Solder circuit board skill 1:
The process flow of selective soldering includes: flux spraying, circuit board preheating, immersion soldering, and drag soldering. The flux coating process plays an important role in selective soldering.
At the end of welding heating and welding, the flux should have sufficient activity to prevent bridging and prevent oxidation of the circuit board. Flux spraying is carried by X/Y robotic arms to the circuit board through the flux nozzle, and the flux is sprayed onto the soldering position of the PCB circuit board.
Solder circuit board skill 2:
The key to microwave peak selection welding after reflow soldering is accurate flux spraying, and the micro hole spraying method will not contaminate the area outside the solder joint.
The diameter of the solder dot pattern for micro dot spraying is greater than 2mm, so the position accuracy of the solder deposited on the circuit board by spraying is ± 0.5mm to ensure that the solder always covers the soldered area.
Solder circuit board skill 3:
The process characteristics of selective soldering can be understood by comparing it with wave soldering. The obvious difference between the two is that in wave soldering, the lower part of the circuit board is completely immersed in liquid solder, while in selective soldering, only a specific area is in contact with the solder wave.
Due to the fact that the circuit board itself is a poor heat conduction medium, it will not heat and melt the solder joints in adjacent components and circuit board areas during soldering.
Before soldering, it is also necessary to apply flux in advance. Compared with wave soldering, flux is only applied to the lower part of the circuit board to be soldered, rather than the entire PCB circuit board.
In addition, selective welding is only applicable to the welding of plug-in components. Selective welding is a new method, and a thorough understanding of the selective welding process and equipment is necessary for successful welding.
2. Precautions for soldering circuit boards
1. Reminder: After receiving the PCB bare board, the first step is to conduct a visual inspection to see if there are any short circuits, open circuits, or other issues. Then, familiarize yourself with the development board schematic and compare it with the PCB silk screen layer to avoid any discrepancies between the schematic and the PCB.
2. After the materials required for PCB soldering are fully prepared, the components should be classified, and all components can be classified into several categories according to their size for subsequent soldering. A complete material list needs to be printed. During the welding process, if one item is not welded completely, use a pen to cross out the corresponding option, which facilitates subsequent welding operations.
Before welding, anti-static measures such as wearing static rings should be taken to avoid static electricity causing damage to components. After the equipment required for welding is fully prepared, the soldering iron tip should be kept clean and tidy. For the first soldering, it is recommended to use a flat angle soldering iron. When soldering components such as 0603 packaging, the soldering iron can better contact the solder pad and facilitate soldering. Of course, for experts, this is not a problem.
3. When selecting components for soldering, they should be soldered in the order of components from low to high and from small to large. To avoid inconvenience caused by the welding of larger components that have been welded to smaller components. Prioritize soldering integrated circuit chips.
4. Before soldering integrated circuit chips, it is necessary to ensure that the chip placement direction is correct and error free. For the silk screen layer of chips, rectangular pads generally represent the starting pins. When welding, one pin of the chip should be fixed first, and the position of the component should be fine tuned before fixing the diagonal pins of the chip, so that the component can be accurately connected in position before welding.
6. In surface mount ceramic capacitors and voltage stabilizing circuits, there is no distinction between positive and negative poles for voltage stabilizing diodes, while positive and negative poles need to be distinguished for light-emitting diodes, tantalum capacitors, and electrolytic capacitors. For capacitors and diode components, the end with a prominent label should generally be negative. In the packaging of surface mount LEDs, the direction along the lamp is the positive negative direction. For components labeled as diode circuit diagrams in silk screen printing packaging, the negative terminal of the diode should be placed at one end of the vertical line.
7. For crystal oscillators, passive crystal oscillators generally have only two pins and no positive or negative distinction. Active crystal oscillators generally have four pins, and attention should be paid to the definition of each pin to avoid soldering errors.
8. For the soldering of plug-in components, such as power module related components, the device pins can be modified before soldering. After fixing the components, they are usually melted with a soldering iron on the back and integrated into the front through the solder pads. Soldering does not need to be done too much, but first, the components should be stabilized.
9. PCB design issues discovered during the welding process should be promptly recorded, such as installation interference, incorrect pad size design, component packaging errors, etc., for future improvement.
10. After welding, use a magnifying glass to inspect the solder joints and check for any virtual soldering or short circuits.
11. After the welding work of the circuit board is completed, the surface of the circuit board should be cleaned with alcohol or other cleaning agents to prevent the iron chips attached to the surface of the circuit board from short circuiting the circuit, and also to make the circuit board cleaner and more beautiful.
3. Characteristics of double-sided circuit board
The difference between single-sided circuit boards and double-sided circuit boards is the number of copper layers. EDA365 Electronic Forum Science Popularization: A double-sided circuit board is a circuit board with copper on both sides, which can be connected through vias. And there is only one layer of copper on one side, which can only be used for simple circuits, and the holes made can only be used for plug-ins and cannot conduct.
The technical requirements for double-sided circuit boards are an increase in wiring density, a decrease in aperture size, and a reduction in the size of metalized holes. The quality of the metalized holes relied upon for layer to layer interconnection directly affects the reliability of the printed circuit board.
As the aperture shrinks, impurities that originally had no effect on larger apertures, such as grinding debris and volcanic ash, once left in the small holes, will render chemical copper deposition and electroplating ineffective, resulting in copper free pores and becoming fatal killers of pore metallization.
4. Welding method for double-sided circuit boards
In order to ensure reliable conductivity of the double-sided circuit board, the EDA365 electronic forum suggests that the connection holes on the double-sided board (i.e. the metalized through-hole part) should be soldered with wires first, and the protruding part of the connection wire tip should be cut off to avoid injuring the operator's hand. This is the preparation work for the board's wiring.
5. Welding tips for double-sided circuit boards:
1. Devices that require shaping should be processed according to the requirements of the process drawings; First, shape the product before inserting it.
2. After shaping, the model surface of the diode should face upwards and there should be no inconsistency in the length of the two pins.
3. When inserting devices with polarity requirements, attention should be paid to ensure that their polarity is not reversed. For roller integrated block components, after insertion, whether it is a vertical or horizontal device, there should be no obvious tilt.
4. The power of the soldering iron used for welding is between 25-40W, and the temperature of the soldering iron tip should be controlled at around 242 ℃. If the temperature is too high, the tip is prone to "death", and if the temperature is too low, it cannot melt the solder. The soldering time should be controlled within 3-4 seconds.
5. During formal welding, it is generally operated according to the principle of welding from low to high and from inside to outside. The welding time should be well controlled. If the welding time is too long, it will burn out the components and the copper plated lines on the copper-clad board.
6. Because it is double-sided welding, a process framework for placing the circuit board should also be made to avoid tilting the components below.
7. After the welding of the circuit board is completed, a comprehensive check should be conducted to identify any missing pins or solder joints. After confirmation, excess device pins on the circuit board should be trimmed before proceeding to the next process.
8. In specific operations, relevant process standards should also be strictly followed to ensure the welding quality of the product.
With the rapid development of high technology, electronic products closely related to the public are constantly being updated and replaced. The public also needs high-performance, small-sized, and multifunctional electronic products, which puts forward new requirements for circuit boards.
The double-sided circuit board was born because of this, and due to its wide application, printed circuit board manufacturing has also developed towards lightweight, thin, short, and small.
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