UGPCB LOGO
Product Overview
As PCB manufacturing costs continue to rise, engineers and procurement professionals are actively seeking cost-effective solutions that do not compromise performance. The double-sided carbon oil circuit board offers a compelling answer to this challenge.
UGPCB presents this double-sided carbon oil circuit board with the following specifications:
| Parameter | Specification |
|---|---|
| Base Material | FR4 |
| Board Thickness | 1.6mm |
| Layer Count | 2 layers (double-sided) |
| Board Size | 43.68 × 38.9mm |
| Minimum Hole Diameter | 0.33mm |
| Trace Width / Spacing | 0.32 × 0.36mm |
| Copper Foil Thickness | 35/35μm (1OZ on both sides) |
| Surface Finish | Carbon oil process |
| Solder Mask / Legend | Green solder mask / white silkscreen |
This is a standard rigid double-sided PCB. It combines traditional copper traces with a carbon oil surface finish that forms conductive carbon film pads at designated locations. The result is a board that offers both the excellent conductivity of copper traces and the superior wear resistance of carbon film contacts. This makes it an ideal choice for keypad applications across consumer electronics, automotive electronics, and medical devices.
What Is a Double-Sided Carbon Oil Circuit Board?
A double-sided carbon oil circuit board is a printed circuit board that uses carbon-based conductive ink (commonly referred to as carbon oil or carbon ink) applied through screen printing onto designated areas of the PCB substrate. After curing, this forms a carbon film conductive pattern.
Carbon oil is a conductive ink composed primarily of conductive carbon powder, thermosetting resin, and various additives. It is precisely printed onto specific locations on the circuit board – typically keypad contact areas – using screen printing technology. After high-temperature curing, the carbon oil forms a carbon film with a defined resistance value.
Compared to traditional gold-plated keypads, carbon oil keypads offer a clear advantage. Carbon oil keypads are highly wear-resistant and can withstand repeated operations, generally lasting for hundreds of thousands or even millions of switching cycles. Replacing expensive gold keypads with carbon oil keypads has become a major industry trend.
Carbon oil boards are a common surface treatment method for single-sided and double-sided PCBs. Through a series of processes including inspection, testing, and burn-in testing, these PCBs achieve reliable long-term operation.
Design Guidelines
Designing a double-sided carbon oil circuit board requires attention to the following key technical aspects:
Carbon Oil Trace Width and Spacing
According to industry PCB carbon oil manufacturing specifications, carbon oil trace width must be greater than 6mil (approximately 0.152mm). Carbon oil films are identified as “GTC” (top layer) or “GBC” (bottom layer) and are output as positive films.
Due to the excellent conductivity of carbon oil, adequate spacing must be maintained between carbon oil features on the finished board to prevent short circuits. Typical minimum clearances are:
The minimum alignment tolerance for carbon oil is ±6mil.
Carbon Oil Window and Copper Pattern Spacing
To account for alignment tolerances and prevent oil leakage that could expose copper, the carbon oil pad must be larger than the copper pad by:
The same clearance must be maintained between the carbon oil window and surrounding copper patterns to prevent carbon oil from covering adjacent copper traces and causing short circuits.
Carbon Oil Thickness Control
Carbon oil thickness directly affects sheet resistance and wear resistance. Industry standard specifications are as follows:
- Single-print carbon oil thickness: 0.3–1.0mil, tolerance ±0.3mil
- Double-print carbon oil thickness: 1.0–2.0mil, tolerance ±0.4mil
For carbon oil thickness exceeding 1.0mil, a double-print process is required. The carbon oil film for the second print must be 3mil smaller on each side than the first-print film.
Design Standard References
When designing a double-sided carbon oil circuit board, the following international standards should be followed:
- IPC-2221: Generic Standard on Printed Board Design – covers spacing, clearance, and structural design requirements
- IPC-6012: Qualification and Performance Specification for Rigid Printed Boards – defines performance requirements and product qualification criteria
- IPC-TM-650: Test Methods Manual – provides standardized test procedures for carbon oil PCB performance
Working Principle
The double-sided carbon oil circuit board operates on two fundamental levels:
Conduction Principle
The conductivity of carbon oil is determined by its formulation, which consists of conductive carbon powder, thermosetting resin, and various additives. Conductivity is measured by sheet resistance (unit: Ω/□).
According to Ohm’s Law:
R = ρ × L / S
Where R is resistance, ρ is resistivity, L is conductor length, and S is conductor cross-sectional area. For the carbon film conductive layer, sheet resistance is inversely proportional to film thickness: Rs = ρ / t (where t is carbon film thickness).
Carbon oil boards generally achieve sheet resistance of ≤20Ω/□, offering excellent cost-performance. Some low-resistance carbon oil formulations also incorporate a small amount of silver oil to further enhance conductivity.
Keypad Switching Principle
In keypad applications, the double-sided carbon oil circuit board works as follows: when a rubber keypad (or other elastic element) is pressed, the conductive carbon particles on the bottom of the keypad contact the carbon oil pad on the PCB, completing the circuit. The carbon oil pad serves as the keypad’s contact conductor, replacing traditional gold-plated contacts.
The advantages of carbon oil contacts include:
- High surface hardness and excellent wear resistance
- Low contact resistance and fast response time
- Chemical inertness, offering corrosion resistance and oxidation resistance
Applications
Double-sided carbon oil circuit boards are widely used across various industries:
Consumer Electronics
- Remote controls: TV remotes, air conditioner remotes
- Calculators: Keypad panels
- Keyboards: Computer keyboards, membrane keyboards
- Gaming devices and electronic keyboards
Automotive Electronics
- Window switch control boards
- Air conditioning control panels
- Instrument clusters and seat controllers
Medical Devices
- Medical equipment operator panels
- Blood glucose test strip electrodes
Industrial and Communications
- Industrial control equipment panels
- Communication equipment
- Smart label RFID
Cost-Effective Alternatives
Carbon oil circuit boards can replace the following traditional processes:
- Copper-plated through-hole processes
- Silver paste via-filling processes
- Copper paste via-filling processes
Environmentally friendly, low-cost carbon paste via-filling is a mainstream industry trend. Microsoft has adopted carbon oil for via-filling on power boards, completely replacing double-layer boards with copper-plated holes.
Product Classification
Double-sided carbon oil circuit boards can be classified along the following dimensions:
By Conductivity Type
- Low-resistance carbon oil boards: Sheet resistance ≤20Ω/□ – suitable for keypad contacts
- High-resistance carbon oil boards: Suitable for potentiometers and resistor layers
- Carbon oil via-filled boards: Use carbon oil to fill via holes for interlayer connections
By Base Material
- FR4 carbon oil boards: Glass fiber/epoxy substrate (this product)
- CEM-1 carbon oil boards: Composite epoxy material substrate
- Polyimide carbon oil boards: Flexible substrate
By Layer Count
- Single-sided carbon oil boards
- Double-sided carbon oil boards (this product)
- Multilayer carbon oil boards
By Surface Finish Combination
- ENIG + carbon oil
- HASL + carbon oil
- OSP + carbon oil
- Carbon oil + immersion tin (currently not suitable for mass production due to tin surface contamination issues)
Materials
Base Material – FR4
This product uses FR4 as the base material. FR4 is the industry designation for glass fiber-reinforced epoxy resin copper-clad laminate, offering:
- Excellent mechanical strength and dimensional stability
- Superior electrical insulation properties
- Good moisture and heat resistance
- Dielectric constant of approximately 4.2–4.3
FR4 is the most commonly used base material for rigid PCBs and offers good adhesion compatibility with carbon oil.
Conductive Layer – Copper Foil
Both sides use 35μm (1OZ) copper foil, ensuring excellent conductivity and current-carrying capacity.
Carbon Oil Material
Carbon oil is a carbon-based conductive ink composed of:
- Conductive carbon powder: 30%–50% content – determines conductivity
- Thermosetting resin: Provides adhesion and curing properties
- Various additives: Regulate flow characteristics and adhesion performance
Carbon oil must exhibit strong adhesion, peeling resistance, and wear resistance.
Solder Mask and Legend
- Solder mask: Green ink – protects circuits and prevents short circuits
- Legend: White silkscreen – identifies component locations and board numbers
Performance Specifications
Electrical Performance
| Parameter | Specification | Reference |
|---|---|---|
| Sheet resistance | ≤20Ω/□ | Industry standard |
| Resistance change rate | ≤10% (after 1 million cycles) | Industry standard |
Mechanical Performance
Dimensional Accuracy
| Parameter | Specification |
|---|---|
| Minimum hole diameter | 0.33mm |
| Minimum trace width | 0.32mm |
| Minimum trace spacing | 0.36mm |
| Board thickness | 1.6mm |
Environmental Reliability
Per IPC-TM-650 standard requirements:
- Damp heat test (IPC-TM-650 2.6.3.7): 1000 hours at 40°C / 90% RH – carbon oil sheet resistance change ≤20%
- Temperature cycling test (IPC-TM-650 2.6.7): 100 cycles from -40°C to 85°C – no carbon oil cracking, on-resistance fluctuation ≤15%
- Abrasion resistance test (IPC-TM-650 2.5.9): After 1000 cycles – abrasion coefficient ≤5mg
Product Structure
The typical structure of a double-sided carbon oil circuit board (top to bottom):
- Legend layer (white): Component reference designators
- Solder mask layer (green): Circuit protection and short-circuit prevention
- Carbon oil layer: Conductive carbon film at designated keypad locations
- Top copper traces (35μm): Conductive circuitry
- FR4 substrate (1.6mm): Insulating support
- Bottom copper traces (35μm): Conductive circuitry
- Solder mask layer (green): Bottom layer circuit protection
- Legend layer (white): Bottom layer identifiers
The carbon oil layer is applied only to designated keypad contact areas or conductive zones, not across the entire board.
Key Features
Significant Cost Advantage
The carbon oil process eliminates certain copper plating and etching steps, reducing material costs by 15%–25%. Compared to hard gold plating, carbon ink costs are 40%–60% lower.
Long Wear Life
Carbon oil keypads offer exceptional wear resistance, lasting over 1 million switching cycles with a resistance change rate of ≤10%.
Simple, Efficient Process
Carbon oil application uses screen printing rather than lengthy electroplating processes. Production lead times can be shortened by over 30%.
Excellent Environmental Resistance
Carbon oil is chemically inert, offering better acid and alkali resistance than copper. It performs more reliably in humid or dusty environments. Carbon film surface hardness reaches 3H (pencil hardness test), outperforming unprotected copper surfaces (1–2H).
Flexible Design Options
Carbon oil can be printed directly to form resistors, keypads, and other functional modules, reducing solder joint counts by 30%–50%.
Manufacturing Process
The typical manufacturing process for a double-sided carbon oil circuit board:
- Material cutting: Cut FR4 copper-clad laminate to working dimensions
- Drilling: Drill via holes and mounting holes (minimum diameter 0.33mm)
- Surface cleaning: Chemical cleaning to remove oxides, oils, and contaminants
- Electroless copper / electroplating: Plating via holes for interlayer connections
- Pattern transfer: Exposure and development to form circuit patterns
- Etching: Remove copper foil from non-circuit areas
- Strip: Remove dry film resist
- Solder mask printing: Apply green solder mask ink
- Solder mask curing: High-temperature curing of solder mask
- Surface finish – carbon oil printing:
- Screen print carbon oil at designated locations (single or double print)
- Pre-bake (80°C × 30 minutes)
- Main cure (150°C × 30–60 minutes)
- Legend printing: Print white silkscreen identifiers
- Profiling: Router routing, V-cut, etc.
- Electrical testing: Continuity testing, insulation testing
- Final inspection and packaging
For PCBs with ENIG + carbon oil combination finish, carbon oil printing is typically performed after ENIG, with blue tape applied to protect carbon oil areas.
Application Scenarios
Remote Controls and Appliance Control Panels
Carbon oil contacts offer long press life and low contact resistance for fast response. Typical applications include TV remotes, air conditioner remotes, and washing machine control panels.
Automotive Electronic Control Modules
Carbon oil circuits inside vehicle door panels offer strong corrosion resistance, performing reliably in complex automotive environments and solving the oxidation and breakage issues common with traditional solutions.
Medical Device Operator Panels
Carbon oil-printed touch sensing areas maintain stable operation in oily, humid environments. Carbon oil materials also offer excellent biocompatibility.
Industrial Keyboards and POS Systems
In POS terminals and industrial keyboards, circuits combining carbon oil with FR4 substrates adapt to temperature variations and deliver stable, reliable performance.
Why Choose UGPCB for Your Double-Sided Carbon Oil Circuit Board?
✅ International Standard Compliance
Product design and manufacturing strictly follow IPC-2221 (Generic Standard on Printed Board Design), IPC-6012 (Rigid Board Performance Specification), and IPC-TM-650 (Test Methods Manual).
✅ Precision Manufacturing Capabilities
- Minimum hole diameter 0.33mm – supports high-density designs
- Minimum trace width/spacing 0.32 × 0.36mm – industry-leading precision
- Carbon oil sheet resistance ≤20Ω/□ – excellent conductivity
✅ One-Stop Service
UGPCB provides end-to-end services from PCB design and manufacturing to PCBA assembly.
✅ High Reliability Guarantee
Carbon oil contacts offer wear life ≥1 million cycles with resistance change rate ≤10%, ensuring long-term stable operation.
Request a Quote Today
The double-sided carbon oil circuit board is a highly cost-effective solution for keypad applications in consumer electronics, automotive electronics, and medical devices.
Whether you need PCB prototyping, volume production, or turnkey PCBA services, UGPCB delivers professional technical support and competitive pricing.
📧 Contact us today for a free technical consultation and custom quote!
- Website: www.ugpcb.com
- One-stop services: PCB design, manufacturing, and PCBA assembly
Data Source Declaration
The technical data and standards cited in this article are derived from the following authoritative sources:
- IPC (Association Connecting Electronics Industries) Standards:
- IPC-2221 – Generic Standard on Printed Board Design
- IPC-6012 – Qualification and Performance Specification for Rigid Printed Boards
- IPC-TM-650 – Test Methods Manual
- Industry Standard Technical Specifications:
- PCB carbon oil manufacturing requirements (carbon oil trace width ≥6mil, film naming conventions, etc.)
- Carbon oil screen printing thickness control specifications (single print: 0.3–1.0mil; double print: 1.0–2.0mil)
- Industry Technical Literature:
- Carbon oil PCB manufacturing specifications – three key dimensions
- Carbon oil process PCB technical analysis
- Carbon oil PCB vs. standard PCB comparison
