Circuit board inspection and maintenance editing.
A chip with the program 1 EPROM chips are generally not suitable for damage.
Because this chip needs ultraviolet light to erase the program, it will not damage the wifi microscope in the test to carry out the circuit board inspection sequence. However, it is reported that due to the material used to make the chip, it may be damaged even if it is not used (mainly referring to the program).
So back up as much as possible. 2。
EEPROM, SPROM and so on. And RAM chip with battery are easy to destroy programs. Whether this chip destroys the program after scanning the VI curve with the tester is still inconclusive.
Nevertheless, colleagues should be careful in this situation. I have done many tests, the main reason may be: maintenance tools (such as testers, soldering irons, etc.) shell. ) is caused by electric leakage.
3。 Do not easily remove the chip with battery from the circuit board.
Two. Reset circuit 1.
When there is a large-scale integrated circuit on the circuit board that needs to be repaired, we should pay attention to the reset problem. 2。
Before testing, it is best to put it back on the equipment and try it again and again. And press the reset button several times.
Three. Function and parameter test 1.
The tester can only reflect the cutoff zone, amplification zone and saturation zone when testing devices. But it can't measure the working frequency and speed, and the volume value of the circuit board is detected by a portable microscope.
2。 Similarly, for TTL digital chips, only the high and low level output changes can be known.
It is impossible to find the speed of its rising edge and falling edge. Four.
Crystal oscillator 1. Usually, it can only be tested by oscilloscope (the crystal oscillator needs to be powered on) or frequency meter, and the multimeter can't measure it, otherwise it can only be replaced.
2。 The common faults of crystal oscillator are: a.
Internal leakage, B. internal open circuit C.
Deteriorating frequency offset D. Leakage of peripheral connection capacitor.
The leakage phenomenon here should be detected by VI curve of the tester. 3。
There are two ways to judge the whole board test: a. During the test, the chip around the crystal oscillator fails.
B no other fault points were found except the crystal oscillator.
4。 There are two kinds of crystal oscillators: a.
Two feet. B.
Four feet, the second of which is electric. Be careful not to short circuit at will. Five.
Fault phenomenon distribution portable microscope detection circuit board 1. Incomplete statistics of circuit board fault parts: 1) chip damage 30%, 2) discrete component damage 30%, 3) connecting wire (copper wire coated on PCB) broken 30%, 4) program damage or loss 10% (with an upward trend).
2。 As can be seen from the above, when the wiring and program of the circuit board to be repaired have problems, there is no good board, so I am not familiar with its wiring and can't find the original program.
This board is unlikely to be repaired. Electromagnetic compatibility of circuit board compatibility design editor refers to the ability of electronic equipment to work harmoniously and effectively in various electromagnetic environments.
2. What is a PCB?
What is a PCB board? Many people have heard the English abbreviation "PCB".
But what does this mean? In fact, it is very simple, that is, printed circuit board, PCB). It will appear in almost every kind of electronic equipment.
If there are electronic parts in a device, they are all embedded in PCB of different sizes. In addition to fixing various small parts, PCB is mainly used to provide electrical connection between the above parts.
With the increasing complexity of electronic equipment, more and more parts are needed, and the lines and parts on PCB are more and more dense. A bare board (with no parts on it) is also often called a "printed circuit board (PWB)".
The substrate of the board itself is made of insulating and heat-insulating materials that are not easy to bend. The fine wiring material visible on the surface is copper foil. Originally, the whole board was covered with copper foil, but a part of it was etched away in the manufacturing process, and the rest became fine wiring in a mesh.
These wires, called conductor patterns or wiring, are used to provide circuit connections for components on the PCB. Usually the color of PCB is green or brown, which is the color of solder mask.
It is an insulating protective layer, which can protect copper wire and prevent parts from being welded to incorrect places. The screen will be printed on the solder mask.
Usually words and symbols (mostly white) are printed on it, indicating the position of each part on the chessboard. Screen printing surface is also called legend.
3.3. What does PCB mean?
PCB is a printed circuit board. Printed circuit board (PCB) is the support of circuit components and devices in electronic products.
It provides an electrical connection between circuit elements and equipment. With the rapid development of electronic technology, the density of PCB is higher and higher.
The quality of PCB design has a great influence on anti-interference ability. Practice has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, it will have a negative impact on the reliability of electronic products.
Printed circuit board, also called printed circuit board, or printed circuit board, or printed circuit board (PCB) or printed wiring board (PWB), is made by cutting an insulating board into a certain size, attaching at least one conductive pattern on it, and having holes (such as component holes, fastening holes, metallized holes, etc.). ) distributed to replace the chassis of electronic components in the past, and realize the interconnection between electronic components. Because this kind of board is made by electronic printing, it is called "printed" circuit board.
It is inaccurate to call "printed circuit board" as "printed circuit" because there are no "printed components" on the printed circuit board, only wiring. It is an important electronic component and its support.
4. Find the basic knowledge of pcb
The basic concept of PCB design 1 and the concept of "layer" are different from those introduced in word processing or many other softwares to realize the nesting and synthesis of pictures, characters and colors. Protel's "layer" is not virtual, but the actual copper foil layer of printed circuit board material itself.
Nowadays, due to the intensive installation of electronic circuit components. In order to meet the special requirements of anti-interference and wiring, some newer electronic products use printed boards with not only upper and lower sides for wiring, but also sandwich copper foil that can be specially processed in the middle of the board. For example, the printed boards used for computer motherboards are all made of more than four layers of materials.
Because these layers are relatively difficult to process, they are mostly used to set up simple power wiring layers (such as ground Dever and power Dever in software), and they are often wired in a large area (such as external AIP1a11e and filling in software). The so-called "Via" mentioned in the software is used to communicate the upper and lower surface layers and the middle layer.
With the above explanation, it is not difficult to understand the concepts of "multilayer pad" and "wiring layer setting". For a simple example, many people have finished wiring, and when they print it out, they find that there are no pads at the terminals of many lines. In fact, this is because they ignored the concept of "layer" when adding device libraries, and did not define their own packaged pad features as "Mulii-layer".
Need to be reminded that once the number of printing plate layers is selected, those unused layers must be closed to avoid trouble and detours. 2. Via In order to connect the lines between layers, a male hole is drilled at the intersection of the lines to be connected in each layer, which is a via.
In technology, a layer of metal is plated on the cylindrical surface of the hole wall through chemical deposition to connect the copper foil in the middle. The upper and lower sides of the hole are made into ordinary pad shapes, which can directly connect the wires on the upper and lower sides or not. Generally speaking, when designing lines, there are several principles to deal with vias: (1) Use as few vias as possible. Once the vias are selected, the gaps between them and the surrounding entities, especially the gaps between wires and vias, must be properly handled. These gaps are not connected to vias that are easily overlooked in the middle layer. If the wiring is automatic, you can select "On" in the "Via minimization 8" submenu to automatically wire.
(2) The larger the current carrying capacity, the larger the size of the required vias, such as vias used to connect the power supply layer and the stratum with other layers. 3. Screen printing layer (Overlay) In order to facilitate the installation and maintenance of the circuit, the required logo patterns and text codes are printed on the upper and lower surfaces of the printed board, such as component labels and nominal values, component outline shapes, manufacturer marks, production dates, etc.
Many beginners only pay attention to the neat and beautiful arrangement of characters and symbols when designing the related content of screen printing layer, while ignoring the actual PCB effect. On the printed circuit board they designed, characters were either blocked by components or intruded into the soldering area, and some labeled components were on the adjacent components. All these designs will bring great inconvenience to assembly and maintenance.
The correct character arrangement principle of screen printing layer is: "unambiguous and beautiful." 4, the particularity of SMD Protel package library has a large number of SMD packages, that is, surface soldered devices.
In addition to its small size, the biggest feature of this device is the single-sided distribution of component pinholes. Therefore, when selecting this kind of device, the surface of the device should be clear to avoid "missing needle".
In addition, the related text labels of such components can only be placed together with the surface of the component. 5. Outer plane and filling are the names of both. The mesh filling area is to treat a large area of copper foil into a mesh, and the filling area is only a complete copper foil.
Novices often can't see the difference between them on the computer during the design process. In essence, as long as the drawing is enlarged, it will be clear at a glance. It is precisely because it is not easy to see the difference between the two, so we do not pay attention to the difference between the two when using it. It should be emphasized that the former has a strong inhibitory effect on high-frequency interference in circuit characteristics, and is suitable for places where large areas need to be filled, especially when some areas are used as shielding areas, dividing areas or power lines with large current.
The latter is mostly used in places where small areas need to be filled, such as general line ends or turning areas. 6.Pad pad is the most common and important concept in PCB design, but beginners often ignore its selection and modification and use circular pads in design.
When selecting the pad type of the component, the factors such as the shape, size, layout, vibration heating and stress direction of the component should be considered comprehensively. Protel gives a series of pads with different sizes and shapes in the bag library, such as circle, square, octagon, square and positioning pad, but sometimes this is not enough and needs to be edited by yourself.
For example, pads with high heat, stress and current can be designed as "tears" by themselves. In the well-known pin pad design of line output transformer of color TV PCB, many manufacturers have adopted this form. Generally speaking, in addition to the above points, you should also consider the following principles when editing your own pad: (1) When the shapes are inconsistent, the width of the connecting line and the specific side length of the pad should not be too different; (2) When it is necessary to route between the lead angles of components, it is often twice the result with half the effort to select pads with asymmetric lengths; (3) The size of each component pad hole should be edited and determined according to the thickness of the component pin. The principle is that the size of the hole is 0.2- 0.4 mm larger than the pin diameter.
7. Various masks These films are not only necessary in the PcB manufacturing process, but also necessary for component welding. According to the position and function of adhesive film, adhesive film can be divided into two types: welding aid film on component surface (or welding surface) (top or bottom surface and component surface (or welding surface) and adhesive film on top or bottom surface.
Flux film, as its name implies, is a film coated on the pad to improve solderability, that is, a light circular spot slightly larger than the pad on the green board. The situation of solder mask is just the opposite. In order to make the circuit board adapt to wave soldering and other welding forms, it is required that the copper foil on the non-pad of the circuit board should not be stained with tin, so the parts outside the pad should be painted with a layer of paint.