Introduction

Printed circuit board (PCB) fabrication involves multiple steps to turn a design into a physical board. PCB etching is the fundamental process used to selectively remove copper from a copper-clad laminate to form the conductive traces and pads.

In this comprehensive guide, we walk through the end-to-end PCB etching process step-by-step including:

• Circuit design
• Transferring the layout
• Preparing the copper board
• Photolithography
• Etching the copper
• Drilling holes
• Finishing the board

By understanding the complete etching workflow, electronics hobbyists, students, and engineers can develop quality printed circuit boards right in their own workshop. Let’s get started!

Step 1 – Design the Circuit

Schematic Capture

The first stage is developing the electronic circuit schematic. This involves:

• Placing symbols for each component – resistors, capacitors, ICs etc.
• Connecting them together to form the desired circuit topology.
• Assigning component values and characteristics.
• Annotating the schematic with any needed text, labels, dimensions etc.

Several PCB Design tools like Eagle, KiCad, OrCad, Altium etc. provide a graphical schematic editor to draft the circuit schematic. Attention should be paid to the schematic clarity and readability.

PCB Footprint Association

Next, each schematic symbol is linked to a PCB footprint. The footprint defines the land pattern on the PCB needed to mount that component. This linkage allows moving from schematic to board layout.

Component footprints for common parts like resistors and caps are available in component libraries. Unique or custom footprints may need to be created for specialized components.

Design Rules and Constraints

Circuit performance depends heavily on board layout. Thus, certain design rules and constraints are defined upfront for the PCB:

• Trace widths – Thicker traces for high current lines
• Clearances – Minimum distances between copper elements
• Critical lengths – Matching trace lengths for differential signals
• Impedance – Controlled trace impedance for high-speed signals
• Board thickness – Thin for flexibility, thick for rigidity

These parameters influence the routing and ensure the PCB behaves electrically as intended.

Step 2 – Transfer the Layout

With the circuit schematic finalized, the physical PCB layout can be designed. There are several techniques to transfer the layout onto the copper board:

Laser/Inkjet Printing

The PCB layout software can directly print the layout onto paper or film. For better resolution, laser printers, inkjet photoplotters, or high-end imagesetters are used.

Advantages:

• Fast and easy for prototyping

Disadvantages:

• Lower resolution and accuracy

Photolithographic Film

A glass photomask is produced that exactly replicates the PCB layout. Shining UV light through the mask selectively exposes a photoresist coating on the PCB.

Advantages:

• High resolution and fidelity
• Suitable for mass production

Disadvantages:

• Requires longer lead time and higher cost

CNC Machining

A computer numerical control (CNC) milling machine selectively mills away copper to physically etch circuits into the PCB material.

Advantages:

• Fast and precise material removal

Disadvantages:

• High equipment cost
• Limited resolution

Toner Transfer

A laser print of the PCB layout is ironed onto the copper board using heat. The toner sticks to the copper serving as an etch resist.

Advantages:

• Simple and inexpensive

Disadvantages:

• Lower consistency and resolution

Step 3 – Prepare the Copper Board

The raw PCB material is a dielectric laminate with copper foil on one or both sides. The copper foil gets etched into the desired conductive pattern.

Board Type

Common laminate materials include:

• FR-4 – Woven fiberglass with flame-resistant epoxy resin. The most common and cost-effective choice.
• CEM-1 – Cotton paper base composite epoxy substrate. Low cost but less durable than FR-4.
• FR-2 – Phenolic resin bonded paper. Obsolete but inexpensive. Absorbs moisture easily.
• GETEK – Woven fiberglass reinforced polyimide film. Withstands very high temperatures.
• Rogers – High frequency circuit materials available in different dielectric formulations. Costlier but better electrical performance.

The laminate material determines key board characteristics like operating frequency, loss tangent, thermal performance etc.

Copper Thickness

PCBs typically use rolled copper foil on the laminate. Common thicknesses are:

• 1 ounce (35um) – Low current wiring, cost effective
• 2 ounce (70um) – Heavier wiring and traces
• 3-6+ ounce (105um+) – High current power distribution

Thicker copper provides higher current capacity but also makes etching slower.

Copper Surface Treatment

For photolithographic etching, the copper foil requires:

• Cleaning – To remove oils and surface contamination
• Microetching – For better photoresist adhesion

A thin chemical oxidation coating helps improve photoresist adhesion and prevent copper oxidization.

Panel Sizing

Raw PCB material is available in standard panel sizes 12″x18″, 18″x24″ etc. The circuit boards are nested inside these panels for fabrication. Individual PCBs are cut from the panel after etching.

Safety Precautions

• Use chemical resistant gloves, eye protection and fume exhausts
• Avoid breathing fumes from chemicals and copper dust
• Follow manufacturer safety precautions for each chemical

Step 4 – Photolithography

Photolithography uses light to selectively expose a photosensitive film applied on the copper laminate. This process involves:

Cleaning

Alkaline cleaners remove dirt, oil and debris from the copper surface. Abrasives can also be used but risk eroding fine copper features.

Microetching

A mild sulfuric acid etchant roughens the glossy copper surface creating micro-roughness for better photoresist adhesion.

Photoresist Application

The printed PCB image serves as a mask for exposing the photoresist coating on the copper foil. Two types of photoresist exist:

• Negative Photoresist – Becomes insoluble on exposure to light. Unexposed areas dissolve away during development.
• Positive Photoresist – Becomes soluble on exposure. Exposed areas dissolve away.

Liquid or dry film photoresist layers can be laminated onto the PCB surface.

Soft Bake

A gentle bake removes residual solvents from the photoresist coating and improves adhesion.

UV Light Exposure

Next, the PCB image mask is placed over the photoresist coating and exposed under a UV light source. The mask selectively blocks UV in some areas.

Developing

A chemical developer dissolves away either the exposed or unexposed photoresist regions depending on whether a positive or negative photoresist was used.

Hard Bake

After developing, a final cure hardens the photoresist and improves chemical resistance for the etching step.

This photolithography process transfers the conductor pattern onto the copper layer creating an etch-resistant mask.

Step 5 – Etch the Copper

With the photoresist mask defined, it’s time to etch away the unwanted copper regions. The main methods include:

Wet Chemical Etching

Immersing the PCB in an etchant liquid dissolves away exposed copper regions. Common etchants include:

• Ferric Chloride – The most common and inexpensive etchant option.
• Ammonium Persulfate – Slower but higher performing etchant.
• Sodium Persulfate – Similar to ammonium persulfate.
• Hydrogen Peroxide and Sulfuric Acid – A strong etchant mixture.

Temperature and agitation accelerate etching. The PCB is rinsed after the unmasked copper fully dissolves.

Dry Etching

Reactive ion etching uses an ionized gas plasma to sputter away copper. Less undercutting but requires vacuum equipment.

CNC Milling

A CNC milling bit directly scrapes away copper not needed in the final layout. No chemicals required but limited resolution.

The goal is anisotropic etching that removes copper vertically with minimal undercutting. This achieves accurate pattern transfer from the photoresist mask.

Step 6 – Drilling Holes

Holes are drilled through the PCB to mount components and provide interconnections between layers in multilayer boards.

Drill Bit Materials

• High Speed Steel (HSS) – General purpose suitable for most PCB materials. Low cost.
• Cobalt-Tungsten Carbide – Far more rigid than HSS. For high accuracy holes in challenging materials.
• Diamond-coated – Extreme rigidity and hardness for highly abrasive substrates like ceramics and glass.

Drilling Technique

• Through-hole drilling fully punctures the PCB. Allows component leads to pass through.
• Blind and buried vias create a cavity between layers. Useful on multi-layer boards.
• Laser drilling can create micrometer scale vias unattainable with drill bits.
• Mechanical punching/routing removes larger irregular slots for connectors etc.

Automated numerically controlled drilling provides the precision and consistency needed for mass PCB production.

Step 7 – Complete Board Fabrication

After etching and drilling, a few final steps complete the PCB fabrication process:

Resist and Copper Stripping

Any remaining photoresist is chemically stripped away. A copper etchant removes the small amount of residual copper underneath the resist. This ensures a clean conductive surface.

Conductive Hole Plating

The interior barrels of any drilled through holes are plated with copper to form a conductive interconnection between layers.

Soldermask Application

A solder mask coats areas of exposed copper that don’t require soldering. This acts as insulation and prevents solder bridges.

Silkscreen Legending

Silkscreen printing applies labels, markings, logos, and component designators.

Surface Finishing

Finished PCBs can be hot air solder leveled (HASL) by depositing a thin layer of solder onto exposed pads and traces. This protects copper traces from oxidation.

Electrical Testing

Each board is electrically tested against open/short circuits and functionality criteria. Debug any failing boards.

Panelization

Individual PCBs are depanelized by sawing/routing/breaking apart the larger fabrication panel.

This completes the PCB ready for population with components!

Conclusion

In summary, PCB etching allows selectively removing copper to form the conductive pattern on printed circuit boards. By mastering essential processes like photomasking, wet etching, precision drilling, and surface finishing both small-scale and high-volume PCB production is achievable. While demanding expertise across chemistry, materials and fabrication disciplines, etching copper-clad laminates remains the cornerstone of transforming electronic circuit designs into functional boards.

PCB Etching FAQs

Here are some common questions about PCB etching:

Q: What are the main advantages of photolithographic PCB etching?

A: Photolithography provides high accuracy and repeatability. It is suitable for mass production volume. Complex trace geometries are achievable.

Q: What etchant chemicals work best for etching copper?

A: The most common options are ferric chloride, ammonium persulfate, and cupric chloride. Ferric chloride is popular being inexpensive and reasonably fast acting.

Q: What are some key parameters that control the etching rate?

A: Etchant temperature, chemistry concentrations, fluid agitation, board/mask alignment, and copper thickness all impact etch rate.

Q: Is dry etching used for high volume PCB production?

A: Dry etching like plasma etching is rarely used due to the high equipment cost. It is primarily used in semiconductor fabrication where better anisotropy and resolution justifies the tradeoff.

Q: What are common steps used to prepare the copper surface prior to photoresist?

A: Cleaning using alkaline solutions followed by a microetch to create surface roughness for photoresist adhesion.

Q: How are holes drilled in circuit boards?

A: Mechanical drilling using carbide or diamond drill bits is most common. Laser drilling offers finer scale vias. Punching is used for odd shaped cutouts.

Q: What are some signs of an under or over etched PCB?

A: Underetched boards have residual copper causing potential shorts. Overetched boards have excessive trace thinning and undercutting.

Q: What safety precautions should be taken when wet chemical etching?

A: Use etching chemicals in a well ventilated area. Wear appropriate PPE – gloves, goggles, mask. Follow safe handling procedures.