Introduction
Printed circuit board (PCB) manufacturing relies on depositing and patterning conductive materials to form the traces and features that carry signals and power. The conductive pastes and liquids used to create these circuit patterns are referred to as PCB inks.
PCB inks consist of conductive particles suspended in an adhesive formulation. After deposition and curing, these inks make up the copper circuitry that connects components on a PCB. This article provides an in-depth overview of PCB inks including:
- Composition and properties of typical PCB inks
- Different ink types and characteristics
- PCB fabrication processes utilizing inks
- New developments in ink formulations
- Advantages of ink-based circuit fabrication
- Considerations when selecting and applying inks
- Examples of practical PCB ink applications
Understanding PCB ink technologies and capabilities allows engineers to consider the optimal materials and processes to fulfill circuit fabrication needs.
Composition of PCB Inks
PCB inks contain two primary constituents:
Metal Particles – Small conductive metal particles, typically copper or silver, make up 50-90% of an ink’s mass. The key properties of the metal particles include:
- High intrinsic conductivity
- Purity – oxygen-free highly pure metals
- Particle shape – spherical, flakes or mixtures
- Particle size distribution – sub-micron to tens of microns
- Oxidation resistance – for stable shelf life
Adhesive Matrix – A polymer binder adheres the metal particles together and bonds the cured ink to the substrate. Key properties include:
- Solvent compatibility – soluble for deposition
- Cure chemistry – thermal, UV, moisture, catalyst
- Flexibility – maintains conductivity during bending
- Adhesion strength – bond to substrates
- Decomposition temperature – for operating range
The proportions and properties of these two components fundamentally define the performance of a PCB ink. Different ink types make trade-offs between these constituents to achieve specific characteristics.
Types of PCB Inks
There are several varieties of PCB inks formulated for different applications and processes:
Silver Flake Inks
- High solid loading (60-85%) of silver flakes
- Low curing temperature
- Best electrical conductivity but higher cost
Copper Particle Inks
- Moderate solid loading (50-60%) of copper
- Low oxidation provides optimal stability
- Lower cost than silver but higher resistance
Nano Copper Inks
- Copper nanoparticles under 100nm
- Enables low-temperature sintering
- High electrical performance
Nano Silver Inks
- Extremely fine (<50nm) silver particles
- Lower curing temperature while maintaining conductivity
- Printable using inkjet methods
Carbon Inks
- Small graphite or carbon black particles
- Lower conductivity than silver/copper
- Often used for resistive elements
Each ink type exhibits trade-offs between performance, process compatibility, shelf life and cost.
PCB Fabrication Processes Using Inks
PCB inks serve different roles across common fabrication processes:
Printed Electronics
- Inkjet, aerosol, screen and pad printing all deposit tracks
- Low-temperature curing forms circuits
Conformal Printed Circuits
- Inks printed on 3D surface
- Enable flexible physiques
Flexible Circuits
- Inks cured on polyimide substrates
- Withstand bending and flexing
Additive Circuit Formation
- Inks printed on PCB substrates
- Add high density interconnects
Selective Plating
- Ink masking layer defines plating areas
- Removed after plating
- Conductive inks add shielding to enclosures
- Protects electronics
Inks provide versatility across diverse PCB manufacturing processes spanning printed electronics to plating to conformal circuits and more.
Advances in PCB Ink Formulations
Ongoing research and development continues expanding PCB ink capabilities through:
Lower Curing Temperature
- Enables printing on temperature-sensitive substrates
- Prevents component damage during curing
Finer Particles
- Reduces required curing temperature
- Provides higher resolution printing
Hybrid Particle Shapes
- Tailor viscosity, densification and sintering
- Optimize printing and electrical characteristics
Graphene Inks
- Imparts flexibility, thermal conduction
- Potential for lower costs
Copper and Nickel Inks
- Alternatives to costly silver inks
- Nearly matching electrical properties
Biocompatible Inks
- Non-toxic inks for medical electronics
- Avoid harm to patients
Innovation in ink chemistry and materials science unlocks new potential applications.
Considerations When Selecting PCB Inks
Key considerations when choosing a PCB ink include:
Printing Method
- Inks are formulated specifically for deposition techniques
- Screen, inkjet, aerosol and other printing
Substrate Material
- Adhesion strength varies with substrates
- CTE matching prevents delamination
Curing Requirements
- Ink curing method must match equipment
- Temperature, UV, moisture, catalyst
Print Resolution
- Particle size distribution affects achievable features
- Finer particles enable smaller printed details
Shelf Life
- Particle oxidation resistance impacts usable life
- Refrigeration prolongs life
Operating Environment
- Temperature, humidity, mechanical stresses
- Ink must maintain performance across conditions
Properly matched ink, process and substrate produces optimal results.
PCB Ink Application Examples
Here are some examples of practical PCB ink applications:
Flexible Circuits
- Silver polymer thick film ink printed on polyimide
- Withstands repeated bending for wearable device
Conformal Antenna
- Aerosol printed copper ink on curved surface
- Enables non-planar smart device communication
Automotive Sensors
- Copper ink printed circuits on polymer housing
- Integrates PCBs into modules
Medical Patches
- Biocompatible silver/graphene ink circuits
- Adheres to skin for health monitoring
EMI Shielding
- Silver ink applied inside device enclosure
- Protects electronics from interference
Heater Elements
- Highly resistive carbon ink printed on glass
- Provides localized transparent heating
These examples illustrate the diversity of possibilities using printed inks for circuit fabrication.
Frequently Asked Questions
Here are some common questions that arise regarding PCB inks:
Q: What is the main trade-off between silver and copper inks?
Silver inks provide higher conductivity but at a higher raw materials cost, while copper offers a more affordable option with slightly higher resistivity.
Q: What are some methods used to deposit PCB inks?
Common methods include screen printing, inkjet printing, aerosol printing, flexographic printing and syringe dispensing.
Q: How small of circuit features can be produced using PCB inks?
The finest features range from 10-50 microns depending on the printing process resolution, particle sizes, and post-printing curing approach used.
Q: Do PCB ink patterns require plating for most applications?
Often yes, plating improves conductivity and durability. But some applications like embedded heating elements can utilize printed ink-only circuits.
Q: What are the shelf life considerations for PCB inks?
Shelf life varies based on formulation, with silver flake inks lasting 1-3 years refrigerated. Use within 6 months is recommended for optimal results.
Conclusion
PCB inks enable versatile printed circuit fabrication techniques spanning printed electronics, EMI shielding, heaters, sensors, and more. As ink formulations and printing processes continue advancing, ink-based circuit manufacturing empowers new design possibilities and applications. Understanding the composition, types, processes and properties of these conductive inks allows engineers to take full advantage of their capabilities and expand the horizons for PCB innovation.
