Types of Printed Circuit Boards You Should Know

 Printed circuit boards (PCBs) provide the foundation for electronics assembly and packaging. PCBs feature conductive traces and pads to mechanically support and electrically interconnect components using pathway materials laminated onto insulating substrates.

There exist numerous varieties of PCBs designed to serve particular fabrication requirements, assembly intents, environmental needs, and system performance targets. Categorizing boards by material compositions and processing methods distinguishes the primary options available.

PCB Types by Materials and Composition

PCB properties depend significantly on the materials laminated and layered together, including conductors, insulators and reinforcements. Key material types include:

FR-4 Boards

The most common and inexpensive boards consist of layers of woven fiberglass cloth combined with an epoxy resin binder (FR-4). These rigid laminates clad with copper foil traces handle most low-frequency consumer and light industrial applications. FR-4 provides reasonable performance across thermal, chemical, mechanical and electrical properties at low cost.

CEM Boards

Composite epoxy material (CEM) boards offer increased resilience versus FR-4 boards for more demanding flexible and rigid-flex circuit needs. CEM-1, CEM-2, and CEM-3 laminates involve glass fabric composites (CEM-1) or non-woven aramid fiber mats (CEM-3) combined with epoxy resin and additives for flexibility and heat resistance.

Ceramic Boards

Ceramic substrate boards made from aluminum oxide, aluminum nitride, or other mineral compounds provide high thermal conductivity plus environmental and temperature resilience for specialty applications such as military systems. However, ceramics tend to be expensive and brittle. Ceramic PCBs typically combine with other insulating and conductive materials in hybrid constructions.

Metal Core Boards (MCPCB)

MCPCBs utilize a base metal layer like aluminum or copper instead of FR-4 laminate to spread and dissipate heat extremely well while providing rigidity. These boards offer thermal management to handle power devices. Insulating dielectric separates external copper layers from inner metal cores.

PCB Types by Number of Layers

PCBs classify into single, double, and multilayer boards by the number of conductive copper foil circuit layers bonded by insulating dielectric substrates:

Board Type	Layer Description
Single Layer	One layer of circuitry
Double Layer	Two conductive layers with traces
Multilayer	Three or more circuit layers stacked

Additional layers enable increased component densities and routing channels even in tight spatial confines to handle complex, miniaturized circuit needs. High layer counts allow splitting ground planes and signals across parallel board layers. However, each extra layer adds fabrication cost and thickness. High-density interconnect (HDI) methods can produce 20+ thin layers PCBs.

PCB Types by Supported Components

Boards tailor towards compatibility with leaded or leadless components via plated through holes, pads and other mounting provisions:

Through-Hole Tech Boards

These traditional boards accommodate inserted component leads secured by soldering through-hole terminations on the reverse side. Boards feature punched or drilled holes with plated sidewalls for conductivity. Through-hole mounting supports large leaded elements and socketed ICs.

Surface Mount Tech (SMT) Boards

Designed primarily for leadless components that solder directly onto surface pads instead of through holes. Dense surface mount boards enhance miniaturization with placement on both sides. Common for consumer products. May combine with some through-hole parts.

Mixed Technology Boards

Incorporate provisions for both leaded and leadless component mounting onto the same PCB. Utilized when transitioning from through-hole to higher density SMT production.

PCB Types by Supported Components

PCB properties and constructions suit different environmental performance levels from light duty to robust demanding settings:

Industrial Boards

Durable, heavy duty PCBs withstand harsh temps, shakes, gases and moisture in industrial environments. Utilize glass reinforced epoxy composites with high temperature rated materials certified to IPC standards for reliable operation.

Aerospace/Defense Boards

Feature heavyweight copper and high performance dielectric materials to meet rigorous temperature cycling, vibration resistance and quality adherence per military/avionics protocols.

Consumer/Appliance Boards

Use affordable, volume FR-4 materials to survive home/office operating conditions while minimizing costs for high volume production. Prioritize reduced Pb/halogens to meet environmental compliance standards like RoHS/REACH.

Automotive Boards

Employ high temp resins and verification testing to perform reliably across vehicle operational standards. Emphasize reduced outgassing to counter atmosphere/corrosive agents in engines and cabins over device lifespan.

Specialized and Custom PCB Solutions

Innovative PCB technologies cater towards niche applications with special attributes:

Rigid-Flex Boards

Combine rigid and flexible materials bonded together in one circuit package. Flexible connectors link separate rigid board modules closely to optimize space efficiencies and electrical connections. Space/aerospace suited.

Metal Core Boards

Incorporate metal plate or substrate to spread heat loads across large surface areas underneath dielectric and traces rather than FR-4 cores. Power electronics and LED boards dissipate energy.

Flexible Circuits

Use thin, bendable polyimide film materials able to twist dynamically and provide connections in tight spaces near hinges or cases. Wearable devices and consumer products leverage flexibility.

Stretchable Circuits

Experimental conductive traces printed on elastic polymer substrates that can expand over 30% in length then retract electrically intact. Emerging soft robotics and sensors stretch then resume unchanged conductivity.

The wide selection of PCB materials, constructions, and technologies available allows matching specific design wants to optimal board solutions.

Conclusion

Understanding the functioning and capabilities of the primary PCB varieties assists engineers with choosing boards suited for application requirements. Material compositions, layer structures, feature densities and environmental resilience distinguish common printed circuit board technology options spanning affordable commodity to advanced specialty layouts. Matching project needs to board attributes ensures secure functionality.

Frequently Asked Questions

What are the most widely used PCB types?

FR-4 circuit boards with fiberglass-reinforced epoxy resin laminates cladding copper traces can manage the majority of low-cost consumer and commercial electronics assembly needs sufficiently. Multilayer boards stacking insulated conductive layers enable complex, dense layouts.

What PCB types suit high power applications?

Metal core PCB (MCPCB) constructions utilizing thick copper heat sinks and dielectrics rated for high temperatures withstand power device demands well. Ceramic based boards also supply thermal stability for power electronics and energy conversion systems.

HDI Design Basics and the HDI PCB Manufacturing Process

 

Introduction to HDI Technology

High-density interconnect (HDI) printed circuit boards contain finer lines/spaces and higher connection densities than conventional PCBs. HDI technology enables:

• More routing channels for high I/O components
• Miniaturization of products
• High-speed signals with tightly matched lengths
• Improved electrical performance

With HDI PCBs, integrated circuits with over 2,000 I/O and 0.4mm pitch BGAs can assemble onto boards using stacked microvias and thin dielectric build-up layers.

Key drivers for utilizing HDI technology include mobile devices, wireless communications, automotive electronics, and medical systems seeking superior electrical performance within severely constrained volumes.

HDI Design Guidelines and Rules

To leverage the benefits of HDI, engineers must adopt new design methodologies. Some key HDI guidelines per IPC-2226A:

Layer Stacks and Materials

• Stackups with 3 to 12+ layers
• Thin core and prepreg dielectric
• Low-loss, tight-tolerance laminates

Line Width/Space

• Lines/clearances under 8 mil (0.20mm)
• Spaces as fine as 2 mil (0.05mm)

Via Structures

• Microvias with under 8 mil diameters
• Stacking/tiering microvias for routing

Copper Weights

• Core metal over 2 oz (70 μm) thickness
• Build-up layers 0.5-1 oz (15-35 μm) metal

HDI Build-Up Layer Technology

HDI PCBs utilize additional thin circuit layers bonded onto cores using dielectric laminates. This allows trace geometries finer than conventional methods permit.

Dielectric Build-Up Layers

Thin insulating prepreg or film adhesives clad with copper foil. Common materials:

• Polyimide
• FR-4 composites
• Iso-material blends
• Liquid polymer coatings

These laminates coat over cores with build-up metal layers on either side of boards. Dielectrics possess tight thickness tolerances.

Pattern Plating Process

A semi-additive process selectively deposits and etches copper circuits onto laminate layers, which plates then get etched to isolate individual traces.

High density lines/spaces result from precision lithography to define plating areas combined with thin resist and copper. The additive plating avoids etched undercuts.

Microvia Interconnections

Small vias generated by laser drilling connect outer build-up layers through interposers to inner structure layers underneath, either pads or capture vias. The microvias occupy less space versus through-hole vias while permitting layer-to-layer connections.

Laser via diameters scale below 8 mils thanks to precise beam accuracy. Plating then fills holes to form barrel interconnects.

Core Layer and Final Metal Considerations

In addition to build-up technology, factors like core layer designs and outer surfaces influence HDI performance.

Core Dielectric Layers

Low-loss, tight-tolerance prepreg core materials maintain signal quality under high speeds, frequencies and layer counts:

• Thermoset resins (FR-4, BT Epoxy)
• Thermoplastics (LPC, PTFE composites)
• Ceramics (Alumina, Aluminum Nitride)

Final Surfaces

Final metal layers deposited on the outside of an HDI PCB serve vital electrical and assembly functions:

Outer Signal Layers: Accommodate fine delay matching lines.

Solder Masks: Help define solderable areas. Low coefficient of thermal expansion (CTE).

Surface Finishes: Protect pads while enabling solderability. Examples: ENIG, Immersion Silver, OSP.

Legend Ink: Identify components. Resistance to cleaning processes.

HDI Manufacturing Process Overview

Fabricating HDI boards with thin dielectrics, high layer counts and fine geometries involves specialized sequential building steps:

1. Core Lamination

Start with an insulating dielectric layer clad with copper foils bonded onto both sides that will form a robust inner foundation.

2. Drilling and Plating

Mechanically drill holes for through vias in the core then electroplate conductive barrel surfaces.

3. Outer Layer Imaging

Photoinage and etch CU circuits on either side of the core layers.

4. Build-Up Layer Bonding

Adhere sequential dielectric film/prepreg layers. Repeat to build HDI layer count. Laser drill microvias.

5. Build-Up Metalization

Semi-additively plate and define traces on build-up layers. Repeat to form multilayer interconnects.

6. Final Surfaces

Deposit solder mask, legend, and metal finish onto the HDI stackup.

7. Route and Test

Electrically test circuits then depanelize individual boards.

The combination of thin, sequential laying with laser drilling and precision imaging enables HDI PCBs containing dense interconnects with fine features.

Below outlines a few key advantages and limitations when working with HDI technology during PCB design and fabrication:

Advantages	Limitations
Enables miniaturization of electronics	More fabrication process steps
Finer lines and spaces increase routing channels	Tight tolerance materials controls
Microvias provide high I/O density	Narrow process margins for thin layers
Layer counts over 20 possible	High development costs

Conclusion

In summary, high-density interconnect PCB technology facilitates complex multilayer boards with stacked microvias and fine line geometries not achievable with conventional manufacture approaches. This permits far greater interconnect densities and electrical performance needed in compact, cutting-edge electronic devices and assemblies. However, the powerful HDI techniques also introduce tighter design rules, material requirements and fabrication sequences to manage versus standard PCBs.

Frequently Asked Questions

What are the main benefits of HDI PCB technology?

The principle benefits of HDI are increased routing channels through additional thin circuit layers, smaller via structures enabling higher I/O component densities, and the ability to tightly match electrical lengths across layers to maximize high-speed signal integrity.

What are some key HDI design considerations?

When leveraging HDI technology, designers must follow tighter constraints and rules regarding line/space dimensions, layer dielectrics, microvia implementation, and lamination sequences. Adhering to HDI design guidelines ensures manufacturability and interoperability.