BT Epoxy Resin as an Alternative PCB Substrate Material

When you look at the sets of materials used in HDI PCBs, you will find at least 11 options for buildup layers. These are selected based on the various capabilities required to fabricate the PCB, and these materials can provide extremely fine line widths in HDI buildup layers. Finally, these materials double as packaging materials, specifically in multilayer substrates and as dielectric insulators supporting redistribution layers (RDLs).

One of these alternative material sets is constructed from bismaleimide-triazine (BT) resin, or BT epoxy. This thermoset material offers structural rigidity as the core layer for HDI PCBs, or it can be used as an outer HDI buildup layer for advanced boards. In addition to being a suitable substrate material for HDI PCBs, it can be used as a core or buildup layer in IC packaging. Some examples of builds involving BT epoxy are shown below.

BT Resin Material Properties Overview

The primary application of BT epoxy is in multilayer circuit board builds. It performs as an acceptable or somewhat more advanced alternative to many FR4 grade materials, as outlined in the table below. This material set is also commonly available from multiple vendors, although the number of boards produced specifically on BT epoxy laminates is lower compared to other resin-based FR4 materials.

The material properties for an example BT epoxy-based laminate are outlined in the table below.

These values differ somewhat from those most-often cited for FR4-grade laminates. In particular, the Z-axis CTE value and thermal conductivity tend to be of higher interest in many systems. For RF systems, the lower Dk value is of interest when BT epoxy-based laminates are used on the outer layer of the PCB for microstrip printed circuits.

Core or Prepreg in HDI?

BT epoxy laminates can be used as either a core layer and an outer buildup layer in an HDI PCB (i + N + i stackups). A common strategy for its use in HDI buildup layers is in the inner conventionally-built region. This region will use a conventional build process with a mechanically drilled buried via spanning the entire set of inner layers, and these inner layers will be built from the usual alternating prepreg and core layer materials.

BT epoxy could then be used as the outer layer if available in a thin enough prepreg. However, the outer layer will usually be one of the typical HDI buildup materials, such as:

• Laser-drillable prepregs
• Glass-reinforced epoxies
• Liquid crystal polymer (LCP)
• Hardened polyimide
• Ajinomoto build-up film (ABF)

Image showing the usage of BT epoxy as the internal core layer for a 3 + 1 + 3 HDI PCB with ABF build-up layers.

Because the BT epoxy layer is suitable as a core in an HDI build, it can also be used in packaging, specifically in chip-scale packaging (BGA) or as the substrate for a BGA.

Why BT Epoxy?

BT epoxy is most useful compared to FR4 when cost needs to be balanced with greater reliability. Based on the material properties listed above, BT epoxy can be used as a higher reliability substitute for FR4-based materials due to the following primary characteristics:

• Higher thermal conductivity
• Lower CTE value (z-axis)
• High Tg value

The lower CTE value below Tg is important for reducing CTE mismatch. This means that, when the PCBA is thermally cycled, there will be less stress on copper structures, specifically the copper wrap plating on via lands.

Electromigration

There is another quality of BT epoxy that illustrates its higher reliability compared to FR4-based materials: electromigration resistance. Electromigration refers to the movement of material (in this case, copper or plating materials) under the influence of a high electric field between two points in the PCB. In PCB substrates, electromigration can take the form of dendrite growth between two conductors along the PCB material caused by the gradual transfer of momentum of the conducting electrons.

Electromigration locations on electronic components mounted on a PCB. Read more about these processes in Corrosion Science.

BT epoxy is a good choice when the changes of electromigration need to be reduced. In the context of PCBs, electromigration can lead to the formation of dendrites, which can cause circuit failure through short or open circuits. BT epoxy is one of the tools used to help prevent electromigration, including in PCBs running at higher voltages.

Engineers who build advanced PCBs and IC packages with BT epoxy trust the complete set of PCB design tools in Allegro PCB Designer. Allegro 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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