How to Produce a Printed Circuit Board Assembly (PCBA) I
01. Bare Board Loading
The first step in assembling a circuit board is to neatly arrange the bare boards and place them onto a magazine rack. The machine will then automatically feed the boards one by one into the SMT (Surface Mount Technology) production line.
02. Solder Paste Printing
As the printed circuit board (PCB) enters the SMT line, the first operation is solder paste printing. Solder paste is applied to the pads on the PCB where components will be soldered. Later, during the reflow process, this solder paste will melt and bond the electronic components to the board.
03. Solder Paste Inspection (Optional)
To ensure quality, some SMT factories inspect the solder paste right after printing using optical inspection equipment. Boards with poor printing will be rejected, and the paste will be cleaned off and reprinted, or excess paste will be removed for correction.
04. High-Speed Pick and Place Machine
Small components such as resistors, capacitors, and inductors are first mounted onto the PCB using a high-speed pick and place machine. These components are temporarily held in place by the solder paste. Larger components are not suitable for this machine, as they can slow down the process and may shift due to the board’s fast movement.
05. General-Purpose Pick and Place Machine
Also known as the “slow-speed machine,” this step involves placing larger components such as BGA ICs and connectors. These components require high precision. A camera is used to confirm the component's position before placement, resulting in a slower process compared to the high-speed machine.
06. Manual Component Placement or Visual Inspection
Before the board enters the reflow oven, there is usually a checkpoint where operators visually inspect for misaligned or missing components. In some smartphone PCB production lines, an AOI (Automated Optical Inspection) is also used before reflow to ensure quality.
07. Reflow Soldering
The purpose of reflow soldering is to melt the solder paste and form intermetallic compounds (IMC) between the solder and component leads. The temperature profile during this process—heating and cooling curves—significantly affects solder quality. Typical reflow ovens have zones including preheat, soak, reflow, and cooling zones to optimize the soldering outcome.
For lead-free processes using SAC305 solder paste, the melting point is around 217°C. Therefore, the reflow oven must reach at least this temperature to re-melt the solder paste. However, the maximum temperature should generally not exceed 250°C, as some components may deform or melt if exposed to excessive heat.
Once the PCB passes through the reflow oven, the assembly process is essentially complete—except for any hand-soldered components. The remaining steps involve inspecting and testing the board for defects or functional issues.