Introduction

Printed circuit boards (PCBs) are essential components in modern electronics. They provide the foundation on which components are mounted and connected. With the steady miniaturization of electronics, PCBs have become more complex and advanced. A key trend is the use of thicker copper and more layers to accommodate higher current loads and improved signal integrity.

One such PCB is the 4-layer high TG (thermal grade) thick copper immersion gold board in purple color. As the name suggests, it has the following key features:

• 4 conductive layers
• High TG or thermal grade copper
• Thick copper thickness
• Immersion gold surface finish
• Purple soldermask color

In this article, we will take a deep dive into this PCB configuration. We will look at the benefits of 4-layer, thick copper, thermal grade copper, immersion gold plating, and purple soldermask. We will also explore the applications that can benefit from this robust PCB platform.

Benefits of 4-Layer PCBs

The number of layers in a PCB influences factors like current capacity, noise immunity, and routing flexibility. 2-layer boards provide the simplest and most cost-effective solution for simple, low component density designs.

4-layer boards provide an optimal balance of cost and performance for many applications. Here are some of the benefits of 4-layer PCBs:

• More routing channels: The two internal layers mean more routing channels are available. This helps accommodate higher component densities and complex routing connections.
• Good signal isolation: The additional layers provide more isolation between critical signals. One layer can be used as a ground plane to shield sensitive analog signals on another layer.
• Controlled impedance: Having two reference planes allows the implementation of controlled impedance routing for high-speed signals. This improves signal integrity at high frequencies.
• Lower layer count: 4 layers provides better performance than 2 layers without the cost of 6 layers or more. This makes it a versatile option suitable for many applications.
• Redundancy: Critical signals can be routed on multiple layers for redundancy. This improves reliability and resilience to faults.

In summary, the combination of cost, performance, and flexibility makes 4-layer PCBs a practical solution for many industrial, telecom, automotive, medical, and consumer electronics applications.

Advantages of Using Thick Copper

Copper thickness is a critical design choice for PCBs. The most common options are 1 oz (35 μm), 2 oz (70 μm), and 3 oz (105 μm) copper. However, some applications require even thicker copper.

Using a 4-layer board with thicker copper offers several benefits:

• Higher current capacity: Thicker copper traces can carry more current due to lower resistivity. This reduces power loss and heating.
• Lower voltage drop: The lower resistance results in less voltage drop across traces. This allows maintaining tighter voltage regulation.
• More power layers: More copper provides the opportunity to use entire layers or large areas as power planes. This further increases power distribution capabilities.
• Shorter traces: Higher conductivity means achieving lower resistances even with shorter trace lengths. This provides more routing flexibility.
• Better thermal performance: The extra copper helps dissipate heat from components more efficiently. This improves the PCB’s overall thermal management.
• Higher reliability: Thicker copper is more resistant to corrosion and mechanical wear over time. This enhances long-term reliability.
• Lower impedance: Thicker traces have lower impedance, which helps maintain signal integrity at high speeds and frequencies.
• EMI/RFI shielding: Additional copper helps block electromagnetic interference (EMI) and radio frequency interference (RFI).

For applications with high power demands, multiple voltage rails, fast switching speeds, and high component densities, using thicker copper on a 4-layer board provides substantial performance and reliability benefits.

What is Thermal Grade Copper?

Standard copper foils used in PCB fabrication are referred to as electrodeposited or ED copper. However, some applications require copper foil with enhanced thermal properties. This is referred to as thermal grade or TG copper.

Here are some key properties of thermal grade copper:

• Higher purity: TG copper contains 99.9% pure copper versus 99.4-99.8% for standard ED copper.
• Smoother surface: TG copper is processed to have a smoother surface finish and lower roughness.
• Higher thermal conductivity: TG copper has a thermal conductivity of 400 W/mK compared to around 380 W/mK for ED copper.
• Lower CTE: The coefficient of thermal expansion (CTE) is lower at 17 ppm/K versus 18 ppm/K for ED copper.
• Higher LTTR: The longer-term thermal resistance (LTTR) spec measures how long it takes the copper to heat up. TG copper has a higher LTTR indicating slower heating.

The higher purity, smoothness, thermal conductivity, and lower CTE of TG copper provide several benefits:

• Cooler operation: The faster heat spreading means components run cooler. This improves reliability and extends component life.
• Lower warping/stress: Lower CTE means less warping and physical stress on components and solder joints when heated.
• Finer lines & spaces: The smooth surface allows etching finer trace widths and smaller clearances. This enables higher circuit densities.
• Better DFM: TG copper permits tighter tolerances during fabrication. This results in better design for manufacturability (DFM).
• Higher current: The excellent thermal characteristics allow TG copper foils to carry more current than standard ED copper.

TG copper is recommended for high-power PCBs to manage heat dissipation. The high purity and smooth foil also make it suitable for RF and microwave circuits. Overall, it provides better thermal management, higher reliability, easier manufacturability, and extended PCB lifetimes.

Benefits of Immersion Gold Surface Finish

PCB surface finishes serve critical functions like solderability, corrosion resistance, and contact protection. There are various plating options like HASL, ENIG, Immersion Silver etc. Immersion Gold or IAuG is an excellent surface finish for many applications due to following advantages:

Excellent solderability

• IAuG provides superb wetting when soldering components. The gold layer dissolves rapidly into the solder, ensuring reliable solder joints.

High corrosion resistance

• Gold is inert and does not oxidize or corrode. This allows PCBs to withstand harsh environments like high humidity, temperature cycling, and corrosive atmospheres.

Durable shelf life

• IAuG maintains solderability and contact integrity over long storage periods. PCBs can be fabricated well in advance of assembly with no degradation.

Good wire bondability

• The gold layer allows robust wire bonding for interconnections in microelectronics packaging. This is suitable for photovoltaic cells, MEMS, and semiconductor dies.

Biocompatibility

• Gold is bio-inert making IAuG suitable for medical devices and implants where the PCB contacts body tissues and fluids.

High lubricity

• The gold layer acts as a lubricant preventing wear of metal contacts through insertions and matings. This extends the life of connectors and contacts.

Excellent conductivity

• Gold has the highest conductivity which helps maintain signal integrity at high frequencies.

In summary, immersion gold is an advanced finish ideal for high-reliability applications in demanding environments with extended shelf lives. It is suitable across industrial, medical, telecom, automotive, aerospace and defense sectors.

Benefits of Purple Soldermask

The solder mask or solder resist is the protective colored layer on the PCB surface. It defines the solderable areas and prevents bridging between conductors. Although various colors are available, purple soldermask offers some unique advantages:

High contrast

• The purple mask provides high visual contrast with the gold traces/pads. This makes it easier to accurately position and inspect components during assembly.

Reduced eyestrain

• Studies show purple requires less eye accommodation compared to black. This reduces eyestrain for assemblers over long shifts.

Defect detection

• Voids and imperfections are easier to identify on the purple mask background. This aids quality control during fabrication and assembly.

Aesthetic appeal

• For consumer products, the purple color provides a modern high-tech look. This can fit well with product designs.

Improved OCR

• Purple color offers better optical character recognition for automated optical inspection (AOI) systems over darker masks like black.

Enhanced thermal dissipation

• Compared to darker colors, the lighter purple mask is believed to reflect more heat. This helps with thermal management.

UV blocking

• The purple dye used in the mask inherently blocks UV light. This provides protection for photosensitive components.

While green and red masks are also common, the purple variant combines improved inspection, easier assembly, aesthetic appeal, and potential thermal advantages. This makes it an attractive choice for many applications.

Applications of 4-Layer High TG Thick Copper Immersion Gold boards

With its robust construction and advanced material properties, this PCB configuration is well suited for demanding applications such as:

Power conversion

• Switch-mode power supplies
• Motor drives
• Solar inverters
• Battery chargers
• DC-DC converters

The thick copper and TG properties provide low resistance and efficient thermal management to handle high currents. 4 layers allow separation of power and signals together with low EMI/RFI emissions.

Telecom infrastructure

• Servers
• Routers
• Base stations
• Networking cards

Immersion gold protects against corrosive gases while enabling high-density assembly. This is critical for the reliability of complex telecom systems.

Automotive electronics

• Engine control units (ECUs)
• Infotainment modules
• ADAS modules
• Controllers

4 layers and thick copper help manage multiple power and ground domains within vehicles. Robust construction withstands vibration while purple mask aids optical inspection.

Medical equipment

• Imaging systems
• Analyzers
• Monitoring devices
• Implantable devices

Immersion gold and biocompatibility allows safe body contact for medical devices. Enhanced thermal performance assists with cooling requirements.

Aerospace/defense

• Avionics
• Radar/communication
• Military-grade electronics

TG copper and immersion gold provides reliability under extreme conditions of temperature, altitude, shock, and vibration. 4 layers support dense circuitry with tightly controlled impedance.

Industrial

• Process control systems
• Motor drives
• Railway electronics
• Power transmission

Thick copper and thermal grade properties enable handling high voltages and currents in factory automation systems and large motors/drives. 4 layers support multi-rail power distribution and isolated control signals.

With its unrivaled thermal performance, excellent reliability, design flexibility, and advanced materials, the 4-layer high TG thick copper immersion gold PCB in purple represents the pinnacle of robust printed circuit board technology. It provides a battle-tested foundation for electronics serving the most demanding applications.

Frequently Asked Questions

Q: What is the typical thickness of thick copper boards?

A: Common thicknesses for thick copper boards are 2oz (70um), 3oz (105um) and 4oz (140um). Some applications utilize copper up to 6oz (210um) or even 8oz (280um) thickness.

Q: What is the difference between FR4 and high TG material?

A: FR4 is the standard glass-reinforced epoxy laminate used in most PCBs. High TG boards use modified epoxy blends with improved thermal conductivity while retaining electrical insulation properties. They manage heat dissipation better than FR4.

Q: Does thicker copper increase the stiffness and rigidity of the PCB?

A: Yes, thicker copper increases board thickness slightly, making the PCB more rigid. The higher stiffness helps reduce warpage and improves dimensional stability during assembly and operation.

Q: Can traces on inner layers handle higher currents than outer layers?

A: Due to better heat dissipation to inner reference planes, traces on inner layers can reliably carry 10-15% higher currents than similar width traces on external layers.

Q: Does immersion gold bonding require heat or ultrasound?

A: No, immersion gold finish allows cold bonding at room temperature using thermosonic or thermocompression methods without ultrasound. This simplifies bonding.

Q: How many reflow cycles can immersion gold withstand?

A: Immersion gold finish retains excellent solderability for over 50 reflow cycles. This permits rework, modifications, and assembly operations over long service life.

Q: Does purple soldermask cost more than standard green or red colors?

A: Traditionally purple mask was more expensive but with volume production, pricing has equalized with other common colors like green and red.

Q: Does the dye in purple soldermask affect thermal or electrical properties?

A: No, the pigments used in small quantities do not degrade thermal or dielectric properties. Material performance is unchanged compared to uncolored masks.

Q: What is the typical thickness of soldermask on PCBs?

A: Typical soldermask thickness is around 30-60um (1.2-2.4 mils). Thicker masks up to 100um provide even better insulation and environmental protection.

Q: Can conformal coating be applied over immersion gold surface finish?

A: Yes, conformal coatings adhere well to immersion gold. The inert finish prevents galvanic corrosion between the coating material and gold layer.