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What should be paid attention to when drawing PCB from the perspective of welding

Aug 24, 2018

With the rapid development of electronic technology, miniaturization, miniaturization, and high-density chips with a pitch of 0.3mm to 0.5mm are becoming more and more popular, and the requirements for electronic soldering technology are getting higher and higher. Although there are now more sophisticated placement machines that can replace manual welding, there are too many factors that affect the quality of the weld. This article will introduce some points that need to be paid attention to during PCB design from the perspective of patch soldering. According to experience, if these requirements are not met, it is likely to cause low soldering quality, soldering and even damage soldering when reworking the PCB. Disk or board.


I. Factors affecting the quality of PCB soldering


From PCB design to the soldering of all components to a high-quality circuit board, there is a need for strict control of the PCB design engineer and even the welding process, the level of the welding workers. There are mainly the following factors: PCB diagram, quality of the board, quality of the device, oxidation degree of the device pins, quality of the solder paste, printing quality of the solder paste, accuracy of the placement of the placement machine, and placement of the placement machine The quality of the assembly, the setting of the temperature profile of the reflow oven, and the like.


The link that the welding factory itself cannot overcome is the link of PCB drawing. Because the people who do circuit design often do not solder the circuit board and can not obtain direct welding experience, do not know the various factors affecting the welding; and the workers in the welding factory do not understand the drawing board, they only complete the production task, no thought, no ability to analyze Causes of poor soldering. Since these two talents perform their duties, it is difficult to integrate them organically.


Second, the recommendations when drawing PCB drawings


Below I will make some suggestions for the design and wiring engineers who draw PCB diagrams in the process of PCB drawing. I hope that in the process of drawing, various bad painting methods that affect the quality of welding can be avoided. It will be mainly introduced in the form of graphic.


1. About positioning holes: Four holes (minimum aperture 2.5mm) should be left at the four corners of the PCB board to locate the circuit board when printing solder paste. The center of the X-axis or Y-axis is required to be on the same axis, as shown below:



2, on the Mark point: for placement machine positioning. Mark points should be marked on the PCB. The specific position: diagonally on the board, can be round, or square pads, not mixed with the pads of other devices. If there are devices on both sides, both sides should be marked.


When designing the PCB, please note the following:

a, the shape of the Mark point is as follows. (up and down symmetry or bilateral symmetry)

b, A size is 2.0mm.

c. From the outer edge of the Mark point within 2.0 mm, there should be no shape or color change that may cause erroneous recognition. (pad, solder paste)

d, the color of the Mark point should have a difference between the color of the surrounding PCB and the color of the surrounding PCB.

e. In order to ensure the recognition accuracy, the surface of the Mark point is plated with copper or tin to prevent surface reflection. For markers with only lines, the spot is not recognized.


As shown below


3. About leaving 5mm side: When drawing PCB, keep the side of not less than 3mm in the long side direction for the placement machine to transport the circuit board. In this range, the placement machine cannot mount the device. Do not place the chip device within this range. Figure:

A circuit board with two sides of the device should take into account that the second over-reflow will remove the soldered side of the device, and in severe cases, the pad will be removed and the board will be destroyed. As shown below:


Therefore, it is recommended that the chip side is not placed on the long side of the chip (usually the Bottom surface) within 5 mm from the side. If it is true that the board area is limited, you can add the process side on the long side. See Article 17 “Recommendations and Adding Process Edges”.图片4.png


4. Do not directly pass the hole on the pad: The defect of the via hole directly on the pad is that the solder paste melts and flows into the via hole after over-flow, causing the device pad to be deficient in tin, thereby forming a solder joint. Figure:

5. Regarding the polarity of diodes and tantalum capacitors: the polarity of diodes and tantalum capacitors should be in accordance with the rules, so as to avoid the wrong direction of welding by workers.


6. About silk screen and logo: Please hide the model number. Especially for boards with high device density. Otherwise, dazzling influences the location of the weld. As shown below:


Also don't just mark the model, no label. As shown in the figure below, it cannot be performed when the placement machine is programmed.


The font size of the silkscreen characters should not be too small to be seen. Character placement should be staggered to avoid misreading.


7. The IC pad should be extended: ICs such as SOP, PLCC, QFP, etc. should extend the pad when drawing the PCB. The pad length on the PCB = IC leg length × 1.5 is suitable, so that it is convenient for manual soldering with soldering iron. The pins are integrated with the PCB pads and tin. Figure:


8. About the width of the IC pad: SOP, PLCC, QFP and other packaged ICs, pay attention to the width of the pad when drawing the PCB, the width of the pad a on the PCB = the width of the IC foot (ie: the Nom. value in the datasheet) ), please do not widen, to ensure that b (ie between the two pads) has sufficient width to avoid continuous welding. Figure:


9. Place the device not to rotate at any angle: Since the placement machine cannot rotate at any angle, it can only rotate 90°C, 180°C, 270°C, 360°C. As shown in Figure B below, 1 °C is rotated. After the placement machine is mounted, the device pins and the pads on the board are staggered by an angle of 1 °C, which affects the soldering quality.


10. Problems to be noticed when shorting adjacent pins: The shorting method in Figure a below is not conducive to workers to identify whether the pins should be connected and not beautiful after soldering. If you draw a picture according to the method of Figure b, Figure c and short-circuit and add solder mask, the effect of soldering is different: as long as each pin is not connected, the chip has no short circuit and the appearance is beautiful.


11. There is a problem with the pad in the middle of the chip: When the chip with the pad in the middle of the chip is drawn in the middle of the chip, it is easy to cause a short circuit. It is recommended to shrink the middle pad to increase the distance between it and the surrounding pin pads, thus reducing the chance of a short circuit. As shown below:



12. The two devices with higher thickness should not be closely packed together: as shown in the figure below, such a layout will cause the device to touch the previously attached device when the second device is mounted, and the machine will detect the danger, resulting in The machine is automatically powered off.

13. About BGA: Because the BGA package is special, its pads are under the chip, and the soldering effect is not visible outside. For ease of repair, it is recommended to make two Hole Size: 30 mil locating holes on the PCB to position the stencil (for solder paste) during rework.


Tips: The size of the positioning hole should not be too large or too small. It is better to make the needle not to be dropped, not shaken, or inserted when the needle is inserted. Otherwise, the positioning is not accurate. As shown below:


It is also recommended to leave a space in a certain area around the BGA to place the device so that the soldering paste can be placed under the stencil.


14, about the color of the PCB: it is recommended not to make red. Because the red circuit board is white under the red light source of the camera of the placement machine, it cannot be programmed, and it is not convenient for the placement machine to perform welding.


15. About small devices under large devices: Some people like to place small devices under the same large device. For example, there are resistors under the digital tube, as shown below:


Such typesetting will cause difficulties in rework, and it is necessary to disassemble the digital tube before rework, and it may cause damage to the digital tube. It is recommended to discharge the resistor under the digital tube to the Bottom surface as shown below:


16. The connection between copper and pad affects the melting of tin: Since copper will absorb a large amount of heat, it is difficult to fully melt the solder, thus forming a virtual solder. as the picture shows:


In Figure a, the device pad is directly connected to the copper; in Figure b, the 50Pins connector is not directly connected to the copper. However, since the middle two layers of the four-layer board are large-area copper, the figures a and b will be covered. Copper absorbs a large amount of heat and causes the solder paste to not melt sufficiently. The body of the 50Pins connector in Figure b is a plastic that is not resistant to high temperature. If the temperature is set high, the body of the connector will melt or deform. If the temperature is set low, the copper will absorb a large amount of heat and the solder paste will not be fully melted. . Therefore, it is recommended that the pads be isolated from large areas of copper. as the picture shows:


17. Suggestions on the puzzle and the process side:


Third, summary


Nowadays, more and more engineers can draw, route and design PCBs with software, but once designed, and can improve welding efficiency, the author believes that it is necessary to pay attention to the above factors. And to cultivate good drawing habits, it is very good to communicate well with the processing factory, every engineer must consider.

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