BT Resin versus ABF Substrate: Material Comparison

Two material systems dominate the package substrate industry, and they are used together as often as they are compared. One is a rigid laminate used as the core of the substrate, and the other is a film laminated onto that core to build fine geometry. Understanding which one sets which property explains most of the design limits.

The Two Materials and Their Roles

BT resin is a bismaleimide triazine laminate, a rigid material reinforced with glass fabric and used as the core of a package substrate. It provides mechanical support, dimensional stability and the coarser routing that connects the fine layers to the ball array.

ABF is a build up film, a resin coated onto a carrier and laminated as a thin dielectric layer over the core. Each film layer is laser drilled and plated to create a wiring layer, and it is the medium in which the fine lines and microvias near the die are formed.

Why They Are Used Together

The combination exists because the requirements differ by layer. Near the die, the pitch is tiny and the routing density is high, so a thin film that can be drilled cleanly and plated with fine lines is needed. Farther from the die, the traces are coarser and the layer has to provide mechanical stiffness, which a thin film cannot do.

A stack therefore alternates: a rigid core in the middle, build up layers above and below it, and the finest geometry in the outermost layers. The layer count on each side of the core is usually balanced to keep the panel flat, which is the constraint that limits how many layers can be added on one side only.

Dielectric Properties

BT resin has a glass transition temperature above one hundred and eighty degrees Celsius in the grades used for substrates, which is what allows the repeated lamination cycles of a build up process. Its dielectric constant is around three point eight to four point two depending on the grade, and its dielectric loss is moderate rather than low. Materials with lower loss are available where the substrate carries high speed signals, and the trade is cost and process difficulty.

ABF films are available in a range of dielectric constants and loss figures, and the choice is made for the layer where the signals actually travel fastest. Because the fine layers are the ones that carry the die-side interfaces, their electrical properties matter most for insertion loss and crosstalk. Where a high speed link passes through the substrate, the loss budget is a system figure, and the comparison of a multilayer PCB for high speed sets out the same reasoning at board level.

Fine Line Capability

BT resin on its own cannot support the finest geometry, because its glass reinforcement and its thickness prevent the resolution needed and the surface is not flat enough at that scale. It is used for lines coarser than the fine layers require.

ABF is designed for the fine layers. Its surface can be planarised, it is thin enough for a laser microvia with a small diameter, and it accepts the semi-additive plating process that produces nearly rectangular conductors. The line widths achievable in an ABF layer are a fraction of those in the core, which is the reason for the whole construction. The plating control this demands is described in electroplating and via filling.

Expansion and Warpage

BT resin has a coefficient of thermal expansion in the region of fourteen to seventeen parts per million in the plane of the board and much higher in the z direction, depending on the glass content and the resin.

ABF film has a higher expansion coefficient than the core and a lower stiffness, so a stack with many film layers tends to warp more. The balance between the two sides of the core, and the copper coverage on each layer, are what keep the finished substrate flat. Warpage is the dominant yield risk in a high layer count substrate rather than an inconvenience, and it is treated as a primary design constraint.

Moisture and Process Behaviour

Both materials absorb some moisture, and the absorbed water turns to steam during reflow. The consequence is delamination between layers or between the film and the core, and the risk rises with the number of lamination cycles and with the total thickness of the stack.

Drying before lamination and reflow is part of the process, and the storage conditions between steps matter as much as the material specification. A stack with many layers accumulates more moisture across more interfaces, which is one of the reasons high layer count substrates are more difficult than the same technology at eight layers.

Where Each Limits the Design

The core limits the mechanical design: its thickness sets the stiffness, its expansion sets the warpage behaviour, and its dielectric properties set the performance of the layers that run through it. Its routing capability is coarser, so the escape routing from the ball array is planned around it.

The film layers limit the electrical design. Line width and spacing, microvia diameter and stack rules, and the number of layers that can be laminated before the yield falls, are all set by the film and the process around it. Designers working at the limit of the technology typically find that the film determines what is possible and the core determines what survives assembly. Where the two conflict, the stack rules used to connect them, described in the material on blind and buried via stack selection, define what the process can actually build.

Choosing Between Them

The question is rarely one or the other. A substrate needs a core for mechanical integrity and film layers for density, and a design that tried to use only one would lose either stiffness or resolution.

The choice that does exist is within each family. A designer selects the core laminate for its expansion and dielectric performance, and the film for its resolution, its expansion and its ability to be processed at the required layer count. Those selections are made with the substrate supplier, because the process is what makes the material work, and the interface between the design and the process is where most of the risk sits.

Applications

Substrates of this kind carry processors, graphics devices, networking chips and the high bandwidth memory interfaces that sit beside them. Chiplet architectures, where several dies share a substrate, increase both the area and the routing density and make the layer count rise further.

Automotive computing and high performance sensors use the same technology at smaller sizes. In each case the driver is the connection count, and the construction follows from the density rather than from a preference for either material.

FAQ

Is BT resin a build up film? No. It is a rigid glass reinforced laminate used as the core. The build up film is a separate material laminated over it.

Can a substrate be made with film only? Coreless constructions exist, with film layers on both sides of a temporary carrier, and they are used where the thinnest possible package is required. They are more difficult to handle and more sensitive to warpage.

Which material limits the layer count? Usually the film and the process around it. Each additional layer pair adds lamination cycles, more opportunities for misregistration and more residual stress, so yield falls as the count rises.