The assembly method of SMT and its process flow mainly depend on the type of surface mount component (SMA), the types of components used and the conditions of assembly equipment. Generally, SMA can be divided into three types: single-sided mixed assembly, double-sided mixed assembly, and full-surface assembly, totaling 6 assembly methods. Different types of SMA have different assembly methods, and the same type of SMA can also be assembled differently.
Choosing an appropriate assembly method according to the specific requirements of the assembled products and the conditions of the assembly equipment is the basis for efficient and low-cost assembly production, and is also the main content of the SMT process design.
1. Single-sided hybrid assembly
The first type is single-sided hybrid assembly. That is, SMC/SMD and through-hole plug-in components (THC) are distributed on different parts of the PCB and assembled together, but the soldering surface is only single-sided. This type of assembly method uses single-sided PCB and wave soldering (currently double wave soldering is generally used). There are two specific assembly methods.
(1) Paste first. The first assembly method is called the first-attach method, that is, the SMC/SMD is installed on the B side (welding side) of the PCB first, and then the THC is inserted on the A side.
(2) Post-sticking method. The second assembly method is called post-attachment method, which is to first insert THC on the A side of the PCB, and then install the SMDo on the B side.
2. Double-sided hybrid assembly
The second type is double-sided hybrid assembly. SMC/SMD and THC can be mixed and distributed on the same side of the PCB. At the same time, SMC/SMD can also be distributed on both sides of the PCB. Double-sided hybrid assembly adopts double-sided PCB, double wave soldering or reflow soldering. In this type of assembly method, there is also a difference between SMC/SMD or SMC/SMD. Generally, it is reasonable to choose according to the type of SMC/SMD and the size of the PCB. Usually, the first-sticking method is more adopted. Two assembly methods are commonly used in this type of assembly.
(DSMC/SMD and THC are on the same side. In the third listed in Table 2-1, SMC/SMD and THC are on the same side of the PCB.
(2) Different side modes of SMC/SMD and THC. In the fourth category listed in Table 2-1, the surface mount integrated chip (SMIC) and THC are placed on the A side of the PCB, and the SMC and small outline transistor (SOT) are placed on the B side.
In this type of assembly method, the SMC/SMD is mounted on one or both sides of the PCB, and the leaded components that are difficult to be surface-level mounted are inserted into the assembly, so the assembly density is quite high.
3. Full fading surface assembly
The third type is full-surface assembly. There are only SMC/SMD on the PCB without THCo. Since the current components have not yet fully realized SMT, there are not many such assembly forms in practical applications. This type of assembly method is generally on a PCB or ceramic substrate with a fine-line pattern, using fine-pitch devices and reflow soldering technology for assembly. It also has two assembly methods.
(1) Single-sided surface assembly method. The fifth method listed in Table 2-1 uses a single-sided PCB to assemble SMC/SMD on a single side □
(2) Double-sided surface abalone installation method. The sixth method listed in Table 2-1 uses a double-sided PCB to assemble SMC/SMD on both sides, which has a higher assembly density.
Assembly process
A reasonable process flow is the guarantee of assembly quality and efficiency. After the surface assembly method is determined, the process flow can be determined according to needs and specific equipment conditions. Different assembly methods have different process flows, and the same assembly method can also have different process flows, which mainly depend on the types of components used, the assembly quality requirements of SMA, the conditions of assembly equipment and assembly production line, and the assembly production Actual conditions, etc.
1. Single-sided hybrid assembly process
There are two types of process flow for single-sided hybrid assembly, one using SMC/SMD first paste method (Figure 2-1 (a)), and the other uses SMC/SMD post paste method (Figure 2-1 (6) ). The wave soldering process is used in both processes.
The SMC/SMD first paste method means to mount the SMC/SMD before inserting the THC, and temporarily fix the SMC/SMD on the mounting surface of the PCB with an adhesive. After the THC is inserted, the wave soldering is used for soldering. The SMC/SMD post-attachment method is to insert THC first, and then mount SMC/SMDo
The process point of the SMC/SMD first paste method is that the adhesive is easy to apply and the operation is simple, but it needs to leave room for the bending of the lead when inserting the THC, so the assembly density is low. Moreover, it is easy to touch the mounted SMD when inserting THC, which may cause damage to the SMD or fall off by mechanical vibration. In order to avoid this phenomenon, the adhesive should have a high bonding strength to withstand mechanical impact.
(A) SMC sticking method first; (8) SMC sticking method later.
The SMC/SMD post-attach method overcomes the shortcomings of the SMC/SMD first-attach method and improves the assembly density. But it is more difficult to apply adhesive. This assembly method is widely used in the assembly of PCB components such as TV and VTR.
2. Double retentive hybrid assembly process
There are two assembly methods for double-sided PCB hybrid assembly: one is that SMC/SMD and THC are on the A side of the circuit board (the third method in Table 2.1); the other is that both sides of the PCB have both A and B SMC/SMD, while THC is only on the A side (the fourth method in Table 2.1). The mixed assembly of double-sided PCB generally adopts the SMC/SMD first paste method.
There are two typical process flows for the third assembly method. Figure 2-2 shows one of the typical process flows. This process can be divided into two processes after reflow soldering SMC/SMD and before inserting THC. When it takes a long time to place or complete the THC after the reflow soldering, the process Ao is used because the flux residue left on the component during the reflow soldering, such as the longer standstill time, is the final cleaning It is difficult to remove effectively. For this reason, process A adds a solvent cleaning process to process B. In addition, some THCs are sensitive to solvents, so they need to be cleaned immediately after reflow soldering. However, process B is the short route and low cost of these two processes, and is widely used in highly automated surface assembly processes. Generally, cleanliness testing should be carried out after cleaning to ensure that the circuit components can reach an acceptable cleanliness level.
Another process flow of the third assembly method is shown in Figure 2-3. This assembly process is used to mix and assemble the SMD and THC with gull-wing-shaped leads on the same circuit board. It does not use solder paste, but electroplating solder on the circuit board, using hot rod or laser reflow soldering process to solder SMD. In this process, an integrated assembly and welding equipment that can mount both SMD and solder is often used for assembly.
The typical process flow of the fourth assembly method is shown in Figure 2-4. SMIC and THC are assembled on side A. SMC/SMD are assembled on side B. After the reflow soldering of the SMIC on the A side, the THC is inserted on the A side, then the adhesive is applied on the B side and the SMC/SMD is mounted. This prevents damage to the SMC/SMD on the B side due to the bending of the THC lead, and the mechanical impact when inserting the THC to cause the SMC/SMD bonded on the B side to fall off. If you need to mount the SMC/SMD on the B side first and then insert the THC on the A side, be careful when the leads are bent. Moreover, the adhesive used for mounting SMC/SMD should have a high bonding strength in order to withstand the mechanical impact of THC insertion.
Figure 2-5 shows another typical process flow of the fourth assembly method. The characteristic of this process is to mount SMIC and SMC first, and then insert THCo process A is to mount SMC/SMD on the B side and SMIC on the A side, and then flow soldering on the A side. Then, THC is inserted on the B side and the wave soldering process is carried out. Process B is a process in which the SMIC on the A side and the SMC/SMD on the B side are reflow soldered in sequence, and then THC is inserted and wave soldered. This process can also be used for manual welding of THCo. In addition, heat-sensitive SMICs, such as Quad Flat Packaged Chip Carriers (QFP), etc., can also be welded using local heating methods such as lasers.
3. The assembly process flow of the whole game
The full-surface assembly process flow corresponds to the fifth and sixth assembly methods listed in Table 1-1.
This assembly method is to assemble only surface-mounted components on a single-sided PCB, and insert components without through-holes. Reflow soldering is used. This is the simplest full-surface assembly process.
When assembling the plastic-encapsulated leaded chip carrier (PLCC) on both sides of the circuit board, process A is used. Since the J-type lead and SM1C with wing-shaped lead are easily bridged by double wave soldering, the reflow soldering process is used on both sides of the component. However, the SM1C assembled on the A side has to go through two reflow soldering cycles. When assembled on the B side, the A side is down, and the SMIC that has been installed on the A side is reflowed on the B side. The reflow will be reflowed. And these larger SMICs are prone to shift or even fall off when the conveyor belt vibrates slightly. Therefore, after applying solder paste, it is necessary to use an adhesive to fix it to prevent the formation of chaotic solder connections and SMIC fall off.
