From the welding point of view, what problems should be paid attention to in PCB design?
kinhford Electronics is a professional PCB design company engaged in electronic product circuit board design (layout layout design), mainly undertake multi-layer, high-density PCB design and circuit board design proofing business. Next, from the welding point of view, talk about what points need to pay attention to when designing PCB.
First, the factors affecting the quality of PCB welding
From PCB design to the completion of all components welding, becoming a high-quality circuit board requires the control of PCB engineers, welding processes, welding workers and many other links
There are mainly the following influencing factors: PCB diagram, the quality of the circuit board, the quality of the device, the oxidation degree of the device pin, the quality of the solder paste, the printing quality of the solder paste, the accuracy of the program programming of the SMT machine, the mounting quality of the reflow furnace temperature curve, and so on.
Circuit designers rarely weld circuit boards, can not get rich welding experience, and welding factory workers do not understand the drawing board, just complete the production task, no mind, no ability to analyze the causes of poor welding.
Second, suggestions for drawing PCB
Here are some suggestions to avoid various bad drawings that affect the quality of welding.
The four corners of the PCB board should be left with four holes (minimum aperture 2.5mm) for positioning the circuit board when printing solder paste. The center of the X-axis or Y-axis direction is required to be on the same axis, as shown below: 2. About MARK
Specific location: On the diagonal of the plate, it can be a round or square pad, and do not mix with the pad of other devices. If there are devices on both sides, both sides should be marked.
3, when designing PCB, please pay attention to the following points:
The shape of the Mark point is as follows, symmetrical up and down or symmetrical left and right. The dimensions of b and A are 2.0mm.
c, from the outer edge of the Mark point within the range of 2.0mm, there should be no shape and color changes that may cause misidentification. (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 accuracy of identification, the surface of the Mark point is plated with copper or tin to prevent reflection. The shape is only marked by lines, and the spot cannot be recognized. 4. Leave 5mm edges
When drawing the PCB, leave no less than 3mm of the edge in the long side direction for the placement machine to transport the circuit board, and the placement machine cannot mount the device in this range. Do not place the patch device within this range. The circuit board of the double-sided decoration, considering the secondary reflow welding, the device is rubbed off, the pad is rubbed off, etc
It is recommended that on the less side of the chip, the long side is within 5mm from the side, do not place the patch device, if it is indeed due to the limited area of the circuit board, you can add the process edge on the long side.
5, do not directly in the pad through the hole
When the pad is punched through the hole, solder paste will flow into the hole in reflow welding, resulting in the lack of tin on the device pad and causing virtual welding, as shown in the figure below. Do not directly hole the pad
6. Polarity labeling of diode and tantalum capacitor
The polarity marks of diodes and tantalum capacitors conform to the industry regulations, so as to avoid workers welding wrong direction by experience. As the picture shows:
Polarity labeling of diodes and tantalum capacitors
7. About screen printing and signage
Please hide the device model. Especially the circuit board with high device density. Otherwise, the dazzle affects finding the welding position. As shown below: Do not only mark the model, do not mark the label. As shown in the following figure, resulting in the placement machine programming can not be carried out.
The size of the screen printing character should not be too small, and the character placement position should be staggered through the hole to avoid misreading.
8, about the IC pad should be extended
For IC packages such as SOP, PLCC and QFP, the length of the pad on the PCB = length of the IC foot ×1.5 is appropriate, which is convenient for manual soldering with the soldering iron, the chip pin and the PCB pad and tin are fused into one. As the picture shows:
9, about the width of the IC pad
SOP, PLCC, QFP and other packaging IC, drawing PCB should pay attention to the width of the pad, the width of the pad a on the PCB =IC foot width (Nom in datasheet). Value), it is not recommended to widen, to ensure that the spacing b between the two pads has enough width to avoid continuous welding. As the picture shows:
10. Do not rotate the device at any Angle
Since the mounter cannot rotate at any Angle, it can only rotate 90 ° C, 180 ° C, 270 ° C, 360 ° C.
As shown in Figure B below, if the device pin is rotated by 1℃, the Angle between the device pin and the pad on the circuit board will be staggered by 1℃, thus affecting the welding quality.
11. Problems that should be paid attention to when shorting adjacent pins
The shorting method in Figure a below is not conducive to worker identification, and it is not beautiful after welding.
If the drawing according to the method of figure b, Figure c short-weld and add solder, the welding effect is not the same.
As long as each pin is not connected, the chip has no short circuit, and the appearance is beautiful.
12, about the middle pad under the chip
For chips with belly, it is recommended to reduce the middle pad to increase the distance between it and the surrounding pad to reduce the chance of short circuit. 13. The two devices with higher thickness should not be closely lined up as shown in the following figure, so that the board will cause the SMT machine to touch the device that has been pasted before the second device, and the machine will detect danger, causing the machine to automatically power off.
14, about BGA Because the BGA package is special, the welding pad is under the chip, and the welding effect is not visible outside.
Tips: The size of the positioning hole should not be too large or too small, so that the needle does not fall off after insertion, does not shake, and is slightly tight when inserting, otherwise the positioning is inaccurate. As shown below:
15. About PCB board color
Red is not recommended. The red circuit board is white under the red light source of the chip camera, which cannot be programmed and is not easy for the chip machine to weld.https://www.kingfordpcb.com/yuan.php
16, about the small device under the large device
Some people like to line up small devices under the same layer of large devices, such as: there is a resistor under the digital tube, as shown below:
This layout will cause difficulties to repair, repair must first remove the digital tube, and may cause damage to the digital tube. It is recommended to drain the resistance under the nixie tube to the Bottom surface, as shown below:
17, about the impact of copper coating and pad connected to molten tin
Because the copper coating will absorb a lot of heat, it is difficult to fully melt the solder, resulting in the formation of virtual welding. As shown in the picture:
In Figure a, the device pad is directly connected with the copper coating; Although the 50Pins connector in Figure b is not directly connected with the copper coating, since the middle two layers of the four-layer plate are large-area copper coating, the solder paste in Figure a and Figure b cannot be fully melted because the copper coating absorbs a lot of heat.
The body of 50Pins connector in Figure b is a plastic that is not resistant to high temperature. If the temperature is set high, the connector will melt or deform. If the temperature is set low, the copper coating will absorb a lot of heat, resulting in the solder paste cannot be fully melted. Therefore, it is recommended that the pad be isolated from large area copper coating. As shown in the picture:
Nowadays, more and more engineers can draw, design and route PCB with software, complete the design, and can improve the welding efficiency, the author believes that it is necessary to pay attention to the above elements.