PCB Fabrication Techniques
As a PCB designer, it can be tempting to leave the details of the manufacturing process up to the fabrication house. However a little understanding of the manufacturing process upfront can help prevent headaches further down the line.
In this post we’ll cover the different types of PCB fabrication techniques, and what you as a designer need to know about designing for manufacturing.
Imaging is where the magic happens: your digital PCB design gets transferred onto the physical board. These days photoimaging is the way to go, the general idea being to use a material that hardens upon exposure to ultraviolet light called photoresist. Here’s how it generally works.
A dry film is produced by inkjet printing the negative image of the PCB circuit design.
The copper surface of the board or panel is coated with liquid photoresist.
The film is aligned with the copper surface of the board and subjected to UV light.
Only the exposed areas of the photoresist harden into place, protecting the copper traces underneath.
The remaining liquid photoresist is removed and the board is ready for etching to remove the excess copper leaving the traces behind.
Alternatively, if you’re working with a fabrication house they may have more expensive equipment which allows you to skip the dry film and directly apply the pattern to copper coated with photoresist via a UV laser.
Etching is the process of removing excess metal from a PCB with an industrial solvent. It’s typically performed after the imaging step to remove the excess metal away from the traces protected with hardened photoresist. It is also used to establish a uniform surface. Common etching chemicals include ferric chloride, cupric chloride, alkaline ammonia, and ammonium persulfate.
Your typical PCB consists of multiple layers of copper interspersed with non-conductive substrate (usually epoxy impregnated fiberglass) such as FR4. Lamination involves using heat and pressure to melt together the different layers of a PCB.
Single-layer laminate: One substrate layer with copper laminated on one side.
Two-sided laminate: One substrate layer laminated with copper on both sides, often called a core in multi-layer laminates.
Multi-layer laminate: Typically start with one core, and stack alternating layers of prepreg (substrate without the copper) and copper in both directions to the specified number of layers.
As the PCB designer, it’s important to be aware of the things that can go wrong during the lamination process so that you can be prepared to modify your design accordingly. For a first time fab, the house will typically do a few prototype runs to hone in on an ideal lamination heat profile. Depending on the results, you may need to scale your images to account for any shifting that may occur as a result of the materials and thicknesses used in your build.
Machining can occur multiple times throughout the PCB manufacturing process depending on your design. Let’s take a look at where machining is used in PCB manufacturing:
Through-holes and vias: It’s possible to save time and money by stacking multiple boards together, securing them in place with stakes, and drilling your through holes. In general, you’ll want to drill non-plated holes and vias towards the end of the manufacturing process, after you’ve applied photoresist, soldermask, and silkscreen.
Panelization: Panelization saves time by allowing multiple boards to be fabricated and tested at once as a panelized array. A drill can be used to route channels, machine vscores or create breakaways (mouse bites), to allow easy removal of the boards towards the end of the manufacturing process.
PCB thickness, material choice and types of drilling (mechanical or laser) are important considerations for manufacturability. To drill through fiberglass such as FR4, you need a good tungsten carbide drill bit. If your design has smaller holes it means you’d have to use thinner drill bits. These thin drill bits tend to snap easily, so the more small holes you need to drill into a board, the more a fabrication house will tend to charge you.
It’s also possible to use a laser instead of a drill bit for drilling smaller vias. The laser is less appropriate to use for larger diameters. So, it’s important to use the appropriate machining method for your design.
Need to apply a metal finish (e.g. gold) to your through-holes and vias? You’ll need to plate it onto the desired surface using one of these common techniques:
Electrolytic Plating: Good for high-volume finishing projects. The boards are bathed in a concentrated solution of the plating metal. A current is applied to plate the exposed metal surfaces on the board (your through-holes and vias) via electrolysis.
Electroless Plating: Instead of using a current to plate metals in a solution onto your board, you can also use catalysts and self-reducing agents to apply a finish (no external current required). The main advantage is a more even coating that is not susceptible to anomalies that may be caused by current flowing across an irregular shape.
Plasma/Dry Plating: For fine-line circuit plating, an inert gas plasma is used to remove metal particles from a charged target to be redeposited onto the target surface. The process must be operated under a vacuum.
For most PCB designers, it’s easier and cheaper to send boards to an outside fabrication house to bring their projects to life. Understanding the different fabrication techniques that are at your disposal can help you compare fabrication houses and identify good deals. If nothing else, being mindful that things such as hole size, the number of holes, line widths, spacing, thicknesses, and layers can all impact cost. Designing boards with manufacturability in mind can prevent delays and redesigns. Check out Cadence’s suite of PCB design and analysis tools today.
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