1.Tools for Hand soldering
Electric soldering iron/ stand/ Hot air gun/ Soldering head etc.
Conditions for soldering:
Solderable parts/ Clean soldered metal surface/ Suitable flux/ Proper soldering temperature/ Right soldering time
2.Solder and flux
Any metal or alloy used to fuse two or more metal surfaces into a whole is called solder. The solder mentioned here only refers to the solder used for soldering.
Common soldering materials:
Tubular solder wire/ Anti-oxidation solder/ Silver-containing solder/ Solder paste
Selection of flux:
During the soldering process, a thin oxide film will be produced when the metal is heated, which will hinder the infiltration of solder and affect the formation of solder joint alloys, which is prone to false soldering.
The use of flux can improve soldering performance. The flux includes rosin, rosin solution, solder paste solder oil, etc., which can be selected reasonably according to different welding objects. Solder paste and solder oil are corrosive and should not be used for welding electronic components and circuit boards. After soldering, the remaining solder paste and solder oil should be wiped clean. Rosin should be used as soldering flux when tin-plating component pins. If the printed circuit board has been coated with rosin solution, there is no need to use flux when the components are soldered.
3.Precautions for manual welding
The correct operating posture for holding electric soldering iron:
In order to reduce the harm to people caused by the chemical substances volatilized when the flux is heated, and to reduce the inhalation of harmful gases, under normal circumstances, the distance between the soldering iron and the nose should be no less than 20cm, usually 30cm is appropriate.
Three ways to hold the soldering iron
The reverse grip method is stable, and it is not easy to fatigue after long-term operation. It is suitable for the operation of high-power soldering iron; the upright grip method is suitable for the operation of low-power soldering iron or electric soldering iron with elbow; generally soldering parts such as printed boards on the operating table when using the pen holding method.
Correct operating posture of solder wire:
Since the solder wire contains a certain proportion of lead, and lead is a heavy metal harmful to the human body, you should wear gloves or wash your hands after the operation to avoid ingesting lead dust.
Two ways to hold solder wire
After using the soldering iron, be sure to insert it on the soldering iron stand securely, and pay attention to the wires and other sundries not to touch the tip of the soldering iron to avoid scalding the wires and causing accidents such as leakage.
4.Five steps of manual soldering operation
Master the temperature and soldering time of the electric soldering iron, and choose the proper contact position between the soldering iron tip and the solder joints to get a good solder joint.
Basic operation steps:
a.Ready for soldering: Hold the soldering wire in your left hand and the soldering iron in your right hand to enter the state of preparation for soldering. The soldering iron tip is required to be kept clean, free of oxides such as solder slag, and a layer of solder is plated on the surface.
b.Heating the weldment: The soldering iron head rests on the joint of the two weldments to heat the whole weldment for about 1 to 2 seconds. For soldering components on the printed circuit board, pay attention to making the soldering iron head contact two objects to be soldered at the same time. For example, the wires and terminals, component leads and pads in Figure (b) should be heated uniformly at the same time.
c.Feed the soldering wire: When the soldering surface of the weldment is heated to a certain temperature, the solder wire touches the weldment from the opposite side of the soldering iron. (Note: Do not send the solder wire to the tip of the soldering iron)
d.Remove the soldering wire): When the soldering wire has melted a certain amount, immediately move the soldering wire to the upper left 45°.
e.Remove the soldering iron: After the solder has soaked the pad and the soldering part of the weldment, remove the soldering iron in the upper right direction at 45° to finish the soldering. From the beginning of the third step to the end of the fifth step, the time is about 1 to 2 seconds.
Three-step soldering method:
For weldments with small heat capacity, such as the connection of thinner wires on a printed circuit board, it can be simplified to three steps.
a. Preparation: Same as step one above.
b. Heating and wire feeding: Put the soldering wire into the soldering iron head after placing it on the soldering piece.
c. Remove the wire and remove the soldering iron: After the solder is soaked and diffused on the soldering surface to reach the expected range, immediately remove the solder wire and remove the soldering iron, and pay attention to the time of removing the soldering wire not to lag the time of removing the soldering iron.
For weldments that absorb low heat, the above-mentioned whole process takes only 2 to 4 seconds. The rhythm control of each step, the accurate control of the sequence, and the skillful coordination of movements are all problems that can be solved through a lot of practice and careful experience.
5.Eight methods of manual welding operations
a. Keep the soldering iron tip clean
When soldering, the soldering iron tip is in a high temperature state for a long time, and it is exposed to weak acid substances such as flux, and its surface is easily oxidized and corroded and stained with a layer of black impurities. These impurities form a thermal insulation layer, which hinders the heat transfer between the soldering iron tip and the weldment. Therefore, pay attention to wipe the soldering iron tip with a damp cloth or wet wood fiber sponge at any time. For ordinary soldering iron tips, you can use a file to repair the surface oxide layer when the corrosion pollution is serious. For long-life soldering iron tips, this method must not be used.
b. Accelerate heat transfer by increasing the contact area
When heating, all parts of the weldment that need to be infiltrated by solder should be evenly heated, not just a part of the weldment, and do not use a soldering iron to increase the pressure of the weldment, so as to avoid damage or hidden dangers that are not easy to detect. Some beginners use a soldering iron tip to put pressure on the soldering surface in an attempt to speed up the soldering. This is wrong. The correct method is to choose different soldering iron tips according to the shape of the weldment, or trim the soldering iron tip yourself so that the soldering iron tip and the weldment form surface contact instead of point or line contact. In this way, the heat transfer efficiency can be greatly improved.
c. Use the solder bridge for heat transfer
In non-pipeline operations, the shapes of solder joints are varied, and it is unlikely that the soldering iron tip will be replaced constantly. To improve the heating efficiency, a solder bridge for heat transfer is required. The so-called solder bridge is to keep a small amount of solder on the soldering iron tip as a bridge for heat transfer between the soldering iron tip and the weldment during heating. Since the thermal conductivity of molten metal is much higher than that of air, the weldment is quickly heated to the soldering temperature. It should be noted that the amount of tin used as a solder bridge should not be kept too much, not only because the solder remaining on the soldering iron tip for a long time is overheated, but the quality has actually been reduced, and it may cause misconnection and short-circuit between solder joints.
d. Pay attention to the remove of the soldering iron
e. Cannot move before the solder solidifies
Do not make the weldment move or be subject to vibration, especially when the weldment is clamped with tweezers, be sure to wait for the solder to solidify before removing the tweezers, otherwise it is easy to cause the solder joint structure to be loose or false welding.
f. The amount of solder should be moderate
The tubular solder wire commonly used in manual soldering has already been filled with flux made of rosin and activator. The diameter of solder wire has various specifications such as 0.5, 0.8, 1.0, …, 5.0mm, etc., which should be selected according to the size of the solder joint. Generally, the diameter of the solder wire should be slightly smaller than the diameter of the pad.
As shown in the figure, the excessive solder not only consumes the solder unnecessary, but also increases the soldering time and reduces the working speed. What's more serious is that excessive solder can easily cause short-circuit failures that are not easily noticeable. Too little solder can not form a firm bond, which is also disadvantageous. Especially when welding printed boards to lead wires, it can easily cause the wires to fall off when the amount of solder is insufficient.
g. The amount of flux should be moderate
A proper amount of flux is very beneficial for soldering. Excessive use of rosin flux will inevitably need to erase the excess flux after soldering, and will extend the heating time and reduce work efficiency. When the heating time is insufficient, the defect of "slag inclusion" is easily formed. When soldering switches and connectors, excessive flux will easily flow to the contacts and cause poor contact.
The proper amount of solder should be such that the pine perfume can only wet the parts where the solder joints will be formed, and will not flow away through the through holes on the printed board. For welding using rosin cored wire, basically no need to apply flux.
h. Do not use the soldering iron tip as a tool to transport solder
Some people are used to soldering on the soldering surface, which results in oxidation of the solder. Because the temperature of the soldering iron tip is generally above 300 ℃, the flux in the solder wire is prone to decompose and fail at high temperatures, and the solder is also in a low-quality state of overheating.
6.Solder joint quality and inspection
The quality requirements for solder joints should include three aspects: good electrical contact, firm mechanical bonding and aesthetics. The most important point to ensure the quality of solder joints is to avoid false soldering.
Causes and hazards of false soldering
False soldering is mainly caused by oxides and dirt on the surface of the metal to be soldered. It makes the solder joints a connection state with contact resistance, which causes the circuit to work abnormally, and the unstable phenomenon of good and bad connection occurs, and the noise increases without regularity. which bring major hidden dangers to circuit debugging, use and maintenance.
In addition, there are also some false solder joints that are still in good contact during a long period of time when the circuit starts working, so it is not easy to find. However, under the influence of environmental conditions such as temperature, humidity, and vibration, the contact surface is gradually oxidized, and the contact gradually becomes incomplete. The contact resistance of the false solder joints will cause local heating, and the local temperature increase will further deteriorate the incomplete contact solder joints, and eventually even cause the solder joints to fall off, and the circuit cannot work properly at all.
This process can sometimes take up to one or two years. The principle can be explained by the concept of "galvanic battery": when the solder joints are damp and water vapor penetrates into the gap, the water molecules dissolve the metal oxides and dirt to form electrolytes, and virtual solder joints The copper and lead-tin solder on both sides are equivalent to the two electrodes of the galvanic battery. The lead-tin solder loses electrons and is oxidized, while the copper material gains electrons and is reduced. In such a galvanic cell structure, metal loss corrosion occurs in the virtual solder joints, local temperature rises aggravate the chemical reaction, and mechanical vibrations make the gaps expand, until the vicious circle makes the virtual solder joints finally open.
According to statistics, nearly half of the failures of electronic products are caused by poor welding. However, it is not easy to find the false solder joints that caused the failure in an electronic device with thousands of solder joints. Therefore, false soldering is a major hidden danger of circuit reliability and must be strictly avoided. Particular attention should be paid when performing manual soldering operations.
Generally speaking, the main reasons for false soldering are: poor solder quality; poor reducibility or insufficient amount of flux; the surface of the soldered area is not cleaned in advance and the tin plating is not strong; the temperature of the soldering iron tip is too high or too low, There is an oxide layer on the surface; the soldering time is not well controlled, too long or too short; when the solder is not solidified during welding, the soldering components are loose.
Requirements for solder joints
Formation and appearance of typical solder joints
On single-sided and double-sided (multi-layer) printed circuit boards, the formation of solder joints is different: as shown in the figure, on a single-sided board, the solder joints are only formed above the pads on the soldering surface; but on double-sided boards or On the multilayer board, the molten solder not only infiltrates the top of the pad, but also penetrates into the metalized hole due to capillary action. The area formed by the solder joint includes the top of the pad on the soldering surface, the metalized hole and part of the pad on the component surface.
Refer to the figure to see a typical solder joint directly from the outside. The requirements for it are: the shape is similar to a cone and the surface is slightly recessed, showing a sloping slope, with the welding wire as the center, symmetrically spreading out in a skirt shape. The surface of the false solder joints often bulge outwards and can be identified.
On the solder joint, the connecting surface of the solder presents a concave natural transition, the junction of the solder and the soldering piece is smooth, and the contact angle is as small as possible; the surface is smooth with metallic luster; there are no cracks, pinholes, and slag inclusions.
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