PCB Tech - 3 kinds of PCB wiring sharing and detection methods

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:

1. 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.

2. 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 value, the larger the Lp, and 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.

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 model 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.

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.

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 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%.