Role of Prepreg in Multilayer PCB Manufacturing

A PCB stackup arrangement includes layers, planes, cores, base, substrate, laminate, and prepreg. From Core we can say FR4, Aluminum, Rogers and all. On the other hand prepregs are the dielectric material between the adjacent cores or the core and a layer. In multilayer PCBs, prepreg is a vital part that holds the PCB core and layers together. Once the difference between prepreg and core is clear, what exact material should you use for your application? How do the important electrical parameters change during plating, etching, and curing?

The best prepreg material for a given multilayer PCB depends on the thickness, layer structure, and impedance. The three types of prepreg available based on the content of resin present are high resin (HR), medium resin (MR), and standard resin (SR). This article discusses multilayer boards and PCB stackups as well as what prepreg is and the role it plays.

1.    Prepreg in PCB Manufacturing?

Prepreg is short for pre-impregnated composite fiber, which refers to fiberglass cloth that has been pre-impregnated with a partially cured resin system. As its name reveals, a prepreg is nothing more than an insulating film. The heat transfer allows prepreg a dielectric material to be inserted between two cores or a copper foil and core in the preparation of PCB, to provide the required insulation. It acts as the bonding layer between copper-clad laminates in multilayer PCBs for both electrical insulation and physical support.

Prepreg is responsible for tuning the desired electrical properties such as controlled impedance depending on their dielectric constants. The mechanical properties are also reinforced to keep the PCB reliable and durable. The prepreg acts as an adhesive that bonds the layers together while providing electrical insulation. Because the resin is only partially cured, when heat and pressure are applied during lamination, the resin fully cures and solidifies, creating a strong, uniform bond.

2.    How does Prepreg Selection Influence the PCB?

Prepreg selection plays a critical role in determining the quality, integrity of lamination bond and heat dissipation on PCBs. At high-frequency in a design both impedance and signal losses can be influenced by the relative permittivity. This is important to be sure that the resin will flow properly between each lamination. Choosing prepreg with the right Glass Transition Temperature (Tg) for thermal management allows PCB consumed operating heat, not bending or cracking during use.

3.    Types of Prepregs Used in PCB Manufacturing

There are various types of prepregs used in the manufacturing of multilayer PCBs, each suited for different requirements:

1. Epoxy Prepreg:

Epoxy prepregs are the most commonly used in PCB manufacturing due to their excellent adhesion properties, high mechanical strength, and low dielectric constant. Epoxy prepregs are ideal for applications requiring high reliability and performance, such as consumer electronics and telecommunications.

2. Polyimide Prepreg:

Polyimide prepregs are often used for high-temperature applications due to their superior thermal stability and resistance to extreme environmental conditions. These prepregs are ideal for aerospace, automotive, and military applications.

3. Halogen-Free Prepreg:

For environmentally conscious designs, halogen-free prepregs are an alternative to conventional materials. These prepregs help in reducing the environmental impact of the PCB manufacturing process and are used in industries that require environmentally safe and sustainable electronics, such as medical devices.

4.    Manufacturing Process of Prepreg:

The manufacturing process must be both comprehensive and exact to assure the final prepreg for PCB meets the standard requirements.

1. Impregnation Process

This process makes a prepreg, which also acts as an adhesive and dielectric by coating the fibreglass fabric with resin. Because the high-grade polyester resin coats a fibreglass fabric, impregnation must be carefully controlled to ensure an even depositing of resin across each layer.

Fibreglass Fabric Preparation: The operation described here is focused on what is called the reinforcement material or support fabric which gives mechanical strength; This fabric is generally woven to a density dictated by designed mechanical properties for the prepreg product.

Bath of epoxy resin: The fibreglass passes through a bath of epoxy resin with perhaps some additives to improve the properties, such as flame resistance or flexibility. Although the composition of resin can vary depending on application, for optimal insulating and bonding epoxy is most popular.

Partial Curing (B-staging): Resin is partially cured (to the B-stage). At that point, the resin is tacky and not completely cured which permits it to flow in a laminating press during PCB assembly that helps make strong interlayer connections.

2. Drying and Curing

After impregnation, the next steps applied for drying and curing to ensure that resin is well cured and maintained its bonding ability in lamination. This is an essential stage in the regulation of a perfect final prepreg material.

Controlled Drying: The drying concerns the controlled one to which occurs a quick entrance of these resins. It extends on this wire passing through an oven specifically designed for this process. The temp and time inside the drying oven are controlled to be unvarying all over the prepreg for partial curing.

Partial Cure Monitoring: It is important to understand the level of cure as overcure makes a metamorphic transition during lamination difficult, meanwhile under-curing will not provide bond strength. Hence, accuracy in this stage is crucial for the overall quality of prepreg.

3. Cutting and Packaging

After the resin has been cured to specification, it is trimmed and packaged into rolls of prepreg for sale. This pre-shipment is intended to prepare the materials for correct use in PCB manufacturing.

Precision Cutting: The next stage is precision cutting of the prepreg which has dried by now and partially cured (solid-state polymerization). Precise cutting is necessary to enable the prepreg sheets to be aligned perfectly with one another during PCB layering so defects will not appear in the finished product.

Sterile Prepreg Sheets: Prepreg sheets are tagged with epoxy resin content, fibreglass type and thickness. Now for the prepreg, we need to pack it as well in order that we make it clean and uncontaminated before PCB fabrication.

4. Controlling Resin Content

When making a prepreg, one of the most important variables to control is resin content as it has a significant impact on mechanical and electrical properties. Having the right resin-to-fibre ratio allows you to adjust your prepreg for specific designs on PCBs.

Adjusted Resin Amount: The amount of resin is precisely controlled in the impregnation stage since this will affect significantly on final properties of prepreg. More resin offers greater adhesion and insulation, so this is used for high-frequency or higher-speed operations.

Strength and Flexibility: Similarly, on the other extreme is low resin content which makes stiffness so rigid that it can result in less deformation under stress for higher mechanical strength requirements. Developing the resin-to-fibreglass ratio, however not to an excessive extent, is another critical process in PCB production. It gives manufacturers room where they can fine-tune their prepreg for certain mechanical and electrical performance requirements.

This creates a structured system of prepreg manufacture that makes the material ideal for various PCB applications, while ensuring mechanical robustness and electrical performance.

5.    Prepreg Defects in PCB Design:

Delamination: It is a fault where the PCB layers set apart as their grip is not strong enough. Things that are often the cause of this issue: Resin flow on lamination wasn't good enough, moisture contamination or prepreg is the oldest a year. Prevention: store the prepreg properly, control lamination temperature and pressure, use new preparation materials to improve bonding.

Resin Starvation: In this scenario, lack of resin inside the prepreg to fill the regions between layers results into poor adhesion. Prevention: Choose prepreg with a sufficient amount of resin for your type of board and carefully manage how the layers are laminated together so that they get the resin through them properly.

Voids: Voids are little air bubbles caught inside the PCB layers, for the most part brought about by dampness retention or helpless overlay conditions. Prevention: Store prepreg in a desiccant cabinet, pre-acclimate the sheets before the lay-up and be sure to provide even heat and pressure during curing.

Corrosion can be controlled by managing the storage conditions and lamination parameters, which in turn help manufacturers to keep it under shape and thus prevent defects.

6.    Determining PCB Impedance for a Prepreg?

Making sure to choose the correct prepreg thickness for impedance control in PCB designs matters so that signal integrity can remain intact and not degrade with high-frequency applications. Here are key guidelines:

Target Impedance: Begin by defining the target impedance of the design, whether that be 50 ohms for a single-ended trace or perhaps 90 ohms for differential pairs. This will help identify the dielectric constant (Dk) and thickness required.

Calculate the Trace Width and Spacing: A prepreg thickness shall be selected depending on copper trace width spacing. Impedance increases with larger prepreg layers and it decreases when the layer is thin. Having proper trace geometries as well the right prepreg thicknesses are essential for hitting your target impedance requirements.

Choose Accessible Prepreg with a Constant Dielectric: A Change in our prepreg dielectric, the speed of the particular signal differs during propagation. Select a low and stable Dk prepreg material to maintain consistent impedance between different signal layers.

Some may use impedance calculators for PCB: Relate prepreg thickness, trace width and dielectric constant using appropriate simulation model. This helps in getting the proper impedance by adjusting thickness.

7.    What Is the Difference Between Prepreg and Core in PCB?

While both prepreg and core materials are essential components in multilayer PCB manufacturing, they serve different purposes and have distinct characteristics.

Challenges and Considerations

While prepreg plays a significant role in PCB manufacturing, it also comes with certain challenges:

⦁ Quality Control: Prepreg must be carefully controlled during manufacturing to ensure that it meets the required thickness and resin content. Any deviation from the specifications can affect the performance and reliability of the PCB.

⦁ Environmental Sensitivity: Prepreg is sensitive to moisture, which can degrade its properties. Therefore, proper storage conditions are crucial to maintaining its quality and performance.

At JLCPCB, we understand how critical the right prepreg material is to the success of your PCB design，especially when working with high-speed, multilayer boards. That’s why we offer a wide selection of high-quality prepreg and core materials, including FR4, aluminum, and high-frequency substrates like Rogers, to ensure your boards meet both mechanical and electrical requirements.

JLCPCB helps engineers achieve optimal signal integrity and long-term reliability. Whether you’re prototyping or moving to mass production, our advanced manufacturing capabilities and low-cost services make it easier than ever to bring professional-grade PCBs to life.

Conclusion:

Picking the right prepreg is essential in order to maintain a PCB that performs well, meets quality standards and is durable. Prepreg is a bonding and dielectric agent, which enables to keep the boards mechanically stable with proper electrical properties of parts. In high speed design the selection of right dielectric material matters a lot, especially where GHz communication lines are propagating. By understanding the crucial role of prepreg in multilayer PCB manufacturing, engineers and designers can optimize their PCB designs for performance.