CCL-HL835 PCB Material: CAF Resistant Halogen-Free Laminate for Demanding Applications

If your PCB design pushes via spacing to the limits, operates in humid environments, or has to survive years in an automotive ECU or industrial controller, then CAF resistance isn’t just a checkbox — it’s a survival requirement. The CCL-HL835 PCB material from Mitsubishi Gas Chemical (MGC) is purpose-engineered to address exactly this challenge, combining the company’s established halogen-free BT resin heritage with specifically enhanced resistance to Conductive Anodic Filament (CAF) formation. This article digs into what CAF is, why the material matters, and how CCL-HL835 fits into real PCB design and fabrication decisions.

What Is CCL-HL835 PCB Material?

The CCL-HL835 is a halogen-free copper clad laminate (CCL) produced by Mitsubishi Gas Chemical Company (MGC), the Japanese specialty chemicals manufacturer that pioneered BT (bismaleimide triazine) resin systems for the PCB industry. Within MGC’s product naming convention, “CCL” denotes a core laminate, and the “HL” series signifies the halogen-free family. The number 835 places it in the high-performance tier of MGC’s multilayer PWB materials lineup, closely related to the HL832 IC package series but optimized for broader PCB applications where both halogen-free compliance and long-term insulation reliability (specifically CAF resistance) are primary engineering requirements.

CCL-HL835 PCB material sits at a specific intersection that not many laminates occupy cleanly: it is simultaneously halogen-free, high-Tg, and built to resist the electrochemical failure mode that has become increasingly critical as PCB feature densities tighten. For engineers designing boards destined for automotive systems, telecommunications infrastructure, servers, or industrial equipment — all environments where boards live long lives under electrical bias and humidity — this combination of properties is precisely what the specification demands.

Understanding CAF: Why It Matters for High-Density PCB Design

Before evaluating any CAF-resistant PCB material, it is worth understanding the failure mode itself. Many engineers encounter CAF failures in the field without immediately recognizing them, because CAF manifests as intermittent leakage or logic instability long before it causes a hard short.

What Is Conductive Anodic Filament (CAF)?

CAF is a conductive copper-containing salt created electrochemically that grows from the anode toward the cathode subsurface along the epoxy/glass interface. It was first discovered in 1976 and identified as a catastrophic failure mode. The formation process requires three conditions to be present simultaneously: moisture absorbed into the laminate, a voltage differential between adjacent conductors, and a degraded or weak resin-to-glass fiber interface that provides a migration pathway for copper ions.

With the trend of high circuit density demands in organic packages, the pitch of plated through holes (PTHs) in packages should be reduced, and the amount of CAF failures is expected to be significantly higher. In practice, this means that as designers push via-to-via and via-to-trace spacing below 300–400µm on inner layers, CAF risk rises sharply — exactly the territory that modern high-density multilayer boards occupy.

The CAF Formation Sequence

The process unfolds in stages that are invisible until a failure occurs:

Stage	What Happens	Detectable?
1 – Interface Degradation	Resin-glass bond weakens due to drilling stress, thermal cycling, or moisture	Not during inspection
2 – Moisture Ingress	Water enters via cracks or weak glass/resin interface	Not electrically
3 – Copper Dissolution	Copper at the anode dissolves into Cu²⁺ ions under DC bias	Possible with SIR testing
4 – Ion Migration	Cu²⁺ migrates along glass/resin pathway toward cathode	Not during normal operation
5 – Filament Formation	Copper salts precipitate and build a conductive filament	Rising leakage current
6 – Electrical Failure	Filament bridges anode to cathode — short circuit	Yes, catastrophic failure

CAF can cause intermittent leakage, logic instability, or catastrophic short failures — often after months or years in service. This delayed onset is what makes it particularly dangerous in high-reliability applications. Boards that pass initial electrical test and even early life testing can develop CAF failures after extended field service.

Why Standard FR-4 Falls Short on CAF

During the manufacture of conventional epoxy, chlorine is a by-product which is essential for the formation of CAF. Residual ionic chlorine in the resin system creates an electrochemically active environment that accelerates copper ion migration. This is one of the underappreciated advantages of halogen-free laminates: by eliminating halogen-based flame retardants, the resin chemistry also removes one of the key ionic accelerants for CAF growth.

Halogen-free materials eliminate the possibility of remaining hydrolyzable halogen in the synthesis process, thus improving the ion migration resistance. At the same time, due to the low water absorption of halogen-free epoxy resin, the source of ion generation is reduced to some extent, thus improving the CAF resistance of the material.

This is the chemical logic behind why CCL-HL835 PCB material achieves its CAF resistance — it is not simply a mechanical property of the glass weave reinforcement, but a function of the halogen-free resin chemistry combined with engineered improvements to the resin-glass adhesion interface.

CCL-HL835 PCB Material: Core Technical Properties

The CCL-HL835 material belongs to MGC’s high-performance halogen-free family and shares the BT resin heritage that distinguishes the HL series from standard FR-4 modified-epoxy laminates. Based on the MGC HL series profile and industry-documented properties for this class of material, here are the key specifications:

Thermal Properties Table

Property	CCL-HL835 (Typical)	Standard Halogen-Free FR-4	High-Tg Halogen-Free FR-4
Glass Transition Temp (Tg, DMA)	≥185°C	~150–160°C	~170–175°C
Thermal Decomposition Temp (Td)	>340°C	~320°C	~330°C
T288 (time to delamination)	>5 min	~2–3 min	~3–4 min
Z-axis CTE (below Tg)	~45–55 ppm/°C	~60–70 ppm/°C	~55–65 ppm/°C
Moisture Absorption	Low (BT advantage)	Moderate	Moderate

Electrical and Reliability Properties Table

Property	Value (Typical)	Test Method	Significance
Dielectric Constant (Dk)	~3.7–4.0 @ 1 GHz	IPC-TM-650	Lower than standard FR-4 (~4.5)
Dissipation Factor (Df)	~0.010–0.015 @ 1 GHz	IPC-TM-650	Suitable for mid-speed digital
Surface Insulation Resistance	>10⁸ MΩ	IPC-TM-650	CAF-critical metric
Volume Resistivity	>10⁸ MΩ·cm	IPC-TM-650	Long-term reliability indicator
CAF Resistance	Excellent	IPC-TM-650 2.6.25	Core differentiator vs standard FR-4
Flammability	UL94 V-0	UL 94	No halogens, antimony, or phosphorus compound

Mechanical Properties Table

Property	Value	Test Method
Peel Strength (1 oz Cu)	≥1.0 N/mm	IPC-TM-650
Flexural Strength (lengthwise)	≥400 MPa	IPC-TM-650
Flexural Strength (crosswise)	≥350 MPa	IPC-TM-650
Dimensional Stability	Low shrinkage	IPC-TM-650
Stiffness	High (BT resin)	—

How CCL-HL835 Achieves Halogen-Free CAF Resistance

The dual achievement of halogen-free compliance and enhanced CAF resistance in CCL-HL835 PCB material is not coincidental — the two properties reinforce each other at the chemistry level.

The Resin System Advantage

Standard FR-4 laminates use DICY (dicyandiamide) curing with brominated epoxy resins. Brominated compounds achieve flame retardancy but introduce residual ionic species that lower the activation energy for CAF formation. Halogen-free PN curing systems (phenol-formaldehyde curing agent with phosphorus-nitrogen flame retardant chemistry) eliminate these residual halogens, leaving a cleaner resin matrix with fewer mobile ionic species.

As P or N is used to replace halogen atoms, the polarity of the whole epoxy resin will be reduced to a certain extent, so the electrical insulation of halogen-free epoxy resin will be better than that of halogen-based epoxy resin, and the dielectric loss will be lower than that of conventional materials.

MGC’s BT resin system (bismaleimide triazine) takes this further. The BT resin cross-linking structure creates a denser, more thermally stable polymer network than standard epoxy, which better resists the formation of micro-voids and the degradation of the resin-glass interface under thermal cycling stress — the very degradation that seeds CAF pathways.

Glass Fiber Interface Engineering

Poor adhesion between the resin and glass fibers in the PCB can create a path for CAF to occur. This may depend on parameters of the silane finish applied to the glass fibers, which is used to promote adhesion to the resin. CCL-HL835 PCB material benefits from MGC’s decades of optimization in resin-glass coupling chemistry, a direct extension of the same engineering that makes the HL832 family the industry reference for IC package substrates.

CCL-HL835 PCB Material Applications

The specific combination of properties in CCL-HL835 points toward a clear set of application environments. These are boards that need green compliance, long service life, dense via structures, and operation in environments where humidity and sustained voltage bias co-exist.

Primary Application Areas

Application Sector	Why CCL-HL835 PCB Material Fits	Key Requirements Met
Automotive ECUs & ADAS	Long life under temperature cycling, humidity, sustained DC bias	High Tg, CAF resistance, halogen-free
Industrial Controllers	Continuous operation in variable humidity environments	CAF resistance, T288, Td
Telecommunications Equipment	Dense multilayer designs, RoHS compliance for global markets	Halogen-free, dimensional stability
Server Motherboards	High layer count, tight via spacing, thermal reliability	High Tg, low CTE, CAF resistance
Medical Devices	Strict environmental compliance, long-term reliability	Halogen-free, RoHS, CAF resistance
Power Electronics	Heavy copper builds, thermal cycling exposure	High Tg, thermal stability
5G Infrastructure	Dense multilayer boards, long field service life	CAF resistance, halogen-free compliance

Why Automotive is the Critical Battleground for CCL-HL835

The automotive application is worth discussing specifically. Modern vehicle electronics demand boards that survive 10–15 year service lives, operating temperature ranges of –40°C to 125°C or beyond, and high-humidity conditions inside engine compartments or exposed chassis positions. These are textbook CAF-activation conditions: thermal cycling degrades the glass-resin interface over time, condensation introduces moisture, and always-on ADAS systems maintain persistent DC bias across densely packed vias.

There has been a significant increase in concerns about the effect of CAF on board reliability due to the reduction of inter-feature spacing caused by increased circuit density, electronic circuits being subjected to increasingly harsh environments especially in high reliability and safety critical applications, and higher soldering temperatures associated with lead-free solders which have the potential to affect laminate stability. CCL-HL835 PCB material addresses all three of these concerns simultaneously.

How CCL-HL835 Compares to Competing PCB Materials

Specifying engineers often evaluate CCL-HL835 alongside several competing material options. Here is how it positions in a practical comparison:

Material	CAF Resistance	Halogen-Free	Tg	Primary Use Case	Relative Cost
CCL-HL835 (MGC)	Excellent	Yes	≥185°C	Automotive, industrial, server	Higher
Standard Halogen-Free FR-4	Moderate	Yes	~150°C	General green electronics	Baseline
High-Tg Halogen-Free FR-4	Moderate	Yes	~170°C	Lead-free PCBs	Moderate premium
Standard FR-4 (brominated)	Poor	No	~135°C	General electronics	Lowest
BT Epoxy (HL832 family)	Excellent	Yes	~185°C	IC package substrates	Higher
Doosan PCB halogen-free	Good	Yes	~170–185°C	PWB, automotive, server	Comparable

The key differentiator for CCL-HL835 PCB material versus generic high-Tg halogen-free FR-4 is the resin system quality — specifically the BT chemistry heritage, the optimized resin-glass adhesion, and the proven track record in demanding package substrate environments that translates directly to improved CAF resistance in dense multilayer boards.

Compliance and Environmental Profile of CCL-HL835 PCB Material

The halogen-free designation of CCL-HL835 carries specific, quantified meaning. Per IPC-4101B and JPCA-ES-01-2003 standards:

Parameter	Limit	CCL-HL835 Status
Bromine (Br) content	< 900 ppm	Compliant
Chlorine (Cl) content	< 900 ppm	Compliant
Total Br + Cl content	< 1500 ppm	Compliant
Antimony compounds	None	Compliant
Phosphorus flame retardant	None (inorganic filler used)	Compliant
UL94 Flammability	V-0	Certified
RoHS Directive	Compliant	Yes
WEEE Directive	Compliant	Yes
IEC 61249-2-21	Halogen-free laminate standard	Compliant

The halogen free materials achieve a flammability rating of UL94V-0 without using halogens, antimony, or phosphorus compound. The substitution of an inorganic filler as the flame retardant has the additional benefits of improving the small hole CO₂ laser drilling properties and lowering the CTE.

This is a genuine environmental advantage: at end-of-life, CCL-HL835 boards do not release dioxins or furans during incineration, which is the key health and environmental concern driving halogen-free adoption under WEEE processing requirements.

PCB Fabrication Considerations for CCL-HL835 PCB Material

Drilling

The inorganic filler flame retardant system in CCL-HL835 has a practical process benefit: CO₂ laser drilling performance is improved compared to organically flame-retarded systems. For blind via drilling at the fine geometries typical in dense multilayer designs (75–100µm laser-drilled vias), this translates to cleaner hole walls and better resin smear control — both of which directly support CAF resistance by reducing the void and crack pathways at the via barrel.

Lamination

Low resin shrinkage during lamination cycling is an important characteristic for high layer count boards where dimensional accuracy across many lamination cycles determines via registration. The BT resin system in CCL-HL835 exhibits low shrinkage properties, supporting tight registration tolerances in 10+ layer multilayer builds.

Lead-Free Soldering Compatibility

The high Tg (≥185°C) and long T288 time (>5 minutes) make CCL-HL835 PCB material fully compatible with SAC305 and other lead-free solder profiles that peak at 260°C. Pre-bake before assembly (120°C for 4+ hours minimum) is recommended to remove absorbed moisture from storage, particularly for high-layer-count boards where moisture-induced delamination risk is elevated.

Storage

BT-based prepregs and cores are moisture-sensitive materials. Sealed storage per manufacturer recommendations with desiccant is standard practice. Boards produced with CCL-HL835 should be assembled within the moisture sensitivity window or re-baked prior to soldering.

CAF Testing Standards for CCL-HL835 PCB Material Qualification

Engineers qualifying CCL-HL835 for a new application should be aware of the test methods that directly characterize CAF resistance:

Standard	Test Method	What It Measures
IPC-TM-650 2.6.25	CAF Resistance Test (X-Y Axis)	Resistance to CAF between horizontal via pairs
IPC-TM-650 2.6.16	Pressure Vessel Method	Glass-epoxy laminate integrity, moisture absorption
IPC-TM-650 2.6.14.1	Surface Insulation Resistance (SIR)	Ionic contamination and leakage pathways
IEC 61189-2	SIR under temperature/humidity	Long-term electrochemical reliability
JEDEC JESD22-A101	Temperature/Humidity Bias (THB)	Accelerated CAF growth under bias and humidity

Accelerated CAF testing typically uses conditions of 65°C / 85% RH with voltage bias for 500+ hours. Materials like CCL-HL835 that are specifically engineered for CAF resistance will show a significantly longer time-to-failure (or no failure within the test window) compared to standard FR-4 under these conditions.

Useful Resources for Engineers Working with CCL-HL835 PCB Material

Resource	Description	Link
MGC BT Materials Lineup	Official MGC product page for halogen-free laminate series	mgc.co.jp
MGC Halogen-Free BT for PWB	Specific HL series halogen-free materials overview	mgc.co.jp/hfbt
UL Product iQ (File E81340)	UL certification database for MGC laminate series	productiq.ulprospector.com
IPC-TM-650 2.6.25 (CAF Test)	Standard CAF resistance test method for PCB laminates	ipc.org
IPC-4101 Specification	Base materials specification for rigid and multilayer boards	ipc.org
NPL CAF Research	UK National Physical Laboratory CAF testing and research	npl.co.uk
Isola CAF White Paper	CAF formation characterization across resin systems	isola-group.com
Wikipedia: CAF	Overview of CAF failure mechanism and mitigation	en.wikipedia.org
Leiton PCB Material Database	Filter and compare 350+ PCB base material datasheets	leiton.de
Sierra Circuits Material Selector	Interactive PCB material selector including CAF-resistant filter	protoexpress.com

5 Frequently Asked Questions About CCL-HL835 PCB Material

Q1: What is the difference between CCL-HL835 and CCL-HL832 PCB material?

The two products belong to MGC’s same halogen-free BT resin family but target different primary applications. The CCL-HL832 series (particularly the NX variants) is optimized for IC package substrates — ultra-thin cores, coreless process support, and CSP/BGA/flip chip geometries. CCL-HL835 PCB material is positioned for multilayer PWB (printed wiring board) applications where the higher design priority is long-term CAF resistance in dense via structures under sustained field conditions. Both share the high-Tg, halogen-free, BT-resin heritage, but HL835 is engineered for the board-level reliability requirements of automotive, industrial, and server applications rather than the ultra-thin IC package geometries of HL832.

Q2: Can CCL-HL835 PCB material completely eliminate CAF risk?

No PCB material can completely eliminate CAF risk — it is a system-level failure mode that depends on design spacing, drilling quality, fabrication cleanliness, and operational environment, not just material choice. What CCL-HL835 does is substantially raise the threshold at which CAF initiates and dramatically slow filament growth, giving designers much more margin before failure occurs. Good design dramatically reduces CAF risk; good manufacturing eliminates the remaining vulnerabilities. CCL-HL835 PCB material provides the material-level foundation; proper via spacing design rules, drilling process control, and ionic cleanliness are equally important.

Q3: Is CCL-HL835 PCB material suitable for high-frequency applications?

Its Dk (~3.7–4.0 at 1 GHz) and Df (~0.010–0.015) are more favorable than standard FR-4 and make it usable in mid-frequency digital designs, but it is not a purpose-designed low-loss material for RF applications above 5 GHz. For mmWave 5G, radar, or high-speed SerDes applications at multi-gigabit rates with tight insertion loss budgets, dedicated low-loss halogen-free materials (Dk < 3.5, Df < 0.005) would be more appropriate. For the dense multilayer digital boards in automotive, server, and telecom switching equipment where CAF resistance is the concern, CCL-HL835 provides adequate electrical performance.

Q4: What via spacing design rules should be used with CCL-HL835 PCB material to prevent CAF?

Even with a CAF-resistant material like CCL-HL835, good spacing practice remains essential. General industry guidelines recommend a minimum via-to-via spacing of 400µm center-to-center on inner layers for applications with sustained DC bias above 50V in humid environments, with 300µm being a practical minimum for less demanding conditions. Staggered via arrangements are more CAF-resistant than straight-line in-line arrays. Avoid straight parallel runs of opposite-polarity conductors on inner layers, and be cautious with orphan copper pads that serve no functional purpose but add ionic contamination sites. Your fab house should be able to provide specific design rules for CCL-HL835 based on IPC-TM-650 2.6.25 test data for this material.

Q5: How does CCL-HL835 PCB material handle multiple lamination cycles in high layer count builds?

This is an area where the BT resin heritage of CCL-HL835 is a real advantage. The low resin shrinkage and high thermal stability of BT-based systems means that dimensional accuracy is maintained better through multiple lamination cycles compared to standard modified-epoxy halogen-free FR-4. For boards with 16+ layers requiring sequential lamination, the accumulated registration error from resin flow and shrinkage is lower with BT-heritage materials. The high T288 time (>5 minutes) also means the material tolerates the thermal excursions of multiple lamination press cycles without delamination risk.