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PCB Layout Guide for HDI Circuit Board Factory

HDI STACK

HDI stack diagram in HDI circuit board factory

Introduction to HDI Boards

HDI boards (High Density Interconnect) are circuit boards with relatively high circuit density that use micro blind and buried vias technology. HDI boards have inner and outer layers of circuits, and by using drilling and metalization processes, connections between different layers of circuits are achieved.

HDI boards are generally manufactured using the lamination method, and the more layers there are, the higher the technical level of the board. Ordinary HDI boards are basically single-layer laminates, while advanced HDI boards use technologies such as double-layer or multi-layer lamination, as well as advanced PCB technologies like stacked vias, electroplated filling, and laser direct drilling.

When the density of a PCB exceeds eight layers, manufacturing it using HDI can be more cost-effective than traditional complex pressing processes. HDI boards facilitate the use of advanced assembly technologies, offering higher electrical performance and signal accuracy than traditional PCBs. Additionally, HDI boards provide better improvement in areas such as radio frequency interference, electromagnetic interference, electrostatic discharge, and heat conduction.

As electronic products continue to move towards high density and high precision, the term “high” not only refers to improving machine performance but also reducing machine size. High Density Interconnect (HDI) technology enables the design of end products to be more compact while meeting higher standards of electronic performance and efficiency. Many popular electronic products today, such as smartphones, digital cameras, laptops, and automotive electronics, use HDI boards. With the continuous upgrading of electronic products and market demand, the development of HDI boards is expected to be rapid.

HDI Board Factory Discusses Common PCB Routing Rules

  • Pre-define digital, analog, and DAA signal routing areas on the PCB.
  • Separate digital and analog components and their corresponding traces as much as possible and place them within their respective routing areas.
  • Keep high-speed digital signal traces as short as possible.
  • Keep sensitive analog signal traces as short as possible.
  • Properly distribute power and ground.
  • Keep DGND, AGND, and real ground separate.
  • Use wide traces for power lines and critical signal traces.
  • Power lines and ground lines should radiate as much as possible, and signal lines should not form loops.
  • Place digital circuits near parallel buses/serial DTE interfaces, and DAA circuits near telephone line interfaces.
  • For small discrete devices, symmetric routing is required, and for SMT pads with close spacing, leads should be connected from the outside of the pad and not directly in the middle.
  • Give priority to key signal lines: power, small analog signals, high-speed signals, clock signals, and synchronous signals, among others.
  • Follow the principle of routing density priority: Start routing from the devices with the most complex interconnections on the board and from the areas with the densest wiring.
  • Avoid sharp angles and right angles in PCB design to prevent unnecessary radiation and manufacturing process issues.
  • There should be no through-holes on SMT pads to prevent solder paste loss and component soldering defects. Important signal lines are not allowed to pass between pin legs.

PCB High-Frequency Circuit Routing

  • Choose the number of PCB layers wisely. Using intermediate power layers (Vcc layer) and ground layers (Gnd layer) can shield and effectively reduce parasitic inductance and capacitance, shorten routing lengths, and reduce signal interference.

  • Routing method: Must be at 45° angles, not 90° angles.

  • Inter-layer routing direction: Should be perpendicular to each other. If the top layer is horizontal, the bottom layer should be vertical to reduce signal interference.

  • Grounding: Ground important signals to significantly improve their anti-interference ability, and ground multiple interfering signals to prevent them from interfering with other signals.

  • Decoupling capacitors: Add decoupling capacitors to the power supply terminals of ICs.

  • High-frequency choke: When there are common grounds like digital ground and analog ground, add high-frequency choke devices between them, usually using ferrite beads with wire holes in the center.

  • Copper filling: Increasing the grounding area can also reduce signal interference. (Dead copper needs to be removed during copper filling)

  • Routing length: The shorter the routing length, the better, which reduces interference. However, not all routing needs to be short. For example, DDR routing requires equal lengths for clock, address, and data lines, so you will see many intentionally lengthened serpentine traces.

Routing for Special Components

  • High-frequency components: Keep the connections between high-frequency components as short as possible to reduce distributed parameters and mutual electrical interference; sensitive components that are prone to interference should not be placed too close to each other.
  • Components with high voltage differences: Increase the distance between components with high voltage differences and their connections to avoid accidental short circuits and component damage. To prevent creeping currents, the distance between copper traces with a voltage difference of 2000V should be greater than 2mm.
  • Heavy components: Heavy components should be fixed with brackets.
  • Heating and thermally sensitive components: Heat-generating components should be kept away from thermally sensitive components, and heat-generating components should be evenly distributed.

Important Parameters for PCB Routing Design

  • Copper trace (Track) width: 0.3mm for single-sided boards, 0.2mm for double-sided boards;
  • Minimum gap between copper traces: 0.3mm for single-sided boards, 0.2mm for double-sided boards;
  • Minimum distance from copper traces to the edge of the PCB: 1mm, minimum distance from components to the edge of the PCB: 5mm, minimum distance from solder pads to the edge of the PCB: 4mm;
  • The solder pad diameter for mounting components with general through-holes is twice the inner diameter of the solder pad.
  • Electrolytic capacitors should not be placed near heat-generating components, such as high-power resistors, transformers, high-power transistors, three-terminal voltage regulators, and heat sinks. The distance between electrolytic capacitors and these components should be greater than 10mm.
  • Screw holes should not have copper traces (except for ground) or components within 5mm outside radius.
  • In large-area PCB designs (over 500m²), to prevent PCB bending during reflow soldering, a 5mm to 10mm wide gap should be left in the middle of the PCB without placing components for placing pressure bars to prevent PCB bending.
  • Use a hollow arrow to indicate the direction of reflow soldering for each PCB.
  • When routing, the direction of the DIP package IC should be perpendicular to the direction of reflow soldering, not parallel, to avoid tin bridging.
  • When the routing direction changes from vertical to horizontal, it should enter at a 45° angle.
  • The width of the power line should not be less than 18mil; the width of the signal line should not be less than 12mil; the width of the CPU input and output lines should not be less than 10mil (or 8mil); and the spacing between lines should not be less than 10mil.
  • The density of board routing should be appropriate. When the density difference is too large, it should be filled with mesh copper foil with a grid larger than 8mil (or 0.2mm).
  • In areas within 1mm of the edge of the PCB designated for routing and within 1mm around mounting holes, routing is prohibited.
  • Warning marks should be printed on the silk screen layer near components such as fuses, fuse resistors, AC 220V filter capacitors, transformers, etc.
  • The distance between the live and neutral lines of the AC 220V power supply should not be less than 3mm. The distance between any wire in the 220V circuit and low-voltage components, pads, and traces should not be less than 6mm, and a high-voltage mark should be printed on them. Low-voltage and high-voltage should be separated by thick wire nets as a warning to maintenance personnel to handle them with care.

The above is a comprehensive guide to PCB wiring knowledge compiled by Highleap Electronic. Hope you can gain something from it! Contact us to learn more about PCB manufacturing processes.

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