PCB News - Precautions for printed circuit board copper wiring

Trace current density: Now most electronic circuits are made of insulating board bound copper. The copper thickness of the commonly used circuit board is 35μm, and the current density value can be taken according to the 1A/mm empirical value for the wiring. For the specific calculation, please refer to the textbook. In order to ensure the mechanical strength of the wiring, the line width should be greater than or equal to 0.3mm (other non-power circuit boards may have a smaller minimum line width). Circuit boards with a copper thickness of 70μm are also common in switching power supplies, so the current density can be higher.

To add, the commonly used circuit board design tool software generally has design specification items, such as line width, line spacing, dry plate via size and other parameters can be set. When designing the circuit board, the design software can automatically execute in accordance with the specifications, which can save a lot of time, reduce part of the workload, and reduce the error rate.

Generally, double-sided PCB boards can be used for circuits or wiring with high reliability requirements. It is characterized by moderate cost and high reliability, which can meet most applications.

Some products in the module power line also use multi-layer boards, which are mainly convenient for integrating power devices such as transformers and inductors, optimizing wiring and power tube heat dissipation. It has the advantages of good process appearance and consistency, and good heat dissipation of the transformer, but its disadvantages are high cost and poor flexibility, and it is only suitable for industrialized large-scale production.

Single-panel, the general-purpose switching power supplies circulating in the market almost all use single-sided circuit boards, which have the advantage of low cost, and some measures in the design and production process can also ensure its performance.

Today, I will talk about some experiences in the design of single-sided printed circuit boards. Because single-sided boards have the characteristics of low cost and easy manufacturing, they are widely used in switching power supply circuits. Because they have only one side of copper bound, the electrical connection of the device and the mechanical fixation are all To rely on that layer of copper, you must be careful when handling it.

In order to ensure a good mechanical structure performance of the welding, the single-sided board pad should be slightly larger to ensure a good bond between the copper skin and the substrate, so that the copper skin will not peel off or break off when subjected to vibration. Generally, the width of the welding ring should be greater than 0.3mm. The diameter of the pad hole should be slightly larger than the diameter of the device pin, but it should not be too large. Ensure that the solder connection distance between the pin and the pad is the shortest. The size of the pad hole should not hinder the normal inspection. The diameter of the pad hole is generally larger than the pin. The diameter is 0.1-0.2mm. Multi-pin devices can be larger to ensure smooth inspection.

The electrical connection should be as wide as possible, and in principle, the width should be greater than the diameter of the pad. In special cases, the wire must be widened when the connection meets the pad (commonly known as teardrop generation) to avoid breakage between the wire and the pad under certain conditions. In principle, the minimum line width should be greater than 0.5mm.

The components on the single-sided board should be close to the circuit board. For devices that require overhead heat dissipation, a sleeve should be added to the pin between the device and the circuit board, which can support the device and increase insulation. It is necessary to minimize or avoid external impact on the pad and pin connection. The impact caused by the strengthening of the firmness of the welding. The heavier components on the circuit board can increase the supporting connection points, which can strengthen the connection strength with the circuit board, such as transformers and power device radiators.

The single-panel welding surface pins can be kept longer without affecting the distance from the shell. The advantage is that it can increase the strength of the welding part, increase the welding area, and the phenomenon of virtual welding can be found immediately. When the pin is long and cuts the leg, the welding part receives less force. In Taiwan and Japan, the process of bending the device pins at a 45-degree angle with the circuit board on the soldering surface and then soldering is often used, and the reason is the same as above. Today, I will talk about some matters in the design of double-sided boards. In some application environments with higher requirements or higher trace density, double-sided printed circuit boards are used. Its performance and various indicators are much better than single-sided boards.

The double-sided board pad has a higher strength due to the metalization of the hole, the solder ring can be smaller than the single-sided board, and the diameter of the pad hole can be slightly larger than the pin diameter, because the solder solution can penetrate to the top layer through the solder hole during the soldering process. Pads to increase soldering reliability. But there is a drawback. If the hole is too large, part of the device may float up under the impact of the jet tin during wave soldering, resulting in some defects.

For the treatment of high-current traces, the line width can be processed according to the previous post. If the width is not enough, it can generally be solved by tinning the traces to increase the thickness. There are many methods.

1, set the trace to the pad attribute, so that the trace will not be covered by the solder resist during the circuit board manufacturing, and it will be tinned during the hot air leveling.

2, place a pad on the wiring, set the pad to the shape that needs to be routed, and pay attention to setting the pad hole to zero.

3. This method is the most flexible method to place wires on the solder mask, but not all PCB board manufacturers will understand your intentions, so you need to use text to explain. No solder mask will be applied to the part where the wire is placed on the solder mask

Several methods of tinning the circuit are as above. It should be noted that if the wide traces are all tinned, after soldering, a large amount of solder will be bonded and the distribution is very uneven, which affects the appearance. Generally, a thin strip of tin-plated width is 1~1.5mm, and the length can be determined according to the circuit. The tin-plated part is separated by 0.5~1mm. The double-sided circuit board provides great selectivity for layout and wiring, which can make the wiring more Tends to be reasonable. Regarding grounding, the power ground and the signal ground must be separated. The two grounds can be merged at the filter capacitor to avoid accidental factors of instability caused by large pulse currents passing through the signal ground connection. The signal control loop should be grounded as far as possible. There is a trick, try to put non-grounded traces on the same wiring layer, and finally lay ground wires on another layer. The output line generally passes through the filter capacitor first, and then to the load. The input line must also pass through the capacitor first, and then to the transformer. The theoretical basis is to let the ripple current pass through the filter capacitor.

Voltage feedback sampling, in order to avoid the influence of large current passing through the wiring, the sampling point of the feedback voltage must be placed at the end of the power supply output to improve the load effect index of the whole machine.

The wiring change from one wiring layer to another wiring layer is generally connected by via holes, and it is not suitable to realize through the device pin pads, because this connection relationship may be destroyed when the device is inserted, and when every 1A current passes through, There should be at least 2 vias, and the diameter of vias should be greater than 0.5mm in principle. Generally 0.8mm can ensure processing reliability.

Device heat dissipation. In some low-power power supplies, the circuit board traces can also serve as heat dissipation. Its characteristic is that the traces are as wide as possible to increase the heat dissipation area. No solder resist is applied. If possible, the vias can be evenly placed to enhance the thermal conductivity.