Understanding the PCB Fabrication Process

 

Introduction

Printed circuit boards (PCBs) serve as the foundation for connecting and supporting electronic components in virtually every type of electronic equipment. The fabrication of PCBs involves many complex chemical and mechanical processes that must be tightly controlled to consistently produce quality boards. In this article, we will provide a comprehensive overview of the major steps involved in manufacturing printed circuit boards from start to finish.

PCB fabrication combines materials science, chemistry, physics, and precision engineering across a range of processes. Gaining a strong understanding of the nuances of PCB manufacturing can help designers, engineers, and procurement specialists better optimize their board designs for design for manufacturing (DFM) and achieve high yields during volume production. We will cover common terminology, key process steps, critical design considerations, trends in fabrication technology, and more across rigid, flexible, and specialty PCB categories.

PCB Fabrication Process Overview

The major steps involved in fabricating a printed circuit board are:

1. Design and Layout
2. Prototype Engineering
3. Fabrication
4. Assembly
5. Testing and Inspection

This article will focus primarily on detailing the fabrication process itself, as highlighted above. First we will briefly introduce the other stages to understand how fabrication fits into the overall board development workflow.

PCB Design and Layout

Before fabrication can begin, the PCB layout must be designed in software based on schematics and the target electronics application requirements. PCB layout involves placing component footprints, routing traces, assigning power/ground planes, defining layer stackups, and more. The completed design is output as Gerber files that specify all conductive and non-conductive layers and drill holes.

Prototype Engineering

Once the initial board design is complete, prototype boards are often fabricated in small quantities to verify the design and conduct testing. Feedback from prototypes will reveal any needed layout revisions. Multiple prototype iterations may be produced before moving to full production fabrication.

PCB Assembly

After bare PCB fabrication and any post-processing steps are complete, electronic components are soldered onto the boards through surface mount technology (SMT) and/or through hole assembly processes. Fully assembled boards are what get integrated into finished products.

Testing and Inspection

Each fabricated PCB undergoes electrical testing and visual inspection at multiple stages to catch any defects before the board progresses further. Different sets of tests are conducted on bare boards vs. assembled boards to ensure quality.

With this context of the full development workflow, we will now do a deep dive into the key steps that take place during professional PCB fabrication itself.

PCB Fabrication Process Steps

The major fabrication process steps that transform base materials into completed printed circuit boards are outlined below.

Overview of the major PCB fabrication process steps

We will explore each of these steps in detail across the sections below.

Step 1: Substrate Preparation

The substrate forms the base of the PCB, providing mechanical support and insulating dielectric layers between conductive copper sheets. The material used depends on the PCB type - commonly fiberglass-reinforced epoxy FR-4 for rigid boards or polyimide for flex circuits. The substrate starts as large panels that undergo initial preparation steps:

Cutting - Sheets are cut to size based on the finished PCB dimensions. Allowance is added for an unrouted border.

Cleaning - Substrates are cleaned to remove any dust, oils, or debris that could impact lamination or imaging processes.

Grain Etching - Glass fabric weaves in materials like FR-4 can leave microscratches when cut that impact copper adhesion. A mild glass etch cleans the fabric surface.

Desmear - Some substrates are processed with a chemical desmear operation to remove residue in drilled holes and improve adhesion.

Proper substrate preparation sets the stage for laminating high quality metal foil layers.

Step 2: Metallization

In this step copper foil is laminated onto the substrate to form the conductive layers of the PCB. There are several metallization sub-steps:

Foil Cutting - Copper foil sourced on large rolls is cut into sheets sized for the PCB substrate. 1 oz/ft2 (35 μm) foil is common but thicker and thinner foils are also used.

Lamination - The copper foil sheet is aligned over the substrate and laminated using heat and pressure to bond them together into a foil-clad panel. The thickness of the bond line adhesive is tightly controlled.

Oxide Treatment - The copper surface is treated to create a bonding oxide layer that improves laminate strength. Common methods are brown, black, or AC/DC plating oxides.

Surface Preparation - Chemical steps like microetching and pickling prepare the foil for coating while optimizing anchor profile.

Multilayer boards go through repeated cycles of layup, lamination, and drilling/metallization to build up alternating layers of copper and dielectric.

Step 3: Imaging

The conductive traces, pads, vias, and other features that make up a PCB layout are formed by selectively removing unwanted copper through an imaging process. Photolithography is the most common technique. The key sub-steps include:

Coating - The copper layer is coated with a light sensitive photoresist material by dipping or spraying. The resist thickness depends on the copper weight.

Pre-Baking - The board is baked at low temperature to drive out solvents from the liquid photoresist and improve coating adhesion.

Exposure - The PCB layer is aligned under the desired phototool image and exposed to UV light. Exposed areas of the resist chemically change to become soluble.

Developing - The PCB is sprayed with a chemical developer solution. Exposed resist is dissolved away while unexposed areas remain, leaving a patterned resist mask.

Etching - Exposed copper not protected by resist is etched away by a copper etching chemistry to form the conductive pattern. Common etchants include ammonium persulfate and ferric chloride.

Stripping - The remaining photoresist is stripped away using a chemical solution, leaving just the desired copper traces on the substrate.

Photolithography transforms the raw copper-clad laminate into a patterned PCB layer with the layout artwork features defined.

Step 4: Hole Formation

Holes are made through the board where component leads or vias will interconnect between layers. The hole formation techniques used depend on the hole size and PCB complexity.

Mechanical Drilling - For standard through holes above 0.3mm diameter, high speed CNC mechanical drilling is commonly used. Drills are arranged on multi-spindle racks.

Punching - Hollow tip punches can cut out large holes up to 6mm for connectors and other through-hole components.

Laser Drilling - Laser drilling machines use pulsed UV lasers to ablate small microvias <=0.15mm diameter through layers. Zero mechanical force is advantageous.

Plasma Etching - Plasma can selectively remove dielectric in multilayers to selectively form blind/buried vias.

Good hole wall quality and tolerances are critical for achieving reliable interconnects between layers.

Step 5: Metallization and Plating

The board layers must be conductively plated to form connections in the drilled through holes and build up copper thickness. Common steps include:

Desmear - The hole walls are chemically scrubbed to remove drilling debris and improve plating adhesion.

Electroless Copper - An auto-catalytic electroless copper process coats the hole walls and any bare laminate with 1-2 μm of copper.

Electrolytic Copper - Thicker copper up to 30 μm is built up over the initial electroless layer using an electrolytic copper plating bath. Current is applied to the PCB functioning as a cathode.

Outer Layer Plating - Outer conductive layers are also thickened beyond the starting 1 oz foil through additional electrolytic copper plating.

Plating Resist - A plating resist with openings where copper is desired is applied prior to electrolytic plating steps to localize the copper buildup.

Metallization makes the PCB layers and drilled holes conductively continuous.

Step 6: Coating

A solder mask coats the entire PCB providing insulation and mechanical protection, with openings left at pads and traces for components and attachments. Screen printing or photoimageable processes can be used to pattern the mask. Common solder mask materials are epoxies or acrylics loaded with insulation pigments.

In addition, a surface finish is applied to exposed copper pads. Common finishes are organic solderability preservatives (OSPs), immersion tin, immersion silver, and electroless or electrolytic gold. These finishes protect the copper and promote soldering.

Step 7: Legend Printing

Printed silkscreen legends on the PCB identify components, polarity, connections, board reference information, and other markings. White epoxy ink legend printing relies on etched artwork films. Inkjet printing offers greater flexibility particularly for prototyping.

Step 8: PCB Routing

Once fully fabricated, the individual PCBs are separated from the larger panels by mechanically routing around the board perimeter. Diamond cut router bits cut through the substrate laminate. Stress relief routing and other edge preparations may also be incorporated.

This completes the fabrication process, resulting in finished bare PCBs ready for assembly.

Key PCB Materials and Chemistries

We will briefly highlight some of the critical materials, chemical processes, and chemistries involved across the fabrication steps outlined above. These collectively enable the transformation of raw substrates into high performance functional boards.

• Substrate Materials - FR-4, polyimide, PTFE composites, ceramic-filled PTFE, hydrocarbons, etc.
• Metal Foils - Rolled copper, electroplated foils, zinc-coated foils
• Photoresists - Liquid, film, and dry film variants for photolithography patterning
• Etchants - Ammonium persulfate, ferric chloride, cupric chloride for copper etching
• Plating Solutions - Electroless copper, electrolytic copper, tin, nickel, gold, and silver plating baths with brighteners and additives
• Solder Masks - Solvent-based liquid solder masks and UV-exposed photoimageable masks
• Legend Inks - Solvent resistant epoxy two-part ink systems
• Bonding Adhesives - FR-4 prepregs, polyimide adhesives, z-axis conductive films

The interaction between this vast array of chemicals, metals, polymers, and substrates enables the intricate fabrication processes fundamental to producing quality PCBs.

PCB Fabrication Methods by Board Type

While the overall fabrication flow is generally similar across PCB categories, some processes and capabilities differ significantly between rigid boards, flex/rigid-flex boards, and more specialized board types. We will briefly compare some of the fabrication distinctions.

Rigid PCBs

Made from stiff insulating substrates like FR-4, rigid boards represent the bulk of PCB production through well-established fabrication processes. Multilayer boards with 6-36+ layers are common. Key characteristics include:

• Multiple drilling/metallization cycles to form vias between layers
• High layer counts to achieve component density
• Fine line photolithography down to 25 μm traces
• Plated through hole (PTH) interconnections between layers
• Automated optical inspection (AOI)

Flexible Circuits

Flex PCBs fabricated from polyimide substrates bent and flexed in use require some modified fabrication approaches:

• Flex-specific adhesives for bonding metal foils
• Tight control of material thickness for flexibility
• Coverlay laminates over traces for insulation and protection
• Flex-to-rigid interfaces to connect with rigid boards
• Limited use of plated through holes
• Electrical testing of flexure endurance

Rigid-Flex PCBs

Offering both rigid board areas and flexible interconnections in one assembly, rigid-flex manufacturing combines aspects of both:

• Integration of flexible polyimide sections into rigid FR-4 boards
• Layer buildup processes tailored for each section's materials
• Careful structural design to avoid stresses at the rigid-flex junction
• Advanced processes to accommodate thickness changes
• Precise alignment during lamination

HDI PCBs

High density interconnect (HDI) boards with trace pitches below 100 μm have additional demands:

• Thinner dielectrics, 1 oz and 0.5 oz copper foils
• Tight material tolerance control
• Microvias formed by laser drilling
• High-precision laser ablation patterning
• Buildup dielectric technologies
• Sequential lamination processes
• Sophisticated process control algorithms

Metal Core PCBs

Using thermally conductive metallic substrates like aluminum or copper:

• Dielectrics must adhere to metal cores
• Via formation methods suit non-glass substrates
• Careful CTE matching to avoid delamination
• Modified imaging processes
• Vias help dissipate heat into the metal core

There are a wide range of PCB types optimized for different applications, with each presenting unique fabrication opportunities and challenges.

Design for Manufacturing Guidelines

To achieve high fabrication yields and optimal quality printed circuit boards, designers should follow design for manufacturing (DFM) guidelines and rules. Some key DFM considerations include:

Trace Width and Spacing - Match trace dimensions to fabrication capabilities. Excessively thin traces or tight spacing will experience high defect rates.

Via Styles - Select via types compatible with layer count and board thickness like blind, buried, microvias.

Pad Styles - Use appropriate pad shapes/sizes for lead types - through-hole, J-lead, gull-wing, BGA, etc.

Annular Rings - Maintain sufficient annular rings on pads for reliable solder joints without shorting.

Planes - Use power, ground, and voltage reference planes properly spaced from signals.

Impedances - Control trace impedances through width tuning and material selection.

Thermal Relief - Include thermal relief connections on high current traces/pads.

Legend and Silkscreen - Ensure legend text size, stroke width, and location is manufacturable.

Panel Utilization - Lay out PCBs to maximize use of panel area and minimize scrap between boards.

Tooling and Fiducials - Incorporate board fabrication tooling holes and fiducial markings.

Adhering to DFM principles avoids design choices that lead to manufacturing defects and reliability risks.

Quality Control and Testing

To deliver high functioning printed circuit boards, rigorous quality control steps are implemented throughout the fabrication process:

• Visual inspection of raw materials
• Monitoring of chemical bath parameters
• Statistical process control tracking
• Test coupons to validate processes
• First article inspections on new designs
• In-process electrical testing
• Automated optical inspection (AOI)
• Destructive physical analysis on sample boards
• Assembled board functional testing
• Documentation at every fabrication step

Testing and inspections ensure any defects or non-conformances are caught as early as possible to avoid additional value-add processing on faulty boards.

PCB Fabrication Technology Trends

There are several emerging trends in fabrication processes and capabilities:

Additive Processes - Additive semi-additive and fully-additive processes build up circuitry rather than etching away copper. This avoids etching waste and enables finer features.

Inkjet Solder Mask - Replacing screen and photoimageable printing with inkjet deposition allows faster mask revisions and improves yields.

Direct Metallization - Technologies like modified semi-additive can plate dielectrics and traces sequentially without traditional laminates.

3D Printing - Additive 3D printing using conductive pastes can generate multilayer PCBs without conventional fabrication.

Automation - Increasing automation with algorithms and analytics to model processes improves quality control.

High Frequency - Fabrication innovations to tightly control tolerances and materials continue advancing high frequency PCBs above 50 GHz.

Conclusion

• PCB fabrication involves the precise sequencing of many chemical, mechanical, and material processes to transform base substrates into functional boards.
• Key steps include substrate preparation, metallization, imaging, hole formation, plating, coating, legend printing, routing, and testing.
• The process flow varies for board types like rigid, flex, rigid-flex, HDI, and metal core PCBs based on their specialized requirements.
• Following design for manufacturing guidelines avoids design choices that lead to fabrication defects.
• Extensive quality control steps and testing validates fabrication processes and finished boards meet specifications.
• Advances in additive processes, direct metallization, automation, and high frequency manufacturing capabilities are emerging.

Understanding the nuances of professional PCB fabrication leads to improved design optimization. As fabrication technology continues evolving through innovations in materials, processes, and quality control, PCBs will meet the increasing demands of emerging electronics.

Frequently Asked Questions

What are some key benefits of using a photoimageable solder mask?

Photoimageable solder masks offer tighter resolution and alignment vs. screen printed masks. They also enable easier and lower cost mask revisions since new images can be directly patterned on panels rather than requiring new screens.

What is the difference between HDI and conventional PCB fabrication?

HDI PCBs use thinner dielectrics, smaller microvias, and finer traces/spaces below 100 μm. This requires much tighter process control and advanced technologies like laser drilling and buildup dielectric deposition to achieve the high density interconnects.

Why is surface preparation important prior to laminating copper foil?

Proper surface preparation through steps like chemical etching optimizes the roughness profile of the substrate and foil surfaces. This improves adhesion strength during lamination to achieve robust copper-dielectric bonding free of delamination defects.

How does electroless copper plating work?

The electroless copper process is auto-catalytic where a chemical reducing agent reacts to deposit copper onto catalytic surfaces. No external current is required