Printed circuit boards (PCBs) are laminates that provide mechanical support and electrical interconnections for electronic components using copper traces on an insulating substrate. While the standard color for PCBs is green from brominated FR-4 glass-reinforced epoxy, specialized “yellow PCBs” utilizing alternative materials also exist.

This article explores yellow PCB technology including materials, properties, applications, design considerations, manufacturability, reliability factors, and comparisons to FR-4 PCBs. We’ll examine when yellow PCBs are utilized and what benefits they provide for electronics durability and high frequency performance.

What Makes a PCB Yellow?

The yellow color of specialized PCBs stems from the substrate material rather than the copper traces. While the copper conductors remain the same, the yellow PCB gains its color from the insulating dielectric layer.

Some common PCB materials that produce a yellow board include:

• Polyimide
• Arlon substrates
• Isola 370HR
• FR-4 with phenolic resin

The pigments and resin systems used in these materials differ from standard FR-4 glass-reinforced epoxy, altering the dielectric color from the traditional green to yellow.

The yellow color itself does not directly affect board function. Rather, the special materials are selected for properties like flexibility, heat resistance, and controlled impedance at high frequencies that incidentally produce the yellow hue.

Understanding PCB material properties helps determine when selecting a yellow PCB material can benefit an application.

Polyimide PCBs

Polyimide is a flexible polymer material capable of retaining mechanical and electrical properties across a wide temperature range. As a PCB substrate, some key traits of polyimide include:

• Flexible even at thin layers down to 25 microns
• Withstands temperatures exceeding 260°C
• Low dielectric constant around 3.5
• Excellent chemical resistance

The ability to flex repeatedly without cracking makes polyimide ideal for dynamic applications. Polyimide’s heat tolerance also suits harsh environments. The low dielectric constant helps maintain signal integrity at high frequencies.

Polyimide laminates from suppliers like Dupont Kapton are widely used for flexible PCB applications:

Flexible yellow PCBs enable custom cabling, dynamic flexing joints, and conformal circuits closely following device contours. Polyimide’s resilience makes flex PCBs ideal for consumer electronics and wearables.

Arlon PCB Material

Arlon is a high-performance thermoset material available in a yellow/orange color. It incorporates a modified epoxy resin reinforced with woven fiberglass. Key attributes include:

• Low dielectric constant of 3.3
• Low dissipation factor
• Lead-free assembly compatible
• High frequency stability
• Halogen-free for reduced toxicity

Arlon’s electrical properties and thermal stability make it well suited for robust high-speed digital and RF designs. It behaves similarly to high-performance FR-4 but with lower loss. The low dielectric constant helps maintain signal integrity.

Common Arlon grades used for PCB fabrication are SI-600, SI-1000, and CLTE-MW. These feature low moisture absorption, controlled thermal expansion, and tight thickness tolerance.

Isola 370HR

Isola 370HR is an epoxy resin PCB material notable for its high temperature rating above 150°C for extended periods. Key features include:

• High Tg of 190°C
• Lead-free assembly compatible to 280°C
• Low Z-axis expansion
• Low dielectric loss
• Available with controlled dielectrics

The high temperature resistance suits 370HR for automotive, aerospace, oil and gas, and other demanding environments. Thermal cycling reliability exceeds standard FR-4.

The yellow hue comes from the flame retardant additive used to achieve 370HR’s UL 94 V-0 flammability rating, the highest classification. This proves invaluable for safety critical electronics.

Phenolic Paper Based FR-4

Standard FR-4 glass-reinforced epoxy is actually available in multiple resin formulations that alter the color:

• Brominated epoxy resin – Green FR-4
• Cyanate ester resin – Red FR-4
• Phenolic resin – Yellow FR-4

The phenolic resin system retains the base FR-4 characteristics – good thermal capability, reasonable cost, and lead-free assembly compatibility. The specific resin chemistry produces the yellow color while meeting fire safety standards.

Phenolic FR-4 offers a lower cost option to achieve yellow PCBs without the premium performance or cost of advanced substrates. It provides a direct replacement for conventional green FR-4, with similar electrical and physical properties aside from color.

Design Considerations for Yellow PCBs

Despite the base material differences, yellow PCBs can be designed similarly to standard FR-4 boards. Some considerations when working with yellow PCB materials include:

Dielectric Properties – Dielectric constant and loss tangent vary for each material. Consult datasheets to obtain electrical properties for impedance matched designs. Polyimide provides the best high frequency loss characteristics.

Flexibility – Polyimide allows small bend radii flex circuits. Other materials remain rigid like FR-4.

Lead-free Assembly – Most yellow PCB dielectrics withstand lead-free soldering up to 280+°C. Ensure components are also lead-free compatible.

Adhesion – The resin system affects copper foil adhesion strength. Specify proper copper weights and finishes to avoid delamination.

Moisture Resistance – Some materials like polyimide absorb more moisture than FR-4. Consider conformal coating in humid environments.

Thermal Management – The thermal conductivity and coefficient of thermal expansion differs from FR-4. Ensure thermal design accommodates the material properties.

Aside from accounting for these dielectric traits, PCB layout and fabrication follow standard methods.

Manufacturing and Reliability Considerations

Yellow PCBs utilize many of the same manufacturing processes as standard FR-4 PCBs. Some considerations for manufacturing using yellow PCB materials include:

Stackup – Layer counts and arrangements are possible as with FR-4, though flexibility may limit layer counts for polyimide. Other materials support high layer counts.

Imaging – Photoresist coating, exposure, and etching techniques are similar. Polyimide flexibility may require special handling.

Drilling – Standard mechanical and laser/plasma processes apply. Ensure hole wall quality.

Metallization – Electroless copper achieves high-reliability interconnect plating.

Solder Mask – Liquid photoimageable (LPI) masks stand up to repeated flexing on bendable boards.

Assembly – SMT and through-hole assembly processes are shared. Ensure parts meet temperature requirements.

Testing – Continuity testing, in-circuit test, and flying probe checks verify assembly quality.

Reliability Testing – Rigorous testing like thermal shock, temperature cycling, humidity, and flexure tests help validate robustness.

While specialized handling may be required in some steps, yellow PCBs utilize much of the same PCB production and test processes as familiar green boards.

Cost Considerations

In general, specialized yellow PCB materials come at increased cost compared to standard FR-4 laminates. Some considerations regarding yellow PCB pricing:

Materials – Advanced engineered dielectrics like polyimide and Arlon cost more than baseline FR-4 prepregs per square foot, increasing materials cost.

Fabrication – More complex processing like handling flex circuits may require additional steps or lower yields, increasing production costs.

Testing – Extensive qualification testing for thermally robust designs increases testing expenses.

Volume – As with any PCB, higher volume production runs amortize costs lowering per unit pricing.

Changes – Design revisions requiring altered tools at any production stage drive costs upward.

Location – Geographic region and shipping considerations factor into overall budget.

While cost remains higher than FR-4, technical requirements often justify the premium for advanced yellow PCB materials providing enhanced capabilities.

Applications Using Yellow PCBs

Some common application examples utilizing yellow PCB technology include:

• Flex circuits for dynamic cabling and interconnections
• Wearable devices requiring mechanical flexibility
• Automotive under-hood circuits surviving high temperatures
• Aerospace and defense electronics in rugged environments
• Low-loss microwave boards for radar and communications
• Medical devices needing safety and reliability

New materials continue to emerge, expanding the possibilities for yellow PCBs providing electrical and mechanical performance surpassing traditional green substrates.

Conclusion

While most PCBs appear green due to standard FR-4 material, yellow PCBs utilizing advanced dielectric materials also offer targeted benefits. Properties like flexibility, thermal robustness, and controlled impedance at high frequencies make selecting yellow PCB materials advantageous for specialized designs, despite increased cost. With progress in material science, engineers can choose from a growing palette of yellow PCB substrates to enable electronics supporting future capability needs across industries.

Frequently Asked Questions

Does the yellow color affect functionality or reliability?

No, the yellow color itself does not influence the PCB’s operation. It stems from alternative resin systems that incidentally produce yellow hues while providing beneficial properties.

Can I solder components to yellow PCBs like standard boards?

Yes, the same soldering methods apply. Ensure parts and boards are suited for lead-free temperatures. Flexible boards may need special fixturing.

Are there drawbacks to yellow PCBs versus green FR-4?

Mainly increased cost currently. Processing can also require special handling, like with flexible circuits. Electrical performance is generally improved, aside from potentially higher loss tangents.

What fabrication challenges are unique to flexible yellow PCBs?

Handling thin flexible boards, adequate adhesion of flexible layers, dealing with flexing during assembly, and protecting components from strain with adhesives or standoffs.

How are impedance controlled traces implemented on yellow PCBs?

Trace width/spacing relationships must match the specific dielectric constant. This requires verifying electrical properties through modeling and prototype testing to correlate to design values.