PCB Technical - General process and precautions of circuit board plug-in

General process and precautions of circuit board plug-in: circuit board plug-in, immersion tin, method and process of cutting feet

1. The method of circuit board plug-in, immersion tin and cutting feet
1. Board-making (usually looking for a special board-making company to make, the drawings are provided by yourself) and clean.
2. Insert horizontal and straight insert small parts, such as 1/4W resistors, capacitors, inductors and other small-size components close to the circuit board.
3. Insert large and medium-sized components, such as 470μ electrolytic capacitors and AC adapters.
4. Insert IC, such as patch IC, can be soldered in the first step. In principle, the components are arranged from low to high, and from small to large, in which the high-low principle takes precedence over the horizontal size principle.
If it is welded by hand, insert and weld one by one when inserting. If you pass the furnace, just follow the tin furnace operation guide directly. The cutting feet can be cut by hand or can be processed with a special cutting foot machine. The basic process requirement is to just cut off the exposed tin package.
If you want to open a factory for large-scale production, it is better to read and master the relevant national and industry standards first, otherwise no one will care about the products you have worked so hard to make. And mastering the standard process can also help you formulate and sequence the process of making circuit boards.

2. Working principle of dip soldering furnace
The solder in the solder pot is heated and melted by the dipping furnace to reach the specified temperature; the workpiece to be welded or the part to be welded is cleaned and moistened with flux; the workpiece to be welded or the part to be welded is immersed in the immersion In the solder pot of the soldering furnace, the part to be welded is heated above the melting point of the solder; due to the effect of affinity, the solder adheres to the part to be soldered; the workpiece is taken out and cooled, and the dip soldering is completed. The temperature of different types of dip soldering varies greatly, and the blacksmith himself is not good at it. When the water tank is dip soldered with 30 tin, the tin temperature is about 350 degrees. The thermocouple is equipped with a digital display thermostat to control the heating tube.

3. Guidance for dip soldering, cutting feet, and wave soldering operations
1. Production equipment and raw materials
Soldering furnace, exhaust fan, air compressor, clamp, scraper, circuit board with components inserted, flux, tin bar, thinner, cutting machine, diagonal pliers, wave soldering machine.

2. Preparation
1. Turn on the power switches of the soldering furnace and wave soldering machine as required, set the temperature to 255-265 degrees (high in winter and low in summer), and add appropriate tin bars.
2. Adjust the flux and thinner according to the ratio requirements of the process card, and start the foaming machine.
3. Adjust the height and width of the foot cutter to the corresponding position, the width and flatness of the conveyor belt are consistent with the circuit board, and the height of the cutting foot is 1-1.2mm. Turn the power switch of the conveyor belt and cutter of the foot cutter to ON Location.
4. Adjust the speed of the upper and lower pipelines, and turn on the exhaust equipment.
5. Check the batch number of the material to be processed and related technical requirements, and report the problem to the team leader for processing in advance.
6. According to the wave soldering operating procedures, the whole machine is melted, preheated, cleaned, and conveyed to adjust the speed and the corresponding width of the circuit board until the start light is on.

Three, operation steps
1. Use the right hand to clamp the circuit board with a clamp, and visually check whether each component meets the requirements, and correct the ones that do not meet the requirements with the left hand.
2. Clamp the inserted circuit board with a clip, spray a little flux on the copper surface, scrape off the oxide layer on the tin surface of the tin furnace with a scraper, and immerse the copper surface of the circuit board sprayed with flux into the tin furnace. The board is immersed in about 0.5mm, and the tin immersion time is 2-3 seconds.
3. After the tin is soaked, lift the hand diagonally upwards lightly, and keep it steady, without shaking, to prevent false soldering and not fullness.
4. When it is basically solidified after 5 seconds, put it into the assembly line and flow into the next process.
5. The cutting machine starts to cut the feet, and observe whether the circuit board is lifted or deformed.
6. The height of the cut foot is 1-1.2mm, and it will flow into the automatic wave soldering machine after being qualified.
7. After using the operating equipment, turn off the power.

Fourth, process requirements
1. The flux should be sprayed evenly on the pads of the circuit board.
2. When tinning, the copper surface of the circuit board is just 0.5mm in contact with the tin surface, and no tin dust adheres to the circuit board.
3. The temperature of the tin furnace is 255-265 degrees (high in winter and low in summer), and the tinning time is 2-3 seconds.
4. The solder joints must be smooth and bright, and all solder pads on the circuit board must be tinned.
5. Ensure that the work surface is clean and record the equipment regularly.

Five, matters needing attention
1. The poorly welded circuit must be re-welded, and the second re-welding must be performed after cooling.
2. During the operation, do not touch the tin furnace, and do not allow water or oil stains to fall into the tin furnace to prevent burns.
3. Flux and thinner are flammable materials. Keep away from fire sources during storage and use. The foam tube should be immersed in the flux and not exposed to the air.
4. If it is not used for a long time, the flux should be recycled and sealed. The foam tube should be immersed in a closed container filled with flux.
5. Ventilation should be ensured during welding operation to prevent air pollution. Operators should wear work clothes and masks.
6. The chain claw cleaning liquid storage tank should be added and replaced regularly. The height of the liquid level is 1/2-2/3 of the tank height. Pay attention to adjusting the gap between the brush and the chain claw.
7. When changing tin, pay attention to the safety of the operating staff and avoid burns.
8. Frequently inspect the heating wire to avoid aging and leakage.
9. Pay attention to check the tin level, which should not be less than 20mm from the top of the cylinder.

4. Flux is used when dipping the circuit board of electronic components. So what is flux foaming and what is its function? How to foam?
It can remove the oxides on the solder, making the solder stronger and brighter.

5. Manual tin furnace welding process requirements
Key points of soldering technology
As an operation technique, manual soldering can only be mastered through actual training, but by following the basic principles, learning the experience accumulated by the predecessors and using the correct methods, you can master the operation technique with half the effort. The following points are essential for learning welding technology.
one. Basic soldering conditions
1. Weldability
Not all materials can be connected by soldering. Only some metals have good solderability (strictly speaking, they should be solderable) before they can be connected by soldering. Generally, copper and its alloys, gold, silver, zinc, nickel, etc. have good weldability, while aluminum, stainless steel, cast iron, etc. have poor weldability. Generally, special fluxes and methods are required for soldering.
2. Solder qualified
Substandard lead-tin solder composition or excessive impurities will affect the quality of solder, especially the content of certain impurities, such as zinc, aluminum, cadmium, etc., even the content of 0.001% will significantly affect the wettability and fluidity of the solder, and reduce Welding quality. No matter how smart a chef is, it is impossible to use inferior raw materials to produce delicious dishes. This is obvious.
3. Suitable flux
Different fluxes should be used for welding different materials. Even if the same material is used, different fluxes are often used when the welding process is different, such as manual soldering iron soldering and dip soldering, and different fluxes are required for cleaning and non-cleaning after soldering. . For manual soldering, the use of rosin and active rosin can meet the assembly requirements of most electronic products. It should also be noted that the amount of flux must also be paid attention to, too much or too little is not conducive to soldering.
4. Reasonable solder joint design
Reasonable solder joint geometry is very important to ensure the quality of soldering. As shown in Figure 1 (a), due to the limited strength of lead-tin material, it is difficult to ensure sufficient strength of solder joints, while Figure 1 (b) The joint design has been greatly improved. Figure 2 shows the influence on soldering quality when the lead and hole size of the through-hole mounting component on the printed board are different.

two. Manual soldering points
The following points are derived from the soldering mechanism and proved to be universally applicable by actual experience.
1. Master the heating time
Different heating speeds can be used when soldering. For example, the shape of the soldering iron tip is not good. When soldering large soldering parts with a small soldering iron, we have to extend the time to meet the requirements of the temperature of the tin material. In most cases, extending the heating time is harmful to the assembly of electronic products. This is because
(1) The bonding layer of the solder joint is heated for a long time and exceeds the proper thickness, which causes the performance of the solder joint to deteriorate.
(2) Printed boards, plastics and other materials will deform and deteriorate due to excessive heat.
(3) The performance of components changes or even fails after being heated.
(4) The surface of the solder joint loses its protection and oxidizes due to the volatilization of the flux.
Conclusion: The shorter the time, the better under the premise of ensuring that the solder wets the weldment.
2. Maintain a proper temperature If a high-temperature soldering iron is used to solder the calibration joints in order to shorten the heating time, it will bring another problem: the flux in the solder wire does not have enough time
Overflowing on the surface to be welded causes premature volatilization and failure; too fast solder melting speed affects the performance of the flux; because the temperature is too high, although the heating time is short, it will also cause overheating.
Conclusion: Keep the soldering iron tip in a reasonable temperature range. The general experience is that the temperature of the soldering iron tip is 50°C higher than the melting temperature of the solder.
The ideal state is to shorten the heating time at a lower temperature. Although this is contradictory, in actual operation we can obtain a satisfactory solution through operating techniques.
3. Applying force to the solder joint with a soldering iron tip is harmful
The soldering iron tip transfers heat to the solder joints mainly by increasing the contact area, and it is futile to apply force to the solder joints with a soldering iron. In many cases, the welded parts will be damaged. For example, the welding points of potentiometers, switches, and connectors are often fixed on plastic components. The result of force is likely to cause the original parts to fail.

three. Essentials of Soldering Operation
1. Surface treatment of weldments
The weldments encountered in manual soldering are all kinds of electronic parts and wires. Unless electronic components within the “warranty period” are used under mass production conditions, the weldments encountered generally require surface cleaning Work to remove rust, oil, dust and other impurities that affect the quality of welding on the welding surface. In manual operation, simple and easy methods such as mechanical scraping, alcohol and acetone scrubbing are commonly used.
2. Pre-soldering
Pre-soldering is to wet the leads of the components to be soldered or the conductive welding parts with solder in advance, which is generally called tin plating, tin plating, tin enamel, etc. It is accurate to say that pre-welding is because its process and mechanism are the whole process of soldering-the solder wets the surface of the weldment, and the surface of the weldment is "plated" with a layer of solder after the surface of the weldment is formed by the diffusion of the metal. Pre-soldering is not an indispensable operation for soldering, but it is almost indispensable for manual soldering, especially maintenance, debugging, and research.
3. Do not use excessive flux
The right amount of flux is indispensable, but don't think that more is better. Excessive rosin not only causes the work that needs to be cleaned around the solder joints after soldering, but also prolongs the heating time (the rosin melts, volatilizes and takes away heat), reducing work efficiency; and when the heating time is insufficient, it is easy to be mixed into the solder to form "Slag inclusion" defect; for the welding of switching elements, excessive flux will easily flow to the contacts, resulting in poor contact.
The proper amount of flux should be such that the rosin can only soak the solder joints to be formed, and do not allow the rosin to flow through the printed board to the component surface or socket holes (such as IC sockets). For the welding wire with rosin core, there is basically no need to apply flux.
4. Keep the soldering iron tip clean
Because the soldering iron tip is in a high temperature state for a long time during soldering, and it is exposed to flux and other thermally decomposed substances, its surface is easily oxidized to form a layer of black impurities. These impurities almost form a thermal insulation layer, which makes the soldering iron tip lose its heating effect. Therefore, rub off impurities on the soldering iron stand at any time. Wiping the tip of the soldering iron with a damp cloth or sponge at any time is also a common method.
5. Heating depends on solder bridge
In non-pipeline operations, the shape of the solder joints for one-time welding is varied, and it is impossible for us to constantly change the soldering iron tip. To improve the heating efficiency of the soldering iron tip, it is necessary to form a solder bridge for heat transfer. The so-called solder bridge is a bridge that relies on a small amount of solder on the soldering iron to transfer heat between the tip of the soldering iron and the weldment during heating. Obviously, because the thermal conductivity of molten metal is much higher than that of air, the weldment is quickly heated to the welding temperature, as shown in Figure 4. It should be noted that the amount of tin retained as a solder bridge should not be too much.
6. The amount of solder should be appropriate
Excessive solder not only consumes the more expensive tin unnecessary, but also increases the soldering time and correspondingly reduces the working speed. What's more serious is that in high-density circuits, excessive tin can easily cause undetectable short circuits.
However, too little solder can not form a firm bond, reducing the strength of the solder joints, especially when soldering wires on the board, insufficient solder often causes the wires to fall off.
7. Weldments must be firm
Do not move or vibrate the weldment before the solder solidifies, especially when using tweezers to clamp the weldment, be sure to wait for the solder to solidify before removing the tweezers. This is because the solidification process of solder is a crystallization process. According to the crystallization theory, external force (movement of the weldment) during crystallization will change the crystallization conditions, resulting in coarse crystals, resulting in the so-called "cold welding". The appearance phenomenon is that the surface is dull and has the shape of bean dregs; the internal structure of the solder joint is loose, and it is easy to have air gaps and cracks, which will reduce the strength of the solder joint and poor electrical conductivity. Therefore, the weldment must be kept still before the solder solidifies. In actual operation, various suitable methods can be used to fix the weldment, or reliable clamping measures can be used.
8. Pay attention to the evacuation of the soldering iron
The soldering iron should be handled in a timely manner, and the angle and direction of the withdrawal have a certain relationship with the formation of the solder joint.
Gently rotate when removing the soldering iron to keep the solder joints with proper solder, which needs to be experienced in actual operation.

6. What flux should I choose for the hand immersion tin furnace? SMD is already on the PCB. Doesn't it hurt the SDM?
It is not the flux that hurts the SMD, but the temperature of the tin furnace and the length of the dipping time during the dipping...Of course, if the soldering effect of the flux is good, the dipping time will be relatively shorter. Of course, for SMD components The thermal shock destructiveness is small. Sometimes the red glue used has poor heat resistance. During immersion tin, it will cause the SMD components to fall into the tin furnace. These are also not directly related to the flux, but there are certain Causality...The premise is to find a flux with strong soldering effect and fast soldering but good safety performance, which can help you shorten the soldering time as much as possible, and naturally it will not harm the SMD components to the maximum. .
The activity of the flux is stronger, and naturally its residues are relatively more corrosive. Products with higher requirements for electrical insulation performance should also be considered clearly when choosing a flux... ......