What is Purple PCB?

A purple PCB refers to a printed circuit board that has a purple solder mask color instead of the traditional green. Purple represents an emerging trend in PCB fabrication driven by aesthetics, branding, and technical performance. In this article, we will examine the properties, applications, pros/cons, and methods for obtaining purple colored circuit boards.

Overview of PCB Solder Mask Colors 

The solder mask is a key element on a printed circuit board that serves multiple functions:

• It protects the copper traces from oxidation and potential short circuits.
• It acts as an insulator between closely spaced traces and pads.
• It provides mechanical support by binding to the substrate.
• It offers chemical and moisture protection.
• It allows solder paste to only contact required areas during assembly.

The most commonly used solder mask color has been green, from the early days of PCB fabrication. Green provided sufficient contrast to the copper layers for high visibility. Other standard options include blue, red, white, black, and yellow masks.

More recently, PCB designers have started exploring additional colored solder masks including purple for reasons ranging from aesthetics to improved solder masking performance.

Why Choose a Purple Solder Mask? 

Here are some of the benefits driving the use of purple colored solder masks in PCB fabrication:

Branding and Aesthetics

• Provides a unique and distinctive board appearance for branding.
• Appeals aesthetically to the target demographics in consumer products.
• Allows creativity in PCB coloration beyond just green.

Improved Contrast

• Purple as a complementary color provides excellent contrast with the copper and FR-4 substrate.
• Enhances inspectability and visibility of the circuit patterns.
• Assists automated optical inspection (AOI) systems in defect detection.

Tightened Process Control

• Variations in the purple color help reveal mask thickness inconsistencies.
• Coloration differences make coating flaws easier to identify.

Low Signal Loss

• Purple masks offer excellent dielectric performance with low signal loss.
• Ideal for radio frequency (RF) applications up to 5G ranges.

Lead-Free Compatibility

• Purple withstands the higher soldering temperatures required for lead-free soldering.
• Improved resistance to yellowing at high temperatures compared to green.

For these reasons, purple offers unique technical advantages over traditional green solder masks for certain applications.

Industries Using Purple PCBs 

Some of the industries actively adopting purple solder masks include:

Consumer Electronics

• Smartphones, tablets, wearables, gaming devices.
• Audio-video equipment like smart speakers, home theater systems.
• Peripherals and computer accessories.

Automotive Electronics

• Infotainment panels and head-unit PCBs.
• ADAS system modules for autonomous driving.
• Lighting control boards.

Medical Devices

• Patient monitoring systems and wearable sensors.
• Diagnostic imaging equipment like ultrasound, MRI, CT scanners.
• Therapeutic devices such as defibrillators, infusion pumps.

Aerospace and Defense

• Avionics equipment.
• Radar and communication systems.
• Satellite sub-systems.

Instrumentation

• Industrial sensors and transmitters.
• Process control boards.
• Test and measurement equipment.

IoT Products

• Hardware inside smart home/office devices.
• Environmental sensors and controllers.
• Networking infrastructure like routers and switches.

The appealing aesthetics and branding potential have made purple PCBs quite popular among consumer electronics. But the technical advantages are also driving adoption across other industry verticals such as automotive, aerospace, medical, and instrumentation.

Methods for Producing Purple Solder Masks 

There are two primary methods used to achieve purple colored solder masks on printed circuit boards:

1. Pigments Added to Mask

• Purple pigments are mixed into the liquid solder mask material prior to application on the PCB.
• LED curable masks allow more flexibility in pigments versus UV masks.
• Mask thickness may need to increase to allow sufficient pigment loading for opaque masking.

2. Base Mask Color with Purple Legend Ink

• A base mask color like white is applied to the board.
• Purple legend ink is then printed onto the mask in the areas requiring solder protection.
• Provides vivid purple color while minimizing thickness.
• Allows different colors in different board areas.
• Requires accurate legend ink printing registration.

Both approaches allow creation of purple solder masks. The preferred method depends on the exact purple shade required, mask thickness constraints, and capabilities of the PCB manufacturer.

Design Considerations for Purple PCBs 

Engineers should keep the following guidelines in mind when designing boards with purple solder masks:

Mask Thickness

• Additional mask thickness from pigments may require trace spacing adjustments.
• Ensure final mask measured thickness meets IPC standards.

Legend Printing Alignment

• Tight print registration tolerances are needed with legend ink masks.
• Account for potential misregistration in the design rules.

Component Contrast

• Some component markings may blend into the purple mask. Use black or white markings if possible.

Copper Contrast

• Watch for trace corners or fine patterns with poor contrast against purple.

Solderability Testing

• Verify purple masks meet fully cured requirements and allow adequate solder wetting.

AOI Optimization

• Tune lighting, thresholds, and algorithms to work optimally with the new mask color.

With attention to these constraints, purple solder masks can be implemented smoothly. Verify the PCB manufacturer has qualified the process sufficiently before full production.

Pros and Cons of Purple Solder Masks 

Potential Benefits

• Unique aesthetic appeal for branding
• Excellent contrast for improved inspectability
• Tight process control with color variation sensitivity
• Low signal loss at RF/microwave frequencies
• Withstands lead-free soldering temperatures without yellowing

Possible Drawbacks

• Limited color consistency between PCB suppliers
• Potential registration issues with legend ink approach
• Higher cost than standard green masks
• Component marking contrast problems
• Design constraints from thicker masks if pigments used

The advantages can make purple worthwhile for niche applications. But most boards still utilize the proven quality and cost-effectiveness of standard green solder masks.

Finding a Manufacturer for Purple PCBs 

Here are some tips for finding a capable PCB supplier to produce purple colored boards:

• Ask prospective suppliers for examples of prior purple PCBs to assess color rendition and mask quality.
• Review their process qualifications for purple masks and minimum requirements like legend printing tolerances.
• Validate they can meet applicable IPC standards such as IPC-SM-840 for mask thickness.
• Lean towards suppliers using higher-end inkjet or aerosol printing versus older screen printing processes.
• Evaluate surface finish options compatible with purple masks if planning ENIG, immersion gold, or other special finishes.
• Prioritize suppliers with strong engineering support teams to assist with DFM and resolving any potential issues.
• Get pricing for small initial test batches to validate quality before longer production runs.

Thanks to the flexibility of modern digital printing, many PCB manufacturers can now accommodate requests for custom colors like purple solder masks. Discuss your specific performance and cosmetic requirements upfront to determine the best fabrication methods.

Frequently Asked Questions 

Below are some common questions related to purple colored PCBs:

Does a purple solder mask affect PCB performance or reliability? 

No inherent technical issues arise from using a purple mask vs standard green, assuming the PCB supplier has qualified the purple masking process correctly.

What thickness variation occurs with pigmented purple masks? 

The mask thickness may increase around 3-5 microns typically when using embedded pigments. This must be considered in trace spacing design rules.

How repeatable is the purple color between PCB suppliers? 

Some minor variation in shade should be expected when changing suppliers. Work with the PCB partner to define an acceptable range through color sample reviews.

Can different board areas have different solder mask colors? 

Yes, using legend ink printing allows custom colors in selected board regions. Mixed color masks require tight registration control.

Does purple mask increase the cost of PCB fabrication? 

Yes, expect around a 10-15% premium for purple over standard green masks due to additional process steps, more stringent requirements, and lower production volumes.

How is purple legend ink printing aligned to the base solder mask? 

Printing registration targets on the bare boards are optically detected and used to precisely align the legend ink mask printing.

Can purple masks be applied to flexible PCBs? 

Yes, flexible circuit boards can also leverage purple solder masks. Polyimide flex materials may have different color rendition versus FR-4 substrates.