Multilayer circuit board is a structural element composed of dielectric layer and wires, and the circuit is arranged on the surface and inside of the dielectric material. When engaging in design work, there must be common dimensional planning guidelines, otherwise most electronic components on the market cannot reach commonality. Such wiring design rules are PCBA board design rules.

1. Grid

The position of all components on the circuit board is a relative coordinate relationship. The layout of the circuit on the original circuit board is to allocate blocks on the plane of the circuit board based on the assumed grid lines. The early specifications were 1/10 inch as a grid, while the metric unit was 2.5mm as a grid, and the British equivalent was 100mil. Based on this, subdividing different pitches and making the positional arrangement of holes and copper pads is a traditional design principle of through-hole components. However, after SMT-Surface Mount Technology became popular, it was impractical to arrange holes on the grid. Although there are grid points in the design, the actual design is no longer limited by grid points. The holes are more and more used for conduction. As for blind and buried holes, they are more independent of grid points.

The most influential part of this change is electrical testing. Since the traditional electronic component contacts are based on grid points, the design of drilling or soldering points is based on grid points. Therefore, the circuit boards that follow the grid design can use the so-called Universal Tool for electrical testing. However, after the grid principle is broken, the test must move to a denser contact form. Therefore, a small number of products began to use the so-called Flying Probe test, and a large number of products Production is tested using the Dedicate Tool.

2. Line width spacing

Thin line design has become an inevitable trend for the development of high-density circuit boards, but the design of thin lines must take into account factors such as resistance changes and characteristic impedance changes of thin lines. The size of the line spacing is limited by the dielectric properties of the dielectric material. For organic materials, 4 mil can be selected as the target value. Due to the product requirements and the progress of process technology, products with a pitch of about 2mil or even smaller have also entered practical applications. Faced with the further reduction of the line spacing of semiconductor packaging boards, how to maintain the proper insulation, fortunately, the operating voltage of most high-density packaging boards has also been relatively reduced.

3. Micropore diameter and copper pad diameter

The diameter of the copper pad is generally designed to be between 2.5 and 3 times the aperture. When the circuit board is mainly designed for surface adhesion, the plated hole is still used for plug-in functions in addition to the interlayer connection.

The structure in the table includes through holes and blind buried holes. The holes buried in the board are called IVH (Interstitial Via Hole). It is a circuit board that is formed by continuously laminating the inner layer plate with plated through holes to form a micro-hole interlayer connection. These micro-holes are designed with a small diameter to play a space-saving function. Generally, it is more economical to drill holes with a hole diameter larger than 8mil. Although it is now known to be able to make products <4 mil, the cost is too high and it is not practical.

Limited by mechanical aperture and production rate, circuit boards using the build-up method will not only use micro-hole technology for surface holes, but also design smaller through-holes to increase density. The reduction of the aperture greatly increases the freedom of circuit configuration, and thus the high-density build-up circuit board is popularized.

4. Interlayer structure

The design of the number of layers of a multilayer circuit board is mainly determined by the allowable wiring density. In the past, most of the circuit boards were four-layer boards, mainly due to the need for electromagnetic shielding of signal lines, not from the need for winding density. Due to the increase in the complexity of electronic components, the original winding density and hierarchical design can no longer meet the needs, so the level has gradually increased. However, because increasing the number of layers will increase the production cost, in the initial design, you want to try to reduce the number of layers. Therefore, using more micro-holes and fine wires, the component connection can still be achieved with a limited number of layers. Even so, with the advancement of semiconductor components, the overall number of layers of circuit boards is still gradually increasing.

In terms of circuit structure, as the overall power of electronic products continues to increase and the transmission rate continues to increase, under the condition of limited space and the need to maintain the cross-sectional area of the conductor, many designs will require higher line thicknesses but more thin lines. There are also severe restrictions on the thickness control of interlayer dielectric layers and their tolerances, so the configuration of the inner substrate and film will become very important. Generally, the pressing structure of the circuit board adopts a symmetrical design, which is a consideration for reducing uneven stress.

For products with strict electrical characteristics, the correct integration of the characteristic impedance, the allowable tolerance between the power supply and the ground layer thickness will become more stringent. Therefore, many production procedures are to make the key thickness layers with the substrate first, and the less important layers are given to the film to complete, because the substrate has been hardened in advance, and the substrate that meets the specifications can be selected by screening. Production, which can improve yield and electrical performance.

When designing a high-density layered circuit board, the number of circuit layers should be determined according to the winding density, and the wiring method, interlayer thickness, and circuit width thickness should be determined by electrical characteristics. In order to prevent the plate from bending and warping, a symmetrical pressing design is used as much as possible. Generally, the power and ground layers of high-density build-up boards are mostly set on the inner hard board, and the signal layer is made of build-up lines to integrate impedance characteristics, but this rule may not be followed for higher-level products.

5. How much distance must be left between the part and the board edge?

When designing a PCB, how much distance must be left between the part and the board edge so as not to affect the electrical properties? If the material is FR4, there are SMT parts on both sides, there is a connecting plate design, there are two types of routing and V-CUT processing between the plates, if the width of the annular ring is increased, can the tolerance of the finished aperture be more effectively controlled?

This problem has a lot to do with SMT processing equipment. Most of the equipment needs to keep the board edge at least 0.15 inch as the safety distance for the assembly and transportation of the splint. If using the Panel design, the break between the pieces can be designed with V-cut or stamp holes. It should be noted that the V-cut path has a width of 20-30 mils, and there is no conductor residue on it (calculated by the V-cut center line to both sides). It is ideal. Some manufacturers deliberately design a line on the V-cut path, which can be used as an indicator of whether there is a missing cut during the electrical test.

Increasing the width of the Annular ring can indeed effectively control the concept of the finished hole tolerance:
5.1. In the high-current area or independent holes of the PCBA board plating process, if the Pad can be increased, the average current density can be reduced, and the difference between the hole copper thickness in this area and other areas can naturally be reduced.

5.2. The larger Annular ring can reduce Solder residue in the hole during the tin spraying process. If other electroplated metal surface treatments are used, the current density can be reduced and the difference in pore diameter can be reduced as well as copper plating.