Via holes, blind holes, and buried holes of 4layer PCB
Via: Also called through hole, it is opened from the top layer to the bottom layer. In a four-layer PCB, the via hole penetrates the 1, 2, 3, and 4 layers, which will hinder the routing of irrelevant layers. There are two main types of vias:
1.PTH (Plating Through Hole), the hole wall has copper, usually via holes (VIA PAD) and component holes (DIP PAD)
2.Non Plating Through Hole NPTH, the hole wall has no copper, and is generally a positioning hole and a screw hole.
Blind Via: It is only visible on one of the top or bottom layers, and cannot be seen on the other layer. In other words, blind vias are drilled from the surface, but not through all layers. The blind holes may only go from 1 to 2, or from 4 to 3 (advantage: 1,2 conduction will not affect the 3,4 wiring); while the via hole goes through 1,2,3,4 layers, which is irrelevant The layer routing has an impact, but the cost of blind holes is higher and a laser drilling machine is required. Blind hole boards are used to connect the surface layer and one or more inner layers. One side of the hole is on one side of the board, and then leads to the inside of the board. To put it simply, only one side of the blind hole surface can be seen, and the other side It's on the board. Generally used in PCB boards with four or more layers.
Buried Via: Buried via refers to a via hole in the inner layer. After lamination, it cannot be seen, so it does not occupy the area of the outer layer. The upper and lower sides of the hole are in the inner layer of the board. In other words, it is buried inside the board. . To put it simply, it is sandwiched in the middle. These processes cannot be seen from the surface, neither the top layer nor the bottom layer. The advantage of making buried holes is that it can increase the wiring space. However, the process cost of making buried holes is very high and is not used in general electronic products. It is only used in particularly high-end products. Generally used in PCB boards with six layers or more.
Vias are used in almost all PCB boards and are the most basic and commonly used holes. Therefore, we will not explain them here. We will mainly talk about blind holes and buried holes. First, let’s start with traditional multilayer boards. The structure of a standard multi-layer circuit board contains inner and outer circuits, and then uses drilling and metallization processes in the holes to achieve the internal connection function of each layer of circuits. However, due to the increase in circuit density, the packaging methods of parts are constantly updated. In order to allow the limited circuit board area to place more higher-performance components, in addition to the thinner line width, the aperture has also been reduced from 1 mm for DIP jacks to 0.6 mm for SMD, and further reduced to 0.4 mm or less. However, the surface area will still be occupied, resulting in the production of blind holes and buried holes.
In a four-layer board, the two middle layers are mainly used to carry the power layer (VCC) and ground layer (GND). If the board components are dense and dense, then the inner layer can also carry some ordinary signal lines. The adjacent inner layer on the main component layer (the main component layer is usually on the top layer) is used as the ground layer, the entire layer is used for copper coating, and then the other inner layer is used as the main wiring layer, the secondary component layer (subcomponent layer) Usually on the bottom floor) also take some lines.
Positive film and negative film: Four-layer board, the first thing to understand is the positive film and negative film, which is the difference between layer and plane. The positive film is the wiring method usually used on the top and ground layers. The wiring areas are copper wires, and Polygon Pour is used to fill large blocks of copper. The negative film is just the opposite. It defaults to copper coating, and the routing area is the dividing line. That is, after generating a negative film, the entire layer has been covered with copper. What you need to do is to divide the copper coating, and then set the divided copper coating. network. In the previous version of PROTEL, Split was used to divide, but in the current version of Altium Designer, Line and the shortcut key PL are used to divide. The dividing line should not be too thin. I use 30mil (about 0.762mm) to divide the copper. At this time, just use LINE to draw a closed polygonal box, double-click the box to apply copper and set up the network. Both positive and negative films can be used for the inner electrical layer, and positive films can also be realized through wiring and copper coating. The advantage of the negative is that large blocks of copper are filled by default. There is no need to rebuild when adding vias, changing the size of the copper, etc., thus saving the time of recalculating the copper. When the middle layer is used for the power layer and ground layer, most of the layers are covered with large pieces of copper, so the advantage of using negative film is obvious.
Advantages of using blind vias and buried vias: In non-through hole technology, the application of blind vias and buried vias can greatly reduce the size and quality of PCB, reduce the number of layers, improve electromagnetic compatibility, increase the characteristics of electronic products, and reduce cost, and will also make the design work easier and faster. In traditional PCB design and processing, through holes will cause many problems. First of all, they occupy a large amount of effective space. Secondly, a large number of through-holes densely packed in one place also cause huge obstacles to the inner wiring of multi-layer PCBs. These through-holes take up the space required for wiring, and they densely pass through the power supply and ground. The surface of the line layer will also destroy the impedance characteristics of the power supply ground layer, causing the power supply ground layer to fail. And conventional mechanical drilling will be 20 times more work than using non-through hole technology. In PCB design, although the size of pads and via holes has gradually decreased, if the thickness of the board layer does not decrease proportionally, it will cause the aspect ratio of the through hole to increase, and the increase in the aspect ratio of the through hole will reduce reliability. With the maturity of advanced laser drilling technology and plasma dry etching technology, it has become possible to apply non-through small blind holes and small buried holes. If the diameter of these non-through holes is 0.3mm, the parasitic parameters caused are It is about 1/10 of the original conventional holes, which improves the reliability of PCB. Due to the use of non-through hole technology, there will be fewer large vias on the PCB, thus providing more space for wiring. The remaining space can be used for large-area shielding to improve EMI/RFI performance. At the same time, more remaining space can also be used for partial shielding of devices and key network cables on the inner layer to achieve optimal electrical performance. The use of non-through vias makes it easier to fan out device pins, making it easy to route high-density pin devices (such as BGA package devices), shortening the wiring length, and meeting high-speed circuit timing requirements.
Disadvantages of using blind holes and buried holes: The main disadvantages are the high cost of the board and the complexity of processing. It not only increases the cost but also increases the processing risk, and makes it more difficult to test and measure during debugging. Therefore, it is recommended to avoid blind holes and buried holes as much as possible, unless the board size is limited and it is necessary to use them.
