The names of circuit boards include: ceramic circuit board, alumina ceramic circuit board, aluminum nitride ceramic circuit board, circuit board, PCB board, aluminum substrate, high-frequency board, thick copper plate, impedance board, PCB, ultra-thin circuit board, ultra-thin circuit board, printed (copper etching technology) circuit board, etc. Circuit boards play an important role in miniaturizing and visualizing circuits, as well as optimizing the layout of electrical appliances for mass production of fixed circuits. A circuit board can be called a printed circuit board or a printed circuit board, with English names such as (Printed Circuit Board) PCB, (Flexible Printed Circuit Board) FPC circuit board (FPC circuit board, also known as flexible circuit board). FPC circuit board is a highly reliable and excellent flexible printed circuit board made of polyimide or polyester film as the substrate. It has the characteristics of high wiring density, light weight, thin thickness, and good flexibility. It is also known as a flexible and hard combination plate. The birth and development of FPC and PCB have given rise to the new product of flexible and hard combination plate. Therefore, the combination of soft and hard circuit boards refers to the combination of flexible circuit boards and rigid circuit boards through pressing and other processes according to relevant process requirements, forming a circuit board with FPC characteristics and PCB characteristics.
classification
Circuit boards can be classified into three major categories based on the number of layers: single panel, double-sided board, and multi-layer circuit board.
Firstly, on the most basic PCB, the components are concentrated on one side, while the wires are concentrated on the other side. Because the wires only appear on one side, this type of PCB is called a single-sided circuit board. Single panel manufacturing is usually simple and cost-effective, but its disadvantage is that it cannot be applied to overly complex products.
Double panel is an extension of single panel. When single-layer wiring cannot meet the needs of electronic products, double-sided panels are used. Both sides are covered with copper and have routing, and the wiring between the two layers can be conducted through vias to form the required network connection.
Multilayer board refers to a printed circuit board with three or more layers of conductive patterns and insulating materials sandwiched between them, and the conductive patterns are interconnected as required. Multilayer circuit board is a product of the development of electronic information technology towards high speed, multifunctionality, large capacity, small size, thinning, and lightweight.
If circuit boards are classified according to their characteristics, they can be divided into soft boards (FPC), hard boards (PCB), and soft hard junction boards (FPCB).
Circuit board material
FR-1: Flame retardant copper-clad phenolic paper laminated board. IPC4101 detailed specification number 02; Tg N/A;
FR-4:1) Flame retardant copper-clad epoxy E-glass fiber cloth laminated board and its adhesive sheet material. IPC4101 detailed specification number 21; Tg≥100℃；
2) Flame retardant copper foil modified or unmodified epoxy E-glass fiber cloth laminated board and its adhesive sheet material. IPC4101 detailed specification number 24; Tg 150℃~200℃；
3) Flame retardant copper-clad epoxy/PPO glass cloth laminated board and its adhesive sheet material. IPC4101 detailed specification number 25; Tg 150℃~200℃；
4) Flame retardant copper-clad modified or unmodified epoxy glass cloth laminated board and its adhesive sheet material. IPC4101 detailed specification number 26; Tg 170℃~220℃；
5) Flame retardant copper-clad epoxy E-glass cloth laminated board (used for catalytic addition method). IPC4101 detailed specification number 82; Tg N/A;
Testing and repairing
One. Chip with program
EPROM chips should generally not be damaged. Because this chip requires ultraviolet light to erase the program, it will not damage the program during testing. But there is information that due to the materials used to make the chip, over time (over time), even if not in use, it may still be damaged (mainly referring to the program). So it is important to provide backups as much as possible.
EEPROM, SPROM, and RAM chips with batteries are all highly susceptible to program damage. There is no consensus on whether the use of a<tester>for VI curve scanning on such chips has disrupted the program. Nevertheless, colleagues should be cautious when encountering such situations. The author has conducted multiple experiments, and the main reason may be due to the leakage of the casing of maintenance tools (such as testers, soldering irons, etc.).
3. Do not easily remove chips with batteries from the circuit board.
Two. Reset circuit
When there are large-scale integrated circuits on the circuit board to be repaired, attention should be paid to the reset issue.
2. It is best to reinstall the device before testing, repeatedly turn on and off the machine, and try again. And press the reset button multiple times.
Three. Function and parameter testing
1. The testing instrument can only reflect the cutoff region, amplification region, and saturation region when detecting devices. But it cannot measure specific values such as the working frequency and speed.
Similarly, for TTL digital chips, only high and low level output changes can be known. And it is impossible to determine the speed of its rising and falling edges.
Four. crystal oscillator
Usually, only oscilloscopes (crystal oscillators require power) or frequency meters can be used for testing, and multimeters cannot measure them. Otherwise, the substitution method can only be used.
2. Common faults of crystal oscillators include: a. Internal leakage, b. Internal open circuit, c. Deterioration frequency deviation, d. Leakage of peripheral connected capacitors. The leakage phenomenon here should be detectable using the VI curve of the<tester>.
3. There are two judgment methods that can be used during the whole board testing: a. The chips around the crystal oscillator do not pass the test. b. No other fault points were found except for the crystal oscillator.
There are two common types of crystal oscillators: a. Two legged. b. Four legs, the second leg is powered on, be careful not to short circuit randomly.
Five. Distribution of fault phenomena
1. Incomplete statistics of the faulty parts of the circuit board: 1) 30% chip damage, 2) 30% discrete component damage,
3) 30% of the wiring (copper wire laid on PCB board) is broken, 4% of the program is damaged or lost (with an upward trend).
2. As can be seen from the above, when there are problems with the wiring and program of the circuit board to be repaired, and there is no good board, one is not familiar with its wiring and cannot find the original program. The possibility of repairing this board is unlikely.
Compatible design
Electromagnetic compatibility refers to the ability of electronic devices to work in a coordinated and effective manner in various electromagnetic environments. The purpose is to enable electronic devices to suppress various external interferences, enabling them to operate normally in specific electromagnetic environments, while reducing the electromagnetic interference of electronic devices themselves on other electronic devices.
1. Choosing a reasonable wire width is necessary as the impact interference generated by transient currents on the printed lines of PCB circuit boards is mainly caused by the inductance component of the printed wires. Therefore, the inductance of the printed wires should be minimized as much as possible.
2. Adopting the correct wiring strategy and using equal routing can reduce wire inductance, but the mutual inductance and distributed capacitance between wires increase. If the layout allows, it is best to use a cross shaped mesh wiring structure. The specific method is to horizontally route one side of the printed board and vertically route the other side, and then connect them with metalized holes at the cross holes.
3. In order to suppress crosstalk between PCB circuit board wires, it is advisable to avoid long-distance equal routing as much as possible when designing wiring, and to widen the distance between wires as much as possible. Signal wires should not intersect with ground wires and power wires as much as possible. Setting a grounded printed line between some signal lines that are highly sensitive to interference can effectively suppress crosstalk.
form
The circuit board is mainly composed of solder pads, vias, mounting holes, wires, components, connectors, fillers, electrical boundaries, etc. The main functions of each component are as follows:
Solder pad: A metal hole used for soldering pins of electronic components.
Through holes: There are metal through holes and non-metal through holes, where metal through holes are used to connect component pins between layers.
Installation hole: used to fix the circuit board.
Wire: An electrical network copper film used to connect component pins.
Connector: A component used for connecting circuit boards.
Fill: Copper coating used for ground wire networks can effectively reduce impedance.
Electrical boundary: used to determine the size of the circuit board, and all components on the circuit board must not exceed this boundary.
Main categories
Circuit board systems are classified into the following three types:
Single panel
Single-Sided Boards
We just mentioned it, so we call this type of PCB Single sided. Because there are many strict restrictions on the design of single panel circuits (because there is only one side and the wiring cannot intersect and must follow separate paths), only early circuits used this type of board.
Double sided board
Double-Sided Boards
This type of circuit board has wiring on both sides. However, to use wires on both sides, there must be appropriate electrical connections between them. The ‘bridge’ between these circuits is called a via. A through-hole is a small hole filled or coated with metal on a PCB, which can be connected to wires on both sides. Because the area of a double-sided panel is twice as large as a single panel, and because the wiring can be staggered (can be wound to the other side), it is more suitable for use in more complex circuits than a single panel.
Multi-layer board
In complex application requirements, circuits can be arranged in a multi-layer structure and pressed together, and through-hole circuits can be built between layers to connect the circuits of each layer.
Inner layer circuit
The copper foil substrate is first cut into sizes suitable for processing and production. Before laminating the substrate, it is usually necessary to roughen the copper foil on the surface of the board using methods such as brushing, grinding, and micro etching. Then, the dry film photoresist is tightly attached to it at an appropriate temperature and pressure. Send the substrate with the dry film photoresist attached to it into a UV exposure machine for exposure. The photoresist in the transparent area of the film will undergo polymerization reaction after being exposed to UV radiation (the dry film in this area will be retained as an etching resist in later development and copper etching steps), and the line image on the film will be transferred to the surface dry film photoresist. After tearing off the protective film on the film surface, first use sodium carbonate aqueous solution to develop and remove the areas on the film surface that have not been exposed to light, and then use a mixed solution of hydrochloric acid and hydrogen peroxide to corrode and remove the exposed copper foil, forming a circuit. Finally, the dry film photoresist that has been successfully removed will be washed away with an aqueous solution of sodium hydroxide. For inner layer circuit boards with six or more layers, use an automatic positioning punching machine to punch out riveting reference holes for inter layer circuit alignment. Multi-Layer Boards
In order to increase the area that can be wired, multi-layer boards use more single or double-sided wiring boards. Multiple layers of double-sided boards are used, and a layer of insulation is placed between each board and then firmly bonded (pressed together).
The number of layers on a board represents the number of independent wiring layers, usually even and including the outermost two layers. Most motherboards have a structure of 4 to 8 layers, but technically it is possible to achieve PCB boards with nearly 100 layers. Most large supercomputers use quite a few layers of motherboards, but because these types of computers can already be replaced by clusters of many ordinary computers, super multilayer boards are gradually no longer being used. Because each layer in the PCB is tightly integrated, it is generally not easy to see the actual number, but if you carefully observe the motherboard, you may be able to see it.
The automatic detection technology of circuit boards has been applied with the introduction of surface mount technology, which has led to a rapid increase in the packaging density of circuit boards. Therefore, even for low-density and general quantity circuit boards, automatic detection of circuit boards is not only basic, but also economical. In complex circuit board testing, two common methods are needle bed testing and dual probe or flying needle testing.
Tester editor
Based on the material characteristics and wide application fields of circuit boards, in order to effectively save volume and achieve a certain degree of accuracy, the three-dimensional space characteristics and thin thickness are better applied to digital products, mobile phones, and laptops. The recommended instruments for circuit board (FPC) testing include the MUMA200 all aluminum alloy optical imaging measuring instrument, the three-axis fully automatic optical imaging measuring instrument VMC250S, the VMC four axis fully automatic optical imaging measuring instrument, the VMS series optical imaging measuring instrument, and so on.
test method
Needle bed method
This method involves connecting probes with springs to each detection point on the circuit board. The spring provides each probe with a pressure of 100-200g to ensure good contact at each detection point, and such probes arranged together are called “needle beds”. Under the control of the detection software, the detection points and signals can be programmed. Figure 14-3 shows a typical structure of a needle bed tester, where the tester can obtain information about all the testing points. In fact, only the probes for the test points that need to be tested are installed. Although the needle bed testing method may be used to test both sides of the circuit board simultaneously, when designing the circuit board, all testing points should still be placed on the soldering surface of the circuit board. Needle bed testing equipment is expensive and difficult to maintain. Needles are selected with different arrangements of probes based on their specific applications.
A basic general-purpose grid processor consists of a drilled board with center to center spacing of 100, 75, or 50mil between pins. Pin serves as a probe and makes direct mechanical connections using electrical connectors or nodes on the circuit board. If the solder pads on the circuit board match the test grid, the polyethylene film punched according to the specifications will be placed between the grid and the circuit board to facilitate the design of specific probes. Continuity detection is achieved by accessing the end points of the grid (defined as the x-y coordinates of the solder pads). Since every network on the circuit board undergoes continuity testing. In this way, an independent detection is completed. However, the proximity of the probe limits the effectiveness of the needle bed testing method.
observation
The circuit board has a small volume and complex structure, so professional observation instruments must also be used to observe the circuit board. Generally, we use a portable video microscope to observe the structure of the circuit board. Through the video microscope camera, we can clearly see the microscopic structure of the circuit board from the microscope, which is very intuitive. Through this method, it is relatively easy for us to design and inspect circuit boards. The portable video microscopes used on site in the factory, MSA200 and VT101, are more convenient than traditional microscopes because they can achieve “real-time observation, real-time detection, and multi person discussion”!
Flying needle test
The flying needle tester does not rely on the pin pattern installed on the fixture or bracket. Based on this system, two or more probes are mounted on tiny magnetic heads that can move freely in the x-y plane, and the test points are directly controlled by CADI Gerber data. The dual probes can move within a range of 4mil from each other. The probes can move independently and there is no real limit to how close they are to each other. The tester with two arms that can move back and forth is based on the measurement of capacitance. Press the circuit board tightly onto the insulation layer on a metal plate as another metal plate for the capacitor. If there is a short circuit between the lines, the capacitance will be larger than at a certain point. If there is an open circuit, the capacitance will decrease.
Testing speed is an important criterion for selecting a testing instrument. The needle bed tester can accurately test thousands of test points at once, while the flying needle tester can only test two or four test points at a time. In addition, when using a needle bed tester for single-sided testing, it may only cost 20-305, depending on the complexity of the board, while a flying needle tester requires Ih or more time to complete the same evaluation. Shipley (1991) explained that even though manufacturers of high-yield printed circuit boards consider mobile flying pin testing technology slow, this method is still a good choice for manufacturers of complex circuit boards with lower production volumes.
For bare board testing, there are specialized testing instruments available (Lea, 1990). A more cost-effective approach is to use a universal instrument, although initially more expensive than specialized instruments, its initial high cost will be offset by a reduction in individual configuration costs. For general grids, the standard grid for boards with pin components and surface mount devices is 2.5mm. At this point, the test pad should be greater than or equal to 1.3mm. For the grid of Imm, the test pad design should be larger than 0.7mm. If the grid is small, the test needle will be small, brittle, and easily damaged. Therefore, it is best to use grids larger than 2.5mm. Crum (1994b) stated that the combination of a universal tester (standard grid tester) and a flying pin tester can make the detection of high-density circuit boards both accurate and economical. Another method he suggested is to use a conductive rubber tester, which can be used to detect points that deviate from the grid. However, the height of the solder pads treated with hot air leveling is different, which will hinder the connection of the test points.
Usually, the following three levels of detection are carried out:
1) Bare board inspection;
2) Online detection;
3) Functional testing.
By using a universal type of tester, it is possible to test a certain style and type of circuit board, as well as for special application testing.
Maintenance knowledge
Circuit board repair is an emerging repair industry. The automation level of industrial equipment is increasing, so the number of industrial control boards in various industries is also increasing. After the industrial control board is damaged, the high cost of replacing the circuit board (ranging from a few thousand yuan to tens of thousands or hundreds of thousands of yuan) has become a headache for enterprises. In fact, the vast majority of these damaged circuit boards can be repaired in China, and the cost is only 20% -30% of purchasing a new board, and the time required is much shorter than ordering a board abroad. Below is an introduction to the basic knowledge of circuit board maintenance.
Almost all circuit board repairs do not have drawing materials, so many people have a skeptical attitude towards circuit board repairs. Although various circuit boards vary greatly, what remains unchanged is that each type of circuit board is composed of various integrated blocks, resistors, capacitors, and other devices. Therefore, circuit board damage must be caused by the damage of one or some of the components. The idea of circuit board repair is based on the above factors. Circuit board maintenance is divided into two parts: inspection and repair, with inspection occupying a very important position. Test the basic knowledge of repairing each component on the circuit board until the faulty part is found and replaced, and then a circuit board is repaired.
Circuit board inspection is the process of identifying, determining, and correcting faults in every electronic component on a circuit board. In fact, the entire testing process is a thinking process and a testing process that provides logical reasoning clues. Therefore, the testing engineer must gradually accumulate experience and continuously improve their level during the maintenance, testing, and repair of the circuit board. General electronic devices are composed of thousands of components. During maintenance and repair, it would be time-consuming and difficult to detect problems by directly testing and inspecting each component on the circuit board one by one. So the targeted maintenance method from the fault phenomenon to the fault cause is an important maintenance method. Once the problem is detected on the circuit board, maintenance becomes easy. The above is an introduction to the basic knowledge of circuit board maintenance.