be cut off using a rotating blade cutter.
To improve the efficiency of this process, assemblers often prefer to use an automated insertion mount machine. This process involves having the necessary components lined up in reels, which the machine then inserts the components, and cuts off any extra lead and clinches them inwards to prevent misplacements of the components. The last step is to pass the board over to a wave soldering machine where the soldering process is completed.
Surface Mount Technology Assembly
As the density of components on the PCB continues to increase, more and more manufacturers are shifting towards the use of surface mount components (SMC) assembly as it offers several advantages over THC. The size of SMCs is much smaller compared to THC, which means assembly could be completed at a higher density. In addition, since SMC are lead-less components, it prevents the need of having holes to be drilled on the PCBs, which reduces costs significantly.
However, assembling PCBs with SMCs will require the use of new technologies which can handle smaller components, placing them in the right positions, and finally soldering them.
As technology becomes increasingly complex it is almost impossible for PCBAs to only have one type of component mounted on them. This is where boards with both SMCs and through-hole components on them are needed.
Single-Sided PCB Assembly
As the name suggests, these boards will have either SMCs, through-hole components, or both, mounted on only ‘one’ side of a single or multi-layered PCB.
For boards that require both components, the assembly process begins with mounting the through-hole components, which are then soldered using a wave soldering machine. The SMCs are then mounted and soldered using a reflow soldering machine.
For boards with only through-hole components, the process would only require the wave soldering machine to solder them. Similarly, for boards with only SMCs components, the process would only require the use of the reflow soldering machine.
Double-Sided PCB Assembly
Similar to single-sided PCB assembly, the double-sided PCB assembly may include SMCs, through-hole components, or both, but this time it is mounted on both sides of the PCB. Though it is viewed as more appropriate to mount through-hole components on just one side of the board. It’s still possible to mount SMCs on both sides.
PCB Assembly Technology
As the assembly process has become more streamlined over time, new technologies have been introduced to replace the traditional method of manual assembly. These methods are:
- Solder-Paste Deposition
- Reflow Soldering
- Wave Soldering
This technology is used for mounting SMCs, as they have short terminals, and do not require holes in the PCB to mount.
To perform a solder-paste deposition, a small amount of solder paste is applied to each SMD on the PCB. To increase the efficiency of this process, a stencil with the necessary openings is used during the application. The stencil is placed on the PCB while making sure it is properly aligned. Then a rubber squeegee is used to push the solder paste through the openings of the stencil to deposit it on the PCB. Only a small amount of solder paste is deposited each time as the stencils are very thin.
Because of the small size of SMCs, it can be a challenge to mount them manually. This is where Pick-n-Place machines can help. Since manufacturers often provide SMCs arrangements in cassettes, reels, tubes, and trays. The operator must input the necessary parameters into the machine to help identify the correct components to pick up and accurately place them on the PCB where the glue or solder paste has been applied.
The Reflow Soldering machine is designed to solder SMCs. This machine has a chamber that contains a conveyor and an infra-red heater arrangement. This machine operates by feeding PCBs with solder paste and SMCs mounted on them on a conveyor and managing the internal temperature of the chamber to allow the boards to be preheated before the solder paste can be melted. The speed of the conveyor, the temperature of the chamber, as well as the temperature at which the soldering takes place, are all customized depending on the specification of the board and its components.
As mentioned previously, PCB designs may include SMCs on both sides of the board or a mixed assembly with through-hole components on the top and SMCs on the bottom. When SMCs are placed at the bottom of a board, it runs the risk of falling off when the through-hole components are being mounted.
To prevent this from happening, a glue-dosing machine can be used to apply minuscule drops of glue at each point on the underside of the PCB where the SMCs will be placed. After the SMCs have been implemented on the board, the board will then go through a curing process where the glue will dry and be fixed into place. Finally, the SMCs and through-hole components will then be passed on to be wave soldered.
As mentioned previously, auto-insertion is a method that is commonly used for mounting through-hole components. This automated process requires input from the assembler as to how the components should be positioned, orientated, and the span of each component to the specific mounting holes on the PCB. Based on the inputted information, the machine will then arrange the components in the form of a continuous reel.
During the mounting process, the auto-insertion machine utilizes the components from its reel, cuts off the leads with reference to the span information, and places them into their respective positions in the PCB, and finally, clinches the ends of the leads to prevent them from falling off.
You will find this technology typically used for through-hole components. The wave soldering machine has a reservoir of solder which is kept molten with heaters. During the assembly process, the molten solder is pushed through a long open spout by impellers inside the tub, which closely controls the height of the overflowing solder.
The PCB assembly is then carried on a conveyor over the bath, the PCB will slightly graze the top of the overflowing solder. The rest of the machine is preheated to bring the components and board closer to the temperature of the molten solder. Additionally, there is a cooling zone where the temperature is brought down after the soldering process has been completed.
When using a wave soldering machine, the operator has full control of the preheat temperature, the speed of the conveyor, as well as the soldering temperature to achieve optimal soldering. Once the proper parameter has been identified, the operator can store this information as a profile for that specific PCB assembly.