Circuit board welding technology paper

Circuit board welding is an important part of electronic technology. Correct welding joint design and good machining technology are the key factors to obtain reliable welding. Xiaobian for you to finish the circuit board welding technology paper, I hope you like.

Circuit board welding technology paper I
Some understandings on the welding and wiring of printed circuit board

Abstract: PCB welding is an important part of electronic technology. Correct welding joint design and good machining technology are the key factors to obtain reliable welding. “Reliable” means that the solder joint not only has all the properties required when the product is first produced, but also works correctly throughout the life of the electronic product.

Key words: circuit board welding wiring principle

Welding is one of the important links in the manufacturing of electronic products, if there is no corresponding process quality assurance, any well-designed electronic products are difficult to meet the design requirements. The function of electronic products depends on the correct interconnection of electronic components, and the interconnection of these components is mostly based on the circuit board welding. The circuit board welding has been playing an important role in the assembly of electronic products. Even though there are many connection technologies available today, circuit board welding remains the dominant technology.

Although the physical-chemical principles of all welding processes are the same, the welding of electronic circuits has its own characteristics, namely high reliability and miniaturization, which is consistent with the characteristics of electronic products. The welding quality of circuit board is affected by many factors. For example, the type of the substrate material and its surface layer and coating type and thickness, processing technology and method, the surface state before welding, flux composition, welding method, welding temperature and time, the gap size of the substrate to be welded, flux type and performance, welding tools, etc. Not only the oxides on the surface of the lead wires of the components being welded and the intermetallic compounds in the internal structure of the lead wires are the important factors affecting the solderability of the lead wires, but also the oxides on the surface of the printed board are the main factors affecting the solderability of the pads.

The quality of the solder depends mainly on the ability of the solder to wetting the surface of the solder, that is, the wetting and solderability of the two metal materials. If the weldability of the weldment is poor, it is impossible to solder a qualified joint. Weldability refers to the ability of the weldment and solder to form a good combination under the action of appropriate temperature and flux. Not all materials can be connected by tin welding, only some metals have good weldability, generally copper and its alloy, gold, silver, zinc, nickel and so on have good weldability, and aluminum, stainless steel, cast iron and so on are very poor weldability. Special fluxes and methods are generally required to solder.

In order to achieve a good combination of solder and weldment, the weldment surface must be kept clean. Even for weldables with good weldability, if there is oxide layer, dust and grease on the weldments surface, it must be cleaned up before welding, otherwise the formation of alloy layer around the weldments will be affected, thus the welding quality cannot be guaranteed. Manual welding is a traditional welding method. Although the production of bulk electronic products is less manual welding, it is inevitable to use manual welding in the maintenance and debugging of electronic products. The quality of welding also directly affects the maintenance effect. Manual welding is a practical skill, after understanding the general method, to practice more; More practice, can have better welding quality. The circuit board welding wiring should pay attention to several principles I summarized as follows.

1. The edge lines of the input end and the output end should be avoided to be parallel to each other to avoid reflection interference. If necessary, ground wire should be added for isolation. The wiring of two adjacent layers should be perpendicular to each other. Parallelism is easy to produce parasitic coupling.

2. Common ground processing of circuit and analog circuit. Many PCBs are no longer single-function circuits (digital or analog), but consist of a mixture of digital and analog circuits. Therefore, it is necessary to consider the interference between them when wiring, especially the noise interference on the ground wire. The sensitivity of the high frequency digital circuits, analog circuits, the signal wire, high-frequency signal lines as far as possible away from the sensitive analog devices, for the ground, the PCB to the outside world is only one node, so must be within the PCB processing, mold has problem, and inside the plate to digital and analog is actually are divided between them, only Is in the PCB and the outside connection interface (such as plug, etc.). There is a short connection between the digital and the analog. Note that there is only one connection point. There are also on the PCB, this is determined by the system design.

3. The bending of the wire is generally taken as a circular arc, while the right Angle or included Angle will affect the electrical performance in the high-frequency circuit. If a right Angle must be taken, two 135° angles are generally used to replace the right Angle.

4. Handling of connecting legs in large area conductors. In a large area of grounding (electricity), the legs of commonly used components are connected with it. The treatment of the connecting legs needs comprehensive consideration. In terms of electrical performance, it is better for the solder pad of the component legs to be fully connected with the copper surface, but there are some bad hidden dangers for the welding assembly of components, such as: (1) Welding requires a high-power heater. ② easy to cause virtual solder joint. Therefore, taking into account the electrical performance and process needs, we make cross-shaped welding pads, which are called heatshield, commonly known as Thermal welding pads. In this way, the possibility of creating a virtual solder joint due to excessive heat dissipation of the section during welding is greatly reduced. The same is done for the grounding leg of the multilayer board.

The grounding wire should be thickened as far as possible. If the grounding wire is very thin, the grounding potential changes with the current, so that the anti-noise performance is reduced. Therefore, the grounding wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be above 2-3mm.

6. Correct single and multi-point grounding. In the low-frequency circuit, the signal operating frequency is less than 1MHz, its wiring and the inductance between the devices have little influence, and the grounding circuit formed by the circulation has a greater influence on the interference, so a point grounding should be used. When the signal operating frequency is greater than 10MHz, the length of the ground wire should not exceed 1/20 of the wavelength if one point grounding is used, otherwise the multi-point grounding method should be used.

7. When the signal wire is distributed on the electrical (ground) layer and the multi-layer printed board is distributed, because there are not many wires left in the signal wire layer, adding more layers will cause waste and increase the workload of the production, and the cost will increase accordingly. To solve this contradiction, it is possible to consider wiring on the electrical (ground) layer. The power supply layer should be considered first and the formation should be considered second. It is best to preserve the integrity of the formation.

8. As far as possible, shorten the wiring between high-frequency components and try to reduce their distribution parameters and electromagnetic interference between each other. Vulnerable components should not be placed too close to each other, and input and output components should be kept as far apart as possible. There may be a high potential difference between some components or conductors, so the distance between them should be increased to avoid an accidental short circuit. Components with high voltage should be arranged as far as possible in a place that is not easy to touch when debugging.

9. Design rule check. Wiring design is completed, it is necessary to carefully check the wiring design by the designers is in line with the rules, rules at the same time also need to confirm whether accord with the demand of the PCB production process, general examination has the following several aspects: line to line, line with element bonding pad, the line and communicating pores, element bonding pad and communicating pores, through hole and the distance between the through hole is reasonable, whether to meet the production Please? Are the power cords and ground wires of appropriate width and tight coupling between the power and ground wires (low wave impedance)? Is there any place in the PCB where the ground wire can be widened? Are the best practices used for critical signal lines, i.e. minimum length, guard lines, and clear separation of input and output lines? Are there separate ground lines for analog and digital circuits? Will the graphics (such as icon, mark) added to PCB cause signal short circuit? Modify some undesirable lines. Is there any process line on PCB? Is the solder stopper consistent with the manufacturing process, is the solder stopper size appropriate, and is the character mark pressed on the device pad? So as not to affect the quality of Denso. Are frame edges of the power source formation in multilayer panels reduced? If the copper foil of the power supply layer is exposed, it is easy to cause a short circuit.

The purpose of this paper is to explain the process of PCB PCB design and some matters needing attention, to provide design specifications for designers, facilitate the designers to communicate and check each other.

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