The design performance of PCB multi-substrates is largely similar to that of single-substrate or dual-substrate designs. Specifically, it is crucial to avoid cramming too many circuits into too small a space, as this can lead to unrealistic tolerances, excessive inner layer capacity, and potential risks to product quality and safety. Therefore, performance specifications should include a comprehensive evaluation of thermal shock, insulation resistance, solderability, and other critical factors of the inner circuit. The following content outlines key factors to consider in multi-substrate design.
1. Mechanical Design Factors
Mechanical design encompasses selecting the appropriate board size, board thickness, board stacking, inner copper tube dimensions, aspect ratio, and other elements.
1.1 Board Size
The board size should be optimized based on application requirements, the dimensions of the system enclosure, limitations of the circuit board manufacturer, and manufacturing capabilities. Larger circuit boards offer several advantages, such as fewer substrates, shorter circuit paths between components, higher operational speeds, and more input and output connections. Consequently, larger circuit boards are often preferable in many applications, such as in personal computers with larger motherboards. However, designing the signal line layout on a large PCB can be more challenging, requiring additional signal layers, internal wiring, or space, and complicating heat management. Designers must consider various factors, including standard board sizes, manufacturing equipment dimensions, and process limitations. Reference 1PC-D-322 provides guidelines for selecting standard printed circuit board sizes.
1.2 Board Thickness
The thickness of multi-substrates is influenced by several factors, including the number of signal layers, the quantity and thickness of power boards, the aspect ratio of apertures, the thickness required for high-quality punching and plating, the length of component pins for automatic insertion, and the type of connections used. The overall thickness of the circuit board comprises the conductive layers, copper layers, substrate thickness, and prepreg material thickness on both sides of the PCB. Achieving tight tolerances on multi-substrates is challenging, and a tolerance standard of approximately 10% is generally considered acceptable.
1.3 Board Stacking
1. To minimize PCB board distortion and achieve a flat finished board, the layering of multiple substrates should be symmetrical. This means having an even number of copper layers and ensuring that the copper thickness and foil pattern density are symmetrical across the board layers. Typically, the radial direction of the construction material (e.g., fiberglass cloth) used for the laminate should be parallel to the laminate’s edges. If the laminate shrinks radially after bonding, it can distort the circuit board layout, leading to variability and reduced dimensional stability.
2. However, by refining the design, warpage and distortion of multi-layer substrates can be minimized. This can be achieved by evenly distributing the copper foil across all layers and ensuring the symmetry of the multi-layer substrate structure. Specifically, this involves maintaining uniform distribution and thickness of the prepreg material. The copper and laminate layers should be constructed from the center layer of the multi-layer substrate to the two outermost layers. The minimum dielectric thickness specified between two copper layers is 0.080 mm.
3. Experience shows that the minimum distance between two copper layers, which is the minimum thickness of the prepreg material after bonding, must be at least twice the thickness of the embedded copper layer. For instance, if each of the two adjacent copper layers is 30 μm thick, the prepreg material must be at least 2(2×30 μm) = 120 μm thick. This can be achieved using two layers of prepreg material (glass fiber woven fabric), with a typical value being 1080.
4. Inner copper foil:
The most commonly used copper foil is 1 oz (1 oz of copper per square foot of surface area). However, for dense PCB boards where thickness is crucial and strict impedance control is needed, 0.50 oz copper foil should be used. For power planes and ground planes, it is preferable to use 2 oz copper foil or heavier. Etching heavy copper foil can reduce controllability, making it challenging to achieve the desired line width and pitch tolerance. Therefore, special processing techniques are required.
5. Hole:
The diameter of plated through holes is typically maintained between 0.028 and 0.010 in, depending on the component pin diameter or diagonal size. This range ensures sufficient volume for better soldering.
6. Aspect ratio:
The aspect ratio is the ratio of the board thickness to the hole diameter. A 3:1 aspect ratio is generally considered standard, though higher ratios like 5:1 are also common. The aspect ratio is influenced by factors such as drilling, slag removal, etch back, and electroplating. Vias should be kept as small as possible while maintaining the aspect ratio within producible limits.
7. Electrical design factors:
Multi-layer substrates are high-performance, high-speed systems. For higher frequencies, reducing signal rise time makes signal reflection and line length control critical. In multi-layer systems, strict requirements for controllable impedance are essential, and the design must meet these specifications. Impedance is determined by the dielectric constant of the substrate and prepreg material, the spacing of wires on the same layer, the thickness of the interlayer dielectric, and the thickness of the copper conductor. In high-speed applications, the lamination sequence of conductors and the signal net connection sequence are also crucial.
8. Dielectric constant:
The dielectric constant of the substrate material is vital in determining impedance, propagation delay, and capacitance. The dielectric constant of glass epoxy substrates and prepreg materials can be controlled by adjusting resin content percentages. The dielectric constant of epoxy resin is 3.45, while glass has a dielectric constant of 6.2. By controlling the proportions of these materials, the dielectric constant of epoxy glass can range from 4.2 to 5.3. The thickness of the substrate also affects the dielectric constant.
9. The prepreg material with a relatively low dielectric constant is suitable for radio frequency and microwave circuits. Lower dielectric constants result in reduced signal delay and minimized electrical loss in these frequencies.
10. The prepreg material ROR 4403 is a new type of material produced by PCB companies. It is compatible with other substrates used in standard multi-layer (FR-4) structures, such as RO 4003 or RO 4350 used in microwave panels.
1. Mechanical Design Factors
Mechanical design encompasses selecting the appropriate board size, board thickness, board stacking, inner copper tube dimensions, aspect ratio, and other elements.
1.1 Board Size
The board size should be optimized based on application requirements, the dimensions of the system enclosure, limitations of the circuit board manufacturer, and manufacturing capabilities. Larger circuit boards offer several advantages, such as fewer substrates, shorter circuit paths between components, higher operational speeds, and more input and output connections. Consequently, larger circuit boards are often preferable in many applications, such as in personal computers with larger motherboards. However, designing the signal line layout on a large PCB can be more challenging, requiring additional signal layers, internal wiring, or space, and complicating heat management. Designers must consider various factors, including standard board sizes, manufacturing equipment dimensions, and process limitations. Reference 1PC-D-322 provides guidelines for selecting standard printed circuit board sizes.
1.2 Board Thickness
The thickness of multi-substrates is influenced by several factors, including the number of signal layers, the quantity and thickness of power boards, the aspect ratio of apertures, the thickness required for high-quality punching and plating, the length of component pins for automatic insertion, and the type of connections used. The overall thickness of the circuit board comprises the conductive layers, copper layers, substrate thickness, and prepreg material thickness on both sides of the PCB. Achieving tight tolerances on multi-substrates is challenging, and a tolerance standard of approximately 10% is generally considered acceptable.
1.3 Board Stacking
1. To minimize PCB board distortion and achieve a flat finished board, the layering of multiple substrates should be symmetrical. This means having an even number of copper layers and ensuring that the copper thickness and foil pattern density are symmetrical across the board layers. Typically, the radial direction of the construction material (e.g., fiberglass cloth) used for the laminate should be parallel to the laminate’s edges. If the laminate shrinks radially after bonding, it can distort the circuit board layout, leading to variability and reduced dimensional stability.
2. However, by refining the design, warpage and distortion of multi-layer substrates can be minimized. This can be achieved by evenly distributing the copper foil across all layers and ensuring the symmetry of the multi-layer substrate structure. Specifically, this involves maintaining uniform distribution and thickness of the prepreg material. The copper and laminate layers should be constructed from the center layer of the multi-layer substrate to the two outermost layers. The minimum dielectric thickness specified between two copper layers is 0.080 mm.
3. Experience shows that the minimum distance between two copper layers, which is the minimum thickness of the prepreg material after bonding, must be at least twice the thickness of the embedded copper layer. For instance, if each of the two adjacent copper layers is 30 μm thick, the prepreg material must be at least 2(2×30 μm) = 120 μm thick. This can be achieved using two layers of prepreg material (glass fiber woven fabric), with a typical value being 1080.
4. Inner copper foil:
The most commonly used copper foil is 1 oz (1 oz of copper per square foot of surface area). However, for dense PCB boards where thickness is crucial and strict impedance control is needed, 0.50 oz copper foil should be used. For power planes and ground planes, it is preferable to use 2 oz copper foil or heavier. Etching heavy copper foil can reduce controllability, making it challenging to achieve the desired line width and pitch tolerance. Therefore, special processing techniques are required.
5. Hole:
The diameter of plated through holes is typically maintained between 0.028 and 0.010 in, depending on the component pin diameter or diagonal size. This range ensures sufficient volume for better soldering.
6. Aspect ratio:
The aspect ratio is the ratio of the board thickness to the hole diameter. A 3:1 aspect ratio is generally considered standard, though higher ratios like 5:1 are also common. The aspect ratio is influenced by factors such as drilling, slag removal, etch back, and electroplating. Vias should be kept as small as possible while maintaining the aspect ratio within producible limits.
7. Electrical design factors:
Multi-layer substrates are high-performance, high-speed systems. For higher frequencies, reducing signal rise time makes signal reflection and line length control critical. In multi-layer systems, strict requirements for controllable impedance are essential, and the design must meet these specifications. Impedance is determined by the dielectric constant of the substrate and prepreg material, the spacing of wires on the same layer, the thickness of the interlayer dielectric, and the thickness of the copper conductor. In high-speed applications, the lamination sequence of conductors and the signal net connection sequence are also crucial.
8. Dielectric constant:
The dielectric constant of the substrate material is vital in determining impedance, propagation delay, and capacitance. The dielectric constant of glass epoxy substrates and prepreg materials can be controlled by adjusting resin content percentages. The dielectric constant of epoxy resin is 3.45, while glass has a dielectric constant of 6.2. By controlling the proportions of these materials, the dielectric constant of epoxy glass can range from 4.2 to 5.3. The thickness of the substrate also affects the dielectric constant.
9. The prepreg material with a relatively low dielectric constant is suitable for radio frequency and microwave circuits. Lower dielectric constants result in reduced signal delay and minimized electrical loss in these frequencies.
10. The prepreg material ROR 4403 is a new type of material produced by PCB companies. It is compatible with other substrates used in standard multi-layer (FR-4) structures, such as RO 4003 or RO 4350 used in microwave panels.
