Prepregs are useful in PCB design and PCB fabrication. Also, these dielectric materials offer several purposes. PCB manufacturers modify these materials to suit their needs. There are different types of PCB prepregs such as 106, 2116, 7628, and 1080. However, we will focus on prepreg 1080 vs prepreg 2116 and compare their differences.
What is a Prepreg?
Prepreg is a reinforcing fabric pre-impregnated with a resin system. The resin system is an epoxy prepreg that has a proper curing agent. Prepregs are often used in the manufacturing of printed circuit boards. This PCB materials is usually considered as the most advanced system of composite fabrication.
Its production involves adding pressure and heat. The use of prepreg fabrics helps to get rid of the process variation that occurs in traditional layups. Prepregs are widely used due to their high performance in fiber reinforced structures. This prepreg material has revolutionized composite fabrication.
Prepregs comprise a reinforcing fabric combined with a semi-cured resin system that. With a well calculated weight ratio for prepregs, it is possible to achieve light-weight characteristics in a composite laminate. The choice of prepreg depends on characteristics like weave selection, fiber, and tow size. These characteristics are similar to that of traditional fabrics.
Prepregs feature a defined “cured ply thickness” and density. Also, the resin included to the fabric enhances the efficiency of the layup processes. This is because the resin plays a significant role in the layup processes. Prepregs are unique materials as high pressures and temperatures are used in curing them.
What is a Prepreg in PCB Fabrication?
In PCB fabrication, prepregs are commonly used. A prepreg is simply an insulation layer in a circuit board. It is a dielectric materials placed between two cores in a circuit board. A prepreg offers the needed insulation in a circuit board. Therefore, it is also referred to as a binding prepreg material. A prepreg offers bonding between a core and a copper foil or two cores.
Prepregs are useful in PCB design and PCB fabrication. Also, these dielectric materials offer several purposes. PCB manufacturers modify these materials to suit their needs. It is possible to transform a particular section of a prepreg into a conductive region by simply combining some catalysts and additives under a chemical process.
This is useful when drilling holes in the prepreg. The basic functionality of a prepreg in a circuit board is to offer protection from short circuit.
How Prepregs are manufactured
There are two different processes of manufacturing prepregs. These are the solvent dip process and the holt melt process.
Solvent dip process
The solvent dip process is ideal for producing fabric prepregs only. Here, the resin is dissolved in a solvent and the reinforcing fabric is then dipped in the resin solvent. This solvent evaporates from the prepreg with the use of a dry oven.
Hot melt process
The hot melt process is suitable for producing both unidirectional and fabric prepregs. This process involves two different stages. In the first stage, a thin film of the heated resin is coated. Also, the resin and the reinforcement material will interact in the prepreg machine. After applying heat and pressure, the manufacturer impregnates the resin in the fiber. This produces the final prepreg.
What are the Advantages of Prepregs?
Prepregs offer a lot of benefits. They are more beneficial than traditional handup layup.
Process robustness
Prepregs enable easier and cleaner processing. With a simple ply cut of prepregs, it is easy to get ply shapes that can be laid into mold surfaces. Also, you don’t have to worry about a mess of resin while laying up large plys.
Less curing time
After the completion of the heat curing cycle, you can service the part. Therefore, you don’t have to wait for 2 days to enable total cure.
Unparalleled uniformity and repeatability
Consistency and robustness help to fabricate repeatable parts. Also, prepregs are very consistent. All you need is to pull the backing away from the cut plys. After the completion of the layup process, the curing and bagging process is less complex compared to the infusion process.
Great mechanical performance
Prepreg comprises an optimized fiber to resin ration. This helps to enhance the mechanical performance of prepregs. Prepregs offer the best mechanical properties for a composite design.
Secondary cure or co-cure
Prepregs can co-cure several aspects of a composite laminate in a shot. This is a big advantage of using prepregs. Co-curing enhances performance capability since laminates that are co-cured have stronger bonds.
Types of Prepregs
There are different types of PCB prepregs such as 106, 2116, 7628, and 1080. Each prepreg derives its name from the prepreg fiberglass type used. The above mentioned prepregs are glass fabric types. Also, each prepreg comprises several parameters standard size, cured thickness, resin content, and weight among others.
The 7628 prepreg makes use of carbon fibers rather than traditional fibers like polyester. This prepreg is useful in some applications since it integrates more carbon fibers than traditional fibers. For multilayer circuit board manufacturing, prepregs play a crucial role. Prepreg 2116, prepreg 1080, and prepreg 7628 are glass styles. A large number of PCB fabricators use these glass styles.
However, each of these glass styles has its own limitations. It is always advisable to make enquiry from the engineers of your PCB company to know these limitation. The thickness of each prepreg varies. For instance prepreg 1080 has a pressed thickness of 0.0787 mm (0.003”) while prepreg 2116 has a pressed thickness of 0.135 mm (0.005”).
Prepreg 1080 Vs. Prepreg 2116
The prepreg 1080 and prepreg 2116 are commonly used prepregs in PCB fabrication. These prepregs differ in terms of their characteristics. While prepreg 1080 offers thickness between 60 and 70 μm and a 60% resin content, prepreg 2116 thickness ranges between 90 and 110 μm and it has a resin content of 50%.
Also, in terms of storage and shelf life, prepreg 1080 and prepreg 2116 differs. Prepreg 1080 can be stored for a longer period compared to prepreg 2116. Therefore, prepreg 1080 is a better option in terms of storage. Before prepregs are used for fabricating circuit board, it is important to consider their storage conditions, thickness, and resin content.
Prepreg 1080 contains more resin compared to prepreg 2116. The resin content between these two prepregs varies. Resin content refers to the percentage of resin present in the total weight of the prepreg. A burnout is a method used for testing the amount of resin in reinforced prepreg fiberglass. This method involves subtracting the final weight from the original weight after a burnout.
Prepregs can spend 3 months in a room temperature less than 23 degrees Celsius. In terms of cost, the prepreg 2116 is more expensive than prepreg 1080. This can be attributed to the fact that prepreg 2116 has a greater thickness than prepreg 1080.
Which is better Prepreg 1080 Vs. prepreg 2116?
The choice of the prepreg to use for your PCB fabrication depends on the application requirement. There are several factors to consider when choosing between these two prepregs. For instance, cost is an important consideration. Prepreg 2116 is more expensive than prepreg 1080. Aside from cost, the thickness and resin content of a prepreg are factors to consider.
Disadvantages of Using Prepregs
Despite the benefits of using prepregs, these PCB materials have their drawbacks which include:
High cost
Prepregs are more expensive than a resin and a fabric. Asides from the high performance a prepreg offers, prepregs are still the best financial option. Prepregs are costly since they require a lot for their manufacturing. An autoclave system or controlled oven needed to cure prepregs needed for fabricating parts contribute to the high cost of prepregs.
Shelf life
One of the disadvantages of prepregs is that they can only be stored for about six months. It has a short shelf life. Although this doesn’t apply to all prepregs, pregregs have various quality aspects. Regardless, prepreg can be cured with heat and storing them at a warmer temperature can minimize their shelf life. Therefore, it is advisable to keep these materials in a cooler temperature. Freezing is a good way of extending their shelf life.
Heat cure
A prepreg needs a heat source at a minimum. Also, manufacturers must sustain the temperature for at least four hours while achieving a minimum of 270 degrees fahrenheit. In most cases, PCB fabricators make use of autoclaves.
What is a Prepreg Resin Flow?
The prepreg resin melts and flow when it is heated under pressure. This is usually referred to as resin flow. The resin flow depends on the pressure and the heat used by the resin. Resin has a limited time for which it can flow freely. After being a fluid for a particular time, the resin becomes gelled.
The prepreg resin flow is crucial to the lamination operation and the lamination conditions can affect this flow. Also, the actual flow will impact such properties as there forms a bond between the interlaminar bond and the oxide inner layer.
How to Choose Prepreg in PCB Design Stage
You will achieve a smooth surface when your lightweight fiberglass fabrics feature high resin contents. Also, these fabrics offer improved internal copper etched patterns and better filler. Furthermore, fabrics with heavy weight tend to be less expensive and as well offer better dimensional stability. This enables greater thickness at less cost. Integrating heavier fabrics like prepreg 7628, can have a negative impact on the drilling characteristics and smoothness. Most times, heavy fabrics like 7628 are integrated in rigid copper clad laminates to reduce cost. However, this can cause defect in small drills or make them to break.
Prepregs vary in thickness and resin content. For instance, prepreg 1080 vs prepreg 2116 have different thicknesses. Furthermore, resins are available in various versions which include; standard resin, medium resin and high resin. The classification of these versions depends on the resin content. Also, features such as dimensional stability, final PCB thickness, cost, dielectric constant, and cost determine the possible material used.
Prepregs with greater thickness are more expensive. Also, prepregs are available in different glass transition temperature and halogen free types. Therefore, it is advisable to always state the final circuit board thickness if you are ordering for a custom layer buildup.
Other Types of Prepregs
There are other types of prepregs which are thermoplastic and thermoset prepregs.
Thermoplastic prepregs
Thermoplastic prepregs are fiberglass in which thermoplastic resin is used in pre-impregnating them. Examples of resins used for thermoplastic prepregs are PPS, PP, PEEK, PE, and PET. These types of prepregs are available in woven or stitched fabrics. Also, they can be available in unidirectional shape.
There is a difference between a thermoplastic prepreg and a thermoset prepreg, The underlying difference is that thermoplastic prepregs always remain stable when they are at room temperature. Also, these prepregs don’t have a long shelf life.
Thermoset prepregs
Thermoset prepregs are commonly integrated in prepreg composite manufacturing. Epoxy prepreg is the main resin matrix for these prepregs. There are other thermoset resins which are used for making prepregs. These include phenolic resins and BMI. At first, the thermoset resin is a liquid before it impregnates the fiber reinforcement.
Furthermore, there is a need to remove excess resin from the reinforcement. On the other hand, the epoxy resin goes through a partial curing and as such the resin transforms from being liquid to solid. This is usually considered as the “B-stage.”
The prepreg resin cures partially at the B-stage. This process is always tacky. The resin goes back to a liquid state when it is exposed to an elevated temperature after which it becomes completely hardened. Once the resin has cured fully, the thermoset resin in the b-stage becomes cross-linked.
Conclusion
Prepreg plays a crucial role in the fabrication of a printed circuit board. This PCB material is available in different sizes which vary in resin content and thickness. We discussed the importance of prepreg and how they are manufactured in this article. Also, we focused on the difference between prepreg 1080 vs prepreg 2116.
