How to route PCB design?PCB&PCBA board made by Tmpking technology Co.,LTD. Tel&Whatsapp&Wechat:008613418507839

The production process of a professional PCB board is quite complicated. Take a 4-layer PCB board as an example. The PCB of the motherboard is mostly 4-layer. When manufacturing, the first two layers are rolled, cut, etched, and oxidized and plated. These four layers are the component surface, the power layer, the ground layer, and the solder layer. These 4 layers are then rolled together to form a motherboard PCB. Then punch and make holes. After cleaning, the outer two layers of circuits are printed, copper-plated, etched, tested, solder mask, and screen printed. Finally, the entire PCB (including many motherboards) is stamped into PCBs of each motherboard, and then vacuum-packed after passing the test.

PCB&PCBA board made by Tmpking technology Co.,LTD. Tel&Whatsapp&Wechat:008613418507839

If the copper skin is not applied well during the PCB manufacturing process, there will be inadequate adhesion, which is easy to conceal short-circuit or capacitance effects (easy to cause interference). Vias on the PCB must also be noted. If the hole is not in the middle, but is biased to one side, uneven matching will occur, or it will easily contact the middle power layer or ground layer, which may cause potential short circuit or poor grounding.

Copper wiring process

The first step in production is to establish the online wiring between the parts. We use negative transfer to represent the working negatives on metal conductors. The trick is to coat the entire surface with a thin layer of copper foil and eliminate the excess. Additive transfer is another method that is rarely used. This is a method of applying copper wire only where it is needed, but we won't talk about it here. Positive photoresist is made of photosensitizer, which will dissolve under illumination. There are many ways to treat photoresist on copper surfaces, but the most common way is to heat it and roll on the surface containing the photoresist. It can also be sprayed on in liquid form, but the dry film type provides higher resolution and can also produce thinner wires. The hood is just a template for the PCB layer in manufacturing.

Before the photoresist on the PCB board is exposed to UV light, the hood covering it can prevent the photoresist in some areas from being exposed. These places covered by photoresist will become wiring. After the photoresist is developed, other bare copper portions are etched. The etching process can immerse the board in an etching solvent, or spray the solvent on the board. Generally used as an etching solvent is ferric chloride. After the etching is finished, the remaining photoresist is removed.

1. Wiring width and current

General width should not be less than 0.2mm (8mil)

On high-density and high-precision PCBs, the pitch and line width are generally 0.3mm (12mil).

When the thickness of the copper foil is about 50um, the wire width is 1 ～ 1.5mm (60mil) = 2A

The common ground is generally 80mil, so pay more attention to the application with microprocessor.

2.How high is the high-speed board?

When the signal's rising / falling edge time is <3 ~ 6 times the signal transmission time, it is considered a high-speed signal.

For digital circuits, the key is to look at the steepness of the edges of the signal, that is, the rise and fall time of the signal.

According to the theory of a very classic book, High Speed Digtal Design, the signal rise time from 10% to 90% is less than 6 times the wire delay, which is a high-speed signal! Signal, as long as the edge is sufficiently steep, it is the same as a high-speed signal.When wiring, you need to use transmission line theory.

3.PCB board stacking and layering

Four-layer boards have the following lamination sequences. The following describes the advantages and disadvantages of various stacks:

First case

GND

S1 + POWER

S2 + POWER

GND

Second case

SIG1

GND

POWER

SIG2

Third case

GND

S1

S2

POWER

Note: S1 signal wiring layer one, S2 signal wiring layer two; GND ground layer POWER power layer

The first case should be the best of the four-layer board. Because the outer layer is a ground layer, it has a shielding effect on EMI, and the power layer is also very close to the ground layer, which makes the internal resistance of the power supply smaller and achieves the best suburban results. But the first case cannot be used when the density of this board is relatively large. Because in this way, the integrity of the ground of the first layer cannot be guaranteed, and the signal of the second layer will become worse. In addition, this structure cannot be used in the case where the power consumption of the whole board is relatively large.

The second case is the one we use most often. From the structure of the board, it is not suitable for high-speed digital circuit design. Because in this structure, it is not easy to maintain a low power supply impedance. Take a board of 2 mm as an example: Z0 = 50ohm is required. Line width is 8mil. Copper foil thickness is 35цm. In this way, the middle of the signal layer and the ground layer is 0.14mm. The ground plane and power plane are 1.58mm. This greatly increases the internal resistance of the power supply. In this kind of structure, since the radiation is directed to space, a shield plate is needed to reduce EMI.

In the third case, the signal line quality on the S1 layer is the best. S2 is next. Shielded against EMI. However, the power supply impedance is large. This board can be used in cases where the power consumption of the whole board is large and the board is an interference source or close to the interference source.

4. Impedance matching

The amplitude of the reflected voltage signal is determined by the source reflection coefficient ρs and the load reflection coefficient ρL

ρL = (RL-Z0) / (RL + Z0) and ρS = (RS-Z0) / (RS + Z0)

In the above formula, if RL = Z0 then the load reflection coefficient ρL = 0. If RS = Z0 source end reflection coefficient ρS = 0.

Because the ordinary transmission line impedance Z0 should generally meet the requirement of 50Ω, and the load impedance is usually in the range of several thousand ohms to several tens of ohms. Therefore, it is difficult to achieve impedance matching at the load end. However, because the impedance of the signal source (output) is usually small, it is about ten ohms.

So it is much easier to achieve impedance matching at the source. If the resistor is connected in parallel at the load end, the resistor will absorb some signals which is not good for transmission (my understanding). When the TTL / CMOS standard 24mA drive current is selected, its output impedance is approximately 13Ω. If the transmission line impedance Z0 = 50Ω, then a 33Ω source matching resistor should be added. 13Ω + 33Ω = 46Ω (approximately 50Ω, weak underdamping helps signal setup time)

When choosing other transmission standards and drive currents, the matching impedance will vary. When designing high-speed logic and circuits, for some key signals, such as clocks and control signals, we recommend that you add source-side matching resistors.

In this way, the signal will be reflected back from the load, because the source impedance is matched, the reflected signal will not be reflected back.

5. Precautions for power and ground wiring layout

Keep the power cord as short as possible, and keep it straight.

Ground loop problem: For digital circuits, the ground loop current caused by the ground loop is tens of millivolts, and the anti-interference threshold of TTL is 1.2V, and the CMOS circuit can reach 1/2 the power supply voltage. In other words, the ground loop current will not cause any adverse effect on the operation of the circuit. Conversely, if the ground wire is not closed, the problem will be even greater, because the pulsed power supply current generated by the digital circuit during operation will cause the ground potential imbalance at various points. For example, I measured the 74LS161 ground current 1.2A (using 2Gsps oscilloscope measured, the ground current pulse width is 7ns).

Under the impact of a large pulse current, if a branched ground wire (line width 25 mil) is used for distribution, the potential difference between each point of the ground wire will reach the level of hundreds of millivolts. After the ground wire loop is used, the pulse current will be distributed to various points of the ground wire, which greatly reduces the possibility of interference circuits. With the closed ground wire, the maximum instantaneous potential difference of the ground wire of each device is measured to be one-half to one-fifth of the closed ground wire. Of course, the measured data of circuit boards with different densities and speeds are very different. What I said above refers to the level equivalent to the Z80 Demo board that comes with Protel 99SE; for low-frequency analog circuits, I think the power frequency after the ground wire is closed Interference is induced from space, which cannot be simulated or calculated anyway.

If the ground wire is not closed, no ground eddy current will be generated. What is the theoretical basis for what Beckhamtao calls "but the ground-frequency open-loop voltage will be greater." To give two examples, I took over a project of someone else 7 years ago, a precision pressure gauge using a 14-bit A / D converter, but the actual measurement has only 11-bit effective accuracy. After investigation, the ground line has 15mVp-p Frequency interference, the solution is to cut off the analog ground loop of the PCB. The ground wire from the front-end sensor to the A / D is branched with flying leads. Later, the mass-produced model PCB was re-produced according to the flying lead routing. problem appear.

In the second example, a friend loves fever and DIY a power amplifier, but the output always has an AC sound. I suggest that it cut the ground loop and solve the problem. Afterwards, the man checked the PCB drawings of dozens of "Hi-Fi famous machines" and confirmed that none of them used a ground loop in the analog part.

6.Printed circuit board design principles and anti-interference measures

A printed circuit board (PCB) is a support for circuit elements and devices in electronic products. It provides electrical connections between circuit elements and devices. With the rapid development of electrical technology, the density of PGB is getting higher and higher. The quality of the PCB design has a great impact on the ability to resist interference. Therefore, when designing a PCB, you must follow the general principles of PCB design and meet the requirements of anti-interference design.