Although many engineers are familiar with the design and manufacturing processes of printed circuit boards (PCBs), many are still not very clear about the PCB assembly process. So today, let us talk about PCB assembly.
1. Component Lead Forming
In order to arrange electronic components neatly and aesthetically on the PCB and to avoid faults such as cold solder joints, lead forming is a very important step. This is typically done using needle‑nose pliers or tweezers. There are many methods for forming component leads, generally including basic forming, bend‑forming, vertical insertion forming, and integrated circuit forming, among others.
2. Pre‑Soldering Treatment of Component Leads and Wire Ends
To ensure soldering quality, before soldering, the component leads must be cleaned of contaminants and tinned. For insulated wires, cut them to the required length, determine the stripping length based on the connection method, strip the insulation, twist the strands of multi‑strand wires, and tin them. This ensures that the leads have good conductivity after being connected to the circuit and can withstand a certain amount of tension without breaking.
3. Checking Placement and Insertion Height
Axial symmetrical components such as resistors, capacitors, and semiconductor devices are commonly mounted in either horizontal (lying‑down) or vertical (standing‑up) orientations. The choice of insertion method depends on the PCB design and specific requirements. After components are inserted into the board, their leads should protrude through the pads by a certain length, typically about 1‑2 mm. For straight‑through insertion, the leads are not bent after passing through the pads, which makes soldering easy. For semi‑bend insertion, the leads are bent at 45°, providing moderate mechanical strength. For full‑bend insertion, the leads are bent at approximately 90°, offering high mechanical strength, but attention must be paid to the direction in which the leads are bent in the pads.
4. Component Soldering
When soldering the circuit, divide the PCB according to functional circuit blocks, generally starting from the signal input end and soldering in sequence. Solder small components first, then larger ones. When soldering resistors, keep them at a uniform height. Pay attention to the polarity of capacitors – the "+" and "‑" terminals must not be reversed. The anode and cathode of diodes must also be correctly oriented. For transistors, keep the soldering time as short as possible, using tweezers to clamp the leads to help dissipate heat. For integrated circuits, first solder two diagonal pins, then solder the remaining pins from left to right and top to bottom. The amount of solder on the soldering iron tip should be sufficient for about 2‑3 pins at a time. The tip should first contact the copper foil on the PCB; when the solder flows to the bottom of the IC pins, then touch the pins with the tip. The contact time should not exceed 3 seconds, and the solder should evenly cover the pins. After soldering, shake the board to check for missed solder joints, solder bridges, or cold joints, and clean any excess solder from the joints.
5. Soldering Quality Inspection
① Visual Inspection
Visually check whether the soldering quality is acceptable: check for missed joints, flux residue around the joints, solder bridging, pad cracks, whether the solder joints are smooth, and whether there is any solder icicle (sharp tip) formation.
② Hand/Tactile Inspection
Manually touch the components to check for looseness or insufficient soldering. Use tweezers to gently pull the component leads to see if there is any movement. When the component is gently moved, check whether the solder on the joint has detached.
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