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For most electronic components on the circuit board, they are polarized or the pins cannot be soldered wrongly. For example, an electrolytic capacitor will explode when it is energized once it is welded backwards. Generally speaking, when automatic feeding machinery is used to assemble circuit board components, the problem of misplaced components will not occur. However, due to the manufacturer's conditions and the characteristics of the components themselves, not all components can be automatically mounted or inserted.
There are various surface mount transformers, connectors, TO packaged integrated circuits, etc. that need to be manually placed manually. These devices may still have problems with assembly errors. Generally, rework is done manually, and this link is also prone to welding reverse problems. Therefore, it is necessary to explain the corresponding relationship between the positioning method of the components and the component pads and silk screens on PCB.
1. Capacitance
For aluminum through-hole electrolytic capacitors, the positive and negative poles are generally indicated by the long and short legs and the marks on the body. The long leg is the positive pole and the short leg is the negative pole. There are generally white or other stripes parallel to the pins on the negative side shell.
One method is to mark a "+" sign directly on the positive side. The advantage of this method is that it is more convenient to check the polarity after the welding is completed. The disadvantage is that it occupies a larger area of the circuit board. The second method is to use silkscreen to fill in the area where the negative electrode is located. This kind of polarity representation occupies a small area of the circuit board, but it is inconvenient to check the polarity after the soldering is completed. It is often used in applications where the density of circuit board components such as computer motherboards is relatively high. The through-hole tantalum capacitors are generally marked with a "+" sign on the body on the positive side, and some varieties are further distinguished by long and short legs. The marking method of this capacitor on the circuit board can refer to aluminum electrolytic capacitors. For surface-mount aluminum electrolytic capacitors, the side coated with ink is the negative electrode, and the positive side base is generally cut corners. It generally uses the silk screen "+" sign to indicate the positive electrode on the circuit board, and draws the outline of the device at the same time. In this way, the cut-off side can also be used to identify the positive electrode. For surface mount tantalum capacitors, the left side of the three capacitors on the circuit board is the negative electrode, and the right side is the positive electrode. The middle one is the most vivid.
2. Diode
For light-emitting diodes, the long and short legs are generally used to indicate the positive and negative poles, the long leg is positive, and the short leg is negative. Sometimes the manufacturer will cut off a bit on the side of the light-emitting diode. This can also be used to indicate the negative electrode. The circuit board generally also uses the silk screen "+" to indicate the positive electrode.
For ordinary diodes: the left side is the negative electrode, and the right side is the positive electrode, that is to say, silk screen or stained glass is used to indicate the positive and negative polarity. The circuit board generally adopts the following two ways to indicate the positive and negative polarity.
Use the silkscreen on the circuit board to indicate the polarity of the diode. This is more vivid. The other is to draw the schematic symbol of the diode directly on the silk-printed circuit board.
The polarity representation method of surface-mounted light-emitting diodes is very confusing. Sometimes the representations of different package types within a manufacturer are varied. However, it is common to paint dots or color bars on the cathode side of the light-emitting diode, and there are also corners on the cathode side.
Ordinary surface mount diodes also use silk screen or stained glass on the body to indicate the negative electrode. Both of the diodes in the above figure are on the left side as the cathode. In the display graphics of the circuit board, it is generally: the left side is the positive electrode pad, and the right side is the negative electrode pad.
3. Integrated circuit
For integrated circuits with DIP and SO packages whose pins are distributed on both sides, the upper semi-circular notch is generally used to indicate that this direction is the upper side of the chip, and the first pin on the upper left side is the first pin of the chip. It is also indicated by silk screen or laser marking a horizontal line on the top. In addition, there is also a pit directly on the body next to the first leg of the chip with silk screen printing or directly during injection molding. There are also some integrated circuits that are represented by cutting a slanted edge on the body of the starting edge of the first leg. The symbols on the circuit board of this type of integrated circuit are generally marked with a notch on the top.
For the QFP, PLCC, and BGA of the quad package: the integrated circuit of the QFP package generally adopts pits, silk-printed dots, or silk-printed on the body corresponding to the first pin to determine the direction. Some use the method of cutting off a corner to indicate the first leg, and in this case, the counterclockwise direction is the first leg. It should be noted that sometimes there will be 3 pits on a chip, then a corner without pits corresponds to the lower right of the chip. The PLCC package is generally represented by a pit at the beginning of the first leg due to its relatively large body. Some also cut corners on the upper left side of the chip. In addition to the gold-plated copper foil in the lower left corner of the BGA package in the above figure, the first pin is also used to indicate the direction of the first pin with missing corners, pits, and silk-screen dots.
4. Other devices
On the physical object, the connector usually controls the direction by positioning the notch. There are also writing 1 near the first foot or using a triangle to indicate the first foot. Other devices generally draw a silk screen consistent with the actual product on the circuit board to avoid incorrect insertion. For the exclusion of through-hole installation, it is generally indicated by enclosing the common end with silk screen on the circuit board, or write 1 near the first pin. In order to standardize the requirements of pads, silk screens, and solder masks of components on circuit boards, the IPC organization has promulgated two related standards: IPC-7351 and IPC-SM-840. However, in actual use, the device direction marking symbols made by the method of device direction expression defined by IPC are often hidden by the device body after soldering, which is not suitable for inspection. The design of the component pad pattern should be adjusted according to the actual situation.
Summary: In short, discrete devices generally use long and short legs and silk screen or coloring methods for polarity expression in physical objects. For integrated circuits, pits, silk screens, notches, notches, notches, or direct indications are often used to mark the first pin. When making the land pattern, 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 in the form of silk screen to avoid errors in manual assembly and welding. PCB manufacturers should pay more attention.
