PCB News - Basic knowledge and skills of manual welding part of SMT products

With the rapid development of electronic technology, the advancement of the electronic assembly industry, and the continuous changes in component packaging forms, manual soldering technology has become a new topic in the electronics industry again.

In the 1970s, chip packaging basically adopted DIP packaging. At that time, this packaging form was suitable for PCB (printed circuit board) perforation mounting, and was more convenient for wiring and operation. In the late 1970s and early 1980s, the electronic technology personnel began to pay attention to the development of smt technology outside the country. In the early and mid 1980s, I introduced smt production lines on a large scale. Since entering the 21st century, the introduction of Chinese smt has accelerated greatly. Although China's smt/EMS industry has achieved rapid development, there are still many problems objectively.

As the packaging of smt electronic components accelerates, the original in-line type is changed to flat-mount type, and the connection cable is also replaced by FPC soft board. The component resistance and capacitance have passed 1206, 0805, 0603, and 0402. To 0201 flat mount type, BGA package has used Bluetooth technology, which without exception shows that the development of electronics has been towards miniaturization and miniaturization, and the difficulty of manual soldering has also increased. Damage to components, or cause poor welding, so our front-line manual welders must have a certain understanding of welding principle, welding process, welding method, welding quality assessment, and electronic foundation.

One, welding foundation

Hand Soldering: Refers to using the soldering iron tip as the main heat source and other manual equipment to manually heat the solder and the soldered parts (such as component pin solder ends, pads, wires, etc.) for soldering/or dismantling The welding process/operation. It is a basic and effective assembly method for manufacturing electronic products.

1. Wetting: The molten solder spreads on the surface of the base material to be soldered to form an adhesion layer.

There are many examples of this in nature. For example, if a drop of water is dripped on a clean glass plate, the water droplet can be completely spread on the glass plate. At this time, it can be said that the water completely wets the glass plate; if the drop is A drop of oil, the oil drop will form a ball, and it will spread out. At this time, it can be said that the oil drop can wet the glass plate; if a drop of mercury is dropped, the mercury will form a sphere and roll on the glass plate. It shows that mercury does not wet the glass. The wetting and spreading of the base material by the solder is the same. When the solder is melted on the pad without flux, the solder rolls on the pad in a ball shape, that is, the cohesion of the solder is greater than the adhesion of the solder to the pad. At this time, the solder does not wet the pad; when the flux is added, the solder will spread on the pad, which means that the cohesion of the solder is less than the adhesion of the solder to the pad at this time, so the solder can be wetted on the pad. Wet and spread.

2. Wetting angle: refers to the angle between the interface between the solder and the base metal and the tangent of the solder surface after the solder is melted, also known as the contact angle

3. Diffusion: With the progress of wetting, the inter-diffusion phenomenon between the solder and the metal atoms of the base metal begins to occur. Usually atoms are in a state of thermal vibration in the lattice lattice, once the temperature rises. The intensified atomic activity causes the atoms in the molten solder and the base metal to cross the contact surface into each other's lattice lattice. The moving speed and number of atoms are determined by the heating temperature and time.

2. The role of flux

The word flux (FLUX) comes from the Latin word "Flow in Soldering".

The main functions of flux are:

1. Remove oxides

To achieve a good solder joint, the object to be welded must have a completely oxide-free surface, but once the metal is exposed to the air, an oxide layer will be formed. The oxide layer cannot be cleaned with traditional solvents. At this time, flux must be relied on. It plays a chemical role with the oxide layer. After the flux removes the oxide layer, the clean surface of the solder can be combined with the solder.

There are several kinds of chemical projection of flux and oxide:

A, interact with each other to form three kinds of substances;

B, the oxide is directly peeled off by the flux;

C. The above two reactions coexist.

2. Prevent re-oxidation

When the flux is removing the oxide reaction, a protective film must be formed to prevent the surface of the soldered object from being oxidized again until it contacts the solder. Therefore, the flux must be able to withstand high temperatures, and will not decompose or evaporate at the temperature of soldering operations. If it decomposes, it will form solvent-insoluble substances, which are difficult to clean with solvents.

3, reduce the surface tension of the material being welded

During the soldering process, the solder is basically in a liquid state, while the component pins or pads are in a solid state. When the two substances are in contact, the surface tension of the liquid substance will directly cause the contact interface of the two substances to decrease. Our superficial generalization of this phenomenon is "poor tin liquid fluidity" or "small expansion rate". The existence of this phenomenon affects the area, volume or shape of alloy formation. What is needed at this time is the role of "surfactant" in the flux. "Surfactant" usually refers to a substance that can significantly reduce the surface tension of other substances at a very low concentration. It has two molecules at both ends. A group structure, one end is hydrophilic and oily and the other end is lipophilic and water-repellent. It can be seen from its external performance. It is composed of solvent-soluble and solvent-insoluble parts. These two parts are located at the two ends of the molecule, forming a kind of The asymmetric structure, its ability to significantly reduce the surface tension is precisely determined by this special structure.

The amount of surfactant added in the flux is very small, but the effect is very important. It reduces the "surface tension of the material to be welded", which shows a strong wetting effect, which can ensure that the tin liquid is in the welded object. The surface smoothly expands, flows, soaks, etc. Generally, solder joint balling, false soldering, sharpening and other similar defects are related to insufficient surface activity, and this reason is not necessarily because the amount of flux "surfactant" added is too small, but it may also be in the production process. Caused its decomposition, failure, etc., thereby greatly reducing the surface activity.

Three, the structure of the soldering iron

(1) Handle (2) Electric heater (3) Soldering iron tip (4) Temperature control system

The role of each part:

Soldering iron handle--Provide a comfortable and safe handle for the operator.

Electric heater-electric heating conversion, to provide heat for the soldering iron tip.

Soldering iron tip-accepts and stores thermal energy, and transfers the thermal energy or temperature required for soldering to the desired soldering place quickly and effectively.

Temperature control system-to control the soldering iron tip to reach the required soldering temperature and maintain its stability.

Characteristics and parameters of electric soldering iron

1. Input electric power (power consumption) 6. Soldering iron tip leakage voltage

2, electrothermal conversion rate 7, electrical insulation resistance

3, heat capacity 8, service life

4. High soldering temperature 9. Operation and maintainability

5. Reheat rate 10. Price

The basic principle of the selection of soldering iron tip

1. The thickness or mass (ie weight) of the soldering iron tip should match the required heat capacity or soldering temperature of the soldered area.

2. The geometry of the soldering iron tip (especially its head) should be suitable for the spatial orientation to be welded.

3. The geometry of the tip of the soldering iron tip should make the contact area with the welded place large.

4. The handle of the tip of the soldering iron should match the body of the soldering iron used (that is, the inner or outer diameter of the handle should fit properly with the body of the soldering iron without looseness).

5. The service life of the soldering iron tip should be long (such as high temperature resistance, corrosion resistance, not easy to wear, etc.) and the price is appropriate.

Four, welding temperature and time

1. Manual soldering temperature and time

1) The manual soldering temperature is the temperature required for the soldering iron tip and the welded part to contact each other and form a solder joint (that is, the temperature that can be actually obtained at the solder joint or is specifically called "welding temperature"). Generally, the temperature is not higher than the melting point temperature of the tin material itself, which is 38°C or 100°F.

2) The manual soldering time refers to the time required for the soldering iron tip and the part to be welded to contact each other and forming a solder joint (that is, the time that the soldering iron tip stays at the soldering place or specifically called "welding time"). Generally, it should be controlled within 1 to 5 seconds.

Welding time selection:

1～2 seconds: small solder joints, heat-sensitive components, chip components (such as resistors, capacitors), etc.

2～3 seconds: medium solder joints, paper-based or glass fiber-based PCB boards, through-hole plug-in components, multi-pin mount devices, tin enamel and wires, etc.

3～5 seconds: large solder joints, glass fiber-based PCB boards, large soldering areas or fast heat dissipation, shielded wires or thick wires, etc.

2, solder wire diameter selection:

0.8～1.0mm: small solder joints, thermal components, chip components, multi-pin and small-pitch mounting devices, etc.

1.0～1.2mm: middle solder joints, through-hole plug-in components, multi-pin middle and large-pitch mounting devices, tin enamel and wires, etc.

1.0 ～2.0mm: large solder joints, tin enamel, shielded wire, large or fast heat-dissipating grounding, tinning and desoldering, etc.

Generally, the diameter of the solder wire should be approximately equal to 1/2 of the pad directly.

Five, manual welding steps

A, five steps:

1. Preparation: Clean the tip of the soldering iron and dip it with thin tin

2, heating: the soldering iron tip heats the welded parts

3, send tin wire: supply tin wire for the corresponding weldment

4. Remove the tin wire: transfer the soldering iron tip and remove the tin wire

5. Remove the soldering iron tip: Wait and remove the soldering iron tip

B, two-point tinting method

1. Add solder between the soldering iron tip and the welded part to form a thermal bridge.

2. Move the solder wire to the opposite side of the soldering iron tip, continue to add solder, and leave at 45 degrees after reaching a satisfactory solder joint.

Six, ten bad habits common in manual welding:

1. Excessive force (white spots are produced, pads are lifted or deformed, etc.)

2, the solder thermal bridge is not suitable (will produce cold solder joints and insufficient solder flow)

3. Wrong heating head size (If you choose too small a soldering iron tip: it will increase the soldering time, or cause cold solder joints and insufficient solder flow; choosing a too large soldering iron tip: it will damage the board and adjacent devices)

4, the humidity is too high (it will cause the pad to warp and damage the board)

5. Improper use of flux (will increase corrosion, and electron migration will promote the growth of metal whiskers)

6, transfer soldering (referring to plug-in components, which will cause poor wetting)

7, modified rewelding (increasing the growth of the intermetallic layer)