Full Introduction to the PCB Fabrication Process

 Printed circuit boards (PCBs) serve as the foundation for electronics that power countless products we use daily, from phones to cars to medical devices. Fabricating PCBs involves many intricate steps to transform raw materials into the dense interconnects underpinning modern electronics. This article will provide a comprehensive overview of key processing steps, design considerations, and manufacturing techniques used to produce PCBs.

PCB Basics

A PCB mechanically supports and electrically connects components using conductive copper traces etched from copper sheets laminated onto a non-conductive substrate. PCBs provide:

• Mechanical support - Component anchors and structural stability
• Electrical connectivity - Defined conductive paths between components
• Thermal conduction - Heat spreading from hot components
• Environmental protection - Enclosing components from outside elements

PCB Fabrication Process Overview

Major fabrication steps include:

1. Design - Circuit schematics and board layouts are created with EDA software
2. Prototyping - Initial test boards are built and evaluated
3. Fabrication Data Prep - Manufacturing files are generated from electronics design data
4. Board Materials Processing - Raw board materials are manufactured
5. Imaging - Photoresist patterns are selectively exposed onto copper laminates
6. Etching - Exposed copper is chemically dissolved away
7. Drilling - Holes are mechanically drilled for vias and mounting
8. Plating - Conductive surface finishes are plated onto exposed copper
9. Solder mask - Non-conductive coating with selective openings
10. Silkscreen - Printed component legends and identifiers
11. Panelization - Boards are arranged for batch assembly
12. Assembly - Components are soldered onto boards
13. Testing - Quality validation and performance verification
14. Depanelization - Individual boards are routed from panels

These stages require coordination across design, materials science, chemistry, machining, and quality assurance to transform product concepts into functional PCBs. We will explore each step's key considerations in detail.

PCB Design

Before manufacturing begins, the product must be carefully designed using electronic design automation (EDA) software. Key aspects at this concept stage include:

Schematic Capture

Abstractly representing circuit connectivity between components defines logical functionality decoupled from physical implementation. Simulation validates operation.

PCB Layout

Physically placing components and routing traced connectivity subject to manufacturing constraints enables implementation. Design rules, allowable layers, and tolerances guide layout.

Library Management

Central repositories of qualified parts, footprints, symbols, and supplier data streamline incorporating validated ingredients.

Design Rule Checking

Algorithmic analysis against stacked up clearance constraints immediately flags violations. Electrical ( data fidelity prevents incorrect transmission to manufacturing.

With circuit performance verified in simulation and board layouts passing manufacturing checks, design data release kicks off converting concepts into testable prototypes.

Prototyping

Before high volume fabrication, prototyping validation allows assessing and refining design embodiments:

Proof of Concept

Simplified functionality boards using modest components confirm core operation early. Loose tolerances suffice.

Engineering Validation

Adding instrumentation and exercising rigorously against specifications qualifies performance capabilities.

Manufacturing Assessment

Built using intended fabrication processes, tests assess mass production readiness regarding yields, rework rates,calibration iteration cycles to finalize specifications.

Fabrication Data Preparation

Before manufacturing, PCB layouts are processed into detailed fabrication data packages:

Gerber Files

Standard outputs detailing copper layers, solder mask, silkscreen, drill files, and more for photoplotting.

NC Drill Files

Numerical control toolpaths for drilling/routing machines with hole sizes and locations.

Assembly Drawings

Mechanical drawings depicting component placements and identifiers to guide assembly.

Bill of Materials

Listing of every part, supplier, value, and designation referenced on the boards.

Netlists

Connectivity lists capture components between schematic symbols and physical land patterns

3D Models

Photo-realistic board renders assist design reviews. Include assembly models with components.

Careful checking that data correctly conveys design intent is vital before manufacturing work launches.

Raw Board Materials

High performance PCBs require advanced raw materials:

Substrate

The base laminate mechanically supports traces and components. Key properties:

• Dielectric Constant - Impacts signal speeds
• Loss Tangent - Signal lossness
• Thermal Conductivity - Heat transfer ability
• Coefficient of Thermal Expansion - Change with temperature
• Moisture Absorption - Environmental sensitivity
• Z-Axis CTE - Layer bonding compatibility

Conductor

Almost universally copper foils, key attributes are:

• Purity - Iron or organic contamination hurts conductivity
• Surface Roughness - Smoother finish aids finer lines
• Grain Structure - Balance conductivity with flexibility

Prepreg

Pre-impregnated epoxy layers bond stacked laminate sheets under heat and pressure. Consistent resin flow is crucial.

Soldermask

Liquid coatings selectively mask copper traces from shorting or oxidation. Must withstand subsequent processing.

Legend Ink

Printed markings identify components. Durability and flexibility matter.

Plating Deposits

Electrolytic and electroless plating form conductive pads, vias, component finishes, and protective layers from chemical solutions.

Imaging

Photolithographic selective resist patterning transfers the designed layout:

Copper Surface Preparation

Abrasion removes oxides ensuring resist adhesion. Chemical microetch also enhances anchor profile.

Resist Application

Light-sensitive polymer liquid coatings are deposited across sheets by spray coating, roller coating, electrodeposition, or lamination film. Dip coating also used on traces. Consistent known thickness is key.

Mask Alignment

Chromium glass photomasks with transparent layout features precisely align to copper layers through registration holes or fiducials.

Exposure

Light or laser energy polymerizes resist resin in exposed areas rendering solubility change. Ultraviolet wavelengths commonly used.

Developing

Chemical solvent washes away more soluble resist areas after exposure while crosslinked resist remains patterned.

These lithographic processes achieve selective shielding for subsequent etching.

Etching

Chemical etching transfers desired conductor patterns into the copper foil based on resist protection:

Wet Etching

Immersion baths dissolve exposed copper regions not protected by resist leaving only desired traces. Most common technique.

Dry Etching

Reactive ion plasmas can directly etch patterns avoiding resist process. Environmental and cost challenges exist.

Key considerations span etch uniformity across and within panels, achieving target trace geometries without under or overcut, handling thin delicate foils post-etching, and residue cleaning after resist stripping.

Drilling

Holes mechanically generate vias between layers and component holes:

CNC Drills/Routers

Numerically controlled machines direct rotating drill bits along predefined toolpaths at fixed feed rates and spindle speeds matching target hole sizes.

Laser Drilling

Pulsed lasers burn through dielectric layers with small diameter holes unachievable by mechanical bits. Limited depth capability. Used to form cavities and some blind vias.

Punching

Mechanical presses cut through sheet materials using shaped blades for high production rates but larger features.

Stringent process controls maintain hole position and size accuracy. Smearing, charring, debris are key concerns.

Plating

Conductive plating forms solderable surfaces, interlayer connections, and components leads:

Electroless Plating

Autocatalytic chemical reactions selectively deposit metal finish directly onto exposed conductor surfaces without external current flow. Often used first to put down thin initial seed layer.

Electroplating

Electric current flows through electrolyte inducing metal ions to adhere onto circuit board conductive surfaces. Builds up thicker, smoother deposits. Tin-lead historically predominant.

Immersion Plating

Chemical exchange reactions substitute atoms in base metal surface with coating metal deposits without external current. Limited thickness.

Plating uniformity, adhesion strength, and continuity profoundly affect board manufacturability and reliability.

Soldermask Application

Next a thin electrically insulating soldermask coating protects traces permitting soldering:

Liquid Photoimageable Soldermask (LPSM)

Light sensitive liquid resin spun, curtain coated, or screen printed onto boards. Similar lithographic exposure and etching forms openings. Most common approach.

Dry Film Soldermask

Hot roll lamination bonds photoimageable films which are then exposed/developed. Handled as solid sheets though less resolution capability.

Direct Imageable Soldermask

Inkjet deposition eliminates lithography steps by printing only required barrier regions in single pass. Still maturing technology.

Even application, cure integrity, and robustness against soldering, cleaning, and handling is essential.