Soldering iron pre-welding treatment and welding steps (soldering iron welding method)

(1) Pre-soldering treatment steps

Before welding, the components should be leaded There are generally three steps to deal with the welding part of the pin or circuit board: "scraping", "plating" and "testing":

"Scraping": It means to clean the welding part before welding. The commonly used tools are a knife and fine sandpaper to clean the pins of integrated circuits and printed circuit boards to remove dirt. After cleaning, it is generally necessary to apply flux to the components to be disassembled.

"Plating": It means plating tin on the scraped components. The specific method is to dip the rosin alcohol solution on the scraped welding part of the component, then press the hot soldering iron tip with tin on it, and rotate the component to evenly coat it with a thin layer of tin.

"Test": Use a multimeter to test whether all tin-plated components are of reliable quality. If there are components of unreliable quality or damaged, they should be replaced with components of the same specifications.

(2) Welding steps

After completing the pre-welding treatment, formal welding can be carried out.

Different welding objects require different working temperatures of the soldering iron. When judging the temperature of the soldering iron tip, you can touch the soldering iron to the rosin. If there is a "squeaking" sound, it means the temperature is appropriate; if there is no sound and the rosin can only barely melt, it means the temperature is too low; if the soldering iron tip touches the If there is a lot of smoke when applying rosin, it means the temperature is too high.

Generally speaking, there are three main steps in soldering:

(1) First melt a small amount of solder and rosin on the soldering iron head, and align the soldering iron head and solder wire at the solder joint at the same time .

(2) Before the flux on the soldering iron tip has evaporated completely, touch the soldering iron tip and solder wire to the solder joint at the same time to start melting the solder.

(3) When the solder infiltrates the entire solder joint, remove the soldering iron tip and solder wire at the same time.

The welding process generally takes 2 to 3 seconds. When soldering integrated circuits, the amount of solder and flux must be strictly controlled. In order to avoid damage to the integrated circuit due to poor insulation of the soldering iron or voltage induced by the internal heater on the casing, in practical applications, the method of unplugging the power plug of the soldering iron and soldering while it is hot is often used.

Soldering iron soldering and its prevention methods

When welding, each solder joint should be firmly welded and in good contact. The tin points should be bright, smooth and without burrs, and the amount of tin should be moderate. The tin and the object to be soldered are fused firmly and there should be no weak soldering. The so-called virtual soldering means that only a small amount of tin is soldered at the solder joint, resulting in poor contact and occasional disconnection. In order to avoid false welding, the following points should be noted:

(1) Ensure that the metal surface is clean

If there are rust stains, dirt or oxides on the surface of the welding parts and solder joints, they should be Before welding, use a knife or sandpaper to grind until the bright metal is exposed. Only then can the surface of the weldment or solder joint be tin-plated.

(2) Control the temperature

In order to keep the temperature appropriate, a soldering iron with appropriate power should be selected according to the size of the component, and attention should be paid to controlling the heating time. If you use a low-power soldering iron to solder large components or solder ground wires on a metal base plate, it is easy to form a false solder.

When the soldering iron tip is pressed against the welding spot with solder, if after removing the soldering iron, no or very little solder is left on the welded spot, it means that the heating time is too short, the temperature is not enough, or the solder has been soldered. The object is too dirty; if the solder flows down before removing the soldering iron, it means that the heating time is too long and the temperature is too high.

(3) Appropriate amount of tin

Determine the amount of tin dipped in the soldering iron according to the size of the required solder joint, so that the solder can enough wrap the object to be soldered to form a suitably sized and Smooth solder joints. If the tin applied at one time is not enough, it can be replenished, but the soldering iron must be removed after the tin applied last time has been melted.

(4) Select appropriate flux

The function of flux is to improve the fluidity of the solder, prevent the oxidation of the welding surface, and play a role in assisting and protecting the soldering. When soldering electronic components, the use of solder paste should be avoided as much as possible. A better flux is rosin alcohol solution. When soldering, just drop a little on the soldered area.

Reflow soldering process

The reflow oven must be able to provide sufficient heat (temperature) to the entire assembly and all pin locations. Many odd-shaped/through-hole devices are taller and have larger thermal capacities than other SMCs assembled on the assembly. For THR applications, forced convection systems are generally considered superior to IR. Separate top and bottom heating controls also help reduce ΔT on the PCB assembly.

For a computer motherboard with a high-stack 25-pin DSUB connector (1.5in), the component body temperature is unacceptably high. The solution to this problem is to increase the temperature at the bottom and decrease the temperature at the top. The time above liquidus should be long enough to allow the flux to evaporate from the PTH, possibly longer than the standard temperature curve. Cross-section slice analysis may be important to confirm the correctness of the reflow profile. In addition, peak temperatures and thermal gradients across components must be carefully measured and tightly controlled. Therefore, when setting the reflow soldering temperature curve, you must pay attention to:

·Control the generation of voids/bubbles;

·Monitor the temperature distribution on the board and the temperature difference between large and small components;

·Consider the thermal compatibility of the component body;

·Heating rate, time above liquid phase, reflow peak temperature, cooling rate.

An appropriate and stable heating rate is required because during this process, the viscosity of the solder paste decreases due to heating and the volatilization of the flux increases the viscosity of the solder paste. An appropriate and stable heating rate maintains the viscosity of the solder paste. smooth. This is very important when there is solder paste left on the top of the component pins during assembly.

Figure 1 shows that after the temperature curve is optimized, the molten solder paste is completely pulled back into the through hole, forming a good solder joint.

Welding Precautions

In addition to following the soldering essentials when welding printed circuit boards, you should also pay attention to the following points:

(1) Soldering iron One strand should be of internal heating type (20~35W) or temperature-adjusting type (the temperature of the soldering iron should not exceed 300℃), and the soldering iron tip should be small conical.

(2) When heating, try to keep the soldering iron tip in contact with the copper foil and component leads on the printed board. For larger pads (diameter greater than 5mm), move the soldering iron during welding, that is, the soldering iron rotates around the pad. ＇

(3) For welding of metallized holes, not only the solder must wet the pad, but also the hole must be filled with moisture. Therefore, the heating time of metallized holes should be longer than that of single panels.

(4) Do not use the soldering iron tip to rub the solder pad when welding. Rely on surface cleaning and pre-soldering to enhance the solder wetting performance. Components with poor heat resistance should use tools to assist heat dissipation, such as tweezers.

When welding transistors, be careful not to weld each tube for more than 10 seconds, and use needle-nose pliers or tweezers to hold the pins for heat dissipation to prevent burning the transistors. When welding a CMOS circuit, if the leads have been short-circuited in advance, do not remove the short-circuit wire before welding. For soldering irons that use high voltage, it is best to unplug the soldering iron and use the residual heat to solder. When welding integrated circuits, on the premise that wetting can be ensured, the welding time should be shortened as much as possible. Each pin should not exceed 2 seconds per pin.

Welding method

The five-step welding method is a commonly used basic welding method, which is suitable for welding workpieces with large heat capacity, as shown in Figure 14.

(1) Prepare for soldering

Prepare the soldering wire and soldering iron and prepare for soldering.

(2) Heating the soldering piece

Put the soldering iron in contact with the soldering point. Pay attention to keeping all parts of the soldering iron heated by the soldering iron (such as the leads and pads on the printed board) heated. , Secondly, pay attention to let the flat part (larger part) of the soldering iron tip contact the weldment with a larger heat capacity, and the side or edge part of the soldering iron tip contact the weldment with a smaller heat capacity to keep the weldment evenly heated.

(3) Melting solder

After the weldment is heated to a temperature that can melt the solder, place the welding wire on the solder joint, and the solder begins to melt and moisten the solder joint.

(4) Remove the solder

After melting a certain amount of solder, remove the solder wire.

(5) Remove the soldering iron

Remove the soldering iron after the solder completely wets the solder joint. Note that the direction of removing the soldering iron should be approximately 45°.

For workpieces with small welding heat capacity, it can be simplified to a two-step operation: prepare for welding, put the soldering iron and solder wire at the same time, and remove the soldering wire and soldering iron at the same time

Welding requirements

Welding is one of the important links in the assembly process of electronic products. Without corresponding welding process quality assurance, it will be difficult for any well-designed electronic device to meet the design specifications. Therefore, when welding, the following points must be done:

1. The welding surface must be kept clean

Even for weldments with good weldability, harmful oxide films, oil stains, etc. may be produced on the surface due to long-term storage and contamination. Therefore, the surface must be cleaned before welding, otherwise it will be difficult to ensure quality.

2． When welding, the temperature and time should be appropriate and the heating should be uniform.

When welding, the solder and the metal to be welded are heated to the welding temperature, so that the molten solder soaks and spreads on the surface of the metal to be welded and forms a metal compound. Therefore, to ensure that the solder joints are firm, there must be an appropriate soldering temperature.

At a high enough temperature, the solder can be fully wetted and fully diffused to form an alloy layer. Too high temperature is not conducive to welding. The welding time has a great impact on the wettability of the solder, the welded components, and the formation of the bonding layer. Accurate control of welding time is the key to quality welding.

3． The solder joints must have sufficient mechanical strength

In order to ensure that the parts to be welded will not fall off or loosen when subjected to vibration or impact, the solder joints must have sufficient mechanical strength. In order to make the solder joints have sufficient mechanical strength, the method of bending the lead terminals of the components to be soldered before soldering is generally used. However, excessive solder accumulation cannot be used, which may easily cause false soldering and soldering between solder joints. short circuit between.

4. Welding must be reliable and ensure electrical conductivity

In order for the solder joints to have good electrical conductivity, false soldering must be prevented. False welding means that the solder does not form an alloy structure on the surface of the object to be welded, but simply adheres to the surface of the metal to be welded. During welding, if only part of the alloy is formed and the remaining part is not, the solder joint can pass current in a short period of time, and it is difficult to find the problem with instrument measurement. However, as time goes by, the surface where the alloy is not formed will be oxidized, and the phenomenon of on and off will occur, which will inevitably cause product quality problems.

In short, good quality solder joints should be: the solder joints are bright and smooth; the solder layer is uniform and thin, and in proportion to the size of the solder pad, and the outline of the joint is vaguely visible; the solder is sufficient and skirt-shaped. Spreads out; no cracks, pinholes, or flux residue. Figure 8 shows the appearance of a typical solder joint, in which the height of the "skirt" is approximately 1 to 1.2 times the radius of the pad.

Overview of the basic operations of manual welding

In the small-scale production of small electronic products, electrical maintenance personnel carry out maintenance work and electronic enthusiasts learn and experiment, manual welding is inseparable. The basic skills that electronics enthusiasts must master may seem simple, but the correct welding steps are often ignored. Wrong operation methods will directly affect the welding quality, leaving hidden dangers of product failures such as (virtual soldering). Therefore, electronics enthusiasts must master the correct welding methods during learning and practice, and at the same time pay attention to safety during welding operations.

1. Welding operating posture and hygiene

Chemical substances emitted by heated flux are harmful to the human body. If the nose is too close to the soldering iron tip during operation, it is easy to inhale harmful gases. Generally, the distance between the soldering iron and the nose should be at least 30cm, usually 40cm is appropriate.

There are three ways to hold the soldering iron, as shown in Figure 1. The reverse grip method has stable movements and is not suitable for fatigue during long-term operation. It is suitable for the operation of high-power soldering irons. The upright grip is suitable for operating a medium-power soldering iron or an electric soldering iron with an elbow. Generally, the pen holding method is used when welding printed circuit boards and other welding parts on the operating table.

Introduction to how to solder SMD components

Because of their extremely small size, SMD components are difficult to solder with a soldering iron like ordinary components. A special solder paste is required for soldering. To solder patch components under amateur conditions, you can buy patch solder paste in the market. There are two common types on the market now, one is prepared solder paste with the trademark "Shenwel", and the other is made of solder paste. The solder paste blended with a blender is trademarked as "Big Eye Brand".

The soldering method is: place the patch component on the pad, and then apply the adjusted patch solder paste at the contact point between the component pin and the pad (be careful not to apply too much to prevent short circuit), then use a 20W internal heating soldering iron to heat the connection between the pad and the chip component (the temperature should be 220~230°C). When you see the solder melting, you can remove the soldering iron. After the solder solidifies, the soldering is completed. After soldering, you can use tweezers to clamp the soldered chip components to see if there is any looseness. If there is no looseness (it should be very strong), it means the soldering is good. If there is any looseness, re-apply some patch solder paste and re-solder according to the above method.