<< Click to Display Table of Contents >> Navigation:  Background Theory > PCB Design > How are printed circuit boards made? > The PCB Build-Up

The term "PCB build-up" refers to the process and technique of adding layers to create a multilayer PCB (Printed Circuit Board). This term is often associated with high-density interconnect (HDI) PCBs, which are compact boards with a higher circuitry density than traditional PCBs.

In the context of PCB design and manufacturing, a build-up refers to the layer structure of a PCB. A simple two-layer PCB has a "1-2-1" build-up, which means there's one layer of insulating material (substrate), two layers of conducting material (usually copper), and another layer of insulating material.

For multilayer PCBs, the build-up gets more complex. For example, a four-layer PCB might have a "1-2-2-1" build-up. The numbers represent the sequence of conductive and insulating layers, starting from the top of the board and going to the bottom.

The complexity of PCB build-ups has increased with advancements in technology. Many modern electronics, such as smart-phones and high-speed computers, use HDI PCBs that feature multiple layers and advanced build-up techniques.

Here are a few key terms related to PCB build-ups:

Core

This is the base material, usually an FR4 substrate with copper layers on both sides.

In the context of printed circuit boards (PCBs), the term "core" refers to the base material used in the construction of multilayer PCBs. A core is essentially a double-sided PCB (copper on both sides) with a substrate layer sandwiched between. The substrate layer is typically made from FR-4, a type of fiberglass-reinforced epoxy laminate which is excellent for providing mechanical strength and is a good electrical insulator.

The copper on either side of the core serves as the conductive paths (or traces) for electrical signals. This copper is usually applied to the substrate in a very thin layer, which can be etched away in a pattern to form these traces.

In a multilayer PCB, several cores can be stacked together with additional layers of copper and insulating material (known as "prepreg") to achieve the desired number of conductive layers. Each core within the multilayer PCB is separated by a layer of this prepreg.

The cores in a PCB are usually specified by their thickness and the weight of the copper cladding. For example, a "1.6mm, 1oz copper" core has a substrate thickness of 1.6mm and a copper layer weight of 1oz per square foot. The thickness of the core can impact the rigidity and strength of the final PCB, while the weight of the copper affects the electrical characteristics of the board.

Prepreg

Prepreg, short for "pre-impregnated," is a type of material used in the manufacturing of multilayer printed circuit boards (PCBs). It's used as an insulating layer between copper layers and also to bind the layers together. It consists of fiberglass impregnated with resin that is partially cured, making it tacky and malleable under normal conditions.

In the context of PCB manufacturing, prepreg is used as an insulating layer between the conductive layers (or cores) of the PCB. When the PCB is subjected to heat and pressure during the lamination process, the resin in the prepreg flows, sticks to the cores, and then fully hardens, effectively gluing the different layers of the PCB together. The fully cured prepreg also serves as an excellent electrical insulator between the conductive layers of the PCB.

The thickness of the prepreg and the amount of resin it contains can be adjusted to control the final thickness and dielectric properties of the PCB. Several sheets of prepreg might be stacked together to achieve the desired thickness.

Like cores, prepregs are often specified by their thickness and resin content. For example, a common prepreg specification might be "7628," which refers to a type of heavy-weight fiberglass cloth. Other common prepreg types include "1080" (light-weight fiberglass cloth) and "2116" (medium-weight fiberglass cloth).

It's important to note that the handling of prepreg requires careful control over storage conditions (it must be stored in cool conditions to prevent premature curing) and manufacturing processes to ensure a reliable and high-quality PCB.

Sequential Build-up (SBU)

Understanding the build-up of a PCB is crucial for the PCB design process, as it impacts the electrical performance, signal integrity, mechanical strength, and manufacturing cost of the final product.

Sequential Build-Up (SBU) is a method used in the manufacturing of high-density interconnect (HDI) printed circuit boards (PCBs). As the name suggests, in the sequential build-up process, layers are added sequentially, one on top of another, to build a multilayer PCB.

Here's a basic overview of the SBU process:

Start with the Core: The process starts with a conventional double-sided or multilayer PCB as the core.

Add Insulating Layer: An insulating layer (typically prepreg) is laminated onto the core.

Add Copper Layer: A thin layer of copper is added on top of the insulating layer.

Pattern the Copper: The copper layer is then patterned using photo-lithography to create the necessary circuit traces.

Drill and Plate Vias: Microvias are then drilled (usually with a laser) through the insulating layer to the copper layer beneath. The vias are then plated to provide an electrical connection between layers.

Repeat as Necessary: Steps 2-5 are repeated for each additional layer. In most HDI boards, several layers of copper and insulation are added.

Final Lamination: Once all layers have been added, the board goes through a final lamination process to ensure all layers are properly bonded together.

The advantage of the SBU process is that it allows for the creation of very high-density PCBs, with multiple layers of fine circuit traces and small vias. This makes it possible to create smaller, lighter, and more complex PCBs, which is why SBU is commonly used in devices like smart-phones and high-speed computers.

However, SBU is more complex and costly than traditional PCB manufacturing processes. It requires specialized equipment (like laser drilling machines) and careful process control to achieve reliable results. It's also important to work with a PCB manufacturer that has experience with SBU to ensure the best results.