I recently finished up a new activity, the creation of the HDI Handbook. While putting that book together and doing research on HDI materials for chapter five, I collected a lot of information on new materials from Asia.

What Are the New Materials?

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Table 1 shows a summary of the new, thin material that is from the HDI Handbook. The most interesting are the:

• Laser drillable FR-4 prepregs;
• Resin-coated copper foils including a new type with glass-reinforcement; and
• Vacuum laminated dry-film-type dielectrics for very-fine line circuitry.

Laser Drillable Prepregs

We have mobile phone growth to thank for the creation of this new group of woven-glass prepregs. There are now 12 of these new laser drillable prepregs (LDPs). Table 2 describes these in more detail. The one characteristic they all share and differentiates them from conventional prepregs is that the glass bundles and fibers are s-p-r-e-a-d o-u-t and this makes for a more uniform fabric to laser drill and metallize. This also creates a uniform dielectric constant that eliminates the problem with noise caused by the fiber-weave of conventional FR-4s. Figure 1 shows various 50X closeups of these new prepregs, along with the conventional prepregs. Six conventional prepregs—106, 1080, 2112, 2113, 2313, and 2116—now have LDP versions.

Engineered Films

Enlarge this picture Table 2 Physical Characteristics of 12 Available LDPs and a Comparison to Conventional Prepregs Using the Same Glass Type1

The third type of new dielectrics was created by the Japanese for both fine lines and fine vias. Known as ABF for their inventor, Ajinomoto-Build-Up-Film, it is an offshoot of the photo-sensitive dry-film dielectrics and solder masks developed in the late 1990s by DuPont, Enthone, MacDermid, and Hitachi, minus the photoinitiators. These dry-film dielectrics were ALL semi-additive metallization and, as such, could produce very fine lines and spacings (down to 10 µm lines and spacings). In addition, they were vacuum laminated in a conveyorized process, thus increasing throughput compared to batch b-stage lamination. I’ll let you go to the HDI Handbook for the details on vacuum lamination and semi-additive metallization. Table 3 shows more details of these ABF films. Notice that not all of them are epoxy!

New Technology Handbook

The focus and intent of my new HDI Handbook is not intended to introduce PCB technology to the reader that is effectively done by Coombs’ PCB Handbook. It does take up high density interconnects (HDI) and microvias in much more detail from where Coombs left off. The 16 chapters consist of:

Section 1: Introduction to HDI
The widespread use of new electronic components employing ball-grid array (BGA), chip scale packaging (CSP), and other evolving technology form-factors means new fabrication techniques must be used to create PCBs that will accommodate parts with extremely tight lead pitches and small geometries. In addition, extremely fast signal rise times and signal bandwidths challenge systems designers to find better ways to overcome the negative effects of inductance, noise, radio frequency interference, and electromagnetic interference have on their products’ performance. The use of PCBs incorporating microvia circuit interconnects is currently one of the most viable solutions on the market.

Assemblies can be charted by their characteristics. The interaction of interconnect elements, such as assembly, PCBs, and components, are described by their metrics: assembly density, assembly complexity, component complexity, and board density. The HDI Vintage Chart shows the three basic characteristics of HDI architecture.

Enlarge this picture Table 3 Various ABF Build-Up Films and Their Properties1

Section 2: The HDI Interconnect Market
HDI products, size, and market growth rates, examples of different HDI products using the four HDI platforms: substrates and interposers; modules; consumer and mobile phones; and large high-performance boards. History of adoption and worldwide capacity and complexity.

Section 3: Design of Advanced Substrates
As the electronic products industry continues to push the envelope of extreme miniaturization, product development teams are being forced further into the realm of high density interconnect. Design techniques and substrates labeled exotic only a few short years ago are now considered mainstream. In particular, build-up substrate usage has grown dramatically and is now found in a large percentage of high-production electronic products. This section is to educate and inform you on the technologies, needs, issues, and solutions available today for advanced substrate design. The focus will briefly cover the four changes to the PCB design techniques required for HDI and the IPC Design Standard 2226.

Section 4: Electrical Performance
The good-old days of 10 to 16 MHz clock frequencies are gone. The chief design challenge in circuit boards or packages used to be routing all the signals in two layers and getting packages that wouldn’t crack during assembly. The electrical properties of the interconnects were not important because they didn’t affect system performance. But the world has changed in the past 10 years. Clock frequencies on chip now are over 3 GHz and over 800 MHz on board. In most systems, as the clock frequency goes up, the rise time always gets shorter. A shorter rise time means signal integrity problems increase dramatically. Signal integrity is broadly concerned with the problems that arise from how the electrical properties of the interconnects and power distribution affect system performance. External signals can affect noise as well as the return path for signals along planes. HDI provides miniaturization, smaller and shorter vias, shorter interconnect lengths with smaller parts and finer pitch devices, thinner and lower dielectric-laser drillable dielectrics, and embedded components. These can be enabling techniques for improved high-frequency electrical performance.

Section 5: HDI Materials
Many new materials now support HDI and are created to be laser drilled. Many of these are very thin and can have very low dielectric constants as well as lower loss characteristics. Embedded capacitances as well as embedded passives are new materials uniquely suited for laser-drilled blind vias. The material performance and slash sheets from IPC HDI Material Standard 4104 explain many of these.

Section 6: The HDI Manufacturing Processes
Various HDI manufacturing processes and structures are explained. These utilize standard PCB processes but with greater miniaturization and higher density.

Section 7: Small Hole Creation
The machines, processes, quality concerns, and issues with creating small vias.

Section 8: Metallization
Desmear and metallization (electroless), including the materials and processes for paste in vias.

Enlarge this picture Figure 1 Closeups of Various Laser Drillable and Conventional Prepregs (50X)1

Section 9: Fine-Line Imaging and Etching
Image transfer processes, stripping and etching fine lines, registration, equipment, and materials for fine-line image transfer.

Section 10: Via-Fill, Plating, and Finishes
Plating, pulse-plating, small-holes plating and filling, and final finishes.

Section 11: Inspection and Testing
Electrical testing of HDI.

Section 12: Quality, Acceptability, and Reliability
Performance of HDI benchmarking, vendor readiness, qualification, quality issues, lab techniques, and equipment.

Section 13: Related Assembly Topics
Via-in-pad soldering issues, fine pitch, soldering, changes to stencil printing and voids in microvias, and in-circuit testing of limited access HDI boards.

Section 14: Embedded Components
Embedded resistors, embedded capacitors, distributed capacitance and embedded actives, and the materials and design tools.

Section 15: Advanced HDI and Next-Generation Interconnects
The use of more complex components with very high I/O counts and increasing speed has pushed the interconnect into the realm of photonics and opto-electronics. The materials, processes, and test vehicles for optical waveguides in printed circuits are reviewed, as well as other innovative technologies for 3D interconnects.

Section 16: Advanced Packaging and System-in-Packages
The use of more complex components and the scarcity of space have created the need for dense packages and now system-in-packages as well as 3D packaging strategies.

One last comment: For those of you that have not discovered it yet, I have a new blog. You can catch my blog, Holden’s PCB Insights, at communities.mentor.com/mgcx/community/pcb/blogs/happy_holden/.

References

1. Holden, H., ed. HDI Handbook, published by pcb007, www.pcb007.com, Oct. 2008.