Factors Affecting PCB Welding Quality

罗小群

Factors Affecting PCB Welding Quality [Copy link]

Factors Affecting PCB Welding Quality [Oruqi Technology] Professional circuit design, PCB duplication, schematic reverse engineering, electronic product optimization design, etc. With the rapid development of electronic technology, the miniaturization and micro-miniaturization of electronic components and high-density chips with a spacing of 0.3mm~0.5mm are becoming more and more common, and the requirements for electronic welding technology are becoming higher and higher. Although there are more sophisticated placement machines that can replace manual welding, there are too many factors that affect the welding quality. This article will introduce several key points that need to be paid attention to when designing PCBs from the perspective of patch welding. According to experience, if these requirements are not followed, it is very likely to cause poor welding quality, cold welding, and even damage to the pads or circuit boards when reworking the PCB. Factors Affecting PCB Welding Quality From PCB design to welding all components to make 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 diagram, circuit board quality, device quality, oxidation degree of device pins, solder paste quality, solder paste printing quality, placement machine programming accuracy, placement machine placement quality, reflow oven temperature curve setting and other factors. The link that welding factories cannot skip is the PCB drawing link. People who do circuit design often do not solder circuit boards and therefore cannot obtain direct soldering experience, and do not know the various factors that affect soldering; and the workers in welding factories do not know how to draw boards. They only complete production tasks and have no thoughts or ability to analyze the causes of poor soldering. Since the talents in these two areas perform their respective duties, it is difficult to combine them organically. Suggestions for drawing PCB Below I will give some suggestions to the design and wiring engineers who draw PCB drawings, hoping to avoid various bad drawing methods that affect the welding quality during the drawing process. It will be mainly introduced in the form of pictures and texts. Positioning hole 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 should be on the same axis. Mark Used for placement machine positioning. Mark points should be marked on the PCB board. The specific location is at the diagonal corner of the board. It can be a round or square pad. Do not mix it with the pads of other components. If there are components on both sides, both sides must be marked. When designing your PCB, please pay attention to the following points: a. The shape of the mark point. (vertically symmetrical or left-right symmetrical) b. The size of A is 2.0 mm. c. Within 2.0mm from the outer edge of the mark point, there should be no shape or color changes that may cause incorrect identification. (Pad, solder paste) d. The color of the mark point should be different in brightness from the color of the surrounding PCB. e. To ensure the recognition accuracy, the surface of the Mark point is electroplated with copper or tin to prevent surface reflection. For marks with only lines, the light spot cannot be recognized. Leave 5mm edge When drawing a PCB, leave at least 3mm of edge in the long side direction for the placement machine to transport the circuit board. The placement machine cannot place components within this range. Do not place placement components within this range. 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. In serious cases, the soldering pads will be rubbed off and the circuit board will be damaged. Therefore, it is recommended that SMD components should not be placed within 5mm of the long side of the side with fewer chips (usually 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". Do not place vias directly on pads The defect of directly drilling vias on pads is that during reflow, the solder paste melts and flows into the vias, causing the device pads to lack tin, thus forming cold solder joints. Polarity marking of diodes and tantalum capacitors The polarity markings of diodes and tantalum capacitors should comply with industry regulations to prevent workers from soldering in the wrong direction based on experience. Silk screen printing 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 because of the dazzling display. Do not mark the model without the number, as this 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. IC pads should be extended When drawing PCB for ICs packaged like 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 for the chip pins, PCB pads, and tin to fuse together when soldering with a soldering iron. Width of IC pad For ICs packaged in SOP, PLCC, QFP, etc., you should pay attention to the width of the pad when drawing the PCB. The width of pad a on the PCB = the width of the IC foot (i.e., the Nom. value in the datasheet). Please do not widen it to ensure that b (i.e., between the two pads) has enough width to avoid continuous welding. Do not rotate the device at any angle. 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 rotates 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. Issues to be aware of when short-circuiting adjacent pins The short-circuit 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. 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, 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. Do not place two thicker components close together, otherwise the layout will cause the placement machine to hit the previously placed component when placing the second component. The machine will detect the danger and cause the machine to automatically power off. About BGA Since BGA packaging is special, its pads are all under the chip, and the soldering 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. It is also recommended to leave some space around the BGA to avoid placing components, so that there is room for the stencil to scrape the solder paste during rework. PCB board color It is not recommended to make it red, because the red circuit board will appear white under the red light source of the camera of the SMT machine, which makes it impossible to program and inconvenient for the SMT machine to solder. Small device under the big device Some people like to place small devices under large devices on the same layer, for example: there are resistors under digital tubes. 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. The copper cladding is connected to the pad, affecting the melting of tin Since the copper coating absorbs a lot of heat, it is difficult for the solder to fully melt, resulting in cold solder joints. The device pad is directly connected to the copper. Due to the large area of copper, the copper will absorb a lot of heat and the solder paste will not be fully melted. It is recommended to isolate the pad from the large area of copper. Summarize There are many 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 cultivate good drawing habits and be able to communicate well with the processing factory, which is something every engineer should consider. (Transferred)