What should be paid attention to when drawing PCB from the perspective of welding
01. Factors affecting PCB welding quality
From PCB design to the completion of welding of all components into a high-quality circuit board, PCB design engineers and even welding processes, the level of welding workers and many other links need to be strictly controlled. The main factors are: PCB drawing, quality of circuit board, quality of device, degree of oxidation of device pins, quality of solder paste, printing quality of solder paste, accuracy of program compilation of SMT machine, mounting quality of SMT machine, setting of temperature curve of reflow oven and so on.
The link that welding factory itself cannot cross is the link of PCB drawing. Since people who do circuit design often do not solder circuit boards, they cannot get direct welding experience and do not know the various factors that affect welding; and the workers in welding factory do not know how to draw boards. They only complete production tasks and have no mind or ability to analyze the causes of poor welding. Since these two aspects of talents perform their duties, it is difficult to combine them organically.
02. Suggestions for drawing PCB drawings
Below, I will give some suggestions to design and wiring engineers who draw PCB drawings on the link of PCB drawing, hoping to avoid various bad drawing methods that affect welding quality in the process of drawing. It will be mainly introduced in the form of pictures and text.
1. About positioning holes: Four holes (minimum hole diameter 2.5mm) should be left at the four corners of the PCB board to position the circuit board when printing solder paste. The center of the circle in the X-axis or Y-axis direction is required to be on the same axis, as shown in the following figure:
2. About the Mark point: It is used for positioning the placement machine. The Mark point should be marked on the PCB board. The specific location: at the diagonal corner of the board, it can be a round or square pad, and it should not be mixed with the pads of other devices. If there are devices on both sides, both sides should be marked.
When designing PCB, please pay attention to the following points:
a. The shape of the Mark point is as follows. (Symmetrical up and down or symmetrical left and right)
b. The size of A is 2.0mm.
c. There should be no shape or color changes that may cause incorrect identification within 2.0mm from the outer edge of the Mark point. (Pad, solder paste)
d. The color of the Mark point should have a light and dark difference from the color of the surrounding PCB.
e. In order to ensure the recognition accuracy, copper or tin is electroplated on the surface of the Mark point to prevent surface reflection. For marks with only lines in shape, light spots cannot be identified.
As shown in the figure below:
3. About leaving 5mm edge: When drawing PCB, leave no less than 3mm edge in the long direction for the placement machine to transport the circuit board. The placement machine cannot place components within this range. Do not place patch components within this range. As shown in the figure:
For circuit boards with components on both sides, it should be considered that the components on the side that have been soldered will be rubbed off during the second reflow, and in serious cases, the soldering pads will be rubbed off and the circuit board will be damaged. As shown in the following figure:
Therefore, it is recommended that the chip components should not be placed within 5mm of the long side of the side with fewer chips (generally the bottom side). If the circuit board area is limited, you can add a process edge to the long side, see Article 17 of this article "Suggestions on panel assembly and process edge addition".
4. Do not make vias directly on the pad: The defect of making vias directly on the pad is that the solder paste melts and flows into the via during reflow, causing the device pad to lack tin, thus forming a cold solder joint. As shown in the figure:
5. Regarding the polarity marking of diodes and tantalum capacitors: The polarity marking of diodes and tantalum capacitors should comply with industry regulations to prevent workers from soldering in the wrong direction based on experience. As shown in the figure:
6. Regarding silk screen and logo: Please hide the device model. Especially for circuit boards with high device density. Otherwise, it will be difficult to find the soldering position due to the dazzling display. As shown in the following figure:
Do not mark the model number without the number, as shown in the figure below, which will cause the placement machine to be unable to program.
The font size of the silk-screen characters should not be too small to be clearly seen. The characters should be placed in a staggered manner across the vias to avoid misreading.
7. Regarding the extension of IC pads: When drawing PCB for ICs packaged in SOP, PLCC, QFP, etc., the pads should be extended. The pad length on PCB = IC foot length × 1.5 is appropriate. This makes it easier to melt the chip pins, PCB pads, and tin into one when soldering with a soldering iron. As shown in the figure:
8. Regarding the width of IC pads: For ICs packaged in SOP, PLCC, QFP, etc., the width of pads should be noted when drawing PCB. The width of pad a on PCB = the width of IC foot (i.e.: Nom. value in datasheet). Please do not increase the width to ensure that b (i.e. between two pads) has enough width to avoid continuous welding. As shown in the figure:
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9. Do not rotate the device at any angle when placing it: Since the placement machine cannot rotate at any angle, it can only rotate 90°, 180°, 270°, and 360°. As shown in Figure B below, if it is rotated 1°, the device pins and the pads on the circuit board will be offset by 1° after placement by the placement machine, thus affecting the welding quality.
10. Matters needing attention when short-circuiting adjacent pins: The short-circuiting method in Figure a below is not conducive to workers identifying whether the pins should be connected, and the appearance after welding is not beautiful. If you short-circuit and add solder mask according to the methods in Figure b and Figure c when drawing, the welding effect will be different: as long as each pin is not connected, the chip will not have short circuit phenomenon, and the appearance is also beautiful.
11. Regarding the problem of the pad in the middle of the chip: When drawing a chip with a pad in the middle of the chip, if you draw the pad in the middle according to the chip package drawing, it is easy to cause a short circuit. It is recommended to reduce the size of the pad in the middle to increase the distance between it and the surrounding pin pads, thereby reducing the chance of a short circuit. As shown in the following figure:
12. Do not arrange two thicker components closely together: As shown in the figure below, such layout will cause the placement machine to touch the previously placed component when placing the second component. The machine will detect danger and cause the machine to automatically power off.
13. About BGA: Due to the special BGA package, its pads are all under the chip, and the welding effect cannot be seen from the outside. For the convenience of rework, it is recommended to make two positioning holes of Hole Size: 30mil on the PCB board to position the steel mesh (used to scrape solder paste) during rework.
Warm reminder: The size of the positioning hole should not be too large or too small. The needle should not fall off or shake after insertion, and it should be slightly tight when inserted, otherwise the positioning will be inaccurate. As shown below:
It is also recommended to leave space around the BGA to avoid placing components, so that the stencil can be placed to scrape the solder paste during repair.
14. Regarding the color of the PCB board: it is recommended not to make it red. Because the red circuit board appears white under the red light source of the camera of the mounter, it cannot be programmed and is not convenient for the mounter to solder.
15. Regarding small components under large components: Some people like to arrange small components under large components on the same layer, for example: there is a resistor under the digital tube, as shown below:
Such layout will make it difficult to repair, and the digital tube must be removed first during repair, which may also cause damage to the digital tube. It is recommended to arrange the resistor under the digital tube to the bottom side, as shown below:
16. Regarding the connection between copper cladding and pad affecting tin melting: copper cladding absorbs a lot of heat, making it difficult for solder to fully melt, thus forming a cold solder joint. As shown in the figure:
In Figure a, the device pad is directly connected to the copper clad; in Figure b, although the 50-pin connector is not directly connected to the copper clad, since the middle two layers of the four-layer board are covered with large areas of copper, both Figure a and Figure b will cause the copper clad to absorb a large amount of heat and cause the solder paste to not fully melt. The body of the 50-pin connector in Figure b is a plastic that is not resistant to high temperatures. If the temperature is set too high, the body of the connector will melt or deform. If the temperature is set too low, the copper clad will absorb a large amount of heat and cause the solder paste to not fully melt. Therefore, it is recommended to isolate the pad from the large area of copper clad. As shown in the figure:
17. Suggestions on panelization and processing
03. Summary
Nowadays, there are more and more engineers who can use software to draw, wire and design PCBs. However, once the design is completed, it can greatly improve the welding efficiency. The author believes that it is necessary to pay attention to the above factors. And cultivating good drawing habits and being able to communicate well with the processing factory are what every engineer should consider.
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