-
- News
- Books
Featured Books
- design007 Magazine
Latest Issues
Current IssueSimplification
On the subject of simplification, our expert contributors explain how to design PCBs without making them complex and over-constrained—whatever the level of technology. In addition, we discuss the effect your decisions and tradeoffs have on design complexity.
The Standards of Design
Our expert contributors discuss how PCB designers can utilize standards to save time and money, not to mention frustration. We also spotlight the newly updated version of the IPC Checklist, a handy guide that illustrates which standards cover which topics, from front-end design through assembly.
Rigid-flex: Designing in 3D
In this month’s issue, our expert contributors share their best tips, tricks and techniques for designing rigid-flex circuits. If you’re a rigid board designer considering moving into the 3D world of rigid-flex, this issue is just what the doctor ordered!
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - design007 Magazine
UHDI Fundamentals: A Primer on UHDI
September 28, 2023 | Anaya Vardya, American Standard CircuitsEstimated reading time: 4 minutes

There has always been pressure to reduce line and space as we have seen the bleeding edge technology go from 8 to 5 mils and then to 3 mils. The difference between “then” and “now” is that the prior advancements, for the most part, used the same processes, chemistry, and equipment going from 8 mils to 3 mils. But going from 3-mil to sub 1-mil trace and space is a quantum leap in printed circuit board (PCB) technology that requires a whole new set of processes and materials.
High density interconnect (HDI, the predecessor of UHDI) deals with line width and space, but primarily employs via structures to increase density. In broad terms, HDI printed circuit boards are defined as PCBs with one or more of the following via structures: microvias, stacked and/or staggered microvias, buried and blind vias, and all with sequential lamination cycles. Printed circuit board technology has been evolving with changing technology that demands smaller and faster features. HDI boards allow smaller vias, pads, lines and spaces—in other words, higher density. This increased density also allows a reduction in the number of layers and a smaller footprint. For example, one HDI board can house the functionality of multiple standard technology PCBs. Conventional state-of-the-art technology has been stuck at the 3-mil line and space capability for the longest time, but that is just not good enough to meet the increasingly tighter real estate constraints of today’s products. That is where UHDI comes in.
What is UHDI?
As soon as we cross below the 1-mil (0.001") line-width threshold, we need to stop talking in terms of mils and ounces and begin talking in terms of microns. For reference, a 3-mil trace is 75 microns, so a 1-mil trace is 25 microns. In general terms, ultra high-density interconnect (UHDI) refers to lines and spaces on a printed circuit board that are sub-25 micron. As electronics continue to shrink, so does the printed circuit board, not only in the X- and Y-axes, but also the Z-axis. Designers are challenged with reducing the form factor as well as the thickness of printed circuit boards to meet these demands.
Subtractive vs. Additive
Subtractive technology is how we have primarily produced printed circuit boards since their inception. The subtractive process refers to the selective building up of traces and features on a copper-clad substrate through plating processes, and then removing, or subtracting, the base copper to leave the circuit pattern. The limiting factor of the subtractive process is the thickness of the base copper, typically 5-micron, or 2-micron ultra-thin foil. This base copper thickness defines the minimum trace that can be achieved through the etching process. That is the technology that got us as an industry to the 3-mil line and space. The etched sidewall of the trace is tapered, not straight, with the taper getting larger the closer you get to the base substrate. The amount of taper is defined by the copper weight; the higher the weight, the more the taper. This limitation is what drove the development of UHDI technology.
UHDI additive technology starts with unclad substrate and adds an ultra-thin 0.2-micron layer of liquid ink to the substrate. The selective trace pattern is applied and the circuit pattern is then built up through the plating processes. The game changing difference here is only having to etch 2 microns of base material, which results in perfectly straight trace sidewalls.
SAP and mSAP
The semi-additive process (SAP) and modified semi-additive processes (mSAP) have been around for a while and have brought the capability of line width and space down to the 1–3 mil level. mSAP began in the IC substrate industry but has proliferated throughout PCB manufacturing shops for use with HDI products. These processes utilize a layer of base copper on the substrate between 5 and 2 microns to achieve line width reductions. However, production of sub-1-micron traces requires the ultra-thin 0.2-micron layer of liquid ink used in the A-SAP process.
A-SAP
The pioneer of UHDI processing is Averatek, which has brought its A-SAP™ process to market. A-SAP stands for “Averatek Semi-Additive Process” and is the industry leader in this technology. American Standard Circuits has partnered with Averatek on the technology which allows for the manufacture of PCBs with line width and space down to 15 microns. Some of the benefits of using A-SAP are:
- Significant size and weight reduction
- Improved reliability and signal integrity
- Improved RF performance
- Reduced costs
- Biocompatibility
These processes have allowed devices like cellphones to continually shrink in size while increasing functionality. As technology continues to push everything smaller and faster while integrating with other devices, UDHI is here for the foreseeable future.
This article originally appears in the September 2023 issue of Design007 Magazine.
Anaya Vardya is president and CEO of American Standard Circuits; co-author of The Printed Circuit Designer’s Guide to… Fundamentals of RF/Microwave PCBs and Flex and Rigid-Flex Fundamentals. He is the author of Thermal Management: A Fabricator's Perspective and The Companion Guide to Flex and Rigid-Flex Fundamentals. Visit I-007eBooks.com to download these and other free, educational titles.
Suggested Items
HyRel Technologies Selects EQC Southeast as Manufacturers' Representative for Strategic Growth
11/29/2023 | HyRelHyRel Technologies, a global provider of quick turn semiconductor modification solutions, is pleased to announce a strategic partnership with EQC Southeast as its manufacturers' representative.
Inovaxe Named Distributor for VisiConsult’s X-ray Counter in the U.S.
11/29/2023 | InovaxeInovaxe, a world leader and provider of innovative material handling and inventory control systems, proudly announces its role as a distributor for VisiConsult’s innovative X-ray Counter solutions in the United States.
Standard of Excellence: The Perfect PCB Fab—A Vision for Five Years Ahead
11/29/2023 | Anaya Vardya -- Column: Standard of ExcellenceTo sustain a standard of excellence, it’s equally important to look at both what we’re doing today as well as what we’ll do in the future. In other words, plan ahead. In that spirit, I thought it would be prudent to peek into the future and talk about what a printed circuit board fabrication facility with a “standard of excellence” will look like five years from now.
Lockheed Martin Technology Demonstration to Showcase Faster On-Orbit Sensor Calibration
11/28/2023 | Lockheed MartinLockheed Martin will soon launch a unique wideband Electronically Steerable Antenna (ESA) payload demonstrator to show the company's investment in advanced technology to perform missions faster once on orbit. The payload demonstrator, which will launch aboard Firefly Aerospace’s Alpha rocket, extends Lockheed Martin’s significant investment in scalable wideband ESA technology development to showcasing an actual on-orbit capability. This technology is critical to future remote sensing architectures.
Cambridge, Intel and Dell Join Forces on UK’s Fastest AI Supercomputer
11/27/2023 | Cambridge UniversityThe Cambridge Open Zettascale Lab is hosting Dawn, the UK’s fastest artificial intelligence (AI) supercomputer, which has been built by the University of Cambridge Research Computing Services, Intel and Dell Technologies.