UHDI PCBs & IC Substrates Guide

IC Substrates & UHDI PCBs

IC substrates are vital in semiconductor packaging, providing mechanical support and electrical connections between chips and PCBs. As demand for more compact and efficient electronics grows, innovations in materials and manufacturing processes are crucial for IC substrates to meet evolving requirements. These processes are also being brought to PCBs, known as ultra-high density interconnect (UHDI).

NWES is one of the few PCB design service bureaus leveraging its HDI/UHDI PCB design experience for IC substrate design. Like any complex microelectronic product, IC substrate design rules are dictated by manufacturing process limitations. To help customers better understand what drives design and engineering decisions for IC substrates, we compiled this guide on the substrate production process. We hope this informs your approach to your next project.

Copper foils or additively deposited copper are used to create the intricate electrical circuitry that connects the various components of the IC. Electrolytic copper foil, with thicknesses ranging from 1.5 µm to 18 µm, is preferred for its excellent conductivity and ability to form uniform layers through electroplating processes. These materials collectively contribute to the robust performance and reliability of IC substrates, enabling them to meet the stringent demands of modern electronic devices and systems.

Layer-by-Layer Substrate Buildup Steps

Step 1: Preparation of the Substrate Core

The process begins with selecting and preparing the core material, typically epoxy resin or fiberglass-reinforced laminate. This core serves as the foundational layer upon which multiple insulating and conductive layers will be built. The core material must be rigid and stable to support the subsequent buildup process and maintain dimensional stability throughout manufacturing.

Step 2: Pre-Curing for Structural Reinforcement

Pre-curing the laminated build-up layer involves exposing the substrate to a controlled heat process, which partially cures the build-up layer, enhancing its structural integrity and preparing it for subsequent processing steps. This step ensures that the build-up layer is stable and well-bonded to the core, reducing the risk of defects during further processing.

Step 3: Via Formation

Vias, or small holes that enable electrical connections between layers, are formed using laser drilling techniques such as CO2 or UV lasers. Accurate alignment and real-time feedback systems are critical to ensure the vias are drilled precisely in the correct locations without damaging adjacent circuitry. This step is essential for creating reliable interconnections between the substrate layers.

Step 4: Micro-Etching and Adding a Copper Seed Layer

Micro-etching is performed to increase the surface roughness inside the vias, improving copper adhesion. An electroless copper plating process is then used to deposit a thin copper seed layer onto the etched surface. This seed layer is crucial for the subsequent electroplating steps, ensuring uniform copper deposition and strong electrical connections.

Step 5: Photoresist Coating

A photoresist layer is applied to the substrate, either as a dry film through lamination or as a liquid resist via dip or spray coating. The photoresist serves as a protective layer that defines the circuit patterns during the photolithography process. The choice of photoresist and coating method depends on the specific requirements of the product being manufactured.

Step 6: Circuit Patterning through Photolithography

Photolithography is used to transfer the circuit patterns onto the photoresist-coated substrate. This process involves exposing the photoresist to light through a photomask, creating the desired circuit patterns. Advanced UHDI PCBs and IC substrates may use direct imaging (DI) systems or steppers to achieve high precision and fine line patterning.

Step 7: Copper Electroplating

Copper electroplating is performed to build up the thickness of the conductive traces defined by the photolithography process. The substrate is immersed in a copper sulfate solution, and an electric current is applied to deposit copper onto the exposed areas. This step ensures that the conductive pathways are robust and capable of handling the electrical loads.

Step 8: Photoresist Removal

After electroplating, the remaining photoresist is removed using a chemical solution or mechanical process, revealing the underlying copper circuitry. This step is critical for finalizing the circuit patterns and preparing the substrate for further processing or assembly.

Step 9: Final Surface Treatment and Inspection

The final step involves surface treatment processes such as annealing or flash etching to refine the copper traces. Plating layers are also applied to enable soldering and protect traces. The substrate undergoes thorough inspection to identify and rectify any defects, ensuring that it meets the required quality standards before being integrated into the final IC package.

UHDI Processing Capabilities

The design of UHDI PCBs and IC substrates follows very similar processes as HDI PCBs in terms of materials, layout, route planning, and simulation. The manufacturing capabilities for the processes listed above will limit what can be done in these designs, and this requires direct interface with an IC substrate manufacturer to ensure success. Today’s UHDI PCB and IC substrate processing capabilities are listed in the following table.

Feature	Size limit
Linewidth	15 microns (0.6 mil), advanced processing at 10 microns (0.4 mil)
Spacing	15 microns (0.6 mil), advanced processing at 10 microns (0.4 mil)
Through-hole sizes	6 mil/12 mil pad (14/16 pad recommended for Class 2/3)
Microvia hole	As low as 1 mil laser drill
Microvia pad	~3x hole diameter
Via features	Fill & cap (VIPPO)
Plating thickness	Up to 1 mil hole wall
Material thickness for uVia	2 mil maximum
Copper foil thickness	As low as 1/8th oz.

Not all fabricators in North America and Europe can access the capabilities listed in the above table, and many companies have been forced to send their packaging and UHDI projects to Taiwan, Japan, or South Korea. Today, that is changing due to supply chain pressures that became apparent during the COVID-19 era and the resulting Chips and Science Act legislation. UHDI and substrate fabrication capacity in the US and Europe is expanding, currently intending to meet prototyping needs but later to support advanced packaging of the most advanced, highest-value semiconductor chips.

Future trends in IC substrate design and manufacturing are driven by the need for more complex and higher-density interconnections, which demand continuous advancements in materials and processes. Manufacturers must adopt advanced process tools and stringent inspection and metrology procedures to identify and eliminate defects, thereby improving yield rates. Collaboration across the electronics ecosystem, including partnerships between IC substrate manufacturers, designers, and equipment suppliers, is essential for aligning efforts to meet evolving requirements.