Common treatment methods for vias in PCB design

Common treatment methods for vias in PCB design

Via is one of the important components of a multi-layer PCB. The cost of drilling is usually 30% to 40% of the cost of PCB board. From a design point of view, a via is mainly composed of two parts, one is a drill hole in the middle, and the other is a pad area around the hole. The size of the two parts determines the size of the hole. . Obviously, in high-speed, high-density PCB design, the designer always hopes that the smaller the via, the better, so that more wiring space can be left on the board. In addition, the smaller the via, the parasitic capacitance of its own. The smaller, the more suitable for high speed circuits. However, the reduction in the size of the hole also brings about an increase in cost, and the size of the via hole cannot be reduced indefinitely. It is limited by the process techniques such as drilling and plating: the smaller the hole, the smaller the drill The longer the hole takes, the easier it is to deviate from the center position; and when the depth of the hole exceeds 6 times the diameter of the hole, there is no guarantee that the hole wall can be uniformly plated with copper.

Therefore, in terms of integrated design and production, we need to consider the following issues:

1. The inner diameter of the full-passing hole is required to be 0.2mm (8mil) or more in principle, and the outer diameter is 0.4mm (16mil) or more. In difficult places, it must be controlled to an outer diameter of 0.35mm (14mil);

According to the experience, the inner diameter and outer diameter of the commonly used via size of PCB generally follow X*2±2mil (X indicates the inner diameter). For example, a 8 mil inner diameter via can be designed as 8/14 mil, 8/16 mil or 8/18 mil; for example, a 12 mil via can be designed as 12/22 mil, 12/24 mil, 12/26 mil;

1, BGA in the 0.65mm and above design recommendations do not use buried blind holes, the cost will increase significantly. When using a blind hole, a first-order blind hole is generally used (TOP layer - L2 layer or BOTTOM - negative L2). The inner diameter of the via hole is generally 0.1 mm (4 mil) and the outer diameter is 0.25 mm (10 mil). 

1. The via hole cannot be placed on the pad of less than 0402 resistor capacity pad; theoretically, the lead inductance is small on the pad, but when it is produced, the solder paste is easy to enter the via hole, causing the solder paste to be uneven and causing the device to stand. The phenomenon of getting up ('the monument'). The recommended spacing is 4-8 mils

1. The spacing between the vias and the vias should not be too close, and the holes are likely to cause holes. Generally, the hole spacing is 0.5mm and above, 0.35mm-0.4mm is avoided, and 0.3mm and below are prohibited

5, as shown in Figure 4, in addition to the heat dissipation through the hole, ≤ 0.5mm of the through hole, need to plug the hole cover oil (the inner diameter is 0.4mm inside need to block the hole).

1) Especially for devices with a metal case, the principle is not to punch the hole under the body, and the hole with a certain hole should be covered to avoid short circuit between the case and the via hole.

2) According to the production feedback of the board factory, it is often mentioned that the via hole under the BGA is too close to the pad and needs to move the via hole. This situation is caused by the fact that the via holes are not equidistant from the BGA pads. Since the current BAG via holes and the position of the test holes are not equidistant from the BGA pads, PCB designers do not pay attention to this. Leading to engineering problems and a hidden danger to welding quality. Therefore, we directly recommend punching to the center of the two pads, especially in the BGA, because the pitch of the Pitch is small, after the hole is punched, it is necessary to cover the hole of the BGA under the hole, so as to avoid the short circuit of the BGA ball. .

6. The fixed pad of the earphone terminal, button, FPC, etc. is used to prevent the pad copper from falling. When the condition permits, the pad can be used for 1-2 vias (the via is evenly placed), which can effectively improve the "fixation".