Ever wondered what goes into making or producing a multilayered circuit board? The processes range from separating the fiber sheets from the copper clad, as well as transferring the circuit design to the copper clad.
What is a Multilayer PCB?
The name makes it self-explanatory. Ordinarily, a multilayer PCB should be the one that features or has many layers. From the basic design, the structure of these layers includes the top layer, inner layer and the bottom layer.
How Many Layers Does the Multilayer PCB have?
The fact that this type of circuit board uses several layers doesn’t mean that it may support many at the end of the day.
Note that the layer count or the number of layers used in the multilayer PCB is usually above two (2). The reason is simple – there are three major layer positionings for the board. Using only 2 layers therefore doesn’t apply to this.
You can expect the multilayered circuit board to have anything from 4 and 40 layers.
Layer Connection
The interconnection of the layers is a major factor that differentiates it from the other PCB typologies.
The complexities associated with working on the multilayer PCB are resolved first by the compression of all the layers to form a single multilayered circuit board.
With the new form made, the next step is to interconnect all the layers using the copper plated holes.
Getting Ready to Make a Multilayered PCB
If you want to make this type of PCB with multilayers, it will do you a world of good to have all the tools you needed.
Here are some of the tools you need for the job:
- PCB design software, such as OrCAD, Proteus or Eagle.
- Fiber Sheet Base, which is used to create the base for the board.
- The glossy photo paper should not be left out, because that’s where you will make a custom imprinting of the design.
1. Separate the Fiber Sheets
The chances are that the copper clad you are using the for the design job has some fiber sheets. For context, these fiber sheets are usually attached to the copper clad’s base.
To help smoothen the job, you need to separate or remove the fiber sheets from the rest of the board. That way, you will only have a few or even one fiber sheet on the copper clad’s base.
2. Make the Multilayer PCB Layout
The next step is to use any of the available PCB designing software or CAD tool. Examples of these design tools are Eagle, Proteus and OrCAD.
3. Make the Inner Layer’s Core
The inner layer is one of the most important layers in the multilayer PCB stackup. Once the core of the inner layer is made, proceed to treat the laminate of the desired thickness with UV light, copper foil and dry film resist.
4. Stack up the Layers
The primary task here is lamination but that wouldn’t be possible if the multilayer PCB stackup is not created.
The stackup or structure of the board has to do with the number of layers you are creating. The following are some of the things you must know about the multilayered PCB stackup:
- The sheets of material are stacked or placed on the top of one another.
- Holes are also drilled to align these materials to solidify the stacks as they being stacked up.
With the stacking process explained, these are the things you must know about the lamination process:
- The following are laminated – copper foil sheets, inner layer core and prepreg sheets.
- The staking of the multilayered PCB’s sheets must have been structured in a way that the next sheet of material can align with the existing ones and be stacked up.
5. Transfer the PCB Layout Design to the Copper Clad
For the multilayer PCB design, there are two (2) major methods of transferring or moving or transferring the circuit design to the copper clad.
Heatless Toner Method
The primary method of circuit board layout’s transfer to a copper clad is by using the heated process. However, if you don’t want the heat that comes with, going for the heatless toner transfer method is a better way to go about it.
With that being said, here are the steps to using this method to transfer the PCB layout to the copper clad:
- Clean the Copper Clad: use tissue and freshwater to clean the surface of the copper clad.
- Use an Alcohol plus Acetone Mixture: consider using Acetone and alcohol to make an 8:3 ratio of the mixture on the copper clad’s surface.
- Glossy Paper Placement: you are not free to place the glossy paper over the copper clad, now coated with the Acetone and alcohol mixture.
- Time and Precision: the mixture should be allowed for us at least 30 minutes before you will dip the circuit design inside.
- Peeling the Paper: after the 30 minutes has elapses, remove the gloss (photo) paper from the copper clad’s surface. Please note that you can use a tissue paper to clean off the residue caused by the paper.
The Heating Element Toner Transfer Method
This is the second format of transferring the PCB schematics to the copper clad. Below are further pieces of information regarding how this process works:
- Heat Element Usage: it is imperative to use a heat element, that is, something that will help heat up the PCB schematics for the transfer to take place.
- Apply Pressure: you can now apply pressure to enable the transfer of the ink from the glossy paper and printing the same on the Printed Circuit Board (PCB).
- Vacuuming: as you are applying pressure, make sure to vacuum too. The relevance of vacuuming in multilayer PCB layout transfer is that it helps to prevent air from getting trapped between the layers.
Key Considerations for the Heated Toner Method
We will like to point out some of the considerations to make when working on the multilayer PCB.
Glossy Paper’s Thickness
What happens if the glossy paper is thick? You can count on the iron to help you get around it. If the glossy paper’s thickness is very thick, it would be a good idea to increase the iron’s temperature up to the highest operating levels.
Duration
Using the heated toner transfer method, it may take up to 2 hours to complete the transfer of the PCB schematics to the PCB.
Of course, the duration can be higher or reduced, depending on the number of layers being stacked up on the copper clad.
6. Etch the Board
Etching is one of the core finishes to the multilayer PCB. It is also a “curing method” designed to help the board be in the best working condition.
Here’s what you need to know when etching the multilayer circuit board:
- Etch with a Solution: you need a dedicated solution or mixture to etch the multilayer board. The name of the solution is Ferric Chloride or FeCI3.
- Copper Clad Placement: the copper clad containing the imprinted multilayer PCB design can now be dipped into or placed inside the solution and allowed to sit for about 8 minutes.
- Trace Removal: it is very common for the unneeded traces to be removed once the copper clad is placed inside the FeC13 solution.
7. Copper Clad Security
Now that you’ve identified the total removal of the residuals and unwanted traces, you can then proceed to take steps to secure the copper clad.
One of such steps is the precise alignment of each copper sheet and using an epoxy glue to secure them. This process should take less than 5 minutes.
Once dried, use the Acetone, this time, as a cleaner. You may also be interested in using a sanding paper to remove any residential glue that the epoxy glue must have learned before.
8. Define the Traces
Defining or “chasing after” traces in a multilayer PCB is not one of the easiest jobs to do. However, it helps you to define or trace those traces.
The easiest way to define the traces is by using a stencil with 2-part epoxy with oil colours and masking ink.
9. Make the PCB Connections
With the major processes handled with ease, it is now time to use a hand drill to drill the vias. Once the drills are made, proceed to using a thin wire among the layers. This comes in handy when soldering both the vias and the pads.
Multilayer PCB Design Tips
Creating the PCB schematics and having the same transferred to the copper clad is a great step. Now, we want to enlighten you on some of the design tips that could make all the difference in your next project:
- Inner Layer Signal Thru-Hole Pad Reduction: reducing the size of the thru-hole pads in the inner layer will open up more routing channels in the multilayer circuit board.
- Adjacent Routing: it is also a great idea to route the wires and the other components in the other direction of the multilayered PCB. This is an innovative way to reduce the chances of broadside crosstalk problems.
- Ground Plane and Power Layer Usage: make sure to use the power and the ground plane layers to the fullest. Doing so facilitates the creation of a microstrip structure that bolsters the circuit board’s signal integrity. You can leverage this to make an effective distribution of power in the board.
Conclusion
The multilayer PCB offers multiple layers that can be used to improve the signal integrity, create a single connection point and reduce the overall size of the multilayer Printed Circuit Board (PCB).
