Problems to be noticed when drawing PCB drawings from a welding perspective
Abstract: With the rapid development of electronic technology, miniaturization, miniaturization, BGA, and high-density chips with a pitch of 0.3mm to 0.5mm are becoming more and more common, and the requirements for electronic soldering technology are becoming higher and higher. . Although there are now more sophisticated placement machines that can replace manual welding, there are too many factors affecting the quality of welding. This article introduces several points to pay attention to when designing PCBs from the perspective of chip soldering. According to experience, if these requirements are not followed, it is likely to cause poor soldering quality, false soldering, and even damage to soldering during PCB repair. Disk or circuit board.
I. Factors affecting PCB welding quality
From PCB design to the soldering of all components to a high-quality circuit board, PCB design engineers, as well as the welding process, the level of welding workers, and other links have to be strictly controlled. There are mainly the following factors: PCB drawing, the quality of the circuit board, the quality of the device, the degree of oxidation of the device pins, the quality of the solder paste, the printing quality of the solder paste, the accuracy of the programming of the placement machine, and the placement of the placement machine. Installation quality, temperature profile setting of reflow oven, etc.
The link that the welding factory cannot surpass is the link of PCB drawing. Because people who do circuit design often do not solder circuit boards and cannot obtain direct soldering experience, they do not know the various factors that affect soldering. Workers in the welding plant do not understand drawing boards. They only have to complete production tasks without thought and ability. Causes of poor welding. Because these two kinds of talents perform their respective roles, it is difficult to combine them organically.
II. Suggestions when drawing PCB
Next, I will give some suggestions to the design and wiring engineers who are drawing the PCB. I hope that in the process of drawing, I can avoid various bad drawing methods that affect the welding quality. It will be introduced mainly in graphic form.
1. About positioning holes: Four holes (minimum hole diameter 2.5mm) should be left at the four corners of the PCB board, which is used to position the circuit board when printing solder paste. The center of the circle in the X-axis or Y-axis direction should be on the same axis, as shown in the figure below:
2. About Mark point: It is used for positioning of the placement machine. Mark points should be marked on the PCB board, the specific location: the diagonal diagonal of the board can be round or square pads, do not mix with the pads of other devices. If there are devices on both sides, both sides must be marked.
Please pay attention to the following points when designing the PCB:
A, Mark dots have the following pattern. (Symmetry up or down)
The dimensions of b and A are 2.0 mm.
C. Within 2.0mm from the outer edge of the Mark point, there should be no change in shape and color that may cause erroneous recognition. (Pad, solder paste)
D, the color of the Mark point should be different from the color of the surrounding PCB.
E. In order to ensure the recognition accuracy, copper or tin is plated on the surface of the Mark to prevent surface reflection. Marks with only line shapes cannot be identified.
As shown below:
3. Regarding the 5mm edge: When drawing a PCB, the edge with a length of not less than 3mm should be used for the placement machine to transport the circuit board. The placement machine cannot mount devices within this range. Do not place chip devices within this range. As shown in the figure:
For circuit boards with devices on both sides, you should take into account that the soldered side devices will be wiped off during the second overflow. In severe cases, the pads will be removed and the circuit board will be destroyed. As shown below:
It is recommended not to place a chip device within 5mm of the long side of the side with a small number of chips (generally the Bottom side). If the area of the circuit board is really limited, you can add craft edges on the long side.
4. Do not directly via holes on the pads: The defect of vias directly on the pads is that the solder paste flows into the vias after reflow, which causes the device pads to be short of tin, thereby forming a dummy solder. As shown in the figure:
5. Polarity labeling of diodes and tantalum capacitors: The polarity labeling of diodes and tantalum capacitors should conform to the regulations to prevent workers from welding in the wrong direction based on experience. As shown in the figure:
6. About silk screen and identification: Please hide the device model. Especially circuit boards with high device density. Otherwise, dazzling influences finding the welding position. As shown below:
Do not label only the model, not the label. As shown in the figure below, the placement machine cannot be programmed.
The size of silkscreen characters should not be too small to be clear. Characters should be staggered through holes to avoid misreading.
7. IC pads should be extended: SOP, PLCC, QFP and other packaged ICs should be extended when drawing PCBs. The length of the pads on the PCB = IC foot length × 1.5 is appropriate. The pins are fused with the PCB pads and tin. As shown in the figure:
8. Regarding the width of IC pads: SOP, PLCC, QFP and other packaged ICs. Pay attention to the pad width when drawing the PCB. The width of pad a on the PCB = IC foot width (ie, Nom. Value in datasheet ), Please do not widen, and ensure that b (that is, between the two pads) has sufficient width to avoid continuous soldering. As shown in the figure:
9. Do not rotate the device at any angle: As 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, when it is rotated by 1 ° C, the device pins and the pads on the circuit board will be staggered by 1 ° C after the placement machine is mounted, which will affect the soldering quality.
10. Problems that should be paid attention to when adjacent pins are short-circuited: The short-circuiting method in figure a below is not conducive for workers to identify whether the pins should be connected, and it is not beautiful after welding. If the drawing is shorted and soldered according to the methods of Figures b and c, the effect of soldering will be different: as long as each pin is not connected, the chip will have no short circuit and its appearance is beautiful.
11. Regarding the problem of pads in the middle of the bottom of the chip: When drawing a chip with pads in the middle of the bottom of the chip, if you press the middle pad of the package drawing of the chip, it will easily cause a short circuit. It is recommended to reduce the middle pad to increase the distance between it and the surrounding pin pads, thereby reducing the chance of short circuit. As shown below:
12. The two devices with higher thickness should not be arranged close together: as shown in the figure below, the cloth board will cause the placement machine to touch the previously attached device when mounting the second device, and the machine will detect the danger, causing The machine automatically powers down.
13. About BGA: Because the BGA package is quite special, its pads are all under the chip, and no soldering effect can be seen outside. For the convenience of rework, it is recommended to punch two positioning holes of Hole Size: 30mil on the PCB board, in order to locate the stencil (used to scrape solder paste) during rework.
Reminder: The size of the positioning hole should not be too large or too small. It is better to prevent the needle from falling, shaking, and a little tight when inserting, otherwise the positioning is not allowed. As shown below:
And it is recommended to leave a clear area around the BGA to not place the device, so that the stencil scraping paste can be placed during repair.
14. About the color of the PCB: It is recommended not to make it red. Because the red circuit board is white under the red light source of the camera of the mounter, it cannot be programmed, and it is not convenient for the mounter to solder.
15. About small devices under large devices: Some people like to arrange small devices under large devices on the same layer, for example, there is a resistor under the digital tube, as shown below:
This typesetting will cause difficulties in rework. The digital tube must be dismantled during repair, and the digital tube may be damaged. It is recommended to arrange the resistance under the digital tube to the Bottom surface, as shown below:
16. Concerning the connection between copper clad and the pad affects the melting tin: Because copper clad absorbs a large amount of heat, it is difficult for the solder to fully melt, thereby forming a false solder. as the picture shows:
In Figure a, the device pads are directly connected to the copper cladding; although the 50Pins connector in Figure b is not directly connected to the copper cladding, because the middle two layers of the four-layer board are large-area copper cladding, both of Figure a and b Copper absorbs a large amount of heat and the solder paste cannot be fully melted. The body of the 50Pins connector in Figure b is not resistant to high temperature plastic. If the temperature is set high, the body of the connector will melt or deform. If the temperature is set low, the copper clad absorbs a large amount of heat and the solder paste cannot be fully melted. . Therefore, it is recommended that the pads be isolated from large areas of copper. as the picture shows:
17. Suggestions on adding jigsaw and adding craft edges:
Nowadays, more and more engineers can use software for drawing, wiring and designing PCBs. However, once the design is completed and the welding efficiency can be improved very well, the author believes that it is necessary to pay attention to the above factors. And to cultivate a good drawing habits, to be able to communicate well with the processing factory, every engineer must consider.