Some problems needing attention in drawing PCB drawings from welding point of view

- Jul 21, 2017-

With the rapid development of electronic technology, miniaturization of electronic components, miniaturization, BGA, spacing of 0.3mm~0.5mm, high-density chip is becoming more and more common, and the requirements for electronic welding technology are becoming higher and higher. Although there are more sophisticated SMT machine can replace manual welding, but there are too many factors affecting welding quality. This paper introduces the main points of the patch welding, and points to note when PCB was designed according to the experience, if not in accordance with these requirements, it may cause the weld and welding quality is not high, even when PCB damage repair pad or circuit board.

From PCB design to all components welding completed for a high quality circuit boards, PCB design engineers and welding technology, welding workers, and many other aspects of the strict control. The main elements are as follows: PCB diagram, circuit board quality, device quality, device pins oxidation degree and solder paste quality, solder paste printing quality, machine programming accuracy, Mounter mount quality, reflow furnace temperature curve setting etc..

Welding factory itself can not overstep the link is the PCB drawing link. Because people often do not do circuit design PCB to obtain direct welding experience, do not know all the factors of affecting the welding; welding factory workers do not understand the drawing board, they just finished production task, no mind, no ability to analyze the cause of bad welding. Since these two aspects of personnel perform their duties, it is difficult to combine organically.

Below I will PCB drawing links to draw PCB diagram design, wiring engineers put forward some suggestions, hope in the drawing process can avoid the influence of welding quality of all kinds of bad painting.

1, about the positioning hole: PCB Board four corners to leave four holes (minimum aperture 2.5mm), used for printing solder paste when positioning the circuit board. The axis of the X axis or Y axis is required to be on the same axis.

2, about Mark points: used for placement of mounter. PCB board to mark Mark points, specific location: in the plate diagonally diagonally, can be round, or square pad, do not mix with other devices solder together. If there is a device on both sides, both sides should be marked.

3, on the left 5mm side: draw PCB, in the long side direction to stay not less than 3mm edge, used for placement machine transport circuit board, in this range, placement machine can not mount device. Do not place patch devices in this range.

Both sides of the circuit board should have taken into account the second bypass, will be welded side of the device to rub off, in serious cases will rub off the pad, destroy the circuit board.

Therefore, it is recommended that the chip side (usually Bottom), the long edge off the 5mm range, do not place chip devices. If it is indeed limited by the circuit board area, it can be machined on the long side.

4, do not directly through holes on the pad: defect directly on the pad vias in reflow when the paste melts flow to the hole, causing device pad lack of tin, thus forming.

5, a diode, tantalum capacitor polarity mark: polarity labeling diode, tantalum capacitors should comply with the regulations, in order to avoid the wrong direction by experience of welding workers.

6, about silk screen and logo: please hide the device type. Circuit board with high device density, in particular. Otherwise, the dizzying effect of finding the weld position. The characters of the silk screen characters should not be too small to be seen. The position of the characters should be staggered so as not to misread.

7, a IC pad should be extended: SOP, PLCC, QFP package IC PCB painting should extend pad PCB pad length =IC * 1.5 foot length is appropriate, so it is easy to use manual soldering, chip pins and PCB pad, tin three fused into one.

8, on the IC pad width: SOP, PLCC, QFP, IC package, PCB should be paid attention to when the pad width, PCB width of the foot pad a =IC width (datasheet in Nom.), please don't widened, ensure B (two welding disc) is sufficient width to avoid solder.

9. Do not rotate any angle of the device. As the placement machine can not rotate at any angle, it can only rotate at 90, 180, 270, 360 degrees. As shown in figure B, 1 DEG C is rotated. After the mounter is mounted, the pads on the device pins and the circuit board will stagger the angle of 1 DEG C, thus affecting the welding quality.

Nowadays, more and more engineers have been able to use software for drawing, wiring and designing PCB. However, once the design is completed and the welding efficiency can be improved, the author thinks it is necessary to pay attention to the above factors. And to develop good drawing habits, good communication with the processing plant is a good consideration for every engineer.

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