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In high-speed, high-frequency electronic equipment, the PCB substrate serves as the core foundation for electrical conductivity, signal transmission and structural support; the choice of materials and the lamination manufacturing process directly determine the circuit board’s signal integrity, thermal stability and service life. The combination of conductor and insulating dielectric materials, together with standardised lamination processes, is key to ensuring high-performance, highly reliable mass production of PCBs.
The performance advantages of PCB substrates stem from the rational combination of conductors and insulating media, the selection of differentiated materials, and standardised lamination processes. Conventional FR-4, high-end BT resin, and ABF laminated films each offer distinct performance and cost advantages, catering to circuit transmission requirements at various levels; meanwhile, the manufacturing logic centred on the core board—involving layer-by-layer stacking, cyclical processes, and overall lamination—is the cornerstone of precision structural formation in multilayer PCBs.
Compared to metal conductors, which are relatively homogeneous in composition, PCB substrates offer a far wider variety of dielectric materials to choose from. The most widely used of these are thermosetting epoxy resins (thermosetting resins), which cure and set upon heating and cannot be remelted or reshaped.
FR-4 is currently the most widely used general-purpose resin substrate in both the consumer and industrial PCB sectors. The ‘FR’ denotes flame-retardant properties, whilst the number ‘4’ refers to the material specification code defined by the National Electrical Manufacturers Association (NEMA). This material is a glass fibre-reinforced composite, utilising glass fibre cloth as a matrix which is impregnated with epoxy resin and cured to form the final product. It offers comprehensive advantages including flame retardancy, electrical insulation, structural stability and high cost-effectiveness.
For high-end applications involving high frequencies, high temperatures and high reliability, the industry is increasingly adopting BT epoxy resin (bismaleimide-triazine resin). Compared to conventional FR-4 materials, the key advantages of BT resin lie in its heat resistance and high-frequency electrical performance. The industry typically measures the heat resistance grade of resins by their glass transition temperature (Tg), a parameter representing the critical temperature at which the resin loses its rigid structure and begins to soften and flow. BT resin possesses a higher Tg value, offering greater structural stability in high-temperature environments. At the same time, its lower dielectric constant effectively suppresses interlayer signal crosstalk, thereby enhancing the quality of high-speed signal transmission.
Both FR-4 and BT resins have corresponding prepreg products (pre-impregnated materials). The principle behind prepreg preparation involves thoroughly impregnating a glass fibre substrate with epoxy resin, followed by semi-curing to maintain a stable, flexible sheet-like form. During PCB lamination, the prepreg can be directly laid as the interlayer insulating medium; once the entire laminate structure is arranged, high temperature and pressure are applied to achieve complete curing and interlayer bonding. This process is convenient and ensures high lamination consistency.
ABF laminated film (Ajinomoto laminated film) is a new, high-end dielectric material launched by Ajinomoto Co., Inc. of Japan. It outperforms traditional epoxy resin materials in terms of electrical performance and thermal stability, and is specifically designed for high-performance PCB substrate applications involving high-speed, high-frequency and high-density packaging. ABF material is supplied in roll form, with protective films attached to both sides upon delivery. One side features an OPP (orthophenylene terephthalate) film, which is peeled off prior to lamination; the other side features a PET (polyester) film, which is removed once the layers have been positioned. Certain grades of ABF film are pre-coated with copper foil prior to shipment, allowing them to be used directly for the fabrication of fine-pitch circuits.
In addition to lower dielectric loss and superior high-frequency characteristics, ABF’s coefficient of thermal expansion (CTE) is highly compatible with core substrate materials such as copper foil and core boards. Under long-term thermal cycling conditions in equipment, inter-layer stress variations within the substrate are minimised, significantly reducing the risk of defects such as cracking, delamination and circuit failure, and greatly enhancing the long-term reliability of high-end packaging circuit boards. The sole drawback is that the material and manufacturing costs are significantly higher than those of traditional FR-4 and BT resin solutions.
In multi-layer PCB design, it is not necessary to use a single dielectric material throughout the entire board. Engineers can select different types of insulating resin materials based on the signal speed, frequency characteristics and impedance requirements of each layer’s circuitry, thereby achieving a precise balance between performance and cost.
PCB Substrate Lamination Process Flow
Step 1: Substrate Preparation
A double-sided copper-clad, metallised core board is used as the base substrate, serving as the core structural support for the entire multi-layer board.
Step 2: Through-hole Drilling and Plating
1. Depending on the required hole diameter precision, either mechanical drilling or laser drilling is used to machine the holes;
2. The hole walls are degummed, degreased and cleaned to remove drilling residues and adhesive debris, ensuring the walls are clean;
3. A thin copper seed layer is deposited on the hole walls via a chemical copper plating process to create a continuous conductive substrate, providing a current path for subsequent electroplating;
4. The hole walls and board surface are electroplated as a whole to thicken the copper layer, forming a reliable conductive layer.
Step 3: Photolithography of the metal layer (circuit formation)
1. Apply a uniform coat of photoresist to the surface of the copper foil on the board;
2. Using a photolithography exposure unit, precisely transfer the pre-designed circuit pattern onto the photoresist layer;
3. Develop the photoresist to remove it from the corresponding areas, exposing the excess copper foil to be etched away;
4. Remove the exposed copper layer via chemical etching, retaining the circuit pattern required by the design;
5. Peel off all residual photoresist, thoroughly clean the board surface, and complete the fabrication of the single-layer circuit.
Step 4: Multi-layer Stacking
If the product requires additional layers, resin-impregnated laminates and copper foil conductive layers are sequentially stacked on top of the current formed layer structure to expand the number of substrate layers.
Step 5: Cyclical Processing
Repeat the full set of processes—through-hole processing, hole metallisation and circuit patterning—to complete the fabrication of microvias, circuits and interconnections layer by layer, until the entire laminated structure is fully processed.
Step 6: Overall Laminating and Curing
Once all laminated structures have been laid out, a high-temperature, high-pressure lamination process is applied to ensure thorough curing of each dielectric layer and tight interlayer bonding, forming a structurally complete, performance-stable, integrated multilayer substrate.
The quality of a PCB substrate is determined by both the choice of dielectric material and the lamination process. The appropriate selection of dielectric materials and strict control of each manufacturing step can effectively improve the signal transmission quality and service life of the PCB, thereby meeting the requirements for the stable operation of various electronic devices.