The first time I dealt with solder joint failures on a large ceramic chip carrier, I learned a lesson about CTE mismatch the hard way. Standard FR-4 at 17 ppm/°C simply couldn’t match the 6 ppm/°C expansion of the alumina package, and after a few hundred thermal cycles, the solder joints cracked. That’s when I discovered aramid-reinforced laminates and IPC-A-142. This specification defines the woven aramid fabrics—better known by the DuPont trade name Kevlar—that enable PCB laminates with CTE values low enough to match ceramic packages and silicon devices.

What is IPC-A-142?

IPC-A-142 is the “Specification for Finished Fabric Woven from Aramid for Printed Boards,” published by IPC in June 1990. This specification covers finished fabrics woven from aramid yarns intended as reinforcing material in laminated plastics for electrical and electronic applications. All fabrics covered by IPC-A-142 are plain weave construction.

Aramid fibers—specifically para-aramid fibers like DuPont’s Kevlar and Teijin’s Twaron—offer a unique combination of properties that make them invaluable for low-CTE PCB applications. The most remarkable property is their negative coefficient of thermal expansion in the longitudinal direction, typically -2 to -4 ppm/°C. When combined with positive-CTE resin systems, this negative fiber CTE produces composite laminates with very low overall expansion.

IPC-A-142 provides two tolerance classes for physical characteristics, allowing specification of fabric properties based on the precision needed for specific design and performance requirements. The specification establishes nomenclature, definitions, general requirements, and physical requirements that aramid fabric suppliers must meet.

Scope and Purpose of IPC-A-142

IPC-A-142 establishes requirements for woven aramid fabrics used specifically in PCB laminate manufacturing. The specification focuses on plain weave fabrics and defines the quality parameters that ensure consistent material properties for electronic applications.

Key areas covered by IPC-A-142 include:

• Fabric terminology and definitions
• Classification system with two tolerance classes
• Finished fabric physical properties (weight, thickness, thread count)
• Visual requirements and workmanship standards
• Chemical requirements for fabric finish
• Quality assurance and inspection procedures
• Test methods referenced from IPC-TM-650

Note that IPC-A-142 covers woven aramid fabrics only. Non-woven aramid reinforcements, such as DuPont’s Thermount products, are covered under a separate specification: IPC-4411.

Aramid Fiber Properties and Composition

Aramid (aromatic polyamide) fibers possess a unique combination of properties that make them ideal for low-CTE PCB applications. Understanding these properties helps explain why aramid reinforcement enables laminates that can match the thermal expansion of ceramic chip carriers and silicon devices.

Negative Coefficient of Thermal Expansion

The defining characteristic of aramid fibers is their negative CTE in the longitudinal direction, typically -2 to -4 ppm/°C. This means the fibers actually shrink slightly when heated. When combined with positive-CTE epoxy or polyimide resin systems (which expand when heated), the opposing behaviors balance out to produce composite laminates with very low net CTE values of 5-9 ppm/°C in the X-Y plane.

High Modulus and Strength

Kevlar 49 aramid fiber exhibits a modulus of approximately 131 GPa (19 million psi) and tensile strength around 3,000-3,600 MPa. This high modulus is critical because it determines how effectively the fiber can restrain the expansion of the surrounding resin. The higher the fiber modulus relative to the resin, the more the fiber dominates the composite CTE behavior.

Low Density

Aramid fibers have a density of only 1.44 g/cm³, compared to 2.54 g/cm³ for E-glass. This 40% weight reduction translates directly to lighter PCB assemblies—a significant advantage for aerospace and portable electronic applications where weight is critical.

Electrical Properties

Aramid-reinforced laminates offer a lower dielectric constant (Dk around 3.5-4.0) compared to E-glass reinforced FR-4 (Dk around 4.2-4.7). This lower Dk enables faster signal propagation and improved high-frequency performance. Combined with the excellent dimensional stability from low CTE, aramid laminates are well-suited for high-speed digital applications.

Aramid vs Glass Fiber Property Comparison:

IPC-A-142 vs IPC-4411: Woven vs Non-Woven Aramid

Two IPC specifications cover aramid reinforcements for PCB laminates: IPC-A-142 for woven fabrics and IPC-4411 for non-woven para-aramid reinforcements. Understanding the differences helps in selecting the right material for your application.

Woven Aramid (IPC-A-142)

Woven Kevlar fabrics per IPC-A-142 use plain weave construction where yarns interlace at right angles. Commercial laminate products like Arlon 45NK use woven Kevlar reinforcement to achieve in-plane CTE values as low as 5-7 ppm/°C. Woven aramid provides predictable, consistent properties in both warp and fill directions, making it suitable for applications requiring uniform CTE control.

Non-Woven Aramid (IPC-4411)

Non-woven aramid reinforcement, most notably DuPont’s Thermount product line, consists of randomly oriented aramid fibers in a paper-like mat structure. Products like Arlon 85NT (polyimide/Thermount) and 55NT (epoxy/Thermount) achieve X-Y CTE of 7-9 ppm/°C. The non-woven structure offers advantages for laser drilling since there are no glass bundles to cause drilling issues, making it popular for HDI applications.

Note that DuPont announced obsolescence of Thermount fibers in 2006, though replacement para-aramid fibers conforming to IPC-4411 have since become available from other suppliers. Verify current material availability with your laminate supplier.

CTE Engineering with Aramid Laminates

The primary driver for aramid-reinforced PCB laminates is matching the thermal expansion of ceramic chip carriers and silicon devices. Understanding how CTE mismatch causes failures and how aramid helps solve the problem is essential for successful application.

The CTE Mismatch Problem

A leadless ceramic chip carrier (LCCC) has a CTE of approximately 6 ppm/°C, while standard FR-4 has a CTE of 14-17 ppm/°C. When the assembly experiences thermal cycling—whether from power cycling, environmental exposure, or reflow soldering—the PCB expands nearly three times as much as the ceramic package. This differential expansion creates shear stresses in the solder joints. After enough thermal cycles, the solder work-hardens and cracks, causing electrical failures.

The severity of this problem increases with package size. Large ceramic chip carriers (greater than 10mm square) are particularly vulnerable. Some reliability standards prohibit mounting LCCCs with more than 16 pins on standard FR-4 substrates due to the inherent reliability risk.

How Aramid Solves the Problem

Aramid fiber’s negative CTE (-4 ppm/°C) combined with its high modulus (19 msi) gives it the “power” to restrain the positive expansion of the resin system. Woven Kevlar laminates can achieve X-Y CTE values of 5-7 ppm/°C, closely matching ceramic packages (6 ppm/°C) and silicon chips (2.6 ppm/°C). This CTE matching dramatically improves solder joint reliability under thermal cycling.

CTE Matching for Ceramic Chip Carriers:

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

Applications for Aramid-Reinforced PCB Laminates

Ceramic Chip Carrier Mounting

The original and primary application for aramid-reinforced laminates is mounting leadless ceramic chip carriers (LCCCs). Defense and aerospace systems commonly use ceramic packages for their hermeticity and reliability. Aramid laminates enable reliable solder joints even with large ceramic packages that would fail on standard FR-4.

High-Density Interconnect (HDI)

Non-woven aramid reinforcement per IPC-4411 is particularly well-suited for HDI applications. The 100% organic composition (no glass) enables clean laser drilling of microvias without the glass bundle issues that can affect via quality. Products like Arlon 33N and 35N are specifically designed for laser via HDI technology.

Aerospace and Defense

The combination of low CTE, low density (40% lighter than glass), excellent dimensional stability, and compatibility with high-reliability resin systems makes aramid laminates ideal for aerospace and military electronics. Weight reduction is particularly valuable for airborne and space applications where every gram matters.

BGA and CSP Mounting

Ball grid array (BGA) and chip-scale packages (CSP) also benefit from the improved CTE matching provided by aramid laminates, particularly for large packages with many solder joints. The reduced stress per joint extends thermal cycling life and improves overall assembly reliability.

Commercial Aramid Laminate Products

Related IPC Standards and Resources

Frequently Asked Questions About IPC-A-142

What is the difference between IPC-A-142 and IPC-4411?

IPC-A-142 covers woven aramid fabrics (like Kevlar woven cloth), while IPC-4411 covers non-woven para-aramid reinforcements (like Thermount paper-like mats). Woven aramid provides the lowest CTE (5-7 ppm/°C) but is harder to process. Non-woven aramid offers good CTE control (7-9 ppm/°C) with easier processing and excellent laser drillability for HDI applications.

Why does aramid fiber have negative CTE?

Aramid’s molecular structure causes it to contract slightly in the fiber direction when heated. The aromatic polyamide chains are highly oriented along the fiber axis, and thermal energy causes minor reorientation that actually shortens the fiber. This negative longitudinal CTE (-2 to -4 ppm/°C) counterbalances the positive CTE of the surrounding resin, resulting in very low composite CTE.

What CTE is needed to match ceramic chip carriers?

Alumina ceramic chip carriers have a CTE of approximately 6 ppm/°C. For optimal solder joint reliability, the PCB substrate should match as closely as possible. Woven Kevlar laminates like Arlon 45NK achieve 5-7 ppm/°C, providing excellent matching. Non-woven aramid products at 7-9 ppm/°C also provide substantial improvement over standard FR-4 at 14-17 ppm/°C.

Are aramid laminates difficult to fabricate?

Aramid laminates require modified fabrication processes. Mechanical drilling causes more tool wear than glass-reinforced materials, though non-woven aramid is easier to drill than woven. Laser drilling works exceptionally well since aramid is 100% organic with no glass. Lamination requires attention to moisture control since aramid absorbs more moisture than glass. Work with your fabricator to ensure they have aramid processing experience.

Are Thermount aramid laminates still available?

DuPont announced obsolescence of Thermount fibers in 2006, which disrupted the supply of non-woven aramid laminates. However, replacement para-aramid fibers conforming to IPC-4411 are now available from alternative suppliers, and laminate manufacturers like Arlon have qualified these replacements in their products. Verify current availability and any qualification requirements with your specific laminate supplier before committing to a design.

Conclusion

IPC-A-142 defines the woven aramid fabrics that enable PCB laminates with CTE values matching ceramic chip carriers and silicon devices. The unique negative CTE of aramid fibers, combined with high modulus and low density, makes aramid reinforcement the preferred choice for applications requiring thermal expansion control.

For designs mounting leadless ceramic chip carriers, large BGAs, or other CTE-sensitive packages, aramid-reinforced laminates per IPC-A-142 (woven) or IPC-4411 (non-woven) provide the reliability that standard FR-4 cannot deliver. While these materials cost more and require specialized processing, the improvement in solder joint reliability often justifies the investment for high-reliability aerospace, defense, and medical applications.