PCB Blog - Detailed explanation of three special wiring sharing and inspection methods of PCB

Before explaining the inspection work after the PCB board wiring is completed, I will introduce the special wiring skills of the three types of PCB. The routing of PCB layout will be explained from three aspects: right-angle routing, differential routing, and serpentine routing:

1. Right-angle wiring (three aspects)
The influence of right-angle wiring on the signal is mainly reflected in three aspects: first, the corner can be equivalent to a capacitive load on the transmission line, slowing down the rise time; second, the impedance discontinuity will cause signal reflection; third, the right angle is generated From EMI, to RF design fields above 10GHz, these small right angles can become the focus of high-speed problems.

2. Differential traces ("equal length, equal distance, reference plane")
What is a differential signal (Differential Signal) In layman's terms, the driving end sends two signals of equal value and opposite phase, and the receiving end judges the logical state "0" or "1" by comparing the difference between the two voltages. The pair of traces that carry the differential signal is called a differential trace. Compared with ordinary single-ended signal traces, differential signals have obvious advantages in the following three

Aspect:
1. Strong anti-interference ability, because the coupling between the two differential traces is very good. When there is noise interference in the outside world, they are almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals. Therefore, the external common mode noise can be completely canceled.
2. It can effectively suppress EMI. In the same way, because the polarities of the two signals are opposite, the electromagnetic fields radiated by them can cancel each other. The tighter the coupling, the less electromagnetic energy is released to the outside world.
3. Timing positioning, because the switching change of the differential signal is located at the intersection of the two signals, unlike ordinary single-ended signals that rely on two threshold voltages, high and low, so it is less affected by process and temperature, and can reduce timing errors. It is also more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small-amplitude differential signaling technology.

3. Serpentine line (adjustment delay)
Serpentine line is a type of routing method often used in Layout. Its main purpose is to adjust the delay to meet the system timing design requirements. The two key parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when the signal is transmitted on the serpentine trace, coupling will occur between parallel line segments, in the form of differential mode. Small, the larger the Lp, the greater the coupling degree. It may lead to the reduction of transmission delay and greatly reduce the quality of the signal due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk. Here are a few tips for Layout engineers when dealing with snakes:
1) Try to increase the distance (S) of parallel line segments, at least greater than 3H, H refers to the distance from the signal trace to the reference plane. In layman's terms, it is to route around a big bend. As long as S is large enough, the mutual coupling effect can be almost completely avoided.
2) Reduce the coupling length Lp. When the double Lp delay approaches or exceeds the signal rise time, the generated crosstalk will reach saturation.
3) The signal transmission delay caused by the serpentine line of the strip line or the buried microstrip line is smaller than that of the microstrip line. Theoretically, the stripline will not affect the transmission rate due to differential mode crosstalk.
4) For high-speed and signal lines with strict timing requirements, try not to take serpentine lines, especially not to meander in a small area.
5) Serpentine traces with any angle can often be used, which can effectively reduce the coupling between them.
6) In high-speed PCB design, the serpentine line has no so-called filtering or anti-interference ability, and it can only reduce the signal quality, so it is only used for timing matching and no other purposes.
7) Sometimes the spiral routing can be considered for winding. The simulation shows that the effect is better than the normal serpentine routing.

General PCB design drawing inspection items
1) No circuit analysis; no circuit is divided into basic units in order to smooth the signal;
2) Does the circuit allow the use of short or isolated critical leads;
3) Where must be shielded, is it effectively shielded;
4) Make full use of the basic grid graphics without;
5) Whether the size of the printed circuit board is the size;
6) Whether to use the selected wire width and spacing as much as possible;
7) Whether the preferred pad size and hole size are used;
8) Whether the photographic plate and sketch are suitable;
9) Is there less jumper wire used; does the jumper wire pass through components and accessories;
10) Are the letters visible after assembly; are their sizes and models correct;
11) In order to prevent blistering, has the large area of copper foil been opened?
12) Are there tool positioning holes?

PCB electrical characteristics inspection items:
1) Has the influence of wire resistance, inductance and capacitance been analyzed; especially the impact of the critical voltage drop to grounding;
2) Whether the spacing and shape of the wire accessories meet the insulation requirements;
3) Whether the insulation resistance value is controlled and specified at key points;
4) Whether the polarity is adequately identified;
5) Has the influence of wire spacing on leakage resistance and voltage been measured from a geometrical point of view?
6) Has the medium that changed the surface coating been identified?

PCB physical characteristics inspection items:
1) Whether all pads and their positions are suitable for final assembly;
2) Whether the assembled printed circuit board can meet the shock and vibration energy conditions;
3) What is the spacing of the specified standard components;
4) Are the components that are not firmly installed or the heavier components fixed?
5) Is the heat dissipation and cooling of the heating element correct? Or is it isolated from printed circuit boards and other thermal components;
6) Are the voltage dividers and other multi-lead components positioned correctly?
7) Is the arrangement and orientation of components easy to inspect;
8) Whether all possible interference on the printed circuit board and on the entire printed circuit board assembly has been eliminated;
9) Whether the size of the positioning hole is correct;
10) Whether the tolerance is complete and reasonable;
11) Control and sign off the physical properties of all coatings;
12) Whether the ratio of the diameter of the hole to the lead wire is within the acceptable range.

PCB mechanical design factors:
Although the printed circuit board uses mechanical methods to support components, it cannot be used as a structural part of the entire device. On the edge of the printing plate, at least every 5 inches for a certain amount of support. The factors that must be considered in the selection and design of printed circuit boards are as follows;
1) The structure of the printed circuit board - size and shape.
2) The type of mechanical accessories and plugs required.
3) The adaptability of the circuit to other circuits and environmental conditions.
4) Consider mounting the printed circuit board vertically or horizontally according to some factors such as heat and dust.
5) Some environmental factors that need special attention, such as heat dissipation, ventilation, shock, vibration, humidity. Dust, salt spray and radiation.
6) Degree of support.
7) Hold and fix.
8) Easy to take off.

PCB printed circuit board installation requirements:
It should be supported within at least 1 inch of the three edge edges of the printed circuit board. According to practical experience, the spacing of support points of printed circuit boards with a thickness of 0.031--0.062 inches should be at least 4 inches; for printed circuit boards with a thickness of more than 0.093 inches, the spacing of support points should be at least 5 inches. Taking this measure increases the rigidity of the printed circuit board and destroys possible resonances of the printed circuit board. A certain printed circuit board usually takes into account the following factors before deciding on the mounting technology used for them.
1) The size and shape of the printed circuit board.
2) Number of input and output terminals.
3) Available equipment space.
4) The desired ease of loading and unloading.
5) The type of installation accessories.
6) The required heat dissipation.
7) Required shieldability.
8) The type of circuit and its relationship with other circuits.

Dial-out requirements for printed circuit boards:
1) No PCB area required to mount components.
2) The influence of plugging tools on the installation distance between two printed circuit boards.
3) The mounting holes and slots should be specially prepared in the design of the printed circuit board.
4) When the plug-in tool is to be used in the equipment, especially its size should be considered.
5) A plug-in device is required, which is usually fixed to the printed circuit board assembly with rivets.
6) In the mounting frame of printed circuit boards, special designs such as load bearing flanges are required.
7) The adaptability of the plugging tools used to the size, shape and thickness of the printed circuit board.
8) The cost involved in the use of plugging and unplugging tools includes both the price of the tool and the increased expenditure.
9) In order to tighten and use plug-in tools, it is required to have access to the inside of the equipment to a certain extent.

PCB Mechanical Considerations:
Substrate properties that have a significant impact on printed circuit assemblies are: water absorption, coefficient of thermal expansion, thermal properties, flexural strength, impact strength, tensile strength, shear strength and hardness. All of these properties affect both the functionality of the printed circuit board structure and the productivity of the printed circuit board structure. For most applications, the printed circuit board's dielectric backing is one of the following:
1) Phenolic impregnated paper.
2) Acrylic-polyester impregnated glass mat with random arrangement.
3) Epoxy impregnated paper.
4) Epoxy impregnated glass cloth.
Each substrate can be flame retardant or combustible. The above 1, 2 and 3 can be punched. The commonly used material for metallized hole printed circuit boards is epoxy-glass cloth. Its dimensional stability is suitable for use in high-density circuits, and it can reduce the occurrence of cracks in metallized holes. One disadvantage of epoxy glass cloth laminate is that it is difficult to punch in the usual thickness range of printed circuit boards, for this reason all holes are usually drilled and a copy milling operation is used to form the printed circuit board shape.

PCB Electrical Considerations:
In DC or low-frequency AC applications, the important electrical properties of insulating substrates are: insulation resistance, anti-electric isolation and printed conductor resistance and breakdown strength. In high frequency and microwave applications, it is: dielectric constant, capacitance, and dissipation factors. In all applications, however, the current carrying capacity of the printed conductors is important.
Wire pattern: PCB routing path and positioning, under the constraints of specified wiring rules, printed wires should take a short route between components. Limit coupling between parallel wires as much as possible. A good design requires a small number of layers of wiring, and also requires a wide wire and a small pad size corresponding to the required packaging density. Sharp and sharp corners in the wire should be avoided because rounded corners and smooth inner corners may avoid some electrical and mechanical problems that may arise.

PCB width and thickness:
Ampacity of etched copper conductors on rigid printed circuit boards. For 1 oz and 2 oz wire, allow for a 10% reduction of nominal value (based on load current), taking into account normal variations in etching method and copper foil thickness and temperature differences; for protected printed circuit board assembly Components (substrate thickness less than 0.032 inches, copper foil thickness more than 3 ounces) are reduced by 15%; for dip soldered printed circuit boards, a 30% reduction is allowed.

PCB wire spacing: The spacing of the wires must be determined to eliminate voltage breakdown or arcing between adjacent wires. Spacing is variable and depends primarily on the following factors: 1) The peak voltage between adjacent conductors. 2) Atmospheric pressure (working height). 3) The coating used. 4) Capacitive coupling parameters. Critical impedance components or high frequency components are generally placed close together to reduce critical stage delays. Transformers and inductive components should be isolated to prevent coupling; inductive signal wires should be run orthogonally at right angles; components that generate any electrical noise due to magnetic field motion should be isolated or rigidly mounted to prevent excessive vibration.

PCB Wire Pattern Check:
1) Whether the wire is short and straight without sacrificing function;
2) Whether the limitation of wire width is complied with;
3) There is no space between wires, between wires and mounting holes, and between wires and pads that must be guaranteed;
4) Whether the parallel arrangement of all wires (including component leads) is relatively close is avoided;
5) Whether acute angles (90°C or less) are avoided in the wire pattern.

PCB Design Project Check Item List:
1) Check the rationality and correctness of the schematic diagram;
2) Check the correctness of the component packaging of the schematic diagram;
3) The distance of strong and weak electricity, the distance of isolation area;
4) Check the schematic diagram and PCB diagram correspondingly to prevent the loss of the network table;
5) Whether the package of the component is consistent with the actual product;
6) Whether the placement of components is appropriate:
7) Whether the components are easy to install and disassemble;
8) Check whether the temperature sensitive element is too close to the heating element;
9) Whether the distance and direction of the mutual inductance components are appropriate;
10) Whether the placement between the connectors is smooth;
11) Easy to plug and unplug;
12) Input and output;
13) Strong and weak electricity;
14) Whether digital simulation is interleaved;
15) Arrangement of windward and leeward elements;
16) Whether the directional element is wrongly flipped instead of rotated;
17) Whether the mounting holes of the component pins are suitable and whether it is easy to insert;
18) Check whether the empty foot of each component is normal and whether it is a leak;
19) Check whether there are vias in the upper and lower layers of the same network table, and the pads are connected through the holes to prevent disconnection and ensure the integrity of the line;
20) Check whether the upper and lower characters are placed correctly and reasonably, and do not put components to cover the characters, so as to facilitate the operation of welding or maintenance personnel;
21) The connection between the upper and lower layers of the very important line should not only be connected by the pads of the in-line components, but also by the vias;
22) The arrangement of the power supply and signal lines in the socket shall ensure the integrity and anti-interference of the signal;
23) Pay attention to the proper ratio of pads and solder holes;
24) Each plug should be placed on the edge of the PCB board as much as possible and easy to operate;
25) Check whether the component label is consistent with the component, and the components are placed in the same direction as possible and placed neatly;
26) The power and ground wires should be as thick as possible without violating the design rules;
27) Under normal circumstances, the upper layer is horizontal, the lower layer is vertical, and the chamfer is not less than 90 degrees;
28) Whether the size and distribution of the mounting holes on the PCB are appropriate, and minimize the bending stress of the PCB;
29) Pay attention to the height distribution of components on the PCB and the shape and size of the PCB board to ensure easy assembly.