IPC-CF-152 Explained: Guide to CIC and CMC Composite Materials for Low-CTE PCBs

If you’ve ever dealt with BGA packages larger than 35mm or designed boards for aerospace and military applications, you’ve likely encountered the CTE mismatch problem. Standard FR-4 expands at roughly 14-17 ppm/°C, while ceramic chip carriers and large silicon packages expand at only 6-7 ppm/°C. Over thermal cycles, this mismatch shears solder joints and causes field failures. IPC-CF-152 addresses this challenge by specifying the composite metallic materials—specifically copper-invar-copper (CIC) and copper-molybdenum-copper (CMC)—that constrain board expansion and improve thermal management.

This guide covers everything you need to know about IPC-CF-152, from understanding when these materials make sense to specifying them correctly for your high-reliability applications.

What is IPC-CF-152?

IPC-CF-152 (full title: “Composite Metallic Materials Specification for Printed Wiring Boards”) establishes the requirements for procuring copper-invar-copper (CIC) and copper-molybdenum-copper (CMC) three-layer composite foils. The current revision is IPC-CF-152B, released in December 1997.

These aren’t ordinary copper foils. They’re engineered sandwich structures where a low-CTE core material (invar or molybdenum) is metallurgically bonded between two layers of copper. The copper outer layers allow the material to be processed using standard PCB fabrication techniques—lamination, drilling, plating, and etching—while the core provides the CTE control and thermal properties that standard materials can’t achieve.

Why Composite Metallic Materials Exist

The fundamental problem these materials solve is coefficient of thermal expansion (CTE) mismatch. Consider a typical scenario: you’re mounting a 45mm ceramic BGA package with a CTE of 6.5 ppm/°C onto an FR-4 board with a CTE of 16 ppm/°C. When the assembly heats up during reflow or operation, the board expands nearly 2.5 times more than the package. This differential expansion creates stress on every solder joint, and over repeated thermal cycles, those joints fail.

CIC and CMC materials provide constraining cores that anchor the FR-4 layers and dramatically reduce the overall board CTE. With proper design, you can achieve system CTEs of 6-12 ppm/°C—close enough to ceramic packages to ensure reliable solder joints through thousands of thermal cycles.

Understanding Copper-Invar-Copper (CIC)

Copper-invar-copper is the most widely used composite metallic material in the PCB industry. It consists of copper bonded to each side of an invar core.

What is Invar?

Invar is a nickel-iron alloy containing approximately 36% nickel and 64% iron. Its claim to fame is an extraordinarily low coefficient of thermal expansion—approximately 1.5 ppm/°C at room temperature. This property was discovered by Swiss physicist Charles Édouard Guillaume in 1896, earning him the Nobel Prize in Physics in 1920.

The low CTE of invar comes from a phenomenon called magnetostriction, where the magnetic domains in the alloy contract as temperature increases, offsetting normal thermal expansion.

CIC Construction and Properties

The copper layers on CIC serve multiple purposes: they provide the bonding surface for lamination to FR-4 prepregs, they enable electrical connectivity (though CIC is rarely used as a signal layer), and they allow standard PCB processing. The ratio of copper to invar determines the final CTE—more invar means lower CTE but also lower thermal conductivity.

CIC Advantages

Excellent CTE Control: CIC achieves the lowest practical CTE of any PCB-compatible metallic material, making it ideal for ceramic component mounting.

Good Electrical Conductivity: The copper cladding provides reasonable conductivity for power and ground plane applications.

Established Supply Chain: CIC has been used in the PCB industry for decades, with multiple qualified suppliers and well-understood processing.

Bondable to Standard Materials: The copper surfaces bond readily to FR-4 prepregs using standard lamination processes.

CIC Limitations

Poor Thermal Conductivity: At 20-30 W/m·K, CIC is far less thermally conductive than pure copper (385 W/m·K) or CMC. It’s not a good choice when heat spreading is the primary requirement.

Heavy Weight: The nickel-iron core makes CIC denser than most PCB materials, which can be problematic in weight-sensitive aerospace applications.

Difficult to Drill: The invar core is harder than copper and dulls drill bits faster. Small vias (<0.3mm) can be particularly challenging.

Magnetic Properties: Invar is magnetic, which may be a concern in some sensitive electronic applications.

Understanding Copper-Molybdenum-Copper (CMC)

Copper-molybdenum-copper addresses CIC’s thermal conductivity limitation while maintaining good CTE control. It consists of copper bonded to each side of a molybdenum core.

Why Molybdenum?

Molybdenum is a refractory metal with a unique combination of properties: low CTE (approximately 5 ppm/°C), high thermal conductivity (138 W/m·K), and excellent high-temperature stability. These properties make it valuable in applications where both thermal management and CTE control are required.

CMC Construction and Properties

CMC Advantages

Superior Thermal Conductivity: CMC’s thermal conductivity is 6-10 times higher than CIC, making it effective as both a constraining core and a heat spreader.

Good CTE Control: While not quite as low as CIC, CMC’s CTE of 6-8 ppm/°C is still excellent for ceramic component mounting.

High-Temperature Capability: Molybdenum maintains its properties at elevated temperatures better than invar.

Non-Magnetic: Unlike CIC, CMC has no magnetic properties.

CMC Limitations

Higher Cost: CMC typically costs more than CIC due to the molybdenum core material.

Processing Difficulty: Molybdenum is harder than invar and even more challenging to drill. Processing requires careful attention to tool selection and parameters.

Heavier Than CIC: The molybdenum core makes CMC the heaviest of the common PCB materials.

Limited Availability: Fewer suppliers offer CMC compared to CIC, which can affect lead times.

CIC vs. CMC: Selecting the Right Material

The choice between CIC and CMC depends on your application’s primary requirements.

Material Comparison Table

When to Choose CIC

• CTE control is the dominant requirement
• Thermal dissipation needs are moderate
• Cost is a significant factor
• Board weight is less critical
• You’re matching to ceramic packages (LCCC, CBGA)

When to Choose CMC

• Both thermal management and CTE control are critical
• High-power components require heat spreading
• Application is magnetically sensitive
• Operating temperatures are elevated
• You need the material to function as a heat sink

Application Examples

IPC-CF-152 Specification Requirements

IPC-CF-152B includes specification sheets that outline engineering and performance data for procurement purposes. Understanding these requirements helps ensure you receive materials that meet your application needs.

Key Specification Parameters

Material Grades and Classes

IPC-CF-152 establishes grades based on dimensional tolerances and surface quality:

Grade A: Tightest tolerances, highest surface quality. Used for demanding applications where consistent properties are critical.

Grade B: Standard tolerances suitable for most applications. Represents the typical commercial product.

Quality Conformance Testing

Materials supplied to IPC-CF-152 must meet specific test requirements including:

• Visual inspection for surface defects
• Thickness measurements at multiple points
• Cladding ratio verification
• Peel strength testing
• Dimensional stability verification

Read more IPC Standards:

IPC Standards Explained: A Comprehensive Guide to PCB & Electronics Assembly Standards

IPC 2615 Standard Explained: PCB Dimensioning & Tolerancing Requirements

MIL-STD-2000A: Military Soldering Standards for Electronic Assemblies

MIL-STD-1686: ESD Control Program Requirements for Military Electronics

IPC 1791 Standard: Everything You Need to Know About Trusted PCB Certification

MIL-STD-883: Complete Guide to Microelectronics Test Methods

J-STD-048 Explained: Complete Guide to Product Discontinuance Notifications

MIL-STD-750: Semiconductor Device Test Methods Explained

MIL-STD-454: General Requirements for Military Electronic Equipment

IPC 9194: Complete Guide to SPC for PCB Assembly Manufacturing

MIL-STD-202: Electronic Component Testing Methods & Procedures

IPC 4110: Complete Guide to Cellulose Paper Specs for Printed Circuit Boards

Boundary Scan Testing (JTAG) in PCB Design: A Complete DFT Guide

Axiomtek IPC-950 Review: Specs, Features & Performance [2026 Guide]

MIL-PRF-55681: Military Ceramic Chip Capacitor Specification Guide

Designing with CIC and CMC Materials

Successfully incorporating composite metallic materials into your PCB requires attention to several design and processing considerations.

Stack-up Design Principles

Symmetrical Placement: CIC or CMC layers should be placed symmetrically about the board’s neutral axis to prevent warpage. A common approach uses two constraining core layers positioned just beneath the outer layers.

Thickness Considerations: Standard CIC/CMC thickness is 0.15mm (6 mil). Using thinner cores distributes the constraint closer to the surface, while thicker cores provide more anchoring force. The optimal choice depends on your layer count and CTE target.

Typical Stack-up Example (8-layer low-CTE board):

Drilling Considerations

CIC and CMC are significantly harder to drill than standard copper and FR-4:

Reduced Hit Counts: Expect drill bit life to be 30-50% shorter than when drilling standard materials.

Entry/Exit Materials: Use appropriate entry and exit materials designed for hard metals.

Speed and Feed Adjustments: Reduce spindle speed and feed rate compared to standard parameters.

Minimum Via Size: Practical minimum via diameter is typically 0.3mm (12 mil) for CIC and 0.4mm (16 mil) for CMC.

Consider Clearance Holes: Some designers use clearance holes in the CIC/CMC layers rather than drilling through them, simplifying processing.

Electrical Considerations

While CIC and CMC can be used as power and ground planes, there are considerations:

CIC Conductivity: The invar core is much less conductive than copper. For high-current applications, calculate the effective sheet resistance carefully.

CMC for Power: CMC’s better thermal conductivity makes it more suitable for power plane applications where heat spreading is beneficial.

Signal Integrity: Neither material is typically used for signal layers due to the mixed metal construction.

Processing and Fabrication Guidelines

Fabricators working with IPC-CF-152 materials need specialized knowledge and equipment.

Lamination

CIC and CMC laminate well using standard FR-4 prepregs, but attention to press parameters is important:

• Use sufficient pressure to ensure complete resin flow around the harder metal surfaces
• Temperature ramp rates may need adjustment to account for the higher thermal mass
• Post-lamination stress relief baking can improve flatness

Plating

The copper cladding on CIC and CMC plates using standard electroless and electrolytic processes. However:

• Ensure adequate surface preparation to achieve good adhesion
• The interface between copper cladding and core material can be a weak point if not properly cleaned

Etching

Standard alkaline or acidic etchants work on the copper cladding. The core material (invar or molybdenum) requires different chemistry if it needs to be etched, which is rarely the case in typical applications.

Resources for IPC-CF-152 Materials

Official Standards Documentation

• IPC-CF-152B Standard – Purchase from IPC Store
• IPC-TM-650 Test Methods – Referenced test procedures
• IPC-4101 Base Materials Specification – Companion laminate standard

Material Suppliers

Technical References

• IPC-2221 Generic Standard on PCB Design – References IPC-CF-152 for constraining cores
• Electronics Cooling Magazine – Thermal management articles
• I-Connect007 Technical Articles – PCB industry news and technical content

Frequently Asked Questions About IPC-CF-152

What CTE can I achieve using CIC in a multilayer PCB?

The final board CTE depends on your stack-up design, not just the CIC material itself. A typical approach using two 6-mil CIC layers symmetrically placed in an 8-layer FR-4 board can achieve an overall CTE of 9-12 ppm/°C, down from the 14-17 ppm/°C of standard FR-4. To get lower CTEs (6-8 ppm/°C), you need thicker CIC layers, more CIC layers, or a combination with low-CTE laminate materials like Kevlar/Aramid. Work with your fabricator to model the expected CTE based on your specific stack-up.

Can CIC or CMC be used as signal layers?

Technically yes, but it’s not recommended. The mixed-metal construction creates unpredictable impedance characteristics, and the invar/molybdenum cores have much lower conductivity than copper. CIC and CMC are best used as dedicated constraining cores, ground planes, or power planes. If you need them to carry signals, limit it to non-critical, low-frequency connections.

Is IPC-CF-152 material compatible with lead-free assembly?

Yes, both CIC and CMC materials are compatible with lead-free reflow temperatures (typically 260°C peak). The materials themselves easily withstand these temperatures. However, the higher reflow temperatures of lead-free processes create more thermal stress on the board, making CTE control even more important. This is actually one reason why CIC/CMC usage has increased in the lead-free era—the greater thermal excursions during assembly make CTE mismatch failures more likely.

How much does CIC/CMC add to PCB cost?

Expect CIC to add 30-50% to your base material cost, and CMC to add 50-100% or more. Processing costs also increase due to slower drilling and more careful handling. However, for high-reliability applications, the cost is justified by improved yield during assembly and dramatically better field reliability. A single field failure in aerospace or medical applications far exceeds the incremental material cost. Work with your fabricator for accurate quotes based on your specific design.

What’s the minimum practical via size when drilling through CIC or CMC?

For CIC, most fabricators can reliably achieve 0.30mm (12 mil) finished hole diameter with proper tooling and process control. CMC is harder, typically limiting practical minimum hole size to 0.40mm (16 mil). If you need smaller vias, consider designing clearance holes in the CIC/CMC layers so the via only passes through standard materials, or use buried/blind via structures that don’t penetrate the constraining cores. Always consult your fabricator early in design to confirm their capabilities with these materials.

Making the Most of IPC-CF-152 Materials

Composite metallic materials aren’t for every application. They add cost, complicate fabrication, and introduce design constraints. But when you’re facing CTE mismatch challenges with large ceramic packages, high-reliability requirements in harsh thermal environments, or the need to combine thermal management with dimensional stability, IPC-CF-152 materials become essential tools.

The key to success is early engagement with your fabricator. CIC and CMC require specialized processing knowledge, and not every shop has experience with these materials. Find a fabricator with a track record in low-CTE boards, share your requirements early, and work together to optimize the stack-up for manufacturability.

For new designs, consider whether you truly need composite metallic materials or whether alternative approaches—smaller packages, different component types, or thermal design improvements—might solve your problem more economically. But when the application demands the ultimate in CTE control and thermal performance, IPC-CF-152 materials deliver capabilities that no other PCB material can match.