BT Resin vs ABF Substrate: Materials Compared

Two Names, Two Different Things

BT resin and ABF are often compared as if they were competing substrate materials, but they play different roles. BT resin is a core material, the rigid sheet that gives a substrate its mechanical backbone. ABF, which stands for Ajinomoto Build-up Film, is a dielectric film laminated on top of that core to form the fine pitch build-up layers. In a high end package substrate for a processor or a large ASIC, both are present at the same time: a BT core with ABF build-up layers above and below. Understanding the difference explains why one is cheap and widely available while the other is a bottleneck material for advanced packaging.

What BT Resin Is

BT stands for bismaleimide triazine, a thermoset resin used with glass fabric to form a rigid laminate core. It was developed to improve on standard epoxy for package substrates, and it offers a higher glass transition temperature, lower moisture absorption and a coefficient of thermal expansion that sits between the build-up film and the PCB. Those properties make it dimensionally stable through the lamination and reflow cycles a package sees, and it holds fine lines well by substrate standards, typically down to around 10 to 20 microns in production. BT based substrates are the workhorse for wire bonded packages, memory packages, and moderate density flip chip parts, where the density requirement is real but not extreme.

What ABF Is

ABF is a glass free, epoxy based film supplied on a carrier and laminated onto a substrate as the build-up dielectric. Because it contains no glass weave, it can be laser drilled with very small vias and patterned with semi-additive processing to line widths and spaces of a few microns, which is what makes high density flip chip packages possible. Its coefficient of thermal expansion is lower than BT, which helps match silicon, and its electrical properties are well controlled at high frequency. The trade is that ABF is a specialist material with limited suppliers, it is processed in a semiconductor-like environment, and it needs careful control of warpage because a stack of thin build-up layers on a thin core will bow if the copper balance and the cure are not right.

The Key Differences

Role. BT is a rigid core material; ABF is a build-up dielectric film. Composition. BT uses glass fabric reinforcement; ABF is glass free. Line width. BT based substrates hold roughly 10 to 20 micron lines, while ABF build-up layers reach a few microns. Via size. Vias in BT are formed by mechanical or laser drilling at larger diameters; ABF layers use laser microvias of 30 microns or less. CTE. ABF is lower and closer to silicon, which is why the fine layers use it. Warpage. A BT core provides stiffness, which is why it is often retained as the backbone under ABF build-up. Cost. BT is far cheaper and much more widely available; ABF is a constrained, premium material.

How They Work Together

The classic high density substrate is a sandwich. A BT core carries the coarse routing and provides mechanical rigidity, and ABF build-up layers are laminated on both faces and processed with semi-additive methods to create the fine, high density routing that connects to the die. The build-up layers are built one at a time, with laser drilled microvias connecting each layer to the one below, and the process repeats until the required layer count is reached. Some very thin packages use a coreless construction, where the build-up layers alone provide the structure, but that places even more emphasis on controlling warpage and handling, and it is used only where thickness is critical.

Package Types and Applications

BT based substrates. Memory packages, wire bonded controllers, sensors, RF modules, and flip chip packages where the ball pitch and line width requirements are moderate. ABF build-up substrates. High performance processors, graphics chips, large networking ASICs, AI accelerators and any package with a high ball count and a large die that needs fine escape routing. Combined. The majority of high end packages use a BT or similar core with ABF build-up on both sides, which is the structure that most people mean when they talk about an ABF substrate.

Cost and Availability

BT laminate is produced by many suppliers and priced like a specialty PCB material, so it is accessible and predictable. ABF is produced by a small number of suppliers, and its availability has been a constraint on advanced packaging capacity. Its cost per unit area is far higher, and it is processed with tighter cleanliness and inspection requirements, which adds to the effective cost. This is why designs push the fine layers only as far as they must: every additional ABF build-up layer adds cost, cycle time and warpage risk, so the layer count is minimised while still meeting the escape routing requirement.

Substrate material choices sit at the boundary between packaging and the board, so they should be decided with the package and the assembly in mind. Review how PCB manufacturing handles the board side of the interface, keep the fan-out inside achievable fine line rules in your PCB design and layout, and check the design and manufacturing considerations before release. A prototype PCB assembly run with the real package confirms that the BGA footprint and the board fan-out agree.

FAQ

Is ABF a replacement for BT? No. ABF is a build-up film and BT is a core material, and high end substrates commonly use both.

Why is ABF needed at all? Because glass free film can be laser drilled and patterned to micron scale features, which a glass reinforced laminate cannot.

Which is better for high frequency? The build-up film gives better controlled electrical properties at fine geometry, but the whole stack has to be designed for the frequency of interest.

Why is ABF in short supply? Because few suppliers produce it and the process is specialised, so advanced packaging capacity depends on it.

Conclusion

BT resin and ABF are not rivals but partners. BT forms the rigid core that gives a substrate its stability and holds moderate density routing, while ABF forms the fine build-up layers that connect to a modern die. Choose the core for stiffness and cost, add ABF only as many layers as the escape routing demands, and control warpage by balancing the copper and the cure. Done that way in 2026, the substrate will survive assembly and thermal cycling without bowing or cracking.