What is IC Substrate – All You Need to Know

The IC substrate plays a critical role in electronics. It supports semiconductor chips, attaching them to PCBs and allowing the two to communicate. However, the component is one of the less understood parts of a chip’s technology. Below, we unveil the science behind its design, construction, and application. Read on to learn about it in detail.

What is IC Substrate?

An IC substrate is the tiny board at the base of an integrated circuit (IC). It’s typically a scaled-down version of the regular PCB but more complex and filled with micron-level components and interconnects.

The substrate serves various functions. One of them is protecting the IC chip from physical and thermal damage. It also provides the mechanical support the chip needs to maintain its form.

More importantly, it provides the connections that electrically attach the chip to the larger circuit boards. This connection allows the tiny chip to interact with the larger PCBs’ components.

As you can see, the IC substrate PCB is the foundation of modern electronics. Packaging integrated circuits with the substrate allows manufacturers to produce devices for data storage, processing, and other functions.

IC Substrate Characteristics

IC substrates are unique circuit boards. They are subtly different in design and performance from a PCB. A few features characterize them. They include the following

• Small-sized, with typical dimensions being 150 mm by 15 mm
  
• Extremely thin, with profiles ranging from 1 mm to 1.5 mm
  
• High component density and thousands of interconnects
  
• High reliability and dependability, given its critical function in electronics
  
• Higher durability than typical circuit boards
  
• Available as either rigid with a core material in the middle or flexible or flexible and coreless
  

What is an IC Substrate Made Of?

A few parts make up the IC substrate structure. They include a central core, dielectric layers (on both sides), and some conductive paths. There are also tiny holes called vias and tiny contacts made of conductive pads or solder balls.

Supporting Core

In the middle is a supporting core. The core is typically resin mixed with glass fibers or a ceramic material. It can also be glass and, if needed, flexible polyimide. It forms the substrate and holds all other parts, including the laminated layers.

Insulating Layers

The insulating layers are bonded to the core on either side to create a multi-layer board. They are typically thin laminates with a copper layer for the conductive traces.

Conductive Traces

The traces make the substrates circuitry or conductive paths that transmit signals to and from components. The traces are typically created by etching a thin copper layer or any other method, such as laser.

Drilled Holes

Tiny holes called vias connect the different layers, routing traces to allow more interconnections in the multilayered structure of the substrates. The vias are laser-drilled, owing to their tiny diameters.

Contact Pads/Pins

These are microscopic components arranged on the substrate’s surface. They make contact with the chip on one side and the motherboard on the other, allowing the transfer of signals between the IC and application PCB.

IC Substrate Vs. PCB

The IC PCB is a smaller version of standard printed circuit boards used in electronic devices and systems, except for a few differences in size and construction. A look at the variations should paint a clearer picture.

Size and Structure

The IC substrate is many times smaller than the conventional circuit board. It also features a middle core and layers of copper-clad resin on either side. The usual PCB is thicker and has a larger area. It also usually has copper-bonded laminates between pre-preg layers.

Function

Integrated circuit substrates package chips. During use, they connect the chips to larger PCB structures like traces. On the other hand, conventional circuit boards hold many different components, including semiconductor chips.

Fabrication and Cost

The IC substrate fabrication is more complex. The board requires advanced technologies and equipment to design and produce, increasing manufacturing costs. The ordinary board is less complex and comparably cheaper to make.

IC Substrate Classification

IC substrates are available in many different types. We mainly classify them by their material types, packaging technology, and bonding method. Each category has a few subtypes, among other variations. Here are the different IC substrate types explained.

By Material

IC substrate materials are either rigid or flexible. Rigid types have a supporting core in the middle for strength and stability. Flexible types are coreless and instead made of layers of dielectrics.

• Rigid boards are resin and glass fiber, ceramic, or glass. The types of resins include epoxy, Bismaleimide Triazine (BT), and Ajinomoto Build-up Film (ABF)
  
• Ceramic materials can be aluminum oxide, silicon carbide, or aluminum nitride
  
• The flexing type is typically polyimide. This IC packaging substrate material has a higher temperature withstand and other properties
  

By Packaging

This category describes how the chip mounts on the substrate. The chips in this group include the ball grid array (BGA), flip chip (FC), and multi-chip module (MCM). There’s also the chip scale packaging (CSP) type.

• The BGA chip uses solder balls instead of pins. The balls are tiny, allowing high contact densities
  
• The FC type has connections on the upper side. It flips to attach to the PCB, which explains its name
  
• On the other hand, the MCM substrate packages more than one chip, allowing more connections in a smaller space
  
• The CSP version holds a single chip. As such, it has fewer contacts than the multichip type
  

By Bonding Method

Boding means the technology the IC substrate PCB uses to attach to the chip. Methods include wire bonding, tape automated (TAB) bonding, and the flip chip (FP) technique.

• As the name suggests, the wire method involves wires, which can be gold, silver, copper, or aluminum
  
• The TAB method uses a pressure-sensitive adhesive to bond the chip to the board, with conductive bumps forming the electrical connections
  
• FC bonding uses solder balls and conductive bumps on the chip’s upper surface. It flips to bond it with the board
  

How are IC Substrates Made?

IC substrate manufacturers use various techniques to produce the component. They include the subtractive, additive, and modified semi-additive processes. Let’s see how each method works.

Subtractive Process (SP)

This process removes material using chemicals to create the circuit. It begins with plating the core with a thin copper layer before covering the surface with a resistive mask. Later, the exposed parts are etched out, leaving the circuit’s pattern untouched. It has lower accuracy due to lateral etching.

Additive Process (AP)

It involves layering the core with a resisting mask and plating it with copper to create the circuitry. The additive process is more accurate than the subtractive process, making it best for high-resolution circuit requirements.

Modified Semi Additive Process (MSAP)

The MSAP method is an additive process with a bit of etching. It begins with covering the core with a resistive mask and layering it with copper to achieve the desired circuit. The resulting structure is then etched to refine it and create higher resolutions.

IC Substrate Manufacturing Process

IC substrate fabrication is a highly automated system. It uses advanced equipment and technology and occurs in a clean environment to avoid contamination. We traced the manufacturing steps after design completion, from beginning to end.

• Copper-layered laminates are sized and cleaned to remove impurities. These will form the substrate’s core
  
• A layer of resin is bonded to the laminate, and tiny holes are drilled into it using lasers
  
• The laminate goes through a process to rid it of debris from the drilling process
  
• The drilled holes are then plated to make them conductive, allowing them to connect the layers electrically
  
• The drilled cover is layered with a photo-resist mask to prep it for etching
  
• The photosensitive film is exposed to light to harden at desired places before etching
  
• The surface is then etched using chemicals to reveal the circuit’s pattern
  
• The board undergoes a few additional steps, which include flash-etching
  

IC Substrate Technology Challenges and Innovations

Substrates for IC packaging use microscopic structures. This construction makes them complex and more challenging to produce. Performance requirements also keep pushing design and fabrication limits.

For instance, increased connections force manufacturers to create thicker boards or find technologies to shrink components. As a result, high-resolution methods, such as laser direct imaging, are becoming more popular, edging out the conventional lithographic process.

Other changes are in the materials. Today, manufacturers are using thinner materials like Bismaleimide Triazine (BT) to replace ceramics. Fabrication, inspection, and testing technologies are also improving. One of them is X-ray imaging for hidden defects.

IC Substrate Applications

Modern PCB is a multipurpose device with high-speed circuits and other capabilities. As such, the average board will have a minimum of one integrated circuit (IC). Application examples include:

• Process chips
  
• Memory chips and modules
  
• Micro-electromechanical systems (MEMS)
  
• Radio frequency (RF) chips
  
• LED chips
  

The above-mentioned integrated circuit chips are invaluable in electrical and electronic devices or systems today. The following industries depend on them.

• Consumer electronics
  
• Motor vehicle systems
  
• Industrial machinery
  
• Telecommunication equipment
  
• Medical devices
  
• Avionics, including space vehicles
  
• Military or defense electronics.
  

Conclusion

The IC substrate is an essential part of semiconductor chips. It supports, protects, and helps conduct heat away and dissipate it. It also electrically connects the chips to PCBs. As this guide outlines, the board also has many applications in electronics. Uses range from packaging simple ICs for consumer devices to advanced types for computing systems.