Welding process measures and electric soldering iron application method

1. Introduction of electric soldering iron
1. External heating electric soldering iron is
generally composed of soldering iron tip, soldering iron core, shell, handle, plug and other parts. The soldering iron tip is installed in the soldering iron core and is made of copper alloy material with good thermal conductivity copper as the base. The length of the soldering iron tip can be adjusted (the shorter the soldering iron tip, the higher the temperature of the soldering iron tip), and there are different shapes such as chisel type, pointed cone, round face, round, pointed cone and semi-circular groove shape to suit The needs of different welding surfaces.
2. The internally heated electric soldering iron
consists of five parts: connecting rod, handle, spring clip, soldering iron core, and soldering iron tip (also called copper tip). The soldering iron core is installed inside the soldering iron tip (heating quickly, and the thermal efficiency is as high as 85%~%%). The soldering iron core is made of nickel-chromium resistance wire wound on a porcelain tube. Generally, the resistance of a 20W electric soldering iron is about 2.4kΩ, and that of a 35W electric soldering iron is about 1.6kΩ.
Generally speaking, the greater the power and heat of the soldering iron, the higher the temperature of the soldering iron tip. Welding integrated circuits, printed circuit boards, and CMOS circuits generally use a 20W internally heated electric soldering iron. The power of the soldering iron used is too high, it is easy to burn the components (generally the junction temperature of the diode and triode will burn out when the temperature exceeds 200 ℃) and cause the printed wire to fall off the substrate; the power of the soldering iron used is too small, and the solder cannot be melted sufficiently , The flux cannot be volatilized, the solder joints are not smooth and firm, and it is easy to produce false soldering. If the soldering time is too long, the device will be burnt out. Generally, each solder joint is completed within 1.5 ~ 4S.
3. Other soldering irons
1) Thermostatic soldering
iron The soldering iron tip of the thermostatic soldering iron is equipped with a magnet-type temperature controller to control the power-on time and achieve the purpose of constant temperature. When the soldering temperature should not be too high and the soldering time should not be too long, a constant temperature electric soldering iron should be used, but its price is high.
2) The
soldering iron is a desoldering tool that integrates a piston-type solder sucker and an electric soldering iron. It is easy to use, flexible, and has a wide range of applications. The disadvantage is that only one solder joint can be desoldered at a time.
3) Steam soldering iron
A soldering iron that uses liquefied gas or other combustible gas to heat the soldering iron tip. It is suitable for occasions where power supply is inconvenient or unable to supply AC power.
Second, the
selection of electric soldering iron 1. The selection of electric soldering iron generally follows the following principles:
① The shape of the soldering iron tip should adapt to the requirements of the surface of the welded part and the assembly density of the product.
② The temperature of the tip of the soldering iron tip should be compatible with the melting point of the solder, generally 30-80℃ higher than the melting point of the solder (not including the temperature drop when the soldering iron tip touches the soldering point).
③ The heat capacity of the soldering iron should be appropriate. The temperature recovery time of the soldering iron tip should be adapted to the requirements of the surface of the welded part. The temperature recovery time refers to the time required for the temperature at the tip of the soldering iron tip to return to the highest temperature after the temperature drops due to heat dissipation during the welding cycle. It is related to the power and heat capacity of the soldering iron, as well as the shape and length of the soldering iron tip.
2. The power principles for selecting an electric soldering iron are as follows:
① When welding integrated circuits, transistors and other components that are vulnerable to heat, consider using 20W internal heating or 25W external heating electric soldering irons.
② When welding thicker wires and coaxial cables, consider using 50W internal heating or 45-75W external heating electric soldering iron.
③ When welding larger components, such as metal chassis grounding lugs, you should choose an electric soldering iron of 100W or more.
3. The use
of electric soldering iron 1. The gripping method of
electric soldering iron The gripping method of electric soldering iron is divided into three types. ① The reverse grip is to hold the handle of the soldering iron in your palm with five fingers. This method is suitable for high-power electric soldering irons to weld parts with large heat dissipation.
② The forward grip method is suitable for larger electric soldering irons, and generally used for curved soldering iron tips.
③ The method of holding a pen Use the method of holding a pen to hold an electric soldering iron. This method is suitable for low-power electric soldering irons, welding parts to be welded with small heat dissipation, such as welding radios and TV printed circuit boards and their maintenance.
2. Handling of the electric soldering iron before use
Before use, power on the tip of the soldering iron to “paste” the soldering iron. First use a frustrating knife to shape the soldering iron tip into a certain shape as needed, and then connect the power supply. When the temperature of the soldering iron tip rises to melt tin, apply the soldering iron tip to the rosin, and then apply a layer after the rosin smokes. Soldering, repeat this two to three times, so that the blade surface of the soldering iron tip is covered with a layer of tin before use.
Electric power without the use of iron should not be prolonged, so that the iron core is easy to accelerate the oxidation blown, shorten its life, the same
also causes the tip during oxidation due to prolonged heating, and even “burned” no “eat tin “.
3. Precautions for the use of electric soldering irons
① Choose different types of electric soldering irons reasonably according to the welding object.
② Do not knock the tip of the soldering iron arbitrarily during use to avoid damage. The thickness of the steel pipe wall of the internally heated electric soldering iron connecting rod is only 0.2mm, so it cannot be clamped with pliers to avoid damage. Frequent maintenance should be used during use to ensure that a thin layer of tin is hung on the tip of the soldering iron.
Fourth, solder
Solder is a kind of fusible metal, which can connect the lead of the component and the connection point of the printed circuit board. Tin (Sn) is a soft, malleable silver-white metal with a melting point of 232°C. It has stable chemical properties at room temperature, is not easy to oxidize, does not lose its metallic luster, and has strong resistance to atmospheric corrosion. Lead (Pb) is a soft light blue-white metal with a melting point of 327°C. High-purity lead has strong resistance to atmospheric corrosion and good chemical stability, but it is harmful to humans. Adding a certain proportion of lead and a small amount of other metals to tin can be made into low melting point, good fluidity, strong adhesion to components and wires, high mechanical strength, good electrical conductivity, not easy to oxidize, good corrosion resistance, and bright solder joints Beautiful solder is generally called solder.
Soldering tin can be divided into 15 types according to the amount of tin, and divided into three grades of S, A, and B according to the chemical composition of tin content and impurities. Wire solder is commonly used for manual welding.
5. Flux①
Flux
Flux can generally be divided into inorganic flux, organic flux and resin flux, which can dissolve the oxides on the metal surface, and surround the metal surface when welding is heated to isolate it from the air and prevent the metal from oxidizing when heated; It can reduce the surface tension of molten solder, which is conducive to the wetting of solder.
② The solder resist
restricts the solder to solder only on the solder joints needed, covers the surface of the printed circuit board that does not need to be soldered, and protects the panel so that the thermal shock during soldering is small, and it is not easy to foam. To prevent bridging, sharpening, short circuit, and virtual welding.
When using flux, it must be applied in an appropriate amount according to the area and surface condition of the welded part. Too small a dosage will affect the welding quality. Too much flux will corrode the components or deteriorate the insulation performance of the circuit board.
Sixth, the basic requirements
for the solder joints 1. The solder joints must have sufficient mechanical strength to ensure that the welded parts will not fall off or become loose when subjected to vibration or impact. Do not use too much solder to accumulate, which will easily cause false soldering and short-circuit between solder joints and solder joints.
2. Welding is reliable, with good conductivity, and virtual welding must be prevented. Virtual soldering means that the solder and the surface of the welded part do not form an alloy structure. It simply attaches to the surface of the welded metal.
3. The surface of the solder joints should be smooth and clean. The surface of the solder joints should have a good luster. There should be no burrs, voids, no dirt, especially the harmful residues of the flux, and the appropriate solder and flux should be selected.
7. The basic operation method of manual soldering

Prepare the soldering iron and tools such as tweezers, scissors, diagonal pliers, needle-nose pliers, solder, flux, etc. before soldering, tin the soldering iron and the soldering parts, hold the solder in the left hand and hold the right hand The electric soldering iron keeps the solderable state at any time.
• Use a soldering iron to heat the spare weldment.
• Feed the solder and melt the appropriate amount of solder.
• Remove the solder.
• When the solder flows to cover the solder joints, quickly remove the soldering iron.
Master the welding temperature and time. When welding, there must be enough heat and temperature. If the temperature is too low, the solder has poor fluidity, and it is easy to solidify, forming a false solder; if the temperature is too high, the solder will flow, the solder joints are not easy to store tin, and the flux decomposition speed will accelerate, which will accelerate the oxidation of the metal surface and cause the printed circuit board. The pads on the board come off. Especially when using natural rosin as a flux, the soldering temperature is too high, and it is easy to oxidize and peel and cause carbonization, resulting in false soldering.
8. Soldering process of the printed circuit board
1. Preparation before welding.
First, be familiar with the assembly drawing of the printed circuit board to be welded, and prepare the ingredients according to the drawing. Check whether the model, specification and quantity of the components meet the requirements of the drawing, and do a good job before assembly Preparatory work such as component lead forming.
2. Welding order The order of
assembly and welding of components is: resistors, capacitors, diodes, transistors, integrated circuits, high-power tubes, and other components are first small and then large.
3. Requirements for component welding
1) Resistor welding
Place the resistor in the specified position according to the diagram. The mark is required to be upward and the word direction is consistent. After installing the same specification, install another specification, and try to make the height of the resistor consistent. After soldering, cut off all the excess pins exposed on the surface of the printed circuit board.
2) Capacitor welding
. Install the capacitor in the specified position according to the drawing, and pay attention to the polarity of the “+” and “-” poles of the polarized capacitor. The direction of the mark on the capacitor should be easy to see. Install glass glaze capacitors, organic dielectric capacitors, and ceramic capacitors first, and finally install electrolytic capacitors.
3) Diode
welding. Pay attention to the following points when welding diodes: first, pay attention to the polarity of the anode and cathode, and not install it wrong; second, the model mark should be easy to see; third, when welding a vertical diode, weld the shortest lead The time cannot exceed 2S.
4) Transistor welding
Pay attention to the correct insertion of the e, b, and c three leads; the welding time should be as short as possible, and the lead pins should be clamped with tweezers during welding to facilitate heat dissipation. When welding high-power transistors, if you need to install a heat sink, the contact surface should be leveled, polished smooth and then tightened. If it is required to add an insulating film, do not forget to add a film. When connecting the pins to the circuit board, use plastic wires.
5) For IC welding,
first check whether the model and pin position meet the requirements according to the requirements of the drawings. When soldering, first solder the two pins on the edge to position them, and then solder them one by one from left to right from top to bottom.
All excess pins of capacitors, diodes, and triodes exposed on the printed circuit board must be cut off.
9. Desoldering method. Desoldering is required
during debugging and maintenance, or when replacing components due to welding errors. Improper desoldering method will often cause damage to components, breakage of printed wires or fall off of pads. Good desoldering technology can ensure the smooth progress of debugging and maintenance work, and avoid the increase of product failure rate due to improper replacement of components. Desoldering of
common components:
1) Desoldering with suitable medical hollow needles
2)
Desoldering with copper braided wire 3) Desoldering with air bag soldering device
4) Desoldering with special desoldering electric iron
5) Desoldering with suction Desoldering with tin electric soldering iron.

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