With the rapid development of electronic technology, the miniaturization, miniaturization, BGA, and high-density chips with spacing of 0.3mm~0.5mm of electronic components are becoming more and more common, and the requirements for electronic welding technology are becoming higher and higher. Although there are now more sophisticated mounters that can replace manual welding, there are too many factors that affect the quality of welding. This article will from the point of view of patch welding, introduced a few points need to pay attention to PCB design, according to experience, if not in accordance with these requirements, it is likely to cause poor welding quality, virtual welding and even damage the pad or circuit board when repairing the PCB.
1. Factors affecting PCB welding quality
From PCB design to the completion of all components welding into a high quality circuit board, PCB design engineers and even the level of welding process, welding workers and many other links have strict control. There are mainly the following 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 placement machine program, the mounting quality of the placement machine, the setting of the temperature curve of the reflow furnace, etc.
The link that the welding plant itself cannot surmount is the link of PCB drawing. Because people who do circuit design often do not weld circuit boards, they can not get direct welding experience and do not know the various factors that affect welding. The workers in the welding factory do not understand the drawing board, they only complete the production task, and have no mind and no ability to analyze the causes of poor welding. It is difficult to combine these two talents organically because of their respective roles.
2. Suggestions for PCB drawing
Below, I put forward some suggestions to the design and wiring engineers who draw PCB drawings on the link of PCB drawings, hoping to avoid various bad drawings that affect the quality of welding in the process of drawing. Will be introduced mainly in the form of graphics.
(1) About the positioning hole: 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 is required to be on the same axis.
(2). About Mark point: used for placement machine positioning. The PCB board should be marked with the Mark point, the specific position: on the diagonal of the board, 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.
When designing PCB, please pay attention to the following points:
a. Mark the shape of the point. (top and bottom symmetry or left and right symmetry)
b. The dimensions of 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 surface reflection. Marks for shapes are only lines, points of light cannot be recognized.
(3) About leaving 5mm side: When drawing PCB, leave no less than 3mm side 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 with devices on both sides should take into account that the second over-reflux will rub off the device on the side of the soldered side, and the welding pad will be rubbed off and the circuit board will be destroyed.
Therefore, it is recommended that the long side of the less side of the chip (generally the Bottom surface) should not be placed within 5mm from the side. If it is true that the area of the circuit board is limited, the process edge can be added on the long side, see the 17 "Suggestions on the board and the process edge" in this article.
(4). Do not directly pass the hole on the pad: the defect of passing the hole directly on the pad is that the solder paste melts into the hole after over-reflux, resulting in the lack of tin on the device pad, thus forming virtual welding. As the picture shows:
(5) Polarity labeling of diodes and tantalum capacitors: polarity labeling of diodes and tantalum capacitors should comply with industry regulations, so as to avoid workers welding in the wrong direction by experience. As the picture shows:
(6). For screen printing and identification: Please hide the device model. Especially the circuit board with high device density. Otherwise, the dazzle affects finding the welding position. As shown below:
Don't just mark the model, don't mark the label. Cause the placement machine programming can not be carried out. The size of the screen character should not be too small to read clearly. Characters should be staggered through holes to avoid misreading.
(7) About the IC pad should be extended: SOP, PLCC, QFP and other packages of IC PCB should be extended when drawing the pad, the length of the pad on the PCB = length of the IC foot ×1.5 is appropriate, so that when manual soldering iron, chip pin and PCB pad, tin melt into one. As the picture shows:
(8). About the width of the IC pad: SOP, PLCC, QFP and other packages of IC, drawing the PCB should pay attention to the width of the pad, the width of the pad a on the PCB = the width of the IC foot (ie: Nom in datasheet). Please do not increase the width to ensure that b(that is, between the two pads) has enough width to avoid continuous welding. As the picture shows:
(9). Do not rotate the device at any Angle: because 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.
(10) Problems that should be paid attention to 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 it is not beautiful after welding. If the drawing is shorted and welded according to the method of Figure b and Figure c, the effect of welding is different: as long as each pin is not connected, the chip has no short circuit phenomenon, and the appearance is beautiful.
(11). The problem of having a pad in the middle of the chip: If the chip with a pad in the middle of the chip is drawn according to the pad in the middle of the package picture of the chip, it is easy to cause short circuit. It is recommended to reduce the middle pad so that the distance between it and the surrounding pin pad is increased, thereby reducing the chance of short circuit.
(12). The two devices with higher thickness should not be closely lined up together: as shown in the following figure, this will cause the placement machine to touch the previous device when mounting the second device, and the machine will detect danger, causing the machine to automatically power off.
(13). About BGA: Because the BGA package is special, its pad is under the chip, and the welding effect is not visible outside. In order to facilitate the repair, it is recommended to punch two positioning holes with Hole Size:30mil on the PCB board to locate the steel mesh (used to scrape the solder paste) during the repair.
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. Moreover, it is recommended to leave open space to place the device within a certain range around the BGA, so that the mesh board scraping paste can be placed during repair.
(14). About the color of PCB board: 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 chip machine, it cannot be programmed and is not easy for the chip machine to weld.
(15) About the small device under the large device: some people like to line the small device under the same layer of the large device, such as: there is a resistor under the digital tube. 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 resistor under the nixie tube to the Bottom surface
(16). On the impact of copper coating and pad connection on molten tin: Because copper coating will absorb a lot of heat, it is difficult to fully melt the solder, resulting in virtual welding. The device pad is directly connected with the copper coating; Although the 50Pins connector is not directly connected to the copper coating, because the middle two layers of the four-layer plate are large-area copper coating, the solder paste cannot be fully melted because the copper coating absorbs a lot of heat. 50Pins connector body is not high temperature plastic, if the temperature setting is high, the connector will melt or deformation, if the temperature setting is low, the copper coating absorbs a lot of heat, resulting in the solder paste can not be fully melted. Therefore, it is recommended that the pad be isolated from large area copper coating.
(17). Suggestions and processes for assembling boards:
Nowadays, more and more engineers can draw, wire and design PCB with software, but once the design is completed, and can improve the welding efficiency, we think that we need to pay attention to the above elements. And cultivate a good habit of drawing, and be able to communicate well with the processing plant, which is what every engineer should consider.
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