PCB Tech - Circuit board size planning and control

Circuit board size planning and control

Multilayer circuit boards are structural elements composed of dielectric layers and circuits, and the circuits are arranged on the surface and inside of the dielectric material. When engaging in design work, there must be common size planning guidelines, otherwise it will not be possible to achieve commonality among most electronic components on the market. Such wiring design rules are the PCBA board design rules (Design Rule).

1 grid point (Grid)

Because all the component positions on the circuit board are in relative coordinates, the original circuit board circuit layout idea is to allocate blocks on the circuit board plane with the envisaged grid lines. Since the circuit board was first dominated by European and American countries, the early specifications were based on 1/10 inch as a grid size, while the metric unit was 2.5mm as a grid, and the British system was equivalent to 100 mil. Based on this, the different pitches are further divided, and the positions of the holes and the copper pads are configured. This is the traditional design principle of through-hole components. However, after the popularity of SMT-Surface Mount Technology, it is impractical to arrange holes on grid points. Although there are grid points in the design, the actual design is almost no longer restricted by the grid points. The holes are more and more conductive. As for the blind and buried holes, they have nothing to do with the grid points.

The most affected part of this change is electrical testing. Since traditional electronic component contacts are based on grid points, the design of holes or solder joints is based on grid points. Therefore, circuit boards that follow the grid design can use the so-called universal tool (Universal Tool) for electrical testing, but after the grid point principle is destroyed, the test must move to a denser contact form, so a small number of products begin to use the so-called flying Needle equipment (Flying Probe) testing, while mass production uses a dedicated tool (Dedicate Tool) testing.

2 Line width spacing

Thin line design has become an inevitable trend in the development of high-density circuit boards, but the design of thin lines must consider the resistance changes of the thin lines, changes in characteristic impedance and other influencing factors. The size of the line spacing is restricted by the insulation of the dielectric material. For organic materials, about 4 mil can be selected as the target value. Due to product demand and the progress of process technology, products with a pitch of about 2 mils or even smaller have also entered practical applications. In the face of further compression of the line spacing of semiconductor packaging boards, how to maintain due insulation has become an issue that must be worked hard. Fortunately, the operating voltage of most high-density packaging boards is relatively low, which is fortunate.

3Diameter of micro-hole and copper pad diameter

Table 1 shows the current circuit board specification level. The diameter of the copper pad is generally designed to be between 2.5 and 3 times the aperture. When the circuit board is designed with surface adhesion as the main design, the plated holes are still used for plug-in functions in addition to interlayer connections.

The structure in the table has through holes and blind buried holes. The holes buried in the board are called Interstitial Via Hole (IVH) by some people. It is a circuit board that connects the inner layers with plated through-holes to form a circuit board that connects the micro-hole layers. The small-diameter design of these micro-holes can play a space-saving function. Generally, mechanical drilling is more economical to produce apertures larger than 8 mils. Although there are products that claim to be able to produce <4 mils, the high cost is not practical.

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

The design of the number of layers of a multilayer circuit board is mainly determined by the allowable wiring density. In the past, circuit boards were mostly 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 demand, so the level is gradually improved. However, since increasing the number of layers will increase the production cost, we want to try our best to reduce the number of layers in the initial design. Therefore, using more micropores and fine lines, the element can still be achieved in a limited number of layers. Piece link. Even so, with the progress of semiconductor components, the overall number of layers of circuit boards is still gradually rising.

In terms of circuit structure, due to the continuous improvement of the overall power and transmission rate of electronic products, under the condition that space is limited and the conductor cross-sectional area must be maintained, many designs will require higher circuit thickness but have to Thin lines. There are also strict restrictions on the thickness control of the interlayer dielectric layer and its allowable error, so the configuration of the inner layer substrate and the film will become very important. Generally, the press-fit structure of the circuit board will adopt a symmetrical design, which is a consideration for reducing the uneven stress.

For products with strict electrical characteristics, the correct integration of characteristic impedance and the tolerances of the thickness between the power and ground layers will become more stringent. Therefore, many production procedures are to make the key thickness levels with the base material first, and the less important levels are handed over to the film to complete, because the base material has been hardened in advance, and the base material that meets the specifications can be selected by screening. Production, so it can improve the yield and electrical performance.

When designing a high-density build-up 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, try to use a symmetrical pressing design. 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 with build-up circuits to integrate impedance characteristics, but this rule may not be followed for higher-level products.