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. We choose these based on the capabilities needed to fabricate the PCB. 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 serving as a substrate material for HDI PCBs, it can also act as a core or buildup layer in IC packaging. The examples below show some builds that involve BT epoxy.

BT Resin Material Properties Overview

The primary application of BT epoxy is in multilayer circuit board builds. It works as an acceptable, and sometimes more advanced, alternative to many FR4 grade materials. The table below shows this. This material set is also widely available from several vendors. However, fewer boards use BT epoxy laminates than other FR4 resin 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 important. This is true when BT epoxy-based laminates are used on the outer layer of the PCB for microstrip printed circuits.

Core or Prepreg in HDI?

You can use BT epoxy laminates as either a core layer or 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 uses a conventional build process with a mechanically drilled buried via across all inner layers. The inner layers will use alternating prepreg and core materials.

You could then use BT epoxy as the outer layer if you can get it 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 works well as a core in an HDI build, it also suits packaging. You can use it in chip-scale packaging (BGA) or as a BGA substrate.

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 replace FR4-based materials for better reliability 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 is the movement of material such as copper or plating materials, caused by a high electric field between two points in the PCB. In PCB substrates, electromigration can cause dendrite growth between two conductors. They form 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 rely on the full Allegro X tool set. Allegro X is Cadence’s leading PCB design and analysis software. It offers many product design features. It also includes full management and version control tools.