PCB design principles and methods of the current electronic product assembly technology has matured, in terms of SMT equipment, both the printing machine or placement machine has reached a fairly high level of precision, however, in some of the use of high-precision equipment in the factory, its products did not achieve the expected quality, which plagued the quality of one of the reasons for product quality is the problem of PCB design.
Component Layout Layout is in accordance with the requirements of the electrical schematic diagram and the external dimensions of the components, the components are evenly and neatly arranged on the PCB, and can meet the mechanical and electrical performance requirements of the machine. Layout is reasonable or not not only affects the performance and reliability of PCB assemblies and the machine, but also affects the PCB and its assemblies and the ease of processing and maintenance, so the layout try to do the following: Figure 9-25 Wide wiring becomes narrower and then connected to the pads
(1) Uniform distribution of components, rows of components in the same circuit unit should be relatively centralised arrangement, in order to facilitate debugging and maintenance; There are interconnecting components should be arranged relatively close to each other, in order to facilitate the improvement of wiring density and to ensure that the alignment distance is the shortest;
(2) For heat-sensitive components, the arrangement should be far away from the components with high heat generation;
(3) Mutual electromagnetic interference may have components, should take shielding or isolation measures.
Wiring is in accordance with the electrical schematic diagram and wire table, as well as the need for wire width and spacing of printed wires, wiring should generally comply with the following rules:
(1) under the premise of meeting the requirements of use, the order of selection of wiring for single-layer, double-layer and multi-layer wiring.
(2) The wires between the two connection discs are laid as short as possible, and sensitive signals and small signals go first to reduce the delay and interference of small signals. The analogue circuit should be laid next to the input line of the earth wire shield; the same layer of wires should be evenly distributed; the conductive area on each wire should be relatively balanced to prevent the board from warping.
(3) Signal lines to change direction should go diagonal or smooth transition, and the radius of curvature is better to avoid electric field concentration, signal reflection and generate additional impedance.
(4) digital circuits and analogue circuits in the wiring should be separated to avoid mutual interference, such as in the same layer of the two circuits should be the ground system and the power system wires are laid separately, different frequencies of the signal line should be laid in the middle of the ground line to avoid crosstalk.
(5) Circuit components grounded, connected to the power supply should be as short as possible, as close as possible to reduce internal resistance.
(6) X, Y layer alignment should be perpendicular to each other, in order to reduce coupling, do not align or parallel to the upper and lower layer alignment.
(7) High-speed circuits with multiple I/O lines as well as differential amplifiers, balanced amplifiers and other circuits should have equal lengths of I/O lines to avoid unnecessary delays or phase shifts.
(8) pads and larger areas of conductive area connected to the thermal isolation should be a length of not less than 0.5 mm of fine wire, fine wire width of not less than 0.13 mm, as shown in Figure 9-1. But for the need to pass 5 A or more high-current pads can not be used thermal pads. Figure 9-1 Thermal insulation pads.
(9) The wire closest to the edge of the printed board, the size from the edge of the printed board should be more than 5 mm, if necessary, the earth wire can be close to the edge of the board. If rails are to be inserted during the printed board process, the wires should be at least a greater distance from the edge of the board than the depth of the rail slots.
(10) double-sided board on the common power and earth wires, as close as possible to the edge of the board, and distributed on both sides of the board, the graphic configuration to make the power line and the ground for the low impedance between. Multilayer board can be set up in the inner layer of the power and ground layer, through the metallised holes with the power and ground lines of each layer connected to the inner layer of the large area of the conductor and the power line, the ground line should be designed as a mesh, which can improve the bonding of multilayer board layers.
(11) In order to test the convenience of the design should set the necessary breakpoints and test points. The following methods are commonly used:
① Components that may affect or interfere with each other should be kept as far away as possible or shielding measures should be taken in the layout.
② Signal lines of different frequencies, do not parallel wiring close to each other; for high-frequency signal lines, should be shielded on one or both sides of the grounding line.
③ For high-frequency, high-speed circuits, should try to design into double-sided and multi-layer printed circuit boards. One side of the double-sided board laid signal lines, the other side can be designed to ground the ground; multi-layer board can be easily disturbed signal lines arranged in the ground layer or between the power supply layer; for microwave circuits with the ribbon line, the transmission signal line must be laid between the two grounding layer, and the thickness of the dielectric layer between them as needed to calculate.
④ The base printed line of the transistor and the high-frequency signal line should be designed as short as possible to reduce electromagnetic interference or radiation during signal transmission.
⑤ Components of different frequencies do not share the same ground line, and ground and power lines of different frequencies should be laid separately.
⑥ Digital circuits and analogue circuits do not share the same ground line, in connection with the printed circuit board external ground connection can have a common contact.
⑦ The work of the potential difference is relatively large components or printed wires, should increase the distance between each other.
PCB thermal design with the increase in the density of assembly of components on the printed circuit board, if you can not effectively dissipate heat in a timely manner, will affect the operating parameters of the circuit, and even too much heat will make the components fail, so the thermal problems of the printed circuit board, the design must be carefully considered, generally take the following measures:
(1) Increase the area of copper foil on the printed circuit board and high-power components ground plane;
(2) heat-generating components are not mounted on the board, or an additional heat sink;
(3) the inner ground of the multilayer board should be designed as a mesh and close to the edge of the board;
(4) the selection of flame-retardant or heat-resistant type of plate.
(1) PCB no process edges, process holes, and can not meet the SMT device Clamping requirements, which means that it can not meet the requirements of mass production.
(2) PCB shape or size of the shape is too large, too small, the same can not meet the clamping requirements of the equipment.
(3) PCB, FQFP pads around the optical positioning mark (Mark) or Mark points are not standard, such as Mark point around the solder resist film, or too large, too small, resulting in Mark point image contrast is too small, the machine frequently alarm can not work properly.
(4) incorrect size of the pad structure, such as chip components of the pad spacing is too large, too small, pad asymmetry, so as to cause the chip components after welding, crooked, standing monument and many other defects.
(5) There is a hole on the pad, the welding caused by the solder melting through the hole on the pad leakage to the bottom, causing the solder joints too little solder.
(6) piece of component pad size asymmetry, especially with the land line, a part of the line used as a pad, so that when reflow soldering piece of component pads at both ends of the uneven heat, the solder paste melted successively and caused by the monument defects.
(7) IC pad design is not correct, the FQFP pad is too wide, resulting in welding after the bridge, or the pad is too short along the back edge caused by the lack of post-solder strength.
(8) Interconnecting wires between IC pads are placed in the centre, which is not conducive to SMA post-soldering inspection.
(9) Wave soldering IC did not design auxiliary pads, resulting in post-soldering bridging.
(10) IC distribution in the PCB is unreasonable, appearing after welding PCB deformation.
(11) Test point design is not standardised, so that ICT (in-circuit tester) can not work.
(12) SMD gap between the incorrect, later repair difficulties.
(13) Solder mask and character map is not standardised as well as solder mask and character map falls on the pad, resulting in false soldering or electrical circuit breakage.
(14) board design is not reasonable, such as "V" groove processing is not good, resulting in deformation of the PCB reflow soldering. The above errors will appear in the bad design of one or more products, resulting in different degrees of impact on the quality of welding. Designers do not know enough about the PCBA process, especially for components in reflow soldering, there is a "dynamic" process does not understand is one of the reasons for bad design. In addition, the design of the early neglect of process personnel to participate in the lack of the enterprise's design specifications for manufactur ability, are also the cause of bad design.
