Looking back at the history of PCBs, high-density PCB design approaches were based on the same methodologies used in integrated circuit (IC) substrates. These methods were developed prior to heterogeneous integration and extremely fine-line routing.
Fast forward to today, a new systems integration platform has emerged that fuses packaging with traditional approaches to PCB design. This is known as substrate-like PCBs. The design of these platforms is very similar to HDI (High-Density Interconnect) PCB design, as it implements practices from both IC substrate design and printed circuit design.
However, Ultra-HDI design with very fine line routing is driving integration on PCBs without a traditional approach to packaging. Instead, it enables the integration of packaged chips and semiconductor die in one platform. Below, I’ll outline how this design approach works and where it aligns with today’s advanced applications.
What is a substrate-like PCB?
A substrate-like PCB is a type of circuit board that uses design practices found in integrated circuit substrates. In this way, it allows the direct bonding of semiconductor dies to the PCB, essentially following a chip-on-board approach.
The difference from chip-on-board is mostly semantic: chip-on-board often refers to coarser pitch dies or wire-bonded dies, while the term substrate-like PCB refers to assemblies for higher-density components with pitches requiring down to 10-micron line widths and spacing.
The other difference between chip-on-board and substrate-like PCBs comes from the range of packaging that can be used on a substrate-like PCB. Because substrate-like PCBs are designed using IC substrate techniques, materials and stack-up designs, it is conceivable that 2.5D and 3D packages could be placed on substrate-like PCBs.
Another possibility is the placement of components with an interposer; the interposer bonds to the substrate-like PCB, while the semiconductor dies communicate with each other via interconnects in the interposer.
The technology approach used here is not new. According to 3D Insights, the use of substrate-like PCBs is mentioned in a 2019 roadmap for Samsung's smartphones. The roadmap outlines the shift from 20-layer HDI PCBs to substrate-like PCBs in mobile handsets.
Samsung’s 2019 technology roadmap for substrate-like PCB implementation in handsets. Note the image on the right side indicates a stacked SLP assembly. Source: 3D Insights
The image above illustrates the advantage of a substrate-like PCB as opposed to a standard any-layer HDI PCB, although the delineation is quite blurry. The substrate-like PCB is designed with finer line widths and spacings, as well as advanced packages placed directly on the board. Some possible package types include:
- Advanced flip-chip packages
- 3D-stacked die packages
- Wafer-level chip-scale packages
- Interposer-based packages
These packages are placed into the same assembly as standard off-the-shelf components, which may be assembled in a reflow process. Another possibility is to assemble lower density packages into the substrate-like PCB assembly, such as in a chip-on-board assembly or as a wirebonded die.
Stack-ups and materials for substrate-like PCBs
The primary design factor that makes substrate-like PCBs similar to HDI PCBs is the stack-up construction. Substrate designers often use the same layer and via arrangements that are found in HDI PCBs, and standardized stack-up constructions can be used in substrate-like PCBs.
This includes via types spanning up to ELIC (Every Layer Interconnect), where stacked blind and buried vias are used throughout the stack-up. Example stack-up constructions shown in some of my HDI seminars are presented below.
Stack-up images
The material systems used in these Ultra-HDI (UHDI) constructions resemble what you would see in IC substrates rather than HDI PCBs. The typical package materials and their usage in a substrate-like PCB stack-up are categorized in the following table:
This list of build-up film materials enables very thin HDI build-up layers that will be sequentially laminated onto a thicker core layer. The HDI build-up layers are then fabricated using an additive deposition process and a direct metallization process, which results in the very thin trace widths and microvias characteristic of IC substrates. The build-up film layers can also be used to build coreless packages which could potentially be used in a substrate-like PCB.
Applications for substrate-like PCBs
As a systems integration platform for advanced packages, there are many opportunities for usage of substrate-like PCBs, both at low volume and high volume. At low volume, most opportunities for substrate-like PCBs will be found in the defense sector, which commonly needs higher-end assemblies, including high-density assemblies.
At the high-volume end of the spectrum, the most obvious opportunity is for mobile devices that require low z-axis profiles. This would most commonly comprise consumer products like:
- Smartphones
- Tablets
- Wearables
- Televisions
However, new opportunities are becoming clear in commercial areas, such as advanced data center hardware or IoT products. Newer advanced build-up films could also expand the opportunities for substrate-like PCBs in commercial products, which could solve some reliability challenges with advanced packages.
