**Introduce the PCBA Process of Different Types of PCB Boards**
1. **Single-sided SMT Mounting**
Solder paste is applied to the component pads. Once the solder paste printing on the bare PCB is complete, the appropriate electronic components are placed and then subjected to reflow soldering.
2. **Single-sided DIP Cartridge**
For PCBs that require through-hole components, production line workers insert the electronic components before wave soldering. Once the soldering process is complete, the leads can be trimmed, and the board is then washed. However, the efficiency of wave soldering is relatively low.
3. **Single-sided Mixed**
The PCB is first printed with solder paste, and electronic components are attached and secured through reflow soldering. After quality inspection, DIP insertion occurs, followed by either wave soldering or manual soldering. For boards with fewer through-hole components, manual soldering is preferred.
4. **Single-sided mounting and plug-in mixing**
Certain PCB boards are double-sided, with one side mounted and the other side using inserted components. The mounting and insertion process is similar to that of single-sided processing; however, the PCB requires fixtures for reflow and wave soldering.
5. **Double-sided SMT mounting**
To enhance both the aesthetics and functionality of the PCB, some design engineers opt for double-sided mounting. Integrated circuit (IC) components are placed on side A, while chip components are mounted on side B, maximizing PCB space and achieving area miniaturization.
6. **Double-sided mixed**
There are two mixed methods applicable to both sides:
The first method involves three heating cycles during PCBA assembly, which results in lower efficiency and a reduced pass rate for wave soldering using the red glue process, making it not advisable.
The second method is suitable for assemblies with numerous double-sided SMD components and few THT components, where manual soldering is preferred. However, for a higher number of THT components, wave soldering is recommended.
**What issues should be considered in the assembly of PCBA?**
The solder paste printing process primarily addresses the consistency of solder paste volume (filling and transfer), rather than the specific volume needed for each solder joint. Essentially, this process tackles fluctuations in the soldering through rate rather than the extremes of high or low rates. Achieving a consistent pass-through rate hinges on the effective distribution of solder paste. By optimizing the design of pads, solder masks, and stencil openings, solder paste can be allocated to each joint as required. Additionally, the consistency of solder paste volume is influenced by the design itself, as various PCB solder mask designs yield different process capability indices.
1. **Area ratio**
The area ratio is defined as the proportion of the area of the stencil window to the area of the window hole wall.
2. **Transfer rate**
The transfer rate indicates the ratio of solder paste deposited on the pads within the stencil window during printing, calculated as the actual volume of solder paste transferred compared to the volume of the stencil window.
3. **The effect of area ratio on transfer rate**
Area ratio significantly influences solder paste transfer. Typically, an area ratio greater than 0.66 is required in engineering, achieving a transfer rate exceeding 70% under these conditions.
4. **Design requirements for area comparison**
The area ratio imposes design requirements on the steel mesh, particularly impacting fine-pitch components. To maintain the area ratio for micro-pad stencil windows, the stencil thickness must comply with this requirement. Thus, for components needing more solder paste, enlarging the stencil window area is necessary, which requires sufficient deformation space around the PCB pad—a key consideration in component spacing design.
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