Carbon conductive printed material and carbon film printed board (below)

Carbon film printed board

A printed board having a carbon film conductive pattern layer is called a carbon film printed board.

Carbon film printed boards are suitable for low power, low power, high input impedance electronic products. Its production cost is lower than the double-sided, nickel-plated gold one-way plate and its reliable performance reflects the unique and superior parts.

1. Carbon film printed board features

Carbon film printed boards have many of the following features.

(1) The pencil hardness of the film layer of the carbon film printed board is greater than 5H, the mechanical strength is high, the wear resistance is high, the service life is more than 1 million times, and the maximum number is 25 million times, which is far higher than the average life of the equipment used.

(2) Because of the stable chemical properties of carbon, the three defenses (mildew proofing, anti-oxidation, and salt spray resistance) perform well.
(3) Carbon film coating can resist acid, alkali, salt and organic solvents such as trichloroethylene, ethanol, etc.

(4) Two or more layers of wiring may be provided on a single-sided printed board instead of a partial double-sided board.

(5) Some resistors printed directly on the circuit or board can reduce the space and board size, reduce the number of solder joints, improve the tensile, shock and impact resistance of the product, and thus improve the overall reliability.

(6) Carbon film printed boards can also eliminate a large number of environmental pollution and waste water disposal problems caused by double-sided metallized holes and plating processes.

(7) The price is low, and the production cost is 30% to 60% of the double-sided panel, which makes it more competitive in the market.

However, the carbon film printed board also has its limitations. At present, it cannot cover thin lines, small apertures, and other high-precision printed circuit boards; it is also susceptible to processing technology, processing technology, and production environment (especially moldy rain, wet climate). Constraints such as factors, in addition, because the carbon film layer can not be welded, so the assembly process is also limited and affected.

Although the carbon film printed board is not a high-precision product, the production process is relatively complicated. At present, there are no more advanced automatic production lines in China. The production process is relatively long and the process control must be relatively strict in order to ensure product quality. Therefore, it has high processing difficulty and the added value of the product is quite low. These conditions have restricted its development and further technological development, which is unfavorable for the development of carbon film printed boards.

2. Carbon film printed board technical conditions

Carbon film printed board under the technical conditions.

(1) Appearance

The carbon film pattern should be flat and dense, with uniform thickness and no significant deviation of the wire pattern.

(2) Electrical properties

The insulation resistance between the carbon film conductor shall not be less than 1×1010Ω;
Interlayer insulation resistance should not be less than l × 1010Ω;
Carbon film square resistance is not more than 60Ω/□;
Contact resistance should not exceed 100Ω/point.

(3) Coating hardness

The pencil hardness of the carbon film coating should not be lower than 4H.

(4) Coating adhesion

After the transparent 3M pressure-sensitive adhesive tape is glued three times at the same position, the carbon film coating should not be lumped off.

(5) Solvent resistance

When samples are tested with chlorine flux ≤ 0.2% active flux and specified solvents, the carbon film layer should be free from blistering, delamination, dissolution, and obvious discoloration.

(6) Wear resistance

After the carbon film is pressed by 500,000, its resistance value does not change by more than 10% of the initial value.

(7) Flame retardancy

The flame retardancy of the carbon film should not be lower than the flame retardant grade of the coated board substrate.

(8) Climate Environment Adaptability

Temperature changes: Carbon membrane plates were exposed to high temperature of 40°C and low temperature of -10°C for 3 hours and tested for 5 cycles. The square resistance change should be no more than 10% of the initial value.

(9) Constant temperature and humidity When the carbon film board is tested according to the conditions shown in Table 2 and restored at room temperature for 2 hours, its insulation resistance is not less than l×108Ω, and the square resistance change is not more than 10%.

3. Manufacturing process

At present, the main varieties of carbon film printed board are carbon film key printed board and single-sided double carbon film board.

(1) Carbon film key printed board Carbon film key printed board is printed on the key part of the original printed circuit board printed conductive carbon material key pattern to replace the key part of the electroplated nickel gold layer.

Its brief manufacturing process is as follows:
Printed graphic production → Printed solder mask printing material → Press button Print carbonaceous conductive printing material

Although the carbon key PCB manufacturing process is simple, the difficulty is that the accuracy of the carbon film overlay is high, especially in the thin wire/pitch keyboard overprinting the carbon film pattern and the bottom copper foil key pattern alignment Accuracy is required, there must be no offset, and all must be covered. If the position is misaligned and the overprinting fails, the pitch becomes smaller, the insulation performance decreases, and a short circuit occurs; at the same time, the bottom copper foil is not fully covered by the carbon film and is exposed due to the offset. Easily oxidize the surface and increase the resistance or contact failure.

(2) Single-sided double-layer printed board

The single-sided dual-layer printed board is characterized in that an insulating layer is formed on the surface of the copper circuit of the single-sided printed circuit board, and then a carbon film layer is printed on the surface of the insulating layer to form an over-the-wire or bridging conductive pattern. The basic structure of a single-sided double-layer printed circuit board is shown in Figure 4.

Its manufacturing process is as follows:

Printed Circuit Pattern Production → Printed Solder Masks → Printed Insulating Underlayers → Printed Conductive Carbon Film

The conductive pattern, solder resist, base insulating layer, and carbon conductive layer of the single-sided dual-layer printed board can be formed by screen printing and must use thermal curing or ultraviolet (UV) curing of the printing material. Therefore, high dimensional stability is generally adopted. Heat-resistant paper-based phenolic resin copper-clad laminates with low warpage, low warpage, and relatively low cost, and process control in the process are critical to the quality reliability of the carbon film cross-over printed boards.

In terms of process control, when printing solder resist printing materials, it is necessary to prevent the solder resist stained on the solder pads for welding and the solder pads on which the carbon film circuit is to be printed, and to prevent the edge of the underlying copper circuit pattern from being unwelded. Coating on the coating causes the insulation resistance of the interlayer to drop; when the base insulating layer is printed, it is necessary to prevent the bare copper solder joint from being contaminated and the copper wire pattern edge is coated at the end, meaning that uniform insulation of 25-40 μm is ensured. Layer thickness. Therefore, when the solder resist printing material and the insulating layer printing material are printed, the squeegee direction can not be the same (cross printing should be performed). That is, when the direction of the squeegee of the screen printing solder resist printing material is parallel to a certain wire, the screen printing insulating layer is printed. The angle must be rotated by 90°. The direction of the squeegee movement is perpendicular to this wire to prevent squeezing and skipping.

The adhesion between the above layers must be good. In particular, the adhesion between the base copper circuit solder plate and the carbon film conductive film layer determines the rate of change of the resistance value of the carbon film conductive film layer. In addition, it must be noted that the base insulating layer can not affect To the performance of the carbon film conductive layer circuit, the insulation resistance between layers meets the technical specifications.
The conduction function of the carbon film printed board and the reliability of the conductive circuit are related to the quality of the carbon conductive printed material. It is also related to the rationality and correctness of the circuit design in the manufacturing process and the screen printing and screen printing operations. For example, the control of the thickness of the carbonaceous conductive film layer is related to the control of the thickness of the photosensitive film, and an excessively thin emulsion film may affect the conductive properties of the carbon film printed board. In addition, if the storage is poor or exceeds the storage period, the resistance of the printing material will increase.

When the carbon film printed board is subjected to hot air leveling or wave soldering, in order to avoid the influence of flux residue on the conductive carbon film and pollution, it is recommended to use strippable printing material for security protection.

The carbon fiber crossover printed circuit board is a single-sided double-layer printed board with a carbon film conductive pattern on the surface of a single-sided printed board, and so on. The double-sided printed board can also be easily made into two-sided three. Layer or double-sided four-layer printed board. However, if a carbon film conductive circuit is formed on a single-sided double-layer printed board, reliability problems may arise, and thus it is not suitable.

Source: Membrane Switch Technology Forum