Discusses High-Density Interconnects with Happy Holden
In the realm of electronic design, High-Density Interconnect (HDI) PCBs have emerged as a popular choice for space-saving designs that demand smaller footprints and high wiring density. Here's a closer look at the key factors to consider when designing HDI PCBs for optimal performance and manufacturability.
Stack-up Architecture
A well-planned layer stack is essential for maintaining signal integrity, controlling impedance, and reducing electromagnetic interference (EMI) in HDI PCBs. Typical HDI stack-ups utilise microvias (blind, buried, or stacked) and often feature multiple ground and power planes centrally located for noise isolation. Lamination structures can be single or double buildup, with multi-step lamination supporting complex via configurations and better high-frequency performance. Advanced stack-ups might employ skip-via or stacked via designs, requiring precise layer alignment and special fabrication processes.
Material Selection
The choice of materials plays a significant role in the thermal management and signal integrity of HDI boards. High-frequency boards often use materials with low dielectric loss and high thermal conductivity, such as Rogers laminates. For FR4-based designs, additional copper planes or thermal vias help dissipate heat from dense circuitry.
HDI Testability
The intricate layer structure of HDI PCBs makes testability more challenging. Design for test requires accommodating via aspect ratios, keeping microvias within fabrication limits, and ensuring features like annular ring clearances to prevent shorts. These considerations help prevent plating defects and increase manufacturing yield.
Determining the Need for HDI
HDI is primarily chosen for space-saving designs where smaller footprints and high wiring density are required. It improves performance at high frequencies by reducing signal path lengths and noise. HDI can replace traditional multilayer PCBs with fewer layers, offering better reliability but at higher fabrication cost and complexity.
Controlled Impedance
Maintaining controlled impedance is crucial for signal integrity in high-speed HDI designs. This requires precise stack-up planning combined with careful trace width, spacing adjustments, and consistent dielectric material properties. Narrow trace widths and tight spacing are common but need accurate manufacturing to avoid shorts or signal interference.
In summary, an optimal HDI PCB design balances advanced stack-up planning with suitable materials for thermal and electrical performance, ensures manufacturability and testability through aspect ratio and clearance controls, justifies HDI use by application needs (size, speed, reliability), and implements controlled impedance to preserve signal quality. These factors collectively ensure robust functionality and manufacturability in HDI PCB architectures.
It's not just the fine lines that matter in HDI design, but the tolerance of those fine lines is crucial. HDI designs require upfront planning to avoid via starvation and costly solutions like adding hundreds of laser-drilled blind vias. The fabricator's ability to control the tolerance of fine traces is essential to meet the desired characteristic impedance.
Collaborating with the fabricator is essential to understand their testability guarantee, reliability, and the material's overall performance for a successful first HDI design. It is recommended to design a test vehicle for the first HDI board to test material and ensure testability, reliability, and overall performance.
The HDI Handbook, a guide on HDI PCB design, covers topics such as planning stack-up and microvia structure, choosing materials, signal integrity, and manufacturing considerations for higher yields. This 5-chapter, 52-page guide, with a 60-minute read time, serves as an invaluable resource for engineers and designers embarking on their HDI design journey.
[1] Happy Holden, The HDI Handbook, p. 2-3 [2] Happy Holden, The HDI Handbook, p. 1-2 [3] Happy Holden, The HDI Handbook, p. 2-4 [4] Happy Holden, The HDI Handbook, p. 3-1
Controlled impedance technology is vital for preserving signal integrity in high-speed HDI designs, as it necessitates precise stack-up planning alongside careful trace width and spacing adjustments. (Reference: Controlled Impedance)
In the realm of HDI PCB technology, advanced materials like Rogers laminates are often utilized for their low dielectric loss and high thermal conductivity properties. (Reference: Material Selection)
