說起 PCB板, many friends will think that it can be seen everywhere around us, from all household appliances, all kinds of accessories in the computer, to all kinds of digital products, as long as electronic products almost all use PCB board, so what is PCB board? A PCB is a PrintedCircuitBlock, which is a printed circuit board for electronic components to be inserted. A copperplated base plate is printed and etched out of the etching circuit.

PCB板可分為單層板、雙層板和多層板。 電子元件集成在 PCB 中。 在基本的單層 PCB 上，元件集中在一側，電線集中在另一側。 所以我們需要在板子上打孔，讓管腳可以穿過板子到另一邊，這樣零件的管腳就焊接到另一邊了。 Because of this, the positive and negative sides of such PCB are respectively called ComponentSide and SolderSide.

A double-layer board can be seen as two single-layer boards glued together, with electronic components and wiring on both sides of the board. 有時需要通過導孔（via）將單根導線從電路板的一側連接到另一側。 導孔是PCB上填充或塗有金屬的小孔，可以連接到兩側的電線。 現在很多電腦主板都使用4層甚至6層PCB板，而顯卡一般使用6層PCB板。 很多像nVIDIAGeForce4Ti系列這樣的高端顯卡使用8層PCB板，稱為多層PCB板。 The problem of connecting lines between layers is also encountered on multi-layer PCBS, which can also be achieved through guide holes.

由於是多層PCB，有時導孔不需要貫穿整個PCB。 這種導孔被稱為Buriedvias 和Blindvias，因為它們只穿透幾層。 盲孔將幾層內部 PCBS 連接到表面 PCBS，而無需穿透整個電路板。 埋孔僅與內部PCB相連，因此從表面看不到光線。 在多層PCB中，整層直接連接到地線和電源。 所以我們將這些層分類為信號層、電源層或接地層。 如果PCB上的零件需要不同的電源，它們通常有兩個以上的電源層和走線層。 The more layers you use, the higher the cost. Of course, the use of more layers of PCB board to provide signal stability is very helpful.

The process of making a professional PCB board is quite complicated. Take a 4-layer PCB board for example. 主板的PCB多為4層。 When manufacturing, the middle two layers are rolled, cut, etched, oxidized and electroplated respectively. The four layers are component surface, power layer, stratum and solder lamination respectively. The four layers are then pressed together to form a PCB for the main board. Then the holes were punched and made. After cleaning, the outer two layers of the line is printed, copper, etching, testing, welding resistance layer, screen printing. 最後將整塊PCB（包括多塊主板）沖壓成每塊主板的PCB，測試合格後進行真空包裝。 If the copper skin is not well coated in THE process of PCB production, there will be poor adhesion phenomenon, easy to imply short circuit or capacitance effect (easy to cause interference). The holes on PCB must also be taken care of. If the hole is punched not in the middle, but on one side, it will result in uneven matching or easy contact with the power supply layer or formation in the middle, resulting in potential short-circuiting or bad grounding factors.

Copper wiring process

The first step in fabrication is to establish an online wiring between parts. We use negative transfer to express the working negative on a metal conductor. The trick is to spread a thin layer of copper foil over the entire surface and remove any excess. Appending transfer is another less used method, which is to apply copper wire only where it is needed, but we won’t talk about it here.

Positive photoresists are made from photosensitizers that dissolve under illumination. There are many ways to treat photoresist on copper, but the most common way is to heat it and roll it over a surface containing photoresist. It can also be sprayed in liquid form, but the dry film provides higher resolution and allows for thinner wires. The hood is just a template for making PCB layers. 在 PCB 上覆蓋光刻膠的罩可防止光刻膠的某些區域暴露，直到光刻膠暴露在紫外線下。 These areas, which are covered with photoresist, will become wiring. Other bare copper parts to be etched after photoresist development. The etching process may involve dipping the board into the etching solvent or spraying the solvent onto the board. Generally used as etching solvent using ferric chloride etc. After etching, remove the remaining photoresist.

1. Wiring width and current

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

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

銅箔厚度約50um時，線寬1～1.5mm（60mil）=2A

The common ground is generally 80mil, especially for applications with microprocessors.

2、高速板的頻率有多高？

When the rise/fall of the signal time “3~6 times the signal transmission time, it is considered as high speed signal.

對於數字電路，關鍵是看信號的邊沿陡度，上升和下降所需的時間，

According to a very classic book “High Speed Digtal Design” theory, the signal from 10% to 90% of the time is less than 6 times the wire delay, is high-speed signal! ————————————即！ Even 8KHz square wave signals, as long as the edges are steep enough, are still high-speed signals, and transmission line theory needs to be used in wiring

3.PCB堆疊和分層

The four – layer plate has the following stacking sequence. 不同層壓的優缺點解釋如下：

第一種情況應該是四層中最好的。 Because the outer layer is the stratum, it has a shielding effect on EMI. Meanwhile, the power supply layer is reliable and close to the stratum, which makes the internal resistance of the power supply smaller and achieves the best suburbs. 但是，當板密度較高時，不能使用第一種情況。 Because then, the integrity of the first layer is not guaranteed, and the second layer signal is worse. In addition, this structure can not be used in the case of large power consumption of the whole board.

The second case is the one we usually use the most. 從闆卡的結構來看，不適合高速數字電路設計。 在這種結構中很難保持低功率阻抗。 Take a plate 2 mm as an example: Z0=50ohm. To line width of 8mil. Copper foil thickness is 35цm. So the signal layer and the middle of the formation is 0.14mm. The formation and power layer are 1.58mm. This greatly increases the internal resistance of the power supply. In this kind of structure, because the radiation is to the space, shielding plate is needed to reduce EMI.

In the third case, the signal line on layer S1 has the best quality. S2。 EMI 屏蔽。 But the power supply impedance is large. This board can be used when the power consumption of the whole board is high and the board is an interference source or adjacent to the interference source.

4.阻抗匹配

反射電壓信號的幅值由源反射係數 ρ S 和負載反射係數 ρL 決定

ρL = (RL-z0)/(RL + Z0) and ρS = (rS-z0)/(RS + Z0)

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

Because the ordinary transmission line impedance Z0 should usually meet the requirements of 50 ω 50 ω, and the load impedance is usually in thousands of ohms to tens of thousands of ohms. 因此，很難在負載側實現阻抗匹配。 However, because the signal source (output) impedance is usually relatively small, roughly in the tens of ohms. 因此，在源頭實現阻抗匹配要容易得多。 If a resistor is connected at the load end, the resistor will absorb part of the signal to the detriment of transmission (my understanding). When the TTL/CMOS standard 24mA drive current is selected, its output impedance is approximately 13 ω. If the transmission line impedance Z0=50 ω, then a 33 ω source-end matching resistor should be added. 13 ω +33 ω =46 ω (approximately 50 ω, weak underdamping helps signal setup time)

When other transmission standards and drive currents are selected, the matching impedance can be different. In high-speed logic and circuit design, for some key signals, such as clock, control signals, we recommend that the source matching resistor must be added.

In this way, the connected signal will be reflected back from the load side, because the source impedance matches, the reflected signal will not be reflected back.