What is PCB assembly process

Although many engineers are very familiar with the design and manufacturing process of printed circuit boards (PCB), there are still many engineers who are not clear about the assembly process of PCB boards, so today we will talk about the assembly process of PCB boards.

1. Component lead forming
In order to arrange the electronic components neatly and beautifully on the printed circuit board and avoid failures such as false soldering, it is very important to form the leads of the components. Generally, needle-nose pliers or snappers are used. There are many methods for component lead forming, generally divided into basic forming method, bending forming method, vertical insertion forming method, integrated circuit forming method, etc.
2. Pre-welding treatment of component leads and wire ends
In order to ensure the quality of welding, the magazines on the leads must be removed before component welding, and the lead must be dipped in tin. The wire with insulation layer is cut off according to the required length, and the stripping length is determined and stripped according to the connection method of the wire. The strand wires are twisted and tinned, so as to ensure that the lead wires are connected to the circuit and can conduct electricity well and can withstand a certain pulling force without breaking the ends.

3. Detection of displacement
Resistors, capacitors, semiconductor devices and other axially symmetrical components are commonly used in two methods: horizontal and vertical. The insertion method used is related to the design of the circuit board, depending on the specific requirements. After the component is inserted into the circuit board, the lead wire should keep a certain length after passing through the pad, generally about 1-2mm; look at the plug-in type, the pin does not bend after passing through the pad, and it is convenient to insert and weld, half a dozen The bent type bends the leads to 45 degrees, which has a certain mechanical strength. The full-bent type bends the leads to about 90 degrees, which has high mechanical strength, but pay attention to the direction in which the leads are bent in the pad.
4. Welding of components
When welding the circuit, the printed circuit board is divided into unit circuits, generally starting from the signal input end, and welding in sequence, first welding small components, and then welding large components. When welding resistors, make the resistors high and low. Pay attention to the "+" and "-" polarities of the capacitors and the polarity of the diodes. Hold the lead pins to facilitate heat dissipation.
The integrated circuit solders the two pins at the opposite corner, and then solders one by one from left to right, from top to bottom. For the copper foil on the printed circuit board, when the solder enters the bottom of the IC pin, the tip of the soldering iron should touch the pin again. The contact time should not exceed 3 seconds, and the solder should be evenly wrapped around the pin. Shake after soldering to check whether there is any leakage, Touch welding, virtual welding, and clean up the solder at the solder joints.
5. Welding quality inspection
①Visual inspection
From the appearance, check whether the welding quality is qualified, whether there is missing welding, whether there is residual flux around the solder joints, whether there is continuous welding, bridge welding, whether there are cracks on the pad, whether the solder joints are smooth, whether there is any sharpening phenomenon, etc.
②Touch inspection
Touch the components with your hands to see if there is any looseness or weak soldering. Use a camera to clamp the lead wires of the components and pull it gently to see if there is any looseness. When the solder joints are shaken, whether the solder on them falls off.