Most manufacturers still maintain their conventional processes for making PCBs, or Printed Circuit Boards. However, this scenario is now changing. Many are upgrading their fabrication techniques for making PCBs, moving towards additive manufacturing, as they see the advantages. We, at PCB Trace Technologies Inc., a prominent high-quality PCB manufacturer, bring you this article to illustrate the various methods of additive manufacturing available. But first, let us explore the PCB design process and the differences between the conventional and the newer methods. We will then look at the advantages and disadvantages of all the processes for making PCB prototypes.
The PCB Design Process
Whatever the processes used for manufacturing the boards, the method of creating them through design remains the same. The design process is as follows:
- Circuit Design
Designers create a PCB to represent a specific circuit design. For this, they begin by drawing a schematic, which they create using a PCB design software and a computer. They must carefully select components that will allow their circuit to function as intended. Once the circuit is converted into a schematic, they can prepare the layout for the board.
- Board Layout
The designer transfers the footprints of all components in the schematic to a 2-D layer. At this point, they must have finalized the dimensions of the board they want. They rearrange the footprints within the boundary of the board and interconnect them with tracks, routing the tracks such that they follow the same pattern of connection as in the schematic. If it is not possible to make all the interconnections, the designer has the option of adding more layers. They use these new layers to route additional interconnections. To make connections between layers, designers add vias, or plated through holes.
- Gerber Files
Once the board layout is ready, the designer must transfer the entire design into an industry standard format before sending them to the PCB manufacturer. The industry uses Gerber files for encoding vital information like copper layers, apertures, drill drawing, component notations, and so on. The manufacturers also run DFM or Design for Manufacturing checks before starting fabrication.
The Traditional Method of PCB Manufacturing
The traditional method of PCB manufacturing is also called the subtractive method, as the unwanted material is removed or subtracted from the original stock. Typically, the manufacturer begins with an insulating core sheet material (mostly fiberglass) on which they bond a copper film. For a single-sided board, they bond the copper film only on one side, and for a double- or multi-layered board, they carry out the bonding on both sides of the core. The insulated copper-clad board now becomes the foundation on which to transfer the circuit design for making the final PCB.
- Transferring the Circuit Design
The manufacturer generates films of each layer of the board from these Gerber files. They coat the copper-clad board with a photosensitive material and transfer the film onto it. Exposing the film and copper clad board to ultraviolet light fixes the photosensitive material on the copper layer where the film has allowed the light to pass through. Developing the copper clad board covers the copper layer with a tough protective layer where the UV light has fixed it, while the rest of the photosensitive material washes off.
- Drilling
Using the drilling information from the Gerber files, the manufacturer drills the copper-clad board for the necessary holes that include vias. The board then must undergo an electroplating process to create vias and plated through holes.
- Chemical Etching
The manufacturer then uses strong etching chemicals to dissolve the exposed copper, leaving untouched the copper tracks covered under the protective film. Chemically removing the protective film exposes the copper tracks, and the PCB is ready for application of solder mask, surface finish, and silk-screening.
- Milling
Rather than using chemicals to etch out the unwanted copper from a copper clad, automated machines are available that use a milling cutter to remove the unwanted copper. The same machine can also drill holes in the copper clad to generate a complete PCB. This process does not require creating films for transferring the design to the copper clad. Rather, the system takes the design from the Gerber files and operates the milling cutter accordingly. Once the milling and drilling operations are complete, the PCB is ready for application of solder mask, surface finish, and silk-screening.
As the drilling, milling, and/or chemical etching process removes material from the copper clad, the above traditional methods of PCB manufacturing are termed subtractive.
Additive Methods of PCB Manufacturing
There are several popular additive methods of manufacturing PCBs. The simplest of them involves printing conductive traces on a substrate, producing single- or double-sided PCBs.
A little more complicated additive process involves creating multiple layers by depositing conductive traces on thin substrates, and then bonding them to form a multi-layered board.
The next process is the 3D printing system, which uses more advanced steps for creating multi-layered PCBs. This additive process prints to create both the insulating substrate and the conductive elements simultaneously, layer by layer. Therefore, this process does not require pressing and bonding the layers into a single stack. Let us examine these processes in detail.
Printing Conductive Traces on a Substrate:
Typically, the substrate is of a rigid FR4 material. It is cut and shaped to the required board size. The manufacturer first drills locating holes in the substrate for process alignment.
The next step involves drilling all the necessary holes based on the drilling information in the Gerber files. The manufacturer may have to electroplate the board to create plated through holes and vias.
Again, based on the layer information in the Gerber files, a computerized printer prints conductive traces on the substrate surface using nanoparticle inks containing copper or silver particles. The board is then baked in an oven so that the ink traces can solidify. The manufacturer now only has to apply solder mask, surface finish, and silk-screening to the PCB to complete it. In essence, this is a semi-additive method.
Differences From Traditional Methods
- Copper clad is not necessary; only the substrate is required
- Transferring the circuit is not necessary
- Chemical etching or milling is not necessary
- The board must be baked after printing is over
Common Processes With Traditional Methods
- Drilling the board with information from its Gerber files
- Application of protection like solder mask, surface finish, and silk screen printing
Advantages
- Fast process
- Any available insulating substrate can be used
- Suitable for fast-turnaround PCB prototyping
Disadvantages
- Cannot achieve fine traces
- Cannot achieve close spacing
- cannot achieve high component density
- Only single-sided, or at best, double-sided Printed Circuit boards are possible
- Cannot achieve multi-layered boards
Printing Conductive Traces on Thin Substrates:
The process is similar to printing conductive traces on a rigid FR4 substrate, however, the substrates in use here are much thinner, as they have to make up a multi-layered stack.
Each layer is drilled, printed with conductive ink, and electroplated to form vias and through holes. Each layer must be baked to allow the conductive ink to solidify.
The layers are consequently stacked together using prepreg and adhesive layers, and bonded together using pressure and heat. The only processes necessary are the application of solder mask, surface finish, and silk-screening. This method is also semi additive.
Differences From Traditional Methods
- Copper clad is not necessary; only the substrate layers are required
- Transferring the circuit is not necessary
- Chemical etching or milling is not necessary
- Each layer must be baked after printing is over
Common Processes with Traditional Methods
- Drilling the board with information from its Gerber files
- Bonding each layer to form the stack
- Applying solder mask, surface finish, and silk screen printing
Advantages
- Fast process, but slower than making double-sided boards
- Any available insulating substrate can be used
- Suitable for fast-turnaround multilayered PCB prototyping
Disadvantages
- Cannot achieve fine traces
- Cannot achieve close spacing
- Cannot achieve high component density
3D Printing Multilayered PCBs:
This process is substantially different from the earlier two. Here, the printer prints both the substrate and the conductive traces, and prepregs layer by layer. Separate drilling is not necessary, as the substrate and copper traces are printed along with forming the necessary holes and vias with information from the Gerber files. This method is a fully additive process for making multi-layered PCBs.
The additive process typically uses deposition of electroless copper to form plated through holes. Three basic methods are popular for this:
Semi Additive
- PTH formation by this process makes use of the electroplating process with very thin surface copper.
Partially Additive & Fully Additive
- PTH formation is through electroless copper deposition
Differences From Traditional Methods
- Copper clad is not necessary
- Substrate layers are not necessary
- Transferring the circuit is not necessary
- Drilling holes is not necessary
- Chemical etching or milling is not necessary
Common Processes with Traditional Methods
- None
Advantages
- Suitable for fast-turnaround multilayered PCB prototyping
- Can print fine traces with close spacing
- Can print embedded passive components also
Disadvantages
- Slower than partially additive methods, but faster than traditional PCB fabrication techniques.
- More expensive than conventional methods of PCB fabrication.
Additive PCB Manufacturing for Rapid Prototyping
Additive manufacturing is currently becoming more popular for electronic manufacturing, especially for high-complexity but low-volume production of PCBs, and for rapid prototyping. As electronics become more complex, leading to an increase in layer count in PCBs, additive manufacturing is becoming a natural choice, replacing traditional PCB manufacturing techniques, and for rapid prototyping. There are several advantages in using a 3D printing system specialized for complex PCBs:
- Significant reduction in production time
- Significantly low material waste
- Highly competitive costs
- Suitable for rapid prototyping
- Suitable for low-volume production
- Protection of intellectual property
- Immediate prototype testing
- Hastening the R&D process
- Faster time-to-market
Conclusion
PCB Trace Technologies Inc. recommends in-house PCB prototyping with additive manufacturing systems. There are several manufacturers of additive manufacturing systems that offer multilayered PCB manufacturing with short turnaround times. You can quickly manufacture multi-layered boards with complex architectures at lower costs and shorter times.
