Majority of electrical engineers as well as individuals working with PCBs (printed circuit boards) have some knowledge about the FR4 material. FR4 is widely regarded as a backbone material where the building of the majority of rigid circuit boards occurs. Moreover, there are still many people who aren’t aware of FR4, what it offers, as well as why it is the most famous PCB base.
In this article, you will learn a lot regarding FR4 printed circuit boards, like what it is, what they offer and the processes involved in FR4 circuit board manufacturing, and more. Read on as we explain further.
What does FR4 Substrate Material Mean?
FR4 can be described as a standard rating, as well as a name. You can apply this name to the epoxy-laminated fiberglass-reinforced sheets useful in Fr4 circuit board manufacturing. Also, this name serves as a grade useful in rating epoxy laminate sheets.
Essentially, this designation indicates a laminate sheet’s base quality. This means that different sheet designs and materials are categorized under the rating of FR4. “FR” in FR4 means flame retardant, then the 4 helps in differentiating the material from the others categorized together.
This FR4 material is known as a composite structure. Also, this material’s most important layer is the fiberglass that is woven into a cloth-like and thin sheet. In addition, this fiberglass present in FR4 ensures that it has the required structural stability.
The fiberglass layer is then bound and surrounded by an epoxy resin that is flame-resistant. The resin gives this material is rigidity, as well as other important physical properties
Also, the FR4 sheets are well known among the electrical designers and engineers as a base material for printed circuit boards. The material’s versatility and low cost, coupled with the lots of physical properties plays a huge role in its popularity.
In addition, the FR4 sheets serve as electrical insulators that have a high dielectric strength. Also, their strength to weight ratio is high. Furthermore, they resist moisture and are lightweight. If you include this to their temperature resistance, you can be sure that using the FR4 material for your circuit boards will help it perform in the majority of environmental conditions.
What Properties Does the FR4 Substrate Exhibit?
The FR4 substrate has the following properties:
It is flame retardant
The chemicals added to materials to help in delaying or preventing fire spread are called flame retardants. The FR4 substrates are also known to possess great mechanical, electrical and thermal properties. This makes them a great choice for different electronic applications. Furthermore, the flame retardant laminates as well as prepregs are very versatile, and are adaptable to different manufacturing procedures, coupled with delivering predictable results.
It has great electrical properties
A PCB material’s electrical properties are important for impedance consideration and signal integrity. Also, they help in specifying the quick nature of the signal propagates via the material as well as the amount of electrical charge that it will be able to hold within a specified volume.
Its moisture absorption ability is low
Moisture absorption deals with a PCB material’s ability in resisting water absorption anytime it is inserted into water. This is explained by an increase in the circuit board material’s weight as a result of water absorption and under some controlled conditions. In addition, the FR4 materials also provide a low value for moisture absorption at 0.10% anytime it is immersed inside water for about 24 hours.
Steps Involved in FR4 Circuit Board Manufacturing
Many of us don’t have an idea of the different steps and processes involved in fr4 circuit board manufacturing. In this section, we will explain the entire process.
Etching
Most fr4 circuit board manufacturing is achieved via the application of a copper layer over the whole PCB substrate material surface either on the two sides or on just one of the sides. With this, you’ll get a printed circuit board that is blank, with its surface having copper present everywhere. Furthermore, from this stage, unwanted copper is taken off with the help of subtractive methods.
Photoengraving
The process of photoengraving makes use of a photomask or mask, which is usually mixed with chemical etching. This is to help in removing or subtracting the copper areas present in the substrate of the circuit board.
The creation of photomask is achieved using a photoplotter that takes this design from your CAD PCB software. Also, the creation of photomasks with a lower resolution is at times done using a laser printer with the help of a transparency.
Lamination
A good number of fr4 circuit boards are composed of multiple layers. We refer to this as the multilayer circuit boards. Furthermore, they are made of different thin trace layers or etched boards, which are bonded altogether via lamination.
Drilling
Every layer of the fr4 circuit board needs one of the layers to have the ability of connecting to the other. You can achieve this via the drilling of small holes referred to as “VIAS“. The drilled holes need precision with the placement. Also, achieving this is most done by using automated drilling machines. Also, computer files and programs called NCD files or Excellon files help in driving the machines. Furthermore, the file helps in determining the size and position of each of the drills present in the design.
In addition, you can use controlled depth drilling in drilling a circuit board layer instead of drilling through the entire layers. You can achieve this by drilling the layers or sheets of the PCB individually before lamination.
- Buried Vias: This is a situation where the holes can only connect the interior layers, excluding the outer surface
- Blind Vias: Here the holes help in connecting a layer to its outside surface
Each hole’s walls for the multilayer boards are usually copper plated in order to help in forming the plated-through holes, which connects the PCB’s conductive layers.
Solder Plating
Lands and pads which require the mounting of electronic components on them are usually plated. This ensures the components’ solderability. Furthermore, the bare copper isn’t always solderable; therefore there is a need to plate the surface with a material facilitating soldering. Back then, a lead-based tin was utilized in plating the surfaces. However, with RoHS compliance, which was enacted, there are now newer materials that are used like gold and nickel to both provide solderability, as well as comply with the RoHS standards.
Furthermore, those areas that shouldn’t be solderable are usually covered using a material. This helps in resisting soldering. In addition, solder resist has to do with a polymer coating, which serves as a solder mask, therefore preventing the bridging of traces by solder and also creating some short circuits to close component leads.
Silk Screen
Here, whenever there is a need to apply visible information to the circuit board like company part numbers, logos, or instructions, then silk screening becomes important. It is used in applying text onto the circuit board’s outer surface. In cases where spacing allows, the screened text will be able to indicate the component designators, as well as switch the setting requirements and some added features in assisting the process of PCB assembly.
Note that red print has to do with the silk screening of a printed circuit board with one side.
Testing
The circuit boards left unassembled are usually made to pass through a bare board test. Here, verification is done on each of the circuit connections. This is just to be sure that they are correct on the completed circuit board.
Furthermore, for high volume fr4 circuit board manufacturing, a fixture is needed to come in contact with the holes or copper lands present on either one or the two sides of the circuit board. This is to help in facilitating testing. Also, computers are useful in controlling its electrical testing unit. This is to help in sending a little current via each point of contact on that bed of nails as well as verify and confirm that you can detect the current on the right contact points.
Also, for the small and medium volume production, there’s a need for a flying probe tester. This helps in checking the electrical contacts. Furthermore, the flying probes make use of moving heads in making contact with the copper holes and lands. This helps in validating the board’s electrical connectivity, which you are testing.
Tips for Choosing the Best FR4 Material
Before you choose the right FR4 material, you should take a look at the thickness as well as other significant characteristics and features of the laminate. Let’s consider some important tips.
- Done make use of thinner materials for the circuit boards having grooves
- Choose an FR4 laminate of high performance once the application’s operating temperature surpasses 150 degrees centigrade.
- Choose a dielectric material offering a uniform Dk value over different frequencies.
Conclusion
We hope you now understand all the processes involved in fr4 circuit board manufacturing. If you have any questions regarding the topic, please let us know.
