Why Ajinomoto Build-Up Film (ABF) is Used in IC Packaging

Ajinomoto Build-Up Film (ABF) was first used to package ICs by a major semiconductor manufacturer in 1999, and since that time it has remained the product of choice for nearly all high-performance processors. This includes CPUs, SoC, GPUs, and many other advanced components. While it is best-known in the semiconductor industry as a material of choice for use as a build-up layer in substrates and advanced packages.

Although ABF is better known as a packaging material, the same material can be used to support the buildup layers in an HDI PCB. Thanks to its compatibility with additive processing and its capability to support fine traces/linewidths, ABF is also useful as an HDI/Ultra-HDI PCB material with conventional HDI or additive buildup processes.

ABF Material Properties Overview

ABF is a type of film dielectric that comes packaged as a roll; the important ABF film is bondable to other HDI materials or organic resin-based substrates for use in HDI PCBs and IC packages. Specifically, it is packaged between a polyethylene (PET) release film and an ortho-phenylphenol (OPP) protective film on the material roll. Once bonded to an etched copper-coated core material, it functions as a standard dielectric, just as one would expect in a planar substrate with interleaved copper layers.

ABF is also a very thin, but rigid film that enables very high quality and highly reliable microvia drilling with low aspect ratio. The cross-sectional image below shows the incorporation of ABF into the substrate of an IC package, substrate-like PCB, or HDI PCB.

The main properties of ABF that are meaningful for use in HDI PCBs and packages are the dielectric constant (Dk and Df) and the CTE value. The CTE value of this material is much less than that of most FR4 grade PCB laminates with standard woven glass composites, including laser-drillable composites. The CTE value is comparable to copper and many engineered PTFE materials. However, ABF comes in a much thinner roll and is much easier to work with compared to PTFE laminates for RF systems, which is why ABF is preferable for use in packaging.

There are multiple ABF products available with different material property values; some typical values for different part numbers are shown below.

ABF can come packaged with a copper film that can be used for subtractive etching. Otherwise, the material will be available without copper and can be used for additive fabrication.

Fine-Line Fabrication

As ABF is used in HDI/Ultra-HDI systems as the buildup layer, it is primarily being used for fine-line fabrication of very narrow traces. The linewidth/spacing of copper lines etched onto ABF buildup layers can be as low as 5 microns. These very fine lines used in HDI/Ultra-HDI buildup layers can also be used in the redistribution layer connecting between the substrate and a die, or in the redistribution layer between an organic interposer and a die.

Cross-sectional view of fine-line traces (5 microns) etched on ABF. [Source: Ajinomoto]

Fabrication Process With ABF

The fabrication process for a package incorporating ABF involves application of the film and bonding in a low-temperature process. After bonding, the ABF packaging is removed and the copper foil is etched with the required circuit features:

1. The OPP film is removed before ABF application to the package
2. ABF film is applied to an etched substrate as a buildup layer
3. Bonding and curing proceeds in a low-temperature process
4. PET film is released and copper features are fabricated
   • If packaged with copper, the exposed copper is etched
   • If packaged without copper, the copper circuits are selectively deposited
5. ABF film is laser-drilled to a controlled depth
6. Laser-drilled layer transitions are plated to form microvias

Summary

Thanks to the very thin layers available with ABF, the CTE value being comparable to copper, and competitive dielectric properties compared to advanced RF laminates, ABF is a highly popular material used for buildup layers in IC substrates. Low temperature processing enables bonding to core layers in a substrate or organic interposer, and the same properties are very useful as the buildup layers in HDI PCBs. As finer packages and unique builds become more popular, such as substrate-like PCBs, ABF and comparable substitutes will see greater usage.

Engineers who build advanced fine-line HDI PCBs and packages trust the complete set of PCB design tools in Allegro X Design Platform. Allegro X is the industry’s best PCB design and analysis software from Cadence, offering a range of product design features with a complete set of management and version control capabilities. Allegro users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.

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