The design performance of multiple substrates is mostly similar to that of single or double substrates, that is, care is taken not to pack too many circuits into too little space, resulting in unrealistic tolerances, high inner capacity, and possibly compromising the safety of product quality. Therefore, the performance specification should consider a complete assessment of inner line thermal shock, insulation resistance, welding resistance, etc. The following describes the important factors that should be considered in the design of multi-substrate.
Mechanical design includes the selection of suitable plate size, plate thickness, plate lamination, inner layer copper cylinder, aspect ratio, etc.
Board sizes should be optimized based on application requirements, system box sizes, board manufacturer limitations and manufacturing capabilities. Big circuit board has many advantages, such as less substrate, many components between short circuit path, so you can have a higher operating speed, Wells and the input and output of each piece of PCB board can have more connections, so should be the preferred big circuit board, in many applications such as in the personal computer, saw the larger motherboard. However, the design of signal line layout on large PCB boards is more difficult, requiring more signal layers or internal wiring or space, and heat treatment is also more difficult. Therefore, the designer must consider various factors, such as standard board size, the size of the fabrication equipment and the limitations of the fabrication process. Some guidelines for selecting standard printed circuit/board sizes are given in 1PC-D-322.
The thickness of a multi-substrate is determined by many factors, such as the number of signal layers, the number and thickness of power plates, the aspect ratio of aperture to the thickness required for good quality drilling and plating, the length of component pins required for automatic insertion, and the type of connection used. The thickness of the whole circuit board is composed of the conductive layer, copper layer, substrate thickness, and prepreg material thickness on both sides of the PCB board. It is difficult to obtain strict tolerances on synthetic multi substrates, and a tolerance standard of about 10% is considered reasonable.
In order to minimize the chance of PCB board distortion and obtain a flat finished board, the layers of the multi-substrate should be symmetrical. That is, it has an even number of copper layers, and ensures that the thickness of copper and the copper foil pattern density of the plate layer are symmetrical. Usually the radial direction of the construction material used for lamination (e.g., fiberglass cloth) should be parallel to the sides of the laminate. Because the laminate shrinks along the radial direction after bonding, this distorts the layout of the circuit board, showing volatility and low spatial stability.
However, the warpage and distortion of the multi-substrate can be minimized by improving the design. The warpage and distortion can be reduced by the even distribution of copper foil on the whole layer and the symmetrical structure of the multi-substrate, that is, the same distribution and thickness of the prepreg material can be ensured. The copper and roller layers should be made from the central layer of the multi-substrate up to the outermost two layers. The minimum distance (dielectric thickness) between two copper layers is 0.080mm.
As a rule of thumb, the minimum distance between two copper layers, i.e. the minimum thickness of the prepreg material 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 30μm thick, the thickness of the prepreg material is at least 2(2×30μm) =120μm. This can be achieved by using two layers of prepreg material (a typical value for fiberglass weaving is 1080).
The most commonly used copper foil is 1oz (1oz per square foot surface area). However, for dense PCB boards, where the thickness is extremely important and requires strict impedance control, such PCB boards need to be used 0.50z copper foil. For the power layer and ground, it is best to choose 2oz or a bit heavier copper foil. However, etching heavier copper foils results in reduced controllability, and it is not easy to achieve the desired line widths and spacing tolerances of the pattern. Therefore, special processing techniques are required.
Depending on the pin diameter or diagonal, the diameter of the plated through hole is usually kept between 0.028 0.010in. This ensures sufficient volume for better soldering.
The aspect ratio is the ratio of the thickness of the plate to the diameter of the borehole. 3:1 is considered the standard aspect ratio, although a high aspect ratio like 5:1 is often used. Aspect ratio can be determined by drilling, degumming or erosion and electroplating. The perforations should be as small as possible when the aspect ratio is maintained within the producible range.
Multi-substrate is a high-performance, high-speed system. At higher frequencies, the signal’s rise time is reduced, so the control of signal reflection and line length becomes critical. In a multi-substrate system, the controllable impedance performance of electronic components is very strict, and the design should meet the above requirements. The factors that determine the impedance are the dielectric constant of the substrate and the prepreg material, the distance between the conductors on the same layer, and the copper conductor’s thickness. In high-speed applications, the sequence of conductor lamination and signal network connection in multi-substrate is also critical. Permittivity: The dielectric constant of the substrate material is an important factor in determining the impedance, propagation delay, and capacitance. The dielectric constant of the substrate and prepreg material using epoxy glass can be controlled by changing the percentage of resin content.
The dielectric constant of epoxy resin is 3.45 and that of glass is 6.2. By controlling the percentage of these materials, the permittivity of epoxy glass can reach 4.2 to 5.3. The thickness of the substrate is a good indication for determining and controlling the permittivity.
Relatively low dielectric constant prepreg materials are suitable for RF and microwave applications. In RF and microwave frequencies, lower permittivity results in lower signal delay. In the substrate, the low loss factor minimizes the electrical loss.
Prepreg material ROR 4403 is a new material produced by Rogers Company. This material is compatible with other substrates used in standard multi-substrate (FR-4 material) construction (for example, RO 4003 or RO 4350 used in microwave boards).
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