PCBA Tech - PCBA area requirements and PCB identification method

PCBA reflow welding process and regional requirements

In the PCBA processing process, reflow soldering is a particularly important process. PCBA reflow soldering has excellent manufacturability, and there are no special regulations on the specific position, direction and spacing of the layout of electronic components. The layout of the electronic components on the reflow soldering surface mainly considers the regulations on the spacing of the electronic components by the solder paste printing stencil window, the regulations on the inspection and repair area, and the regulations on the reliability of the process flow.

Process flow

The process flow of reflow soldering: printing solder paste, one patch, and reflow soldering.

Regional requirements

1. Prohibited area for surface mount electronic components

1. Transmitting side (side parallel to the conveying direction), the 5mm interval of the side is the forbidden area. 5mm is a range that all SMT equipment can accept.

2. Non-conveying side (the side perpendicular to the conveying direction), the range of 2~5mm away from the side is the forbidden area.

Theoretically, electronic components can be laid out to the side. However, considering the edge effect of the deformation of the steel mesh, it is necessary to set up a forbidden area of 2~5mm or more to ensure that the thickness of the solder paste meets the regulations.

3. No electronic components and their pads of any kind can be placed in the forbidden area of the transmission side. The layout of surface-mounted electronic components is mainly prohibited in the non-transmission-side restricted area. However, if the layout of electronic components is required, the process requirements of anti-wave soldering and upturning tin tooling should be considered.

2. Electronic components should be arranged regularly to the utmost extent

The positive poles of the electronic components with polarity, the IC gaps, etc. are placed upwards and left. The regular arrangement is convenient for inspection and helps to increase the patching speed.

Third, the maximum uniform layout of electronic components

Uniform distribution helps reduce the temperature difference on the board during reflow soldering, especially the centralized layout of large-size BGA, QFP, and PLCC, which will cause local low temperature on the PCB circuit board.

Fourth, the spacing (interval) between components is mainly related to the requirements of assembly and welding operations, inspections, and repair areas.

For special needs, such as the installation area of the radiator and the operation area of the connector, please develop the design concept according to the actual situation.

5. Double-sided reflow soldering boards (such as double-sided full SMD boards, mask selective soldering double-sided boards), usually the side with relatively small number and types of components (Bottom side) is soldered first

This surface has to undergo a secondary reflow soldering process, and it is not allowed to place relatively heavy and relatively high electronic components with few pins. Under normal circumstances, the experience is that for BGA devices placed on the Bottom surface, the maximum gravity that the weld can withstand is 0.03g/mm.

Sixth, avoid the BOA design concept of double-sided mirror mounting to the greatest extent. According to relevant experimental research, this design concept reduces the reliability of solder joints by about 50%.

Seven, reflow soldering solder is a fixed supply, so avoid drilling holes on the pads. If required, the plug hole plating design concept can be adopted.

8. For stress-sensitive devices such as BGA, chip capacitors, crystal oscillators, etc., avoid placing them near the separation side of the imposition or the connecting bridge, which is likely to cause the PCB circuit board to bend during assembly.

PCB circuit board recognition method

Considering the complexity of PCB circuit board diagrams, the following series of methods and techniques can be used to improve the speed of recognizing diagrams.

1. According to the basic characteristics of the appearance and shape of a series of electronic components, these electronic components, such as integrated circuits, power amplifier tubes, switches and transformers, can be found relatively conveniently and quickly.

2. Compared with integrated circuits, a certain basic integrated circuit can be found according to the model specifications on the integrated circuit. Even if there are no basic rules for the distribution and arrangement of electronic components, the electronic components in the same unit circuit are usually clustered together.

3. A series of unit circuits have relatively basic characteristics. According to these basic characteristics, they can be found easily and quickly. For example, there are relatively many diodes in the rectifier circuit, and the power amplifier tube has a heat sink. The filter capacitor has the largest capacity and the largest volume.

4. When looking for the ground wire, the large area of copper foil circuit on the PCB circuit board is the ground wire, and the ground wire on a PCB circuit board is connected everywhere. In addition, the metal shells of some electronic components are grounded. When looking for a ground wire, any of the above can be used as a ground wire. In some machines, the ground wires of each PCB circuit board are also connected to each other, but when the connector between each PCB circuit board is not connected, the ground wire between each PCB circuit board It doesn't work. Pay attention to this when overhauling.

5. In the process of comparing the PCB circuit board diagram with the actual PCB circuit board, draw the same identification direction on the PCB circuit board diagram and the PCB circuit board respectively, so that the PCB circuit board diagram can be the same as the PCB circuit board. Recognizing the direction of the picture, eliminating the need to compare the direction of the recognizing picture every time, so that it can be greatly convenient and quick to recognize the picture.

6. When observing the connection status of the electronic components and the copper foil circuit on the PCB circuit board and observing the direction of the copper foil circuit, it can be illuminated with a light. Place the lamp on the side with the copper foil circuit. On the side where the electronic components are installed, the connection between the copper foil circuit and the electronic components can be clearly and quickly observed, so that there is no need to flip the PCB circuit board. Because constantly flipping the PCB circuit board is not only cumbersome, but also very easy to break the leads on the PCB circuit board.