Design performance requirements for PCB multi-layer

writer: G February 19, 2019

1. Mechanical design factor

Mechanical design includes selecting the appropriate board size, board thickness, board stacking, inner copper tube, aspect ratio, and the like.

1.1. Board size

Board size should be optimized based on application requirements, system box size, board manufacturer limitations, and manufacturing capabilities. Large boards have many advantages, such as fewer substrates, shorter circuit paths between many components, so that you can have higher operating speeds, and each PCB board can have more input and output connections, so Large boards should be preferred in many applications, such as in a personal computer, where a larger motherboard is seen. However, designing the signal line layout on a large PCB board is difficult, requiring more signal layers or internal wiring or space, and the heat treatment is more difficult. Therefore, designers must consider various factors, such as the size of the standard board, the size of the production equipment, and the limitations of the manufacturing process. Some guidelines for selecting printed circuit/board sizes for the standard are given in 1PC-D-322.

1.2. Plate thickness

The thickness of the multi-substrate is determined by various factors such as the number of signal layers, the number and thickness of the power board, the aspect ratio of the aperture and thickness required for high quality drilling and plating, the component lead length required for automatic insertion, and The type of connection used. The thickness of the entire board consists of the conductive layer on both sides of the PCB, the copper layer, the thickness of the substrate, and the thickness of the prepreg. Obtaining tight tolerances on a synthetic multi-substrate is difficult, and a tolerance of about 10% is considered reasonable.

1.3. Stacking of boards

In order to minimize the chance of PCB board distortion, a flat finished board is obtained, and the delamination of the multi-substrate should remain symmetrical. That is, it has an even number of copper layers and ensures that the thickness of the copper and the copper foil pattern density of the plies are symmetrical. The radial (e.g., fiberglass cloth) of the construction material typically used for lamination should be parallel to the sides of the laminate. Because the laminate shrinks in the radial direction after bonding, this can distort the layout of the board, exhibiting variable and low dimensional stability.

However, the warpage and distortion of the multi-substrate can be minimized by improving the design. The purpose of reducing warpage and distortion can be achieved by averaging the average distribution of the copper foil over the entire layer and ensuring the structural symmetry of the multi-substrate, that is, ensuring the same distribution and thickness of the prepreg material. The copper and laminate should be fabricated from the center layer of the multi-substrate to the outermost two layers. The minimum distance (dielectric thickness) specified between the two copper layers is 0.080 mm.

It is known from experience that the minimum distance between the two copper layers, that is, the minimum thickness of the prepreg after bonding must be at least twice the thickness of the embedded copper layer. In other words, two adjacent copper layers, if each layer is 30 μm thick, the thickness of the prepreg is at least 2 (2 x 30 μm) = 120 μm, which can be achieved by using two layers of prepreg (glass fiber weave). The typical value of cloth is 1080).

1.4. Inner layer copper foil

The most commonly used copper foil is 1 oz (1 oz of copper foil per square foot of surface area). However, for dense PCB boards, the thickness is extremely important and requires strict impedance control. This PCB board needs to be used.

0.50z copper foil. For the power and ground planes, it is best to use a copper foil of 2oz or more. However, etching a heavier copper foil results in a decrease in controllability, and it is not easy to achieve a desired pattern of line width and pitch tolerance. Therefore, special processing techniques are required.

1.5. Hole

Depending on the component's lead diameter or diagonal size, the diameter of the plated through hole is typically maintained between 0.028 and 0.010 in. This ensures a sufficient volume for better soldering.

1.6. Aspect ratio

The "aspect ratio" is the ratio of the thickness of the plate to the diameter of the borehole. It is generally considered that 3:1 is a standard aspect ratio, although a high aspect ratio like 5:1 is also commonly used. The aspect ratio can be determined by factors such as drilling, desmear or etch back and plating. When the aspect ratio is maintained within the range that can be produced, the vias should be as small as possible.

2. Electrical design factors

Multi-substrate is a high performance, high speed system. For higher frequencies, the rise time of the signal is reduced, so control of signal reflection and line length becomes critical. In the multi-substrate system, the requirements for the controllable impedance performance of electronic components are very strict, and the design must meet the above requirements. The factors determining the impedance are the dielectric constant of the substrate and the prepreg, the spacing of the wires on the same level, the thickness of the interlayer dielectric, and the thickness of the copper conductor. In high speed applications, the lamination sequence of the conductors in the multi-substrate and the order in which the signal nets are connected are also critical. Dielectric constant: The dielectric constant of the substrate material is an important factor in determining impedance, propagation delay, and capacitance. The dielectric constant of the substrate using the epoxy glass and the prepreg can be controlled by changing the percentage of the resin content.

The dielectric constant of epoxy resin is 3.45, and the dielectric constant of glass is 6.2. By controlling the percentage of these materials, the dielectric constant of epoxy glass may reach 4.2 - 5.3. The thickness of the substrate is a measure for determining and controlling the dielectric constant. Very good description.

Prepregs with relatively low dielectric constants are suitable for use in RF and microwave circuits. In RF and microwave frequencies, the lower dielectric constant results in lower signal delay. In the substrate, low loss factors minimize electrical losses.

The prepreg ROR 4403 is a new material produced by ROGERS. This material is compatible with other substrates used in standard multi-substrate (FR-4 material) constructions (eg, RO 4003 or RO 4350 for microwave boards).