1. PCB board welding process
1.1 Introduction of PCB board welding process requires manual plug-in, manual welding, repair and inspection.
1.2PCB board welding process is classified according to the list: components, plug-ins, welding, trimming, inspection. 2. Process requirements for 2.PCB board welding
2.1 Process requirements for component processing
2.1.1 Before the components are inserted, the solderability of the components must be treated. If the solderability is poor, the pins of the components should be tinned first.
2.1.2 after the pins of components are reshaped, the pin spacing should be consistent with the corresponding pad hole spacing of PCB.
2.1.3 the pin processing shape of components should be beneficial to heat dissipation during component welding and mechanical strength after welding.
2.2 process requirements for inserting components in PCB board
2.2.1 The order of inserting components in PCB board is from low to high, from small to large, from light to heavy, from easy to difficult, from common components to special components, and the installation in the previous process cannot affect the installation in the next process.
2.2.2 after the components are inserted, their marks should be easy to read and read from left to right as far as possible.
2.2.3 polarity of polar components should be installed in strict accordance with the requirements on the drawings, and it is not allowed to be installed by mistake.
2.2.4 components should be inserted and installed evenly on the PCB board, arranged neatly and beautifully, and diagonal rows, three-dimensional intersections and overlapping arrangements are not allowed; It is not allowed to be high on one side and low on the other; It is also not allowed to have a long pin and a short pin.
2.3 process requirements for solder joints of PCB
3. electrostatic protection in PCB welding process
3.1 electrostatic protection principle
3.1.1 electrostatic accumulation should be prevented in places where static electricity may be generated, and measures should be taken to control it within a safe range.
3.1.2 The existing electrostatic accumulation should be eliminated quickly and released immediately.
3.2 electrostatic protection method
3.2.1-leakage and grounding. Ground the parts that may or have generated static electricity, and provide static electricity release channels. The method of
burying ground wire is adopted to establish "independent" ground wire.
3.2.2 elimination of non-conductor static electricity: positive and negative ions generated by ion blower can neutralize the static electricity of static power supply.
4。 Insertion of electronic components The insertion of electronic components requires neatness, beauty and stability. At the same time, it should be convenient for welding and conducive to the heat dissipation of components during welding. 4.1 Classification of components According to the circuit diagram or list, resistors, capacitors, diodes, triodes, transformers, patch cables, seats, wires and fasteners are classified. 4.2 Component Pin Forming 4.2.1 Basic Requirements for Component Shaping All component pins are not allowed to bend from the root, and generally should be left over 1.5 mm.. Try to place the component surface with characters in an easy-to-observe position. 4.2.2 Pin shaping of components Hand-processed components are shaped, and bent pins can be shaped with tweezers or a small screwdriver.
4.3 manual insertion of components in the plug-in sequence should meet the technological requirements. Do not touch the component pins and the copper foil on the printed board directly with your hands when inserting. 4.4 Ways of inserting components Diodes, capacitors, resistors and other components are installed on the printed PCB in a prone position.
5. Main welding tools
Manual welding is a technology that every electronic assembler must master. Correct selection of solder and flux and selection of welding tools according to the actual situation are essential conditions to ensure welding quality.
5.1 Solder and flux
5.1.1 Solder
A fusible metal or alloy that can fuse two or more metals into a whole is called solder. Among the common tin-lead solders, tin accounts for 62.7% and lead accounts for 37.3%. The melting point and freezing point of the solder with this ratio are both 183℃, which can be directly cooled from liquid to solid without semi-liquid, and the solder joint can be solidified quickly, shortening the welding time and reducing the virtual welding. The temperature at this point is called * * * crystal point, and the solder with this component ratio is called * * * crystal solder. * * * crystal solder has the characteristics of low melting point, the melting point is consistent with the freezing point.
Good fluidity, small surface tension, good wettability, high mechanical strength, solder joint can bear large tensile force and shear force, and good electrical conductivity.
5.1.2 Flux
Flux is an auxiliary material for welding, and its functions are as follows: ● Remove oxide film.
● prevent oxidation. ● Reduce surface tension. ● Make the solder joint beautiful.
Commonly used fluxes include rosin, rosin alcohol flux, solder paste, zinc chloride flux, ammonium chloride flux, etc.
39 tin-lead solder wire with rosin flux in the center and 61% tin content is often used in welding, also known as rosin solder wire
5.2 Selection of welding tools
5.2.1 Ordinary electric soldering iron
Ordinary electric soldering iron is only suitable for occasions with low welding requirements. Such as welding wires, connecting wires and the like.
the important feature of the constant temperature electric soldering iron is that it has a constant temperature control device, which makes the welding temperature stable and is used to weld fine PCB boards.
5.2.2 Tin suction device
The tin suction device is actually a small manual air pump. When the pressure bar of the tin suction device is pressed, the air in the cavity of the tin suction device is exhausted. Release the lock button of the pressure bar of the tin absorber, the spring pushes the pressure bar to return to its original position quickly, and negative pressure of air is formed in the cavity of the tin absorber, so that the molten solder can be sucked away.
5.2.3 hot air gun
hot air gun is also called the soldering table for electronic components. It is specially used for soldering and disassembling surface mount electronic components (especially multi-pin SMD integrated circuits).