In foreign countries, flexible PCB has been widely used in the early 1960s. In China, production and application began in the mid-1960s. In recent years, with the globalization of the global economy and the opening of the market, its usage has been increasing. Aiming at this opportunity, some small and medium-sized rigid PCB factories adopt soft and hard manufacturing technology, improve tooling tools and processes with existing equipment, and transform to produce flexible PCBs to meet the increasing demand for flexible PCBs. In order to further understand PCB, this paper introduces the flexible PCB technology.
First, the classification of flexible PCB and its advantages and disadvantages
1. soft PCB classification
According to the number of layers and structure of conductors, flexible PCBs are usually divided into the following categories:
1. 1 single-sided flexible PCB
Single-sided flexible PCB has only one layer of conductor, and its surface may or may not be covered. The insulating base material used varies with the application of the product. Commonly used insulation materials are polyester, polyimide, polytetrafluoroethylene, soft epoxy glass cloth and so on.
Single-sided flexible PCB can be further divided into the following four categories:
1) Single-sided connection without covering layer
The conductor pattern of this flexible PCB is on the insulating substrate, and there is no covering layer on the conductor surface. Like the usual single-sided rigid PCB. This kind of product is the cheapest one and is usually used in non-critical and environmentally friendly applications. Its interconnection is realized by brazing, fusion welding or pressure welding. It was often used in early telephones.
2) One-sided connection with covering layer
Compared with the former type, this type only adds a coating on the conductor surface according to the customer's requirements. When covered, the pad needs to be exposed. Simply put, it can be uncovered in the end region. Where accuracy is required, clearance holes can be used. It is one of the most widely used single-sided flexible PCB, which is widely used in automobile instruments and electronic instruments.
3) Double-sided connection without covering layer
This type of connection pad interface can be connected to the front and back of the wire. In order to do this, through holes are opened in the insulating substrate at the pads, which can be made by punching holes, etching or other mechanical methods at the required positions of the insulating substrate. It is used to install components and equipment on both sides and where welding is needed. There is no insulating substrate in the pad area at the channel, and this pad area is usually removed by chemical method.
4) Double-sided connection with covering layer
The difference between this one and the former one is that there is a covering layer on the surface. However, the cover layer has a through hole which also allows both sides to be terminated and still keep the cover layer. This flexible PCB is made of two layers of insulating materials and one layer of metal conductor. It is used in situations where the covering layer and surrounding devices need to be insulated from each other, and the ends need to be connected with the front and rear sides.
1.2 double-sided flexible PCB
Double-sided flexible PCB with two layers of conductors. This kind of double-sided flexible printed board has the same uses and advantages as single-sided flexible printed board, and its main advantage is to increase the wiring density per unit area. It can be divided into: a whether there are metallized holes and whether there are coatings; B, there are no metallized holes and covering layers; C. there are metallized holes and no covering layer; With metallized holes and covering layers. Double-sided flexible PCB without covering layer is rarely used.
1.3 multilayer flexible PCB
Flexible multilayer printed boards, like rigid multilayer printed boards, can be made into multilayer flexible printed boards by multilayer lamination technology. The simplest multilayer flexible PCB is a three-layer flexible PCB with two copper shielding layers on both sides of a single-sided PCB. This three-layer flexible PCB is equivalent to coaxial conductor or shielded conductor in electrical characteristics. The most commonly used multi-layer flexible PCB structure is a four-layer structure, which uses metallized holes to realize inter-layer interconnection, and the middle two layers are generally a power layer and a ground layer.
The advantages of multilayer flexible PCB are light weight of base film and excellent electrical characteristics, such as low dielectric constant. The multilayer flexible PCB board made of polyimide film is about 1/3 lighter than the rigid epoxy glass cloth multilayer PCB board, but it loses the excellent flexibility of single-sided and double-sided flexible PCB boards. Most of these products have no requirements for flexibility.
Multilayer flexible PCB can be further divided into the following types:
1) forms a multilayer PCB on a flexible insulating substrate, and its finished product is defined as flexibility: this structure usually bonds the two ends of many single-sided or double-sided microstrip flexible PCBs together, but the middle part is not bonded together, so it has high flexibility. In order to have desired electrical characteristics, such as matching the characteristic impedance performance with the rigid PCB interconnected with it, it is necessary to design signal lines for each circuit layer of multilayer flexible PCB elements on the ground layer. In order to have high flexibility, a thin and suitable coating, such as polyimide, can be used on the conductor layer instead of a thick laminated covering layer. Metallized holes make the Z plane between flexible circuits realize the required interconnection. This multilayer flexible PCB is most suitable for designs that require flexibility, high reliability and high density.
2) A multilayer PCB is formed on a flexible insulating substrate, and the finished product can be flexed: this multilayer flexible PCB is made of a flexible insulating material, such as a polyimide film, by lamination. After lamination, the inherent flexibility is lost. This flexible PCB is used when the design requires making full use of the insulation characteristics of the film, such as low dielectric constant, uniform dielectric thickness, light weight and continuous processing. For example, a multilayer PCB made of polyimide film insulation material is about one-third lighter than an epoxy glass cloth rigid PCB.
3) A multilayer PCB is formed on a soft insulating substrate, and the finished product must be moldable, not continuous and flexible: this multilayer flexible PCB is made of soft insulating materials. Although it is made of soft material, it has been formed in the finished application due to the limitation of electrical design, such as the need for thick conductor for required conductor resistance or the need for thick insulation and isolation between signal layer and ground layer for required impedance or capacitance. The term "formable" is defined as: multilayer flexible PCB components have the ability to manufacture required shapes and cannot be bent in applications. Application in internal wiring of avionics. At this time, the conductor of stripline or three-dimensional space design is required to have low resistance, minimum capacitive coupling or circuit noise, and can be smoothly bent to 90 at the end of interconnection. Multilayer flexible PCB made of polyimide film material realizes this wiring task. Because polyimide film has high temperature resistance, flexibility and good comprehensive electrical and mechanical properties. In order to realize all the interconnections of this component part, the wiring part can be further divided into a plurality of multilayer flexible circuit components and bonded together with adhesive tape to form a printed circuit bundle.
1.4 rigid-flexible multilayer PCB
This type is usually on one or two rigid PCBs, including the indispensable flexible PCB. Flexible PCB is laminated in rigid multilayer PCB in order to have special electrical requirements or extend to the outside of rigid circuit, thus having the ability to connect Z-plane circuits. This kind of products are widely used in electronic equipment with compression weight and volume as the key to ensure high reliability, high density assembly and excellent electrical characteristics.
Rigid-flexible multilayer PCB can also press the ends of many single-sided or double-sided flexible PCBs together to become rigid parts, while the middle part is not bonded to become flexible parts, and the Z planes of rigid parts are interconnected through metallized holes. Flexible wiring can be laminated into a rigid multilayer board. This kind of PCB is more and more used in those occasions that require ultra-high packaging density, excellent electrical characteristics, high reliability and strict volume restrictions.
A series of hybrid multilayer flexible PCB components have been designed and used in military avionics. In these applications, weight and volume are very important. In order to meet the specified weight and volume restrictions, the density of internal packaging must be very high. In addition to high circuit density, all signal transmission lines must be shielded to minimize crosstalk and noise. If shielded independent wires are used, it is practically impossible to package them into the system economically. In this way, a hybrid multilayer is used.
Flexible PCB to realize its interconnection. This component contains the shielded signal line in the flat stripline flexible PCB, which is an important part of the rigid PCB. At a relatively high operating level, the PCB forms a 90-degree S-bend after manufacturing, which provides a way for Z-plane interconnection, and can eliminate the stress and strain on the solder joint under the vibration stress of X, Y and Z planes.
2. Advantages
2. 1 flexibility
An obvious advantage of using flexible PCB is that it can be wired and connected in three-dimensional space more conveniently, and it can also be curled or folded for use. As long as it is curled within the allowable radius of curvature, it can withstand thousands to tens of thousands of uses without damage.
2.2 reduce the volume
In the assembly and connection of components, the conductor section of flexible PCB is thin and flat, which reduces the conductor size and can be molded along the shell, making the structure of the equipment more compact and reasonable and reducing the assembly and connection volume. Compared with rigid PCB, it can save 60~90% space.
2.3 Lose weight
Under the same volume and current carrying capacity, the weight of flexible PCB can be reduced by about 70% compared with conductor and cable, and by about 90% compared with rigid PCB.
2.4 Consistency of assembly and connection
Flexible PCB is used for connection, which eliminates the mistake of wiring with wires and cables. As long as the processing drawings pass the proofreading, the winding circuits produced in the future will be the same. When installing the connecting cable, there will be no wrong connection.
2.5 Improve reliability
When flexible PCB is used for connection, the wiring can be arranged in X, Y and Z planes, which reduces the transfer interconnection, increases the reliability of the whole system and provides convenience for fault inspection.
2.6 Electrical parameter design controllability
According to the use requirements, designers can control capacitance, inductance, characteristic impedance, delay and attenuation when designing flexible PCB. Can be designed to have the characteristics of transmission lines. Because these parameters are related to conductor width, thickness, spacing, insulation thickness, dielectric constant, loss tangent, etc., it is difficult to do this when using conductor cables.
2.7 The ends can be welded integrally.
Flexible PCB, like rigid PCB, has terminal pads, which can eliminate the peeling and tin lining of wires, thus saving costs. The terminal pads are connected with components, devices and plugs, and the manual welding of each wire can be replaced by immersion welding or wave soldering.
2.8 The use of materials is optional.
Flexible PCB can be made of different substrates according to different use requirements. For example, in low-cost assembly applications, polyester films can be used. In demanding applications, excellent performance is required, and polyimide sub-films can be used.
2.9 Low cost
Using flexible PCB connection can reduce the total cost. This is because:
1) Due to the consistency of various parameters of the wires of the flexible PCB; The realization of integral termination eliminates the frequent mistakes and rework in cable connection, and the replacement of flexible PCB is more convenient.
2) The application of flexible PCB simplifies the structural design and can be directly attached to components, thus reducing the number of fixtures and their fixing devices.
3) For wires that need shielding, the price of using flexible PCB is lower.
2. 10 processing continuity
Since the flexible foil-covered board can be continuously supplied in rolls, the continuous production of flexible PCB can be realized. This is also conducive to reducing costs.
3. Shortcomings
3. 1 High one-time initial cost
Because flexible PCB is designed and manufactured for special applications, the initial circuit design, wiring and photographic master are very expensive. Unless there is a special need to apply flexible PCB, it is usually best not to use it in a few applications.
3.2 It is difficult to replace and repair flexible PCB.
Once the flexible PCB is made, it must be changed from the base map or the compiled optical drawing program, which is not easy to change. Its surface is covered with a protective film, which needs to be removed before repair and restored after repair. This is a difficult job.
3.3 Limited size
When the flexible PCB is not popular, it is usually manufactured in batches, and it cannot be made very long and wide due to the size of production equipment.
3.4 Improper operation is easy to damage
Improper operation by assemblers is likely to cause damage to flexible circuits, and its welding and rework need trained personnel to operate.
PCB high-speed PCB technology->; Implementation process of PCB board->; Go to message
As we all know, making PCB is to turn the designed schematic diagram into a real PCB board. Please don't underestimate this process. There are many things that are feasible in principle but difficult to achieve in engineering, or things that others can achieve but others can't, so it is not difficult to make a PCB board, but it is not an easy task to do it well.
The two major difficulties in the field of microelectronics are the processing of high-frequency signals and weak signals. In this respect, the production level of PCB is particularly important. The same principle design, the same components, different people produce different results. So how can we make a good PCB? Based on our past experience, I would like to talk about my views on the following aspects:
First of all, we must define the design objectives.
When receiving a design task, we must first make clear its design goal, whether there are common PCB boards, high-frequency PCB boards, small signal processing PCB boards or high-frequency small signal processing PCB boards. If it is an ordinary PCB board, as long as the layout and wiring are reasonable and tidy, the mechanical size is accurate. If there are medium load lines and long lines, certain measures must be taken to reduce the load, and the long lines should be opened more, with the focus on preventing long line reflection. When there are signal lines exceeding 40MHz on the board, special consideration should be given to these signal lines, such as crosstalk between lines. If the frequency is higher, the wiring length will be more strictly limited. According to the distributed parameter network theory, the interaction between high-speed circuit and its wiring is a decisive factor, which can not be ignored in system design. With the increase of gate transmission speed, the opposition on signal lines will increase correspondingly, and the crosstalk between adjacent signal lines will also increase proportionally. Usually, the power consumption and heat dissipation of high-speed circuits are also very large, so we should pay enough attention to it when making high-speed PCB.
Pay special attention to these signal lines when there is a weak signal of millivolts or even microvolts on the board. Small signals are too weak to be interfered by other strong signals, so shielding measures are often needed, otherwise the signal-to-noise ratio will be greatly reduced. Therefore, the useful signal is submerged by noise and cannot be effectively extracted.
The adjustment and measurement of the circuit board should also be considered in the design stage, and the physical location and isolation of the test points can not be ignored, because some small signals and high-frequency signals can not be directly added to the probe for measurement.
In addition, other related factors should be considered, such as the number of layers of the board, the packaging shape of the components used, and the mechanical strength of the board. Before making PCB, you should make clear the design goal of the design.
Two. Understand the functional requirements of components used for layout and wiring.
We know that some special components have special requirements in layout and wiring, such as analog signal amplifiers used by Lottie and APH, which require stable power supply and small ripple. The analog small signal part should be as far away from the power device as possible. On the OTI board, the small signal amplification part is also specially equipped with a shielding cover to shield stray electromagnetic interference. GLINK chip used in NTOI board adopts ECL technology, which consumes a lot of power and generates a lot of heat. Therefore, special consideration must be given to heat dissipation in layout. If natural heat dissipation is used, the GLINK chip should be placed in a place with smooth air circulation, and the heat emitted cannot have a great impact on other chips. If speakers or other high-power devices are installed on the board, it may cause serious pollution to the power supply, which should also be paid enough attention.
Three. Consideration of component layout
One of the first factors to be considered in component layout is electrical performance. Try to put closely connected components together, especially some high-speed lines, as short as possible, and separate power signals from small signal devices. On the premise of satisfying the circuit performance, it is also necessary to consider that the components are neatly arranged, beautiful and easy to test, and the mechanical size of the circuit board and the location of the socket should also be seriously considered.
In high-speed system, grounding and transmission delay time on interconnection lines are also the first factors to be considered in system design. The transmission time on the signal line has a great influence on the speed of the whole system, especially for the high-speed ECL circuit. Although the speed of the integrated circuit block itself is very high, the delay time (about 2ns per 30cm line length) increased due to the use of ordinary interconnection lines on the backplane will greatly reduce the system speed. It is best to put synchronous working elements such as shift register and synchronous counter on the same board. Because the transmission delay time of clock signals to different boards is not equal, the shift register may be wrong. If it can't be put on one board, the clock line length from the common clock source to each plug-in board must be equal, and synchronization is the key.
Fourth, the consideration of wiring.
With the completion of OTNI and star-shaped optical fiber network design, there will be more boards for high-speed signal lines above 100MHz to be designed in the future. Here will introduce some basic concepts of high-speed lines.
1. transmission line
Any "long" signal path on the printed circuit board can be regarded as a transmission line. If the transmission delay time of the line is much shorter than the signal rise time, the reflection of the producer will be drowned during the signal rise. Overshoot, recoil and ringing no longer exist. For most MOS circuits at present, the ratio of rise time to transmission delay time is much larger, so the line length can be measured in meters without signal distortion. Used in fast logic circuits, especially ultra-high speed ECL.
For integrated circuits, due to the increase of edge speed, if there are no other measures, the length of traces must be greatly shortened to maintain signal integrity.
There are two ways to make high-speed circuits work on relatively long lines without serious waveform distortion. TTL uses Schottky diode clamping method to clamp the fast falling edge, so that the overshoot is clamped at a level lower than the ground potential by one diode voltage drop, thus reducing the reverse recoil amplitude. The slow rising edge allows overshoot, but it will be attenuated by the relatively high output impedance (50 ~ 80 Ω) of the circuit at the "H" level. In addition, due to the high immunity of "H" class state, the problem of recoil is not very prominent. For HCT series devices, the improvement effect will be more obvious if Schottky diode clamping and series resistance termination are combined.
When there is fan-out along the signal line, the TTL shaping method introduced above appears to be somewhat insufficient at higher bit rate and faster edge rate. Because there are reflected waves in the line, they tend to merge at high bit rate, causing serious signal distortion and reducing anti-interference ability. Therefore, in order to solve the reflection problem, another method is usually used in ECL system: line impedance matching method. This can control the reflection and ensure the integrity of the signal.
Strictly speaking, transmission lines are not necessary for conventional TTL and CMOS devices with slow edge speed, but not always for high-speed ECL devices with fast edge speed. However, when transmission lines are used, they have the advantages of predicting connection delay and controlling reflection and oscillation through impedance matching. 1
There are five basic factors that determine whether to adopt transmission lines. They are: (1) the edge rate of the system signal, (2) the distance of the line, (3) the capacitive load (fan-out) and (4) the resistive load (the termination mode of the line). (5) Allowable percentage of clearance and overshoot (degree of reduction of AC immunity).
2. Several types of transmission lines
(1) Coaxial cable and twisted pair: They are usually used to connect systems. The characteristic impedance of coaxial cable is usually 50Ω and 75Ω, and the characteristic impedance of twisted pair is usually110Ω.
(microstrip line on PCB
Microstrip line is a strip conductor (signal line). It is separated from the ground plane by a dielectric. If the thickness, width and distance between the line and the ground plane are controllable, then its characteristic impedance is also controllable. The characteristic impedance Z0 of microstrip line is:
Where: er is the relative dielectric constant of the dielectric material of the printed circuit board.
δ is the thickness of the dielectric layer.
W is the width of the line.
T is the thickness of the line.
The transmission delay time of unit length microstrip line only depends on the dielectric constant, and has nothing to do with the width or spacing of the line.
(3) stripline in printed circuit board
A stripline is a copper stripline placed in the middle of a dielectric between two conductive planes. If the thickness and width of the wire, the dielectric constant of the medium and the distance between two conductive planes are controllable, then the characteristic impedance of the wire can also be controlled. The characteristic impedance of the stripline is:
Where: b is the distance between two grounding plates.
W is the width of the line.
T is the thickness of the line.
Similarly, the transmission delay time of unit length strip line is independent of the width or spacing of the line; Only depends on the relative dielectric constant of the medium used.
3. Terminate the transmission line
When the receiving end of the line is terminated with a resistor equal to the characteristic impedance of the line, the transmission line is called parallel termination. Mainly used to obtain the best electrical performance, including driving distributed loads.
Sometimes, in order to save power consumption, a 104 capacitor is connected in series with the termination resistor to form an AC termination circuit, which can effectively reduce DC loss.
A resistor is connected in series between the driver and the transmission line, and the terminal of the transmission line is no longer connected with the terminal resistor. This termination method is called serial termination. The overshoot and ringing of longer lines can be controlled by series damping or series termination technology. Series damping is achieved by connecting a small resistor (usually10 ~ 75 Ω) in series at the output of the driving gate. This damping method is suitable for lines with controlled characteristic impedance (such as backplane wiring, circuit boards without grounding layer and most winding wires).
The sum of the value of the series resistance and the output impedance of the circuit (driving gate) is equal to the characteristic impedance of the transmission line when it is terminated in series. The series terminal has the disadvantages of using lumped load only at the terminal and long transmission delay time. However, this can be overcome by using redundant series termination transmission lines.
4. Unterminated transmission line
If the line delay time is much shorter than the signal rise time, the transmission line can be used without series termination or parallel termination. If the bidirectional delay of the non-terminal line (the time for the signal to go back and forth on the transmission line) is shorter than the rising time of the pulse signal, the backlash caused by the non-terminal line is about 15% of the logic swing. The maximum open route length is about:
Lmax
Address (domicile) of the employer (Party A)
Legal representative (principal responsible person)
Name of employee (Party B)
Id address: