Trouble in Your Tank: HDI and UHDI—Building the Foundation of Advanced Packaging
The electronics industry ecosystem has never been more robust or dynamic. AI is a major factor in this rapid growth, fueling advances in high performance computing (HPC), advanced memory, high speed networking, and optical interconnects. In addition, advanced semiconductor packaging and data center build-out are pushing HDI and UHDI fabrication to circuit densities not seen previously.
The Semiconductor Industry Association Roadmap is driving wafer transistor dimensions smaller, enabling faster rise times, and creating the need for finer-pitch interconnects. TSMC is advancing nodes toward 2 nm. Yet despite these remarkable semiconductor breakthroughs, “chips don’t float.” Silicon still requires a mounting platform, as well as printed circuits and organic IC substrates needed to create functional products.
In North America, there is a growing need for greater HDI capability, yet the adoption of HDI board fabrication has been slow. Three obstacles (or chokepoints) may include:
- Defining density needs
- Process controls for fabricating reliable microvias
- Bare board/in-circuit test (ICT) solutions once HDI is assembled
Defining Density Needs
Industry veteran Happy Holden once explained the interrelationship between component packaging, SMT assembly, and printed wiring boards:
- Assembly complexity: A measure of the difficulty to assemble surface mounted components, measured in parts per square inch and leads per square inch.
- Component packaging: The degree of sophistication of a component, measured by its average leads (I/Os) per part.
- Printed wiring board density: The amount of density (or complexity) of a PCB as measured by the average length of traces per square inch of that board, including all signal layers. The metric is inches per square inch.
Wiring density is defined as the amount of electrical interconnection routed within a given board or IC substrate footprint. The greater the wiring density, the smaller the footprint. This allows for miniaturization of the package and board.
Microvias used in HDI and UHDI designs increase density and functionality, while reducing layer counts and the overall footprint. Where are we today? It wasn’t too long ago that the industry was thrilled to get down to 50-micron lines and spaces and 100-micron blind vias.
Today, with advanced packaging, heterogeneous integration and the use of chiplets, 15– 20-micron lines and spaces, and 45–60-micron diameter blind vias are at the leading edge. Major advances in photolithography, metallization, and etching process, laser via formation, unique registration systems, and online process controls have enabled the technology to create very high-density PCBs and substrates. Of course, this sounds easier than it really is. With increased complexity, yields are challenged. At the end of the day, it is all about yields.
Process Controls for Fabricating Reliable Microvias
The small blind vias, known as microvias, pose distinct challenges for PCB fabricators. Unlike through-holes, microvias are far more difficult to drill, desmear, metallize, and plate. Laser drilling adds further complexity compared with mechanical through-hole drilling.
As shown in Figure 1, there are seven fundamental quality concerns, all of which can only be verified through microsectioning. Therefore, dedicated panel coupons are essential for confirming reliable microvia fabrication. This is more reason for extreme diligence in the via formation operation. With a poorly formed via, even the best plating chemistries will not improve the situation. In a future column, we will address the issues shown in Figure 1 in more detail.
Bare Substrate Testing
The testing of HDI PCB prototypes can be accomplished with flying probe electrical testers, but volume production is still relegated to bed-of-nails-type custom fixturing. Multiple head flying probe testers do have more throughput, but even those models with 12 heads have only limited production capacity. It is the bed-of-nails fixture that will have to handle the production load. Unfortunately, the very high-density nature of HDI boards makes custom fixtures very expensive.
HDI boards have test points that, depending on the application, average 100–2,000+ pins per square inch. As an example, HDI server board may require 150–250 pins per square inch. Semiconductor package substrates exceed 1,000 pins per square inch. This fact requires the test equipment to navigate fine pitch HDI boards below 50-micron lines and spacing and 0.4mm pitch.
The good news is that traditional flying probe and bed-of-nails test equipment are being supplemented with a multitude of advanced electrical test technologies. These include very high speed flying probe testers that not only are capable of testing fine pitch designs, but are able to perform impedance measurements and detect intermittent opens and current leakage.
Panel Coupons
Special artwork designed into the panel edges or in unused space on the panel is essential. For additional traceability, if the artwork shares common features on the assembly sub-panel, there is continuity for whoever performs the SMT assembly. Figure 2 shows a small coupon design that can be used on fabrication panels and assembly sub-panel carriers.
The coupons are detachable and can be tested by production personnel during the fabrication process. These coupons test for:
- Desmear-metallization-plating quality (10 immersions of 10 sec in 288°C solder pot inspection)
- Layer-to-layer registration and movement directions (continuity and cross-section)
- Daisy chain electrical reliability (four immersions at 288°C of 10 seconds followed by 4-wire Kelvin measure)
- First drilled hole of multiple diameters/last drilled hole of multiple diameters (inspection if present)
- Layer-order windows (inspection if present)
- CAF reliability (CAF test chamber)
HDI and UHDI fabrication will continue to push the limits of PCB manufacturing as circuit densities increase and feature sizes shrink. For the fabricator, success will depend on much more than having the equipment to produce smaller features. Via formation, process control, testing, and the proper use of panel coupons all become important as tolerances tighten.
In the next installment, I’ll take a closer look at process control for HDI and UHDI fabrication.
This column originally appeared in the September 2026 issue of I-Connect007 Magazine.
