When the industry shifted to lead-free assembly under RoHS, it didn’t just change the solder alloy — it changed the thermal demands on every laminate in the supply chain. Peak reflow temperatures jumped from around 183°C for eutectic tin-lead to 217–260°°ä for SAC alloys, and standard FR-4 with a Tg of 130–140°°ä suddenly had a lot less margin than it used to. That’s the problem KB-6165F was designed to solve. It’s Kingboard’s lead-free optimized, high Tg FR-4 grade, and if you’re running lead-free production on anything more demanding than a simple single-sided board, it deserves a serious look.
What Is KB-6165F Lead-Free High Tg FR-4?
KB-6165F is a high performance FR-4 laminate manufactured by Kingboard Chemical Holdings, specifically engineered for lead-free PCB assembly processes. The “F” designation in the product name indicates its lead-free compatibility — this isn’t a standard FR-4 grade pressed into lead-free service, it’s a material formulated from the ground up to handle the thermal demands of SAC305 and other lead-free solder alloys.
The core construction follows standard FR-4 architecture: woven glass fabric reinforcement bonded with an epoxy resin system, with copper foil on one or both sides. What differentiates KB-6165F lead-free high Tg material is the resin chemistry — a modified epoxy formulation that achieves a glass transition temperature of 165–175°°ä, compared to 130–140°°ä for standard FR-4. That 30–40°C improvement in Tg is the difference between comfortable thermal margin and operating dangerously close to material limits during lead-free reflow.
For a broader view of how KB-6165F fits within Kingboard’s FR-4 product family, the laminate overview provides useful context across their full grade range.
Why High Tg Matters for Lead-Free Assembly
This is the fundamental question engineers ask when evaluating KB-6165F, and it’s worth answering precisely rather than just saying “higher Tg is better.”
The glass transition temperature is the point at which the epoxy resin transitions from a rigid glassy state to a softer rubbery state. Below Tg, the material has relatively low CTE (coefficient of thermal expansion) and high mechanical stiffness. Above Tg, CTE increases dramatically — particularly in the Z-axis — and mechanical properties degrade significantly.
During lead-free reflow, your board spends time above 217°C (SAC305 liquidus) with a peak temperature of 245–260°°ä. Here’s what that means for different Tg grades:
| FR-4 Grade | Typical Tg | Peak Reflow Temp | Margin Above Tg |
| Standard FR-4 | 130–140°°ä | 260°°ä | 120–130°C above Tg |
| Mid Tg FR-4 | 150–160°°ä | 260°°ä | 100–110°C above Tg |
| KB-6165F High Tg | 165–175°°ä | 260°°ä | 85–95°C above Tg |
| Very High Tg FR-4 | 180–200°°ä | 260°°ä | 60–80°C above Tg |
Every board goes above Tg during lead-free reflow — that’s unavoidable. The question is how far above Tg you go and for how long. The higher the Tg, the less time the material spends in the high-CTE rubbery state, and the less Z-axis expansion stress is applied to plated through-holes and vias during each thermal cycle.
For multilayer boards with high via density, multiple reflow passes, or thick copper layers, the cumulative stress from repeated thermal cycling through the Tg region is what causes PTH barrel cracking and via reliability failures. KB-6165F’s higher Tg directly reduces this stress accumulation.
KB-6165F Lead-Free High Tg FR-4: Full Technical Specifications
| Parameter | KB-6165F Value | Test Method |
| Glass Transition Temperature (Tg) | 165–175°°ä | IPC-TM-650 2.4.25 (DSC) |
| Thermal Decomposition Temperature (Td) | ≥340°°ä | IPC-TM-650 2.4.24.6 |
| Time to Delamination T260 | ≥30 minutes | IPC-TM-650 2.4.24.1 |
| Time to Delamination T288 | ≥5 minutes | IPC-TM-650 2.4.24.1 |
| CTEz (below Tg) | ~50 ppm/°C | IPC-TM-650 2.4.41 |
| CTEz (above Tg) | ~250 ppm/°C | IPC-TM-650 2.4.41 |
| CTExy | ~14–16 ppm/°C | IPC-TM-650 2.4.41 |
| Dielectric Constant (1 MHz) | 4.2–4.6 | IPC-TM-650 2.5.5 |
| Dissipation Factor (1 MHz) | 0.018–0.022 | IPC-TM-650 2.5.5 |
| Volume Resistivity | ≥10⸠MΩ·cm | IPC-TM-650 2.5.17 |
| Surface Resistivity | ≥10⸠MΩ | IPC-TM-650 2.5.17 |
| Flexural Strength (lengthwise) | ≥415 MPa | IPC-TM-650 2.4.4 |
| Flexural Strength (crosswise) | ≥380 MPa | IPC-TM-650 2.4.8 |
| Water Absorption (24h) | ≤0.15% | IPC-TM-650 2.6.2 |
| Peel Strength (1 oz Cu, after thermal stress) | ≥1.4 N/mm | IPC-TM-650 2.4.8 |
| Thermal Stress (288°C solder float) | Pass 20s | IPC-TM-650 2.4.13 |
| Flame Resistance | UL 94 V-0 | UL 94 |
| CAF Resistance | Pass | IPC-TM-650 2.6.25 |
Two parameters here deserve specific attention beyond the Tg. The T288 value of ≥5 minutes means KB-6165F can withstand 288°C for at least 5 minutes without delamination — this is the thermal stress test that simulates lead-free solder float conditions and is a direct measure of lead-free assembly compatibility. Standard FR-4 often fails this test or passes with minimal margin. The Td of ≥340°°ä also provides confidence that the material won’t begin decomposing during even aggressive lead-free assembly processes.
KB-6165F vs Standard FR-4 vs Other High Tg Grades
Understanding where KB-6165F sits relative to alternatives is essential for making the right material selection decision:
| Property | Standard FR-4 | KB-6165F (High Tg) | Very High Tg FR-4 | Halogen-Free High Tg |
| Tg (°°ä) | 130–140 | 165–175 | 180–200 | 150–175 |
| Td (°°ä) | ´Ê300–310 | ≥340 | ≥350 | ≥340 |
| T288 | <1 min | ≥5 min | ≥10 min | ≥5 min |
| CTEz below Tg | ~60 ppm/°C | ~50 ppm/°C | ~45 ppm/°C | ~50–55 ppm/°C |
| Df (1 MHz) | 0.018–0.025 | 0.018–0.022 | 0.018–0.022 | 0.018–0.025 |
| Water Absorption | ≤0.20% | ≤0.15% | ≤0.15% | ≤0.20% |
| Lead-Free Suitability | Marginal | Good | Excellent | Good |
| Relative Cost | Baseline | +15–25% | +30–50% | +20–35% |
| Halogen-Free | No | No (standard) | No (standard) | Yes |
KB-6165F hits the sweet spot for most lead-free production applications. Very high Tg grades (180–200°°ä) offer better thermal performance but at a significant cost premium that’s only justified for the most demanding applications — thick multilayer boards, high-reliability industrial or automotive electronics, or designs with extreme via density. For the broad middle ground of commercial and industrial lead-free PCB production, KB-6165F’s Tg of 165–175°°ä provides adequate margin without the cost premium of the highest-performance grades.
Applications Where KB-6165F Lead-Free High Tg FR-4 Excels
Multilayer PCBs with High Via Density
This is the primary use case that drives KB-6165F specification. As layer count increases and via density grows, the cumulative thermal stress on PTH barrels during multiple reflow passes becomes the dominant reliability concern. A 12-layer board going through two reflow passes and a wave solder cycle has experienced significant Z-axis expansion stress by the time it leaves the assembly line. KB-6165F’s lower CTEz below Tg (~50 ppm/°C vs ~60 ppm/°C for standard FR-4) and higher Tg combine to meaningfully reduce this stress accumulation.
Industrial Electronics and Control Systems
Industrial equipment typically has longer service life requirements than consumer electronics — 10–20 year design life is common, versus 3–5 years for consumer products. The thermal cycling reliability advantage of KB-6165F compounds over time. A board that survives 1,000 thermal cycles on standard FR-4 might survive 3,000+ cycles on KB-6165F, depending on the specific design and operating conditions. For industrial control boards, motor drives, and automation equipment, that reliability difference is worth the material cost premium.
Automotive Electronics (Non-Safety-Critical)
For automotive applications outside the powertrain and safety systems — infotainment, body control modules, comfort electronics — KB-6165F provides a good balance of thermal performance and cost. True automotive-grade applications require AEC-Q200 qualified materials with tighter process controls, but KB-6165F’s thermal characteristics are a reasonable starting point for automotive auxiliary electronics.
Server and Networking Equipment
High-density server boards, network switches, and storage controllers run hot, go through multiple reflow passes during assembly, and are expected to operate reliably for years in data center environments. KB-6165F’s combination of high Tg, low CTEz, and good CAF resistance makes it well-suited for these applications. The CAF (conductive anodic filament) resistance is particularly relevant for high-density designs with fine pitch vias operating at elevated temperatures and humidity.
Power Electronics and Inverter Boards
Power conversion boards — inverters, UPS systems, motor drives — experience significant thermal cycling during operation as power components heat and cool with load changes. The combination of assembly thermal stress and operational thermal cycling makes high Tg material a sensible choice. KB-6165F handles both the lead-free assembly process and the operational thermal environment more reliably than standard FR-4.
Telecommunications Infrastructure
Base station equipment, optical network terminals, and telecom switching hardware all benefit from KB-6165F’s thermal reliability. These systems operate continuously in environments ranging from climate-controlled central offices to outdoor cabinets, and they’re expected to run for years without field failures. The material cost premium is trivial relative to the cost of field failures in deployed telecom infrastructure.
Lead-Free Assembly Process Guidelines for KB-6165F
Reflow Profile Optimization
KB-6165F handles standard lead-free reflow profiles without issues, but optimizing the profile reduces unnecessary thermal stress on the board:
| Reflow Zone | Recommended Parameter | Notes |
| Preheat ramp rate | 1–3°°ä/²õ±ð³¦´Ç²Ô»å | Avoid thermal shock |
| Soak zone temperature | 150–180°°ä | Flux activation, moisture outgassing |
| Soak zone duration | 60–120 seconds | Longer for thick boards or heavy copper |
| Ramp to peak rate | 1–2°°ä/²õ±ð³¦´Ç²Ô»å | Controlled approach to liquidus |
| Peak temperature | 245–260°°ä | SAC305 requires minimum 235°C |
| Time above liquidus (217°C) | 45–75 seconds | Sufficient for wetting, minimize thermal exposure |
| Cooling rate | 2–4°°ä/²õ±ð³¦´Ç²Ô»å | Controlled cooling reduces thermal shock |
For double-sided boards requiring two reflow passes, the second pass applies additional thermal stress to PTH barrels and vias. KB-6165F’s higher Tg provides more margin for this, but minimizing peak temperature and time above liquidus on the second pass is still good practice.
Pre-Bake Requirements
KB-6165F’s water absorption of ≤0.15% is better than standard FR-4, but moisture management before assembly is still important. Absorbed moisture converts to steam during reflow, creating internal pressure that can cause delamination or measling. Pre-bake guidelines:
Bare boards stored in unsealed packaging: 120°C for 2–4 hours minimum
Boards with moisture-sensitive components already placed: follow component MSL requirements
Boards stored in sealed moisture barrier bags with desiccant: pre-bake may not be required if floor life hasn’t been exceeded
Wave Soldering Compatibility
KB-6165F handles lead-free wave soldering (typically using SAC305 or SN100C alloy at 260–270°C pot temperature) without issues. Standard preheat profiles apply — topside preheat to 90–110°C before the solder wave. The higher Tg of KB-6165F provides better resistance to the thermal shock of wave soldering compared to standard FR-4, which is particularly relevant for thick multilayer boards.
Regulatory and Environmental Compliance
| Regulation/Standard | KB-6165F Status | Notes |
| RoHS 2011/65/EU | Compliant | No restricted substances |
| UL 94 | V-0 Rated | UL recognized |
| IPC-4101 Slash Sheet | /126 or /129 | High Tg FR-4 specification |
| REACH SVHC | Verify current list | Request material declaration from supplier |
| IEC 61249-2-21 (Halogen-Free) | Not compliant (standard) | Halogen-free variant available separately |
| CAF Resistance | Pass IPC-TM-650 2.6.25 | Important for high-density designs |
The IPC-4101 slash sheet reference is worth understanding. Standard FR-4 falls under slash sheet /21 or /24. High Tg FR-4 grades like KB-6165F fall under /126 (Tg ≥150°C) or /129 (Tg ≥170°C, enhanced thermal properties). When writing your PCB fabrication specification, referencing the correct IPC-4101 slash sheet ensures your fabricator sources material that meets your actual requirements rather than substituting standard FR-4.
Comparing KB-6165F Against Competing High Tg FR-4 Grades
| Manufacturer | Grade | Tg (°°ä) | Td (°°ä) | T288 | Df (1 MHz) | Notes |
| Kingboard | KB-6165F | 165–175 | ≥340 | ≥5 min | 0.018–0.022 | Strong supply chain, wide availability |
| Shengyi | S1000H | 170–180 | ≥340 | ≥5 min | 0.018–0.022 | Common Asia-Pacific alternative |
| Nan Ya | NPG-170 | 165–175 | ≥340 | ≥5 min | 0.018–0.022 | Taiwan-sourced, good consistency |
| ITEQ | IT-180A | 175–185 | ≥350 | ≥10 min | 0.018–0.020 | Higher Tg option |
| Ventec | VT-47 | 170–180 | ≥340 | ≥5 min | 0.018–0.022 | European distribution strength |
| Isola | IS410 | 175–185 | ≥340 | ≥10 min | 0.018–0.020 | Well-established in North America |
Performance specs across these grades are closely matched in the 165–175°°ä Tg range. KB-6165F’s competitive position comes from Kingboard’s manufacturing scale, supply chain reliability, and consistent material quality — factors that matter as much as raw performance specs in production environments.
Procurement and Qualification Checklist
Before committing KB-6165F to a production design, verify these items:
IPC-4101 slash sheet compliance — confirm your fabricator is sourcing material that meets IPC-4101 /126 or /129 as appropriate for your Tg requirement. Don’t accept “high Tg FR-4” without specifying the slash sheet.
T288 test data — request material certification showing T288 ≥5 minutes. This is the most direct measure of lead-free assembly compatibility and should be in the material certification package.
UL recognition — verify current UL file number through UL Product iQ. KB-6165F carries UL 94 V-0 recognition; confirm the current status before finalizing compliance documentation.
CAF resistance data — for high-density designs with fine-pitch vias, request CAF resistance test data per IPC-TM-650 2.6.25.
Full material declaration — request IPC-1752A or IPC-1752B material declaration for REACH SVHC verification and supply chain compliance documentation.
Useful Resources for Engineers
— governing specification including high Tg FR-4 slash sheets /126 and /129
— test method reference for Tg, Td, T288, and CTEz measurement
— verify current UL recognition status for KB-6165F
— official KB-6165F datasheet and product documentation
— design rules for multilayer PCBs
— reliability requirements for lead-free PCB production
— restricted substances compliance reference
— current SVHC list for supply chain compliance verification
Frequently Asked Questions About KB-6165F Lead-Free High Tg FR-4
Q1: Is KB-6165F necessary for all lead-free PCB production, or only for specific applications?
Not all lead-free boards need high Tg material. Simple single and double-sided boards with low via density, going through a single reflow pass, can often use standard FR-4 with adequate reliability. KB-6165F becomes important when you have multilayer boards (6+ layers), high via density, multiple reflow passes, thick copper layers, or long service life requirements. The cost premium of 15–25% over standard FR-4 is justified when the reliability improvement matters — for high-value boards or applications where field failures are costly, it’s an easy decision. For simple, low-cost boards with minimal thermal stress, standard FR-4 may be adequate.
Q2: How does KB-6165F’s Tg of 165–175°°ä compare to what’s actually needed for lead-free assembly?
The IPC-4101 specification for lead-free compatible FR-4 requires a minimum Tg of 150°C (slash sheet /126). KB-6165F at 165–175°°ä exceeds this minimum by a comfortable margin. For most lead-free applications, 150°C Tg is the minimum acceptable, 165–175°°ä is the practical sweet spot, and 180°C+ is reserved for the most demanding applications. KB-6165