For the vast majority of electronic components, they are polar or pins cannot be missoldered. For example, the electrolytic capacitor once welded backward, will explode when electrified. In general, with the use of automatic feeding machinery for the assembly of circuit board components, there will not be the problem of misplaced components. However, due to the limitations of manufacturers and the characteristics of components themselves, not all components can be automatically mounted or inserted.
Common need TO manually place a variety of table paste transformer, connector, TO package integrated circuit, etc. These devices are still subject to faulty assembly. Generally, repair is carried out manually, this link is also prone to reverse welding problems.
For aluminum through-hole installation of electrolytic capacitor, generally by the length of the foot and the body of the mark to represent the positive and negative. Long feet for the positive, short feet for the negative. There are usually white or other stripes parallel to the pin on the negative side of the housing.
One way is to put a “+” sign directly on the positive side. The advantage of this method is that it is convenient to check the polarity after welding. The disadvantage is that it occupies a large area of the circuit board.
The second method is to use screen printing to fill the negative area. This polarity representation occupies a small area of the circuit board, but it is inconvenient to check the polarity after the completion of welding, which is commonly seen in the computer motherboard and other circuit board devices with high density. Tantalum capacitors installed through the hole are generally marked with “+” on the body on the positive pole side, and some varieties are further distinguished by long and short feet.
The marking method of this capacitor on the circuit board can be referred to as the aluminum electrolytic capacitor. For the surface of the aluminum electrolytic capacitor, the ink coated side for the negative, positive side of the base is generally cut Angle treatment. It generally uses screen printing “+” to represent the positive pole on the circuit board and draws the outline of the device. The angular side can also be used to identify the positive pole. For the tantalum capacitor attached to the table, the left side and the right side of the three capacitors on the circuit board are generally negative and positive.
For led, generally use the long foot to represent the positive and negative, the long foot is positive, the short foot is negative. Sometimes the manufacturer will cut off a little bit on one side of the light-emitting diode, which can also be used to indicate the negative pole. The circuit board is generally used screen printing “+” to indicate the positive pole.
For ordinary diodes: the left side is a negative pole, the right side is a positive pole, that is, the use of silkscreen or stained glass to indicate the positive and negative polarity. The circuit board generally adopts the following two ways to express positive and negative polarity. Use the screen print on the circuit board to indicate the polarity of the diode. This is a comparison. The other is to draw the schematic diagram symbol of the diode directly on the screen printed circuit board.
The polarity representation of tiled LEDs is very confusing. Sometimes there is a wide variety of representations for different types of packages within a manufacturer. However, it is common to paint spots or strips on the cathode side of the LED, and there are also color-cutting corners on the cathode side.
Ordinary table paste diodes are also used on the body of the screen or stained glass to represent the negative. Generally, the representation graph of the circuit board is: positive electrode welding plate on the left and negative electrode welding plate on the right.
For the DIP and SO packaged IC with pins distributed on both sides, a semicircular notch on the top is generally used to indicate that this direction is the top of the chip, and the first foot on the upper left is the first foot of the chip. Also useful for screen printing or laser on the top of a horizontal line to indicate. In addition, there are directly next to the first foot of the chip on the body with silk printing or directly pressed in the injection of a hole. Some IC circuits are also represented by cutting a hypotenuse on the body of the starting edge of the first leg. The symbols on the circuit board of this kind of integrated circuit are generally marked with a notch on the top.
For QFP, PLCC and BGA packaged in four directions: The INTEGRATED circuit packaged in QFP generally adopts concave dot, screen dot, or judge the direction according to the model screen print on the corresponding body of the first foot. Some use the method of cutting off an Angle to represent the first foot, counterclockwise as the first foot. It should be noted that sometimes there will be three pits in a chip, so there is no one Angle of the pits, corresponding to the lower right side of the chip. PLCC encapsulation is usually represented by a pit at the beginning of the first leg due to the large body. Some also chip left upper corner to do cutting processing. In addition to the gold-plated copper foil in the lower-left corner in the figure above, BGA packaging also uses missing angles and concave points and silk-screen dots to indicate the direction of the first foot.
Connectors on the object generally control the direction by locating the gap. There are also those that write a 1 near the first foot or use a triangle to represent the first foot. Other devices generally avoid insertion errors by drawing screen prints on the circuit board consistent with the real thing. For the resistance of through-hole installation, it is generally expressed in the way that the common end is circled by screen printing on the circuit board, or 1 is written near the first foot. In order to standardize the requirements of welding plate, screen printing, and welding resistance of components on the circuit board, the IPC organization has issued two related standards, namely IPC-7351 and IPC-SM-840. However, in actual use, the device direction marking symbols made by means of the device direction representation defined by IPC is often blocked by the device body after welding, which is not suitable for inspection. The graphic design of the device welding plate should be adjusted according to the actual situation.
In general, discrete devices on the physical use of long and short feet and silkscreen or coloring methods for polarity representation. For integrated circuits, concave points, silk-screen, notch, missing Angle, missing edge, or direct indication are often used for marking the first foot. In the production of welding disk graphics, it is generally necessary to draw as much as possible according to the shape of the device, and at the same time reflect as much information related to the positioning of the device shape as possible through the form of screen printing, so as to avoid mistakes in manual assembly and welding.
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