What is Prepreg in a PCB Manufacturing?
Prepregs are integral elements of circuit boards with multiple layers. They serve as insulation layers binding cores and copper foil. No wonder any PCB manufacturing process cannot go without them.
Let’s learn about prepreg materials, their types, and the styles that are available on the market. You will also learn about prepreg qualities and how to select prepregs for your projects properly.
What is Prepreg in PCB?
Prepreg means pre-impregnated with resin. They are defined as the dielectric material sandwiched between the PCB cores or core material and an external layer.
From the technical point of view, the prepreg is a glass weave that was pre-glued and is ready to be hardened. The process requires heat, pressure, the use of fiberglass technologies, UV light, and added compounds, including hardener, activator, and catalyst.
The interesting thing about prepregs is that they can be modified to the needs of the manufacturer. Through using the combination of additives and resin bonding agents, under the chemical process, a section of prepreg can be transformed into a conductive region. This effect is called selective conductivity. It finds its application in connecting the top and the bottom layers of prepregs through drilling holes.
Take a look at what the prepreg manufacturing process looks like in the infographics below.
How are Prepregs Used in PCB manufacturing?
Prepreg stacks are purchased in bulk in order to produce multilayer boards. Since prepregs bond the core material and copper layers, one or even a few of them, depending on the number of PCB layers, are involved in the manufacturing process.
Prepregs are arranged in the PCB stackup, then exposed to high temperature and pressure and pressed together to form a multilayer board. The prepreg softens in the process and flows through the PCB core layers. Once the PCB is called down, the prepreg is solidified. The material binds the layers altogether and keeps the structure uniform.
Commonly Used Prepreg Styles in PCB
Let’s examine two commonly available prepreg styles in PCB:
- Thermoplastic prepreg
These are composite reinforcement materials, which include fiberglass, carbon fiber, aramid, and others. They are pre-impregnated with the use of thermoplastic resin. Some of the commonly available resins are PP, PET, PE, PEEK, and PPS. Such prepregs are often provided in fabrics that are woven to stitched.
The distinct characteristic of the thermoplastic prepreg is that it’s stable at room temperature. They don’t usually have a prolonged shelf life.
- Thermoset prepreg
In their turn, thermoset prepregs use epoxy as their resin matrix. Some other resins may be made with the help of BMI and phenolic resins.
Unlike thermoplastic prepregs, thermoset components come in liquid form, and then they are impregnated with fiber reinforcement. With this method, excessive resin powder is eliminated from the reinforcement. At the same time, the epoxy resin undergoes curing and gardening, i.e., changing from liquid to the solid state — a so-called “B-stage.”
In that phase, the resin is partially cured. Once it’s brought to an elevated temperature, it briefly turns to liquid ones again before hardening completely. Once cured, the thermoset resin is cross-linked permanently.
Thermoset prepregs play a key role in most manufacturing processes because they are easier to handle, store, and use for PCB production.
What’s Resin Content in Prepreg and How to Test?
Prepregs are produced with the involvement of epoxy resin. The amount of the material notably impacts the electrical properties of a prepreg, as well as the Coefficient of Thermal Expansion (CTE) and drilling and etching quality. After all, it forms the insulation layer of the component.
Besides, the higher resin content instantly results in the higher thickness of the laminate piece.
Resin content is the ratio of the resin in the prepreg to the overall weight of the prepreg. Here is a step-by-step guide on how to test the resin content in the prepregs:
- Take a single prepreg sample and measure its weight using the high precision laboratory balances.
- Prepare the glass container of LR-grade sulphuric acid (H2SO4).
- Dip the prepreg sample into the sulphuric acid for several seconds for the resin to get completely dissolved in the acid.
- Cleanse the prepreg sample with water.
- Dry the prepreg sample with hot air and towels.
- Measure the weight of the sample using the same balances.
- Make the calculations based on the difference in the weight of the sample before and after.
Here is the formula for measuring the resin content:
Resin (%) = (Original prepreg sample weight – end prepreg sample weight) / Original prepreg sample weight x 100.
Take a look at the resin content that different prepreg styles have and what different prepreg surfaces look like in the images below.
How to Select Prepreg in Design Stage
As detailed, you have standard resin, medium resin, or high resin prepregs available for choosing from. Most engineers prefer to select appropriate prepregs in the design stage already, based on the desired technical parameters and the type of resin. Let’s review some important considerations that may guide the decision-making.
First is cost efficiency. The more resin a prepreg contains, the more expensive it is to produce, so make a choice wisely.
Electrical properties are also of crucial importance. You definitely want to get an exact value for electrical conductivity and return loss. To get the precise match with the technical requirements, you need to consider:
- Geometric routine.
- Particular frequencies.
- Convention on the law.
- The thickness of the material
The parameters may vary between prepreg types. Consult a supplier for accurate estimations.
After all, it’s also well-advised, so you stick with thermoset prepregs. Unlike thermoplastic, they are easier to manufacture PCBs with and don’t leave much room for mistakes.
Summary
Prepregs are parts you need for any multilayer PCB production. They are the insulation layers that bond the core layers of the circuit boards and help to maintain the integrity of the stack. They also impact some thermoelectrical properties.
Guide your choosing based on the resin content in prepregs, the budget, and the electrical properties you intend to achieve.
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