- Step-by-step breakdown of the circuit manufacturing process
- Common alternative materials for printed circuit boards
- Tips for designing your circuit for reliable manufacturing
Circuit manufacturing is a complex process with a lot of different steps
Printed circuit boards (PCBs) are the single most essential element of modern-day electronics. Regardless of size, chances are if you open up any of your electronic devices, it will have a printed circuit board in it. These boards ensure that the device is working as intended, keeping our modern world running.
While circuit boards are nearly everywhere, the circuit manufacturing process producing these boards can be complex. It is a multi-step process that connects multiple engineering processes, designers, and component manufacturers. Let’s dive into what this process entails.
The General Circuit Manufacturing Process
Soldering can be accomplished through multiple different methods
There’s a lot that goes into circuit manufacturing, depending on the board. Material substrate, intended function, and reliability tolerances can all affect the circuit manufacturing process. We’ll be discussing the most general of circuit manufacturing processes, which forms the foundation that other processes are based on.
Step 1: From Design to Production
Once the design has been finalized, engineers export the design into file formats that manufacturers use, most often a set of Gerber files. These files encode essential information such as copper layers, drill drawings, cover overlay, and more. The fabrication data is then read into a computer-aided manufacturing software or CAM.
Step 2: Initial Design Print
Using a special plotter printer, a “film” or “image” of each layer is made. This film is a photo negative of the board itself. Each layer of the circuit board (both inner and outer) and accompanying solder mask top and bottom get its own layer. After the films are printed, they are lined up and a “registration hole” is punched through for later alignment in the circuit manufacturing process.
Step 3: Inner Layer Copper Print and Etching
The design is printed onto a laminate material and then copper is pre-bonded to it. The copper is then etched away to reveal the traces and copper fill regions as dictated by the Gerber files. This can be done through multiple methods, but a common one is by creating a photo-sensitive film called a resist. The resist uses photo-reactive chemicals that harden after being exposed to ultraviolet light. Areas that are hardened correspond to sections that are kept as copper on the board. Other non-exposed copper sections are removed chemically.
Step 4: Stackup Layer Alignment
After each layer has been printed and cleaned, technicians use an “optical punch” and drive a pin through the layers to align them.
Step 4.5: Quality Inspection
No physical modifications are made to the circuit board in this step, but it is an important one. Here, a machine performs an optical inspection by comparing the PCB with the Gerber files, ensuring there are no defects.
Step 5: Lamination
For multilayer boards, this process consists of applying high heat and pressure to the internal layers of the board while using a photosensitive dry resist.
Step 6: Hole Drilling
Based on the Gerber files, a computer-guided drill is used to create registration, guiding, and other specified holes.
Step 7: Plating
A chemical process is used to create a micron-thick layer of copper over the top layer and the holes that have been drilled, creating vias, whether they be through-hole, blind, or buried.
Step 8: Outer Layer Imaging and Etching
A photoresist is applied to the outside layers and then imaged. They are then plated similarly to the previously discussed inner layers, with an additional layer of tin to help protect the copper during the etching process.
Step 9: Soldermask
A solder mask film (usually green) is applied to the surface of the board. Ultraviolet light is then shined on the board to indicate sections for removal.
Step 10: Silkscreen
The final additive process to the circuit board is the application of the silkscreen or legend. This is used to convey company ID numbers, warning labels, component designators, pin locators, or any other useful information.
Step 10.5: Electrical Reliability Test
A circuit continuity test is performed to check for any unintended disconnections in the copper. Then, an isolation test is performed to check for shorts, ensuring the circuit was manufactured as intended.
Step 12 defines the end of the circuit board fabrication process itself. However, in manufacturing the whole circuit for consumer and other highly standardized boards, the following steps correspond to the application of components for circuit assembly.
Step 13: Solder Application
In using the most common type of soldering method, reflow soldering, a metal stencil generated by the Gerber files is used to apply reflow solder paste onto the terminals of relevant pads.
Step 14: Component Placement
Using a pick-and-place machine, components from reels are picked up and placed on their relevant board spots on top of the applied solder.
Step 15: Soldering
Using one of the major soldering techniques, the components are soldered to the board.
- Wave soldering uses a wave of hot solder liquid
- Reflow soldering uses heated soldering paste
- Soft soldering uses an electric torch or gas to fix metal space filler between the board and the components
- Hard soldering is used primarily for combining two solid metal pieces such as copper, brass, silver, or gold
After this final step, the circuit board is often run through reliability tests to ensure it has been manufactured and assembled correctly.
Circuit Manufacturing Alternative Substrate Materials
Flexible circuits are becoming an increasingly popular alternative to standard FR-4-based rigid circuit boards
When it comes to circuit manufacturing and choosing the right substrate, consider your circuit's intended use case. The default substrate is usually FR-4, but depending on thermal or miniaturization requirements, metal-core or flex boards can be good alternatives.
Nowadays, the most common printed circuit board material is FR-4, a woven fiberglass cloth with epoxy resin. It features low water absorption (around 0.15%), good arc resistance, and strong insulation properties.
Ceramic-based boards have more versatility, durability, stability, and stronger insulation properties compared to typical FR-4. Their high thermal conductivity and coefficient of thermal expansion make them especially useful in environments of high temperature or pressure. Some common compositions include aluminum nitride (AlN), alumina (Al2O3), beryllium oxide (BeO), silicon carbide (SiC), and boron nitride (BN).
Aluminum, other metal core boards, or insulated metal substrates (IMS) are also popular alternatives. They have a thin, thermally conductive dielectric and are most often used for circuits that run extremely hot and require good heat distribution.
Flexible Boards and Flexible Circuit Manufacturing
A fourth increasingly popular alternative is flexible substrates. These include a standalone copper-clad foil, such as kapton polyimide foil, or a laminate with a thin stiffener. These can be used for flexible printed circuits or flexible interconnects and have high heat resistance. Flexible circuits offer multiple advantages such as compact size, reduction in weight, better impedance control, absence of mechanical connectors, and more.
Rigid-flex boards, a hybrid of flexible and standard boards, offer the merits of both. They have a similar fabrication process, with some slight variations. The rigid-flex manufacturing process is as follows:
- A laminate containing copper layer is pre-cleaned.
- After generating a pattern step through screen printing or photo imaging, the circuit pattern is then etched onto the lamination through an etchant solution or spray.
- A machine is used to drill holes, pads, and vias.
- A copper plating process is used to deposit copper on the specified plates and holes.
- A coverlay material, usually made of a polyimide film, is imprinted on the rigid-flex board using screen printing. Additional stiffeners are added here if required by the design.
- The flex board is then cut out of the panel using knives or hydraulic punching.
- Electrical testing and verification is then conducted.
Designing for Successful Circuit Manufacturing
Having an intelligent PCB software, such as Cadence Allegro, will help you design reliable circuit boards
The PCB fabrication process is long and complicated, with lots of room for mistakes to be made. If you’re planning on fabricating your board many times, even one small mistake in design can cost time, money, and even lives, depending on the board use case. For this reason, keeping designing for manufacturing (DfM) in mind and prioritizing ease of fabrication is key.
An especially critical tip to streamline your PCB fabrication process includes setting DRC constraints. It’s easiest to contact your PCB manufacturer and download a DRC or rules file that you can upload to your PCB design application. This way, you can ensure that your design fits the manufacturer’s tolerances.
Run a DRC check before sending any of your files for fabrication, as it’ll be much easier to correct your board sooner rather than later. After sending your printed circuit over to the manufacturer, they’ll likely perform a DRC check themselves.
The best way to create designs for easy circuit manufacturing is to employ an advanced PCB application package capable of real-time DFM checking, such as Cadence Allegro. With Cadence Allegro, you’ll be able to enable all sorts of PCB editor tools that will enable better design decisions, reduce back-and-forth from your manufacturer, and create stronger, more reliable circuit boards.
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