Different PCB designs may need different technology for assembly. The following text introduces three kinds of technologies used in PCB assembly processes respectively.
Through-Hole Technology (THT) PCB Assembly Method
Some large size and heavy components, such as capacitors, need to be fastened on the PCB by lead wire inserted into the PCB hole, which is referred to as the through-hole technology. This type of technology can be utilized on boards that require additional rigidity and support.
What is THT Assembly?
Through-hole assembly is assembling the PCB bare board with components by wave soldering machine or manually. Through-hole components are capable of withstanding high levels of environmental stress, mechanical stress, and voltage. They are also useful for prototyping, military, and aerospace PCBs. Some industries request highly reliable PCBs. It is a must to use the through-hole technique to assemble these PCBs.
THT PCB Assembly Process
TTH assembly processes are very simple:
- Plug the Components into the PTH Holes: The first step in the process of using through-hole technology is to plug the component pins into the PTH holes of the PCB board.
- Wave Soldering: And then solder the components in the through hole of the PCB. The soldering is subjected to an intense process known as wave soldering. After wave soldering, the components can be permanently soldered on the PCB bare board. Of course, manually soldering by soldering iron is also commonly used.
- Thorough Inspection: The 3rd step is a thorough inspection of the assembled components for any defects. This can be done using a visual inspection method or an automatic optical method. However, due to the enhanced capabilities of optical inspection, it can now be done with much accuracy. If the issue is found, it should be fixed immediately.
Surface Mount Technology (SMT) PCB Assembly Method
While traditional through-hole technology solders the pins of the components through the holes of the PCB, surface mount technology (SMT) mounts components on a PCB’s surface. It also enables greater density in PCB design, which is beneficial in manufacturing more integrated chips. This technology has several advantages over traditional through-hole construction, but its size and design are not suitable for all components.
What is SMT Assembly?
SMT assembly refers to assembling PCB components on the surface and fix the components by reflow soldering. Compared to conventional through-hole assembly, SMT components tend to be smaller and require less solder, often more susceptible to mechanical stress than through-hole counterparts. In addition, SMT assembly isn’t suited for components that must be regularly connected or disconnected.
SMT PCB Assembly Process
STM assembly is always done by pick & place equipment. The steps in SMT PCB assembly process are:
- Stencil Preparation
A stencil is used to print solder paste on the PCB pads.
- Solder Paste Printing
Firstly, we need to print solder paste on the pads of the PCB by using a solder stencil. Solder paste can be printed by automatic machine or manually.
- Place the Components on the PCB
After the solder paste is printed on the PCB surface, we need to place the PCB components in the correct place. We will use a high pick & place machine to place to do this. For some special components, we may need to place them manually. However, manual placement has low efficiency and is also not possible for small components.
- Run through the Reflow Oven
When components are all placed correctly. We should run the PCB through the reflow oven. The temperature profile may be different for different PCBs, so experienced engineers are needed. Of course, we need to test the oven temperature frequency to make sure the is no mistake.
- AOI (Automatic Optical Inspection)
We always use AOI for every PCB assembly, to reduce the chance of making mistakes. However, experienced workers also need to recognize if it is a real defect or not.
- Visual Inspection
Visual inspection is always needed since there may be different kinds of defects. We always inspect products according to IPC Class 2.
Mixed PCB Assembly Method
In line with the trend that electronic products are becoming more functional and smaller, PCBs are being designed smaller and smaller, which means that the density of components mounted on the PCB is increasing. At the same time, mixed technology is a need in PCB assembly.
What is Mixed PCB Assembly?
Mixed technology assembly can be a mixture of SMT assembly and PTH assembly or a mixture of single-sided assembly and double-sided assembly. Mixed technology is commonly used in various industries such as power and control, testing and measurement, telecommunication, automotive, medical, etc.
Mixed PCB Assembly Process
Mixed PCB assembly process combines the processes of both THT assembly and SMT assembly. Here is a brief flow chart for your reference.
Conclusion
We have introduced SMT and THT PCB assembly processes above, they are used in different stages. During the prototype stage, the technician prefers the through-hole technology when it comes to designing a board, while the surface mount might be necessary if the board is designed for frequency or high-speed operations. This type of technology can help minimize the effects of corrosion and inductance on the circuits. Since PCB assembly process of mixed technology can be very complex and requires a lot of knowledge and skills, every step of the production process must be conducted properly.
