PCB Tech - Back drilling technology in PCB production

1, Via design in high speed PCB

In high-speed PCB design, multilayer PCB is often needed, and via is an important factor in multilayer PCB design.

Through hole in PCB is mainly composed of hole, pad area around the hole and power layer isolation area.

1. The influence of vias in high speed PCB

In the high-speed PCB multilayer board, the signal transmission from one layer of interconnect to another needs to be connected through vias. When the frequency is lower than 1GHz, via can play a good role in connection, and its parasitic capacitance and inductance can be ignored.

When the frequency is higher than 1 GHz, the parasitic effect of vias on signal integrity can not be ignored. At this time, vias show discontinuity in the transmission path, which will cause signal integrity problems such as signal reflection, delay and attenuation.

When the signal is transmitted to another layer through the via, the reference layer of the signal line also serves as the return path of the through-hole signal, and the return current will flow between the reference layers through the capacitance coupling, causing problems such as earth elasticity.

2. Types of vias

Through holes are generally divided into three types: through holes, blind holes and buried holes.

Blind hole: refers to the top and bottom surface of the printed circuit board, with a certain depth, used for the connection of the surface circuit and the inner circuit below. The depth and diameter of the hole usually do not exceed a certain ratio.

Buried hole: refers to the connecting hole in the inner layer of printed circuit board, which does not extend to the surface of the circuit board.

Through hole: this kind of hole passes through the whole circuit board and can be used for internal interconnection or as the installation positioning hole of components. Because the through-hole is easier to realize in technology and lower in cost, it is generally used in PCB.

3. Via design in high speed PCB

In high-speed PCB design, the seemingly simple via often brings great negative effects to the circuit design. In order to reduce the adverse effects caused by the parasitic effect of vias, we can try our best to do the following in the design:

(1) Select reasonable via size. For multi-layer PCB design with general density, it is better to select 0.25mm/0.51mm/0.91mm (drilling / pad / power isolation area); for some high-density PCB, it can also use 0.20mm/0.46mm/0.86mm For power or ground wire vias, larger sizes can be used to reduce the impedance; (2) the larger the power isolation area, the better. Considering the via density on PCB, generally D1 = D2 + 0.41; (3) PCB In other words, the vias should be reduced as much as possible; (4) the use of thinner PCB is conducive to reducing the two parasitic parameters of via; (5) the pins of power supply and ground should be close to the via, and the shorter the lead between the via and the pin, the better, because they will lead to the increase of electrical inductance. At the same time, the lead wire of power supply and ground should be as thick as possible to reduce the impedance; (6) some grounding vias should be placed near the vias of signal layer change, so as to provide short-distance circuit for signals.

In addition, the via length is also one of the main factors that affect the via inductance. For vias used for top and bottom conduction, the length of vias is equal to the thickness of PCB. As the number of PCB layers increases, the thickness of PCB often reaches more than 5 mm. However, in order to reduce the problems caused by vias in high-speed PCB design, the via length is generally controlled within 2.0 mm. For vias with a length greater than 2.0 mm, the impedance continuity can be improved to a certain extent by increasing the via diameter. When the through hole length is 1.0 mm or less, the through hole diameter is 0.20 mm - 0.30 mm.

2, Back drilling technology in PCB production

1. What PCB back drill?

Back drilling is actually a special kind of hole depth drilling. In the production of multi-layer boards, such as the production of 12-layer boards, we need to connect the first layer to the ninth layer. Usually, we drill through holes (drills) and then sink copper. In this way, the first floor is directly connected to the 12th floor. In fact, we only need to connect the first floor to the ninth floor. Because there is no line connected from the 10th to the 12th floor, it is like a column.

This pillar affects the path of the signal, causing signal integrity problems in communication signals. So drill this extra column (called stub in the industry) from the opposite side (secondary drilling). Therefore, it is called back drilling, but it is generally not as clean as drilling, because a little copper will be electrolyzed in the subsequent process, and the drill point itself is also sharp. Therefore, the PCB manufacturer will leave a small point. The length of the stub left is called b value, which is generally good in the range of 50-150um.

2. What are the advantages of back drilling?

1) Reduce noise interference;

2) Improve signal integrity;

3) The thickness of local plate becomes smaller;

4) Reduce the use of blind holes, reduce the difficulty of PCB production.

3. What is the function of back drilling?

The function of back drilling is to drill out the through-hole section which has no connection or transmission function, so as to avoid the reflection, scattering and delay of high-speed signal transmission, and bring "distortion" to the signal. Research shows that the main factors affecting the signal integrity of signal system are design, board material, transmission line, connector, chip package and other factors, the through-hole has a greater impact on the signal integrity Ring.

4. Working principle of back hole production

The micro current generated when the drill needle contacts the copper foil on the substrate surface to induce the height position of the board surface when the drill needle is running in the hole, and then the drill in is carried out according to the set drilling depth, and the drilling is stopped when the drilling depth is reached. As shown in Figure 2

5. Production process of back drill?

The PCB is provided with a positioning hole, and the positioning hole is used to locate the PCB and drill the hole; the PCB after drilling is electroplated, and the positioning hole is sealed with dry film before electroplating; the outer layer figure is made on the electroplated PCB;

After forming the outer layer pattern, the PCB is electroplated, and the positioning hole is sealed with dry film before the pattern electroplating; the positioning hole used by one drill is used for back drilling, and the electroplating hole requiring back drilling is back drilled with a drill knife; after back drilling, the back hole is washed to remove the residual drilling cuttings in the back hole.

6. What are the technical features of back drilling plate?

1) Most backboards are hard boards

2) The number of floors is generally 8 to 50

3) Plate thickness: more than 2.5mm

4) Large thickness diameter ratio

5) The plate size is larger

6) General first drilling hole diameter > = 0.3mm

7) The outer circuit is less, and most of them are crimp hole array design

8) The back hole is usually 0.2mm larger than the hole to be drilled out

9) Back drilling depth tolerance: + / - 0.05mm

10) If the back drilling requires drilling to layer m, then the medium thickness from layer m to layer M-1 (the next layer of layer m) is 0.17mm7?

PCB backplane is mainly used in communication equipment, large server, medical electronics, military, aerospace and other fields.