Welding method and principle of FPC flat wire

Welding is a science. The principle is to use a heated soldering iron to heat and melt the solid solder wire, and then use soldering flux to make it flow into the metal to be welded. After cooling, a solid and reliable solder joint is formed.

When the solder is a tin-lead alloy and the welding surface is copper, the solder first wets the welding surface. As the wetting occurs, the solder gradually diffuses towards the metal copper, forming an adhesive layer on the contact surface between the two metals. Solder and metal copper firmly bond, so solder is completed through three physical and chemical processes: wetting, diffusion, and metallurgical bonding.

1. Wetting: The wetting process refers to the process of molten solder flowing around small bumps and crystal gaps on the surface of the base metal with the aid of capillary force, thereby forming an adhesive layer on the surface of the base metal to be welded, thereby bringing the atoms of the solder and the base metal closer to each other, achieving a distance of atomic attraction.

Environmental conditions that cause wetting: The surface of the base metal to be welded must be clean and free of oxides or contaminants. (Wetting can be figuratively likened to: Dropping water onto a lotus leaf to form water droplets means that water cannot moisten the lotus. Dropping water onto cotton, water will penetrate into the cotton, which means that water can moisten the cotton.) n2. Diffusion: As wetting proceeds, mutual diffusion between solder and base metal atoms begins to occur. Generally, atoms are in a thermal vibrational state in the lattice, once the temperature rises. Increased atomic activity causes the atoms in the molten solder and the base metal to cross the contact surface and enter each other's lattice. The speed and number of atoms in motion are determined by the heating temperature and time.

Metallurgical bonding: Due to the mutual diffusion between the solder and the base metal, an intermediate layer - a metal compound - is formed between the two metals. In order to obtain a good solder joint, it is necessary to form a metal compound between the base metal to be welded and the solder to achieve a strong metallurgical bonding of the base metal.

2. Function of flux

The word FLUX comes from the Latin word "Flow in Soldering". The main functions of flux are:

1. Chemical activity

To obtain good solder joints, the solder must have a completely oxide free surface, but once the metal is exposed to air, an oxide layer forms. The oxide layer cannot be cleaned with traditional solvents. At this time, it is necessary to rely on the chemical interaction between the flux and the oxide layer. After the oxide layer is removed by the flux, the clean surface of the object to be welded can be bonded to the solder.

There are several chemical reactions between flux and oxide:

1. The third substance interacts with each other chemically;

2. The oxide is directly peeled off by the flux;

3. The above two reactions exist simultaneously.

Rosin flux removes the oxide layer, which is a reaction. The main components of rosin are abietic acid and isomeric diterpene acids. When the flux is heated, it reacts with copper oxide to form copper rosin, which is a green transparent substance that is easily soluble in unreacted rosin and removed together with rosin. Even if there is residue, it will not corrode the metal surface.

The reaction of oxides exposed to hydrogen is a typical second reaction. At high temperatures, hydrogen and oxygen react to form water, reducing oxides. This method is commonly used for welding semiconductor parts.

Almost all organic or inorganic acids have the ability to remove oxides, but most cannot be used for welding. In addition to removing oxides, flux also has other functions. These functions are essential for welding and do not need to be considered.

2. Thermal stability

When removing oxides, the flux must also form a protective film to prevent the surface of the object being welded from being oxidized again until it comes into contact with the solder. Therefore, the flux must be able to withstand high temperatures and not decompose or evaporate at the temperature of the welding operation. If decomposed, it will form solvent insoluble substances that are difficult to clean with solvent. W/W grade pure rosin will decompose at about 280 ℃, which should be noted.

3. Flux activity at different temperatures

Good flux requires not only thermal stability, but also activity at different temperatures. The role of flux is to remove oxides, which usually works well at a certain temperature, such as RA flux. Unless the temperature reaches a certain level, chloride ions will not decompose to remove oxides. Of course, the temperature must be within the temperature range of welding.

When the temperature is too high, its activity may also decrease. For example, when rosin exceeds 600 ℉ (315 ℃), there is almost no reaction. This function can also be used to purify the activity of the flux to prevent corrosion. Particular attention should be paid to heating time and temperature in applications to ensure the purity of the activity.

3. Composition and structure of solder wire

"We use lead SnPb (Sn63% Pb 37%) and lead-free SAC (96.5% Sn 3.0% Ag 0.5% Cu) solder wires that are hollow inside. This design is designed to store flux (rosin), so that the flux can be evenly added while adding the solder.". Of course, there are also lead and tin wires. There are many types of SnPb based on their composition, and their main uses are also different

The function of solder wire: to meet the electrical conductivity requirements of components on the circuit and the fixation requirements of components on the PCB circuit board.

4、 Basic structure of electric soldering iron

Electric soldering iron: (1) handle, (2) electric heating wire, (3) soldering iron head, (4) power cord, (5) thermostatic controller, (6) soldering iron head cleaning rack Electric soldering iron is used as a tool for welding electronic components, hardware wires, and other metal objects.

5. Manual welding process

1. Pre operation inspection

(1) Insert the electric soldering iron plug into the designated socket 3-5 minutes before work every day to check whether the soldering iron is hot. If it is not hot, first check whether the socket is plugged in properly. If it still does not heat, immediately report to the administrator. Do not disassemble the electric soldering iron randomly, let alone directly contact the electric soldering iron head with your hands.

(2) The soldering iron head with uneven oxidation or hook should be renewed: 1. It can ensure good thermal conductivity; 2. Ensure the quality of the welded object. If replacing the soldering iron head with a new one, wipe off the maintenance paint after heating, and immediately repair the tin for maintenance. Cleaning of the soldering iron head should be performed before welding operations. If you do not use the soldering iron for more than 5 minutes, you need to turn off the power supply. The sponge needs cleaning. Dirty sponges containing metal particles or sponges containing sulfur can damage the soldering head.

(3) Check whether the tin sponge has water and is clean. If there is no water, please add an appropriate amount of water (an appropriate amount refers to water leaking out when the sponge is pressed to half the normal thickness. The specific operation is: After the sponge is completely wet, keep it on the palm of your hand, and the five fingers naturally close). The sponge should be cleaned. Dirty sponges containing metal particles, or sponges containing sulfur can damage the soldering head.

(4) Whether the human body and the electric soldering iron are reliably grounded, and whether the human body is wearing an electrostatic ring.

2. Welding steps

The specific operation steps of soldering iron welding can be divided into five steps, known as the five step engineering method. To obtain good welding quality, it is necessary to strictly follow the steps. Step welding is one of the keys to good solder joints. In actual production, it is easy to violate the operation steps by contacting the solder wire rather than the solder head first. The molten solder droplets fall on the preheated part to be welded, which can easily lead to faulty solder joints. Therefore, the solder head must be in contact with the part to be welded. Preheating the welded part is an important means to prevent faulty soldering.

3. Welding essentials

(1) Contact mode between soldering iron head and two weldments

Contact position: The soldering iron head should simultaneously contact two welding parts to be connected (such as welding feet and pads). The soldering iron head is generally inclined by 45 degrees to avoid contacting only one of the weldments. When the heat capacities of two weldments differ greatly, the tilt angle of the soldering iron should be appropriately adjusted. The smaller the inclination angle between the soldering iron head and the welding surface, the larger the contact area between the welding part with larger heat capacity and the soldering iron head, and the enhanced thermal conductivity. For example, the tilt angle for LCD pull welding can be around 30 degrees, and the tilt angle for welding microphones, motors, speakers, etc. can be around 40 degrees. Two parts to be welded can reach the same temperature simultaneously, which is considered an ideal heating state.

Contact pressure: When the soldering iron head contacts the workpiece to be welded, a slight pressure should be applied. The strength of heat conduction is proportional to the applied pressure, but it is based on the principle of not causing damage to the surface of the workpiece being welded.

(2) Welding wire supply mode

The supply of welding wires should be controlled in three aspects, namely, the supply time, location, and quantity.

Delivery time: In principle, when the temperature of the part to be welded reaches the melting temperature of the solder, the solder wire should be sent out immediately.

Power supply position: It should be between the soldering iron and the welded part, as close as possible to the welding pad.

Supply quantity: depending on the welding position and pad size. After covering the pad with solder, the solder can be higher than 1/3 of the pad diameter.

(3) Welding time and temperature settings

A. The temperature is subject to actual use. It is advisable to solder a tin spot for 4 seconds, but not more than 8 seconds. Usually observe the soldering iron tip. When it turns purple, the temperature setting is too high.

B. For general online electronic materials, the actual temperature of the soldering iron head is set to (350~370 degrees); For surface mount materials (SMC), set the actual temperature of the soldering iron tip to (330 to 350 degrees)

C. Special materials and soldering iron temperatures require special settings. Silver containing tin wires should be used for FPC, LCD connectors, and the temperature is generally between 290 degrees and 310 degrees.

D. When welding large component feet, the temperature should not exceed 380 degrees, but the power of the soldering iron head can be increased.

(4) Welding precautions

Before welding, observe whether each welding point (copper sheet) is flat and oxidized.

B. When welding items, pay attention to the solder joints to avoid short circuits caused by poor circuit welding

4. Post operation inspection

(1) After using the soldering iron, use a sponge to wipe off the remaining tin from the soldering iron head.

(2) After work every day, it is necessary to clean the tin beads, slag, dust, etc. on the electric soldering iron rack, and then place the electric soldering iron on the electric soldering iron rack.

(3) Place the cleaned electric soldering iron on the upper right corner of the workbench.

Tin Spot Quality Assessment:

1. Standard tin point

(1) The tin point should be within the inner arc;

(2) Tin spots should be round, smooth, without pinholes or rosin stains;

(3) Must be wired with a length between 1-1.2MM;

(4) The shape of the part foot indicates that tin has good fluidity;

(5) The tin surrounds the entire upper tin position and part foot.

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