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PCB Tech

PCB Tech

PCB Tech

PCB Tech

Detailed explanation of three kinds of PCB special wiring sharing and inspection methods
2021-08-19
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Author:IPCB

Before explaining the inspection work after PCB wiring is completed, I will introduce three special wiring techniques for PCB. The PCB LAYOUT routing will be explained from three aspects: right-angle routing, differential routing, and serpentine routing:


One. Right-angle routing (three aspects)

 

The influence of right-angle wiring on the signal is mainly reflected in three aspects: one is that the corner can be equivalent to a capacitive load on the transmission line, which slows down the rise time; the other is that impedance discontinuity will cause signal reflection; the third is that the right-angle tip is generated In the field of RF design above 10GHz, these small right angles may become the focus of high-speed problems.


Two. Differential wiring ("equal length, equidistant, reference plane")


What is a differential signal? In layman's terms, the driving end sends two equal and inverted signals, and the receiving end judges the logic state "0" or "1" by comparing the difference between the two voltages. The pair of traces carrying differential signals is called differential traces. Compared with ordinary single-ended signal traces, differential signals have the most obvious advantages in the following three aspects:


1) The anti-interference ability is strong, because the coupling between the two differential traces is very good. When there is noise interference from the outside, 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. For the same reason, because the two signals have opposite polarities, the electromagnetic fields radiated by them can cancel each other. The tighter the coupling, the less the electromagnetic energy leaked to the outside world.


3) The timing positioning is accurate. Because the switch change of the differential signal is located at the intersection of the two signals, unlike ordinary single-ended signals, which rely on the high and low threshold voltages to determine, it is less affected by the process and temperature, and can reduce the error in the timing. , But also more suitable for low-amplitude signal circuits. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signaling technology.


Three, serpentine line (adjusting delay)


Snake 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 most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when the signal is transmitted on the serpentine trace, the parallel line segments will be coupled in a differential mode, S The smaller, the greater the Lp, the greater the degree of coupling. It may cause the transmission delay to be reduced, and the quality of the signal is greatly reduced due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk. The following are some suggestions for Layout engineers when dealing with serpentine lines:


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 go 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 is close to or exceeds the signal rise time, the crosstalk generated will reach saturation.


3) The signal transmission delay caused by the serpentine line of the Strip-Line or Embedded Micro-strip is less than that of the Micro-strip. In theory, the stripline will not affect the transmission rate due to differential mode crosstalk.


4) For signal lines with high speed and strict timing requirements, try not to take serpentine lines, especially not winding lines in a small area.


5) Serpentine traces at any angle can often be used, which can effectively reduce mutual coupling.


6) In high-speed PCB design, the serpentine line has no so-called filtering or anti-interference ability, and can only reduce the signal quality, so it is only used for timing matching and has no other purpose.


7) Sometimes the spiral routing can be considered for winding. The simulation shows that the effect is better than the normal serpentine routing.


After talking about the PCB wiring, will the wiring be finished? Obviously, no! Inspection work after PCB wiring is also necessary, so how to check the wiring in the PCB design to pave the way for the subsequent design? Please see below!


General PCB design drawing inspection items


1) Has the circuit been analyzed? Is the circuit divided into basic units to smooth the signal?

2) Does the circuit allow short or isolated key leads?

3) Where must be shielded, is it effectively shielded?

4) Have you made full use of the basic grid graphics?

5) Is the size of the printed circuit board the best size?

6) Do you use the selected wire width and spacing as much as possible?

7) Has the preferred pad size and hole size been used?

8) Are the photographic plates and sketches appropriate?

9) Is the use of jumper wires the least? Do jumper wires pass through components and accessories?

l0) Are the letters visible after assembly? Are their size and type correct?

11) In order to prevent blistering, is there a window on the large area of copper foil?

12) Are there tool positioning holes?


PCB electrical characteristics inspection items:


1) Have you analyzed the influence of wire resistance, inductance, and capacitance, especially the influence of the critical voltage drop on the ground?

2) Does the spacing and shape of the wire accessories meet the insulation requirements?

3) Has the insulation resistance value been controlled and specified in key areas?

4) Is the polarity fully recognized?

5) Is the influence of wire spacing on leakage resistance and voltage measured from a geometrical point of view?

6) Has the medium for changing the surface coating been identified?


PCB physical characteristics inspection items:


1) Are all pads and their positions suitable for final assembly?

2) Can the assembled printed circuit board meet the shock and vibration conditions?

3) What is the required spacing of standard components?

4) Are the components that are not firmly installed or the heavier parts fixed?




5) Is the heating element heat dissipation and cooling correct? Or is it isolated from the printed circuit board and other heat-sensitive elements?

6) Are the voltage divider and other multi-lead components positioned correctly?

7) Is the arrangement and orientation of the components easy to check?

8) Has it eliminated all possible interference on the printed circuit board and the entire printed circuit board assembly?

9) Is the size of the positioning hole correct?

10) Is the tolerance complete and reasonable?

11) Have you controlled and signed the physical properties of all coatings?

12) Is the diameter ratio of the hole and the lead wire within the acceptable range?


PCB mechanical design factors:


Although the printed circuit board adopts mechanical methods to support the 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 when selecting and designing printed circuit boards are as follows;


1) The structure of the printed circuit board-size and shape.

2) Types of mechanical accessories and plugs (seats) required.

3) The adaptability of the circuit to other circuits and environmental conditions.

4) According to some factors, such as heat and dust, consider installing the printed circuit board vertically or horizontally.

5) Some environmental factors that require special attention, such as heat dissipation, ventilation, shock, vibration, and humidity. Dust, salt spray and radiation.

6) The degree of support.

7) Keep and fix.

8) Easy to take off.

ATL

PCB printed circuit board installation requirements:


It should be supported at least within 1 inch of the three edges of the printed circuit board. According to practical experience, the distance between the supporting points of a printed circuit board with a thickness of 0.031-0.062 inches should be at least 4 inches; for a printed circuit board with a thickness greater than 0.093 inches, the distance between the supporting points should be at least 5 inches. Taking this measure can improve the rigidity of the printed circuit board and destroy the possible resonance of the printed circuit board.


Certain printed circuit boards usually have to consider the following factors before deciding which mounting technology they use.


1) The size and shape of the printed circuit board.

2) Number of input and output terminals.

3) Available equipment space.

4) Desired ease of loading and unloading.

5) Types of attachments.

6) Required heat dissipation.

7) The required shieldability.

8) The type of circuit and its relationship with other circuits.


The dialing requirements of the printed circuit board:


1) There is no need to install the printed circuit board area of the component.

2) The influence of the plug-in tool on the installation distance between two printed circuit boards.

3) Specially prepare mounting holes and slots in the printed circuit board design.

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 permanently fixed to the printed circuit board assembly with rivets.

6) In the mounting frame of the printed circuit board, special design such as load bearing flange is required.

7) The adaptability of the plug-in tool used and the size, shape and thickness of the printed circuit board.

8) The cost involved in using plug-in tools includes both the price of the tool and the increased expenditure.

9) In order to fasten and use plug-in tools, it is required to have access to the inside of the equipment to a certain extent.


PCB mechanical considerations:


The properties of the substrate that have an important influence on the printed circuit assembly are: water absorption, thermal expansion coefficient, heat resistance, flexural strength, impact strength, tensile strength, shear strength and hardness.


All these characteristics affect not only the function of the printed circuit board structure, but also the productivity of the printed circuit board structure.


For most applications, the dielectric substrate of the printed circuit board is one of the following substrates:


1) Phenolic impregnated paper.

2) Acrylic-polyester impregnated randomly arranged glass mat.

3) Epoxy impregnated paper.

4) Epoxy impregnated glass cloth.


Each substrate can be flame-retardant or combustible. The above 1, 2, 3 can be punched. The most commonly used material for printed circuit boards with metallized holes is epoxy-glass cloth. Its dimensional stability is suitable for high-density circuits and can minimize the occurrence of cracks in the 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 copied and milled to form a print The shape of the circuit board.


PCB electrical considerations:


In DC or low-frequency AC applications, the most important electrical characteristics of insulating substrates are: insulation resistance, anti-isolation, printed wire resistance, and breakdown strength.


In high frequency and microwave applications, it is: dielectric constant, capacitance, and dissipation factors.


In all applications, the current carrying capacity of the printed wire is important.


Wire pattern:


PCB routing and positioning


Printed wires should take the shortest route between components under the constraints of the specified wiring rules. Limit the coupling between parallel wires as much as possible. Good design requires the minimum number of wiring layers, and also requires the widest wire and the largest pad size corresponding to the required packaging density. Because rounded corners and smooth inner corners may avoid some electrical and mechanical problems that may occur, sharp corners and sharp corners in the wire should be avoided.


PCB width and thickness:


The current-carrying capacity of etched copper wires on rigid printed circuit boards. For 1 ounce and 2 ounce wires, taking into account the etching method and the normal variation of copper foil thickness and temperature difference, it is allowed to reduce the nominal value by 10% (in terms of load current); for the printed circuit board assembly coated with a protective layer For parts (substrate thickness less than 0.032 inches, copper foil thickness more than 3 ounces), the components are reduced by 15%; for printed circuit boards that have been dipped solder, they are allowed to be reduced by 30%.


PCB wire spacing:


The minimum spacing of wires must be determined to eliminate voltage breakdown or arcing between adjacent wires. The spacing is variable, it mainly depends on the following factors:


1) Peak voltage between adjacent wires.

2) Atmospheric pressure (maximum working altitude).

3) The coating layer used.

4) Capacitive coupling parameters.


Critical impedance components or high-frequency components are generally placed very close to reduce the critical stage delay. Transformers and inductive components should be isolated to prevent coupling; inductive signal wires should be laid orthogonally at right angles; components that generate any electrical noise due to magnetic field movement should be isolated or rigidly installed to prevent excessive vibration.


PCB wire pattern inspection:


1) Is the wire short and straight without sacrificing functionality?

2) Did you comply with the restrictions on wire width?

3) Between the wires, between the wires and the mounting holes, between the wires and the pads...Is there any minimum wire spacing that must be guaranteed?

4) Have you avoided all the wires (including component leads) that are relatively close in parallel?

5) Are sharp angles (90°C or less than 90°C) avoided in the wire pattern?


List of PCB design project inspection items:


1) Check the rationality and correctness of the schematic diagram;

2) Check the correctness of the component packaging of the schematic;

3) The distance between strong and weak current, the distance between the 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 matches the physical object;

6) Whether the placement position of the components is appropriate:

7) Whether the components are easy to install and disassemble;

8) 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 current and weak current;

14) Whether digital and analog are interlaced;

15) Arrangement of elements on the upwind side and downwind side;

16) Whether the directional component has been 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 pin of each component is normal and whether it is a missing line;

19) Check whether there are vias in the upper and lower layers of the same net table, and the pads are connected through the holes to prevent disconnection and ensure the integrity of the circuit;

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 very important connection of the upper and lower layers should not only be connected with the pads of the in-line components, it is best to use vias;

22) The arrangement of power and signal lines in the socket should ensure signal integrity and anti-interference;

23) Pay attention to the proper ratio of pads and solder holes;

24) The plugs should be placed on the edge of the PCB board as much as possible and easy to operate;

25) Check whether the component label matches the component, and the components should be placed in the same direction as possible and placed neatly;

26) In the case of not violating the design rules, the power and ground wires should be as thick as possible;

27) Under normal circumstances, the horizontal line is used for the upper layer and the vertical line is used for the lower layer, and the chamfer is not less than 90 degrees;

28) Whether the size and distribution of the mounting holes on the PCB are appropriate to minimize the bending stress of the PCB;

29) Pay attention to the height distribution of the components on the PCB and the shape and size of the PCB to ensure easy assembly.