What is Prepreg?

2019-11-28 15:17Writer: qyadminReading:

what is prepreg

    Prepreg composite materials are becoming common in the composite industry. due to their ease of use. consistent properties, and high-quality surface finish. But, there is much to understand about prepregs. before committing to using this material.
     The term "prepreg" is actually an abbreviation for the phrase pre-impregnated. A prepreg is an FRP reinforcement that is pre-impregnated with a resin. Most often, the resin is an epoxy resin, but other types of resins can used. including the majority of thermoset and thermoplastic resins. Although both are prepregs, thermoset and thermoplastic prepregs are different.
     A prepreg (pre-impregnated) is one of the main materials. used in multilayer boards and is what holds the cores together. It composed of fiberglass impregnated with resin (an epoxy-based material). The layers get pressed together at a temperature to. create the required board thickness.
     The prepreg and core quite often get mixed up. But, they are two separate components of the PCB. The core of the PCB is the FR4 layers of copper traces and glass-reinforced epoxy laminate sheets. Once heated,  the prepreg holds the core of the PCB and the layers together.
    "Prepreg" is the common term for a reinforcing fabric. which has been pre-impregnated with a resin system. This resin system (epoxy) already includes the proper curing agent. As a result, the prepreg is ready to lay into the mold without the addition of any more resin. In order for the laminate to cure, it is necessary to use a combination of pressure and heat.
    A prepreg (from pre-impregnated) is fibreglass impregnated with resin. The resin is pre-dried, but not hardened, so that when it heated, it flows, sticks, and is completely immersed. Prepregs are thus fibreglass strengthened by an adhesive layer (like FR4 material). The name of each prepregs derives from the type of fibreglass used. 
    The prepreg types of different suppliers vary in thickness. in addition there are the versions "SR" Standard Resin. "MR" Medium Resin "and" HR "High Resin.depending on the resin content. The best possible material used depending on the desired final thickness. layer structure or impedance. The mentioned thicknesses are thus to considered only as examples. Contact us for technical advice.
    Prepregs and cores are also available in High-Tg. and halogen-free types, but not always in every possible thickness.

Prepreg composition 

       Prepreg, the reinforcing material within a CCL. is made coming from glass fiber after getting baked. It is also called binding sheet by some individuals. and composed by epoxy resin, glass fiber towel, DMF, 2MI, acetone etc.
  • Classification of Prepreg. Prepreg may classified in to many categories based on different classification standards:
  1). Based on glass fiber towel: 106, 1080, 2112, 2116, 1500, 7628;
  2). According to applied resin and the performance: POLYCLAD Turbo 254/226. ISOLA FR402/FR406, ITEQ IT180, Sheng Yi S1141-140/170;
  • Classification of Resin
  1) Phenolic resin
  2) Epoxy botanical
  3) Polyimide plant
  4) Polytetrafluorethelene resin (PTFE or TEFLON resin)
  5) Bismaleimide triazine (BT)
  • Advantages of Glass Fiber can  summarized into the following aspects:
  1) Large intensity
  2) Heat plus fire resistance
  3) Dampness resistance
  4) Excellent insulation capability
  • Aspects impacting quality of prepreg. Superior quality prepreg causes high-quality CCLs, thereafter, PCB. Thus, PCBCart pays much attention to be able to test and control of prepreg quality. that carried out when it comes to resin content. solution time, resin fluidity, risky content. and dicyandiamide crystallization.
  • Select prepreg centered on cost. When that comes to the price tag on prepreg. glass fiber cloth balances for the majority associated with high cost. In most cases, the price tag on prepreg connected with the thickness of glass fiber cloth. The thinner prepreg is, the greater the cost becomes. Inside addition, prepreg 2112 is usually more expensive than prepreg 1080. because the former is usually seldom applied.

pcb prepreg and core

    Multi-layer PCBs made up by two or more PCBs. which stacked together with reliable predefined mutual connection between them. The space between each layer of the board can be either core or prepreg. Cores made up of a copper-plated glass-reinforced epoxy laminate sheets. Prepreg (also known as PP) composed of resin and strengthening material. that is then classified into glass-fiber cloth, paper base. compound material etc. Its main function is to stack all layers into a whole board. by high temperature stack-up.
     The position of Prepreg & Core in multi-layer PCBs depend on your boards' total layer number. attached is PCBCart's standard PCB Layer stackup for 4-layer PCB & 6-layer PCB. (please kindly noted the PCB Layer stack-up. varies from one fabricator to another fabricator.)
Click image for larger version. 
     Actually, the pattern and thickness of each type of core & prepreg are't always stable. and some adjustments will made to meet specific board thickness need. When making decision, factors including thickness of inner layer, product design thickness need. or technique need, prepreg type, practical performance. and the actual thickness after stack-up trial should taken into consideration.

     People often get confused between prepregs and cores due to their similarities. Allow me to clarify it for you. The core is a product of prepreg and lamination. It is more rigid compared to the prepreg. The core made up of a fiberglass-epoxy laminate. which meets the FR4 (Flame Retardant) grade with copper on either side. The core is usually manufactured by a core manufacturer. and the company is liable for any faults in the material that might hinder the operation of a PCB. Whereas prepregs, as described earlier, are also made of the same material like the core. but dried without lamination and aren’t rigid like the core. Hope this disposes the ambiguity.
     Now, the thickness of the PCB depends on the type of prepregs used. It must noted that the required thickness can achieved by a combination of prepreg plies. If you are wondering what plies are, they are nothing but thin sheets of prepregs placed on top of one another. This helps in achieving the desired thickness of the PCB.
Plies of prepregs
Other than PCBs. prepregs find their applications in aerospace components, automotive parts, A/C ducting, machinery. tooling, honeycomb and foam panels, and many more.
In conclusion, prepregs are insulators that glue together cores. and copper foils and assist in the manufacture of a robust PCB.

Prepregs pros 

      Largest strength properties. In a hand layup, it is difficult to achieve 50% resin content. This means that the finished laminate weight is 50% fabric and 50% resin. Typical hand laminates, even when vacuum bagged. end up with a significant amount of excess resin. Excess resin increases brittleness and reduces properties. But, most prepregs contain around 35% resin. This is ideal for largest cured properties. and generally impossible to achieve in normal hand lamination.
      Part uniformity and repeatability. Without the pitfalls of human lamination techniques. there will be neither resin-rich areas nor dry spots. Thickness will be uniform and every part that comes out of the mold. has a theoretical likelihood of being identical. There is still a margin for error in vacuum bagging techniques. handling, etc., but prepregs reduce these problems .
     Less mess and less waste. Prepregs will bleed excess resin. during the curing process but all the excesses of hand layup. cups of resin, messy rollers, drips – are no longer a problem. Plus Fibre Glast prepregs handled at room temperature. so you are not fighting a clock trying to avoid your resin setting up before you are ready.
     Less curing time. After the heat curing cycle completed, the part is ready for service. You do not have to wait the standard 48 hours to allow a full cure as in a typical hand lamination.
Better cosmetics. Mold preparation and mold release is still required. and will affect the cosmetics of the part like a hand laminate. But, Fibre Glast prepregs drop air bubbles and a smooth, glossy surface is more attainable.

Prepregs  cons 

     Cost. Prepregs are pricey. Even when you add up the cost of the resin, cure and fabric, prepregs still cost more.

    Shelf life. This is less of a problem since Fibre Glast prepregs can stored up to six months at room temperature (75°F). Nonetheless. heat cures prepreg and storage at warmer temperatures will reduce the shelf life. Keeping the material cooler will help and freezing will extend the life .
    Necessary heat cure. There will have to be a heat source and vacuum bagging at a minimum. You must be able to achieve a minimum of 270°F and sustain that temperature for a minimum of four hours. Many advanced fabricators use autoclaves but any source of heat will do.

what is prepreg Types

    Prepregs used in high-performance applications where weight. and mechanical properties take precedence over cost. High-end epoxy resins and carbon fiber, aramid. or other advanced fibers are commonly used in prepreg structures.
        Prepregs are reinforcement materials that have been pre-impregnated with either a thermoplastic. or thermoset resin, hence the name prepreg. The resin impregnation process controls the fiber to resin ratio and ply thickness. Thermoset prepregs produced by saturating a fiber reinforcement with a liquid thermoset resin. Excess resin removed from the reinforcement and the resin undergoes a partial curing. changing from a liquid to a pliable solid state. This known as the “B-stage.” Prepregs in the B-stage need refrigerated storage conditions. The curing process is then activated with the application of heat.
     Thermoplastic prepregs produced by coating fiber reinforcement with a thermoplastic matrix. An advantage to thermoplastic prepregs is the ability to reheat. and reform the material many times by heating. above the melting point of the specific thermoplastic matrix. Unlike thermoset prepregs, thermoplastic prepregs can stored at room temperature. In this case, the thermoplastic prepreg formed by heat. and becomes solid when cooling to room temperature.
      Plies of prepregs materials laid-up in a tool by hand or with automated equipment. The laminate stack is then consolidated under the pressure of a vacuum bag. The curing process triggered by the application heat in a heated tool, in an oven. or in an autoclave where heat and high pressure applied. Thermoset prepregs need a controlled heat cycle for curing. that allows for the appropriate resin flow. within the laminate and then polymerization to the cured state. This involves a controlled heating rate. and soak at a specified temperature with a controlled cooldown. Thermoplastic prepregs do not rely on a crosslinking reaction and can cycle more . Both thermoset and thermoplastic prepregs create lightweight high strength composite laminates.
Thermoplastic Prepregs
     Thermoplastic prepregs are composite reinforcements. (fiberglass, carbon fiber, aramid, etc.) that are pre-impregnated with thermoplastic resin. Common resins for thermoplastic prepregs include PP, PET, PE, PPS, and PEEK. Thermoplastic prepregs can provided in unidirectional tape. or in fabrics that are woven or stitched.
The primary difference between thermoset. and thermoplastic prepreg is that thermoplastic prepregs are stable at room temperature. and generally, do not have a shelf life. This is a direct result of the differences between thermoset and thermoplastic resins.
Thermoset Prepregs
       More used in prepreg composite manufacturing is thermoset prepregs. The primary resin matrix used is epoxy. Other thermoset resins made into prepregs including BMI and phenolic resins.
     With a thermoset prepreg. the thermosetting resin starts as a liquid and impregnates the fiber reinforcement. Excess resin removed from the reinforcement. Meanwhile, the epoxy resin undergoes partial curing. changing the state of the resin from a liquid to a solid. This known as the "B-stage."
     In the B-stage, the resin cured, and usually tacky. When the resin brought up to an elevated temperature. it often returns to a liquid state before hardening completely. Once cured, the thermoset resin which was in the b-stage is now cross-linked.

what is prepreg and core in pcb

Multi-Layered Boards
     The prepreg in a multi-layered board is what holds the layers together. Once you stack all the layers of the board. you fuse them together by exposing the board to high temperatures. The thickness of the prepreg will be by the thickness of the board.
    The prepreg should have specific characteristics to it. when used on a multi-layered board. The cloth surface should look and feel smooth. and be free of any oil, stains, defects, foreign matter, cracks, or excessive resin powder.
Complex PCBs
     If you have a complex PCB, you’ll likely have a more complex prepreg. You’ll need to use different types of the prepreg to achieve the. required thickness of both the prepreg and the board itself.

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