Selecting Appropriate HDI PCB Materials: A Guide
High-density interconnect (HDI) PCBs are revolutionizing the electronics industry, particularly in applications where miniaturization, signal integrity, power distribution, and thermal management are paramount. These advanced PCBs feature microvias, finer trace widths, and sophisticated stack-up techniques, enabling higher functional density while maintaining reliable electrical performance.
## Key Properties and Characteristics
The success of HDI PCBs relies on several critical factors. Fine-tuned impedance control is essential for high-speed data (exceeding 25 Gbps), achieved through careful selection of trace geometry, stack-up configuration, and dielectric materials, targeting tolerances within ±5%. Enhanced power distribution networks (PDNs), often featuring embedded capacitance and decoupling patterns, minimize voltage drop and noise.
Thermal management is another crucial aspect, with high component density leading to localized hotspots. Advanced cooling solutions such as thermal vias, heat spreaders, and onboard microfluidic cooling are incorporated to manage heat dissipation. EMI mitigation is managed with guard traces, via fencing, and shielding, reducing interference by 25–40 dB.
Standard HDI PCBs often use high-performance FR4 for cost-effective performance, while more demanding applications use advanced materials like Rogers boards or Panasonic Megtron series for superior signal integrity and lower losses. The precision required for microvias and fine traces increases complexity, cost, and production time compared to conventional PCBs.
## Differences Between HDI PCB Categories
HDI PCBs are categorized into Standard HDI, Ultra HDI, and Mixed Material HDI. Standard HDI balances performance and cost, suitable for most consumer and communication electronics with high-density requirements. Ultra HDI pushes the limits of miniaturization and performance, using advanced laminates and higher manufacturing precision for applications like high-speed data transfer, RF, and military electronics. Mixed Material HDI combines different materials in the stack-up to achieve specific electrical, thermal, or mechanical properties for specialized applications.
## Material Selection
The choice of dielectric materials for HDI PCBs is crucial. High-speed and low-loss materials such as EM-528, I-Tera MT40, Megtron6 R-5775, EM-890K, Isola Tachyon 100G, and Astra MT77 are well-suited for applications operating in the 10 to 30 GHz band. To prevent thermal issues, laminates with a high glass transition temperature (T) greater than the maximum withstanding temperature and the soldering temperature should be chosen. Prefer RoHS-compliant substrates to avoid the use of toxic materials and implement eco-friendly practices.
To minimize fiber weave effect, opt for densely woven fiberglass weaves such as 1078, 1035, 3313, and 1067. When using different types of laminates in your circuit board stack-up, select materials with similar dielectric constants to avoid signal integrity issues.
In high-density applications, select substrates with a low D to reduce signal attenuation. Employing low-profile copper can minimize conductor loss in HDI and high-frequency designs. Select materials suitable for laser drilling, such as flat glass clothes with high resin content like 1035, 1067, and 1086. Very high speed and very low loss (RF/microwave) materials are suitable for applications operating at 20-60GHz.
## Conclusion
HDI PCBs offer a unique blend of miniaturization, signal and power integrity, thermal management, and EMI mitigation capabilities. The choice between standard, ultra, and mixed material HDI categories is dictated by the specific requirements of the application—balancing cost, performance, and manufacturability. As technology continues to advance, so too will the capabilities and applications of HDI PCBs.
Thestackup designer ensures controlled impedance in HDI PCBs by meticulously selecting trace geometry, stack-up configuration, and dielectric materials to meet tolerances within ±5%.
Advanced materials like Rogers boards and Panasonic Megtron series, chosen by the stackup designer, are essential for applications demanding superior signal integrity and lower losses in HDI PCBs.
