The hottest carbon conductive printing material an

2022-07-24
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Carbon conductive printing materials and carbon film printed boards (lower)

carbon film printed boards

printed boards with carbon film conductive graphic layers are called carbon film printed boards

carbon film printed boards are suitable for electronic products with low power, low power consumption and high input impedance that can give polypropylene better optical performance. Its production cost is lower than that of double-sided plate and nickel plated gold unidirectional plate, and its performance is reliable, which reflects its unique non corrosive medium; Superior parts

1. characteristics of carbon film printed boards

carbon film printed boards have the following characteristics

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

(2) due to the stable chemical properties of carbon element, the three prevention (mould proof, oxidation proof and salt fog proof) performance is good

(3) the carbon film coating can resist the corrosion of acid, alkali, salt and organic solvents such as trichloroethylene and ethanol

(4) two or more layers of circuits can be set on single-sided printed boards to replace some double-sided boards

(5) some resistors can be directly printed on the circuit or board surface, which can reduce the space position and board surface size, reduce the welding points, and improve the tensile, seismic and impact resistance of the product, so as to improve the reliability of the whole machine

(6) carbon film printed boards can also eliminate a large number of environmental pollution and wastewater treatment problems caused by double-sided metallized holes and electroplating processes

(7) the price is low, and the production cost is 30% ~ 60% of that of double-sided plate, so it has great competitiveness in the market

however, carbon film printed boards also have their own limitations. At present, they can not be involved in the field of thin wire, small aperture and other high-precision printed boards; It is also easily restricted by processing technology, processing technology, production environment (especially mildew, rain, humid climate) and other factors. In addition, the carbon film cannot 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 complicated. At present, there is no more advanced automatic production line in China. The production process is too long and the process control must be relatively strict to ensure the product quality. Therefore, it has high processing difficulty and the added value of the product is quite low, which restricts its development and further technical development, which is unfavorable to the development of carbon film printed boards

2. technical conditions of carbon film printed boards

technical conditions of carbon film printed boards

(1) appearance

carbon film pattern shall be flat and dense, with uniform thickness and no obvious deviation of conductor pattern

(2) electrical performance

the insulation resistance between carbon film conductors shall not be less than 1 × 1010Ω;

insulation resistance between layers shall not be less than l × 1010Ω;

carbon film square resistance value shall not be greater than 60 Ω/□

the contact resistance shall not be greater than 100 Ω/point

(3) coating hardness

pencil hardness of carbon film coating shall not be less than 4H

(4) coating adhesion

after the transparent 3M pressure-sensitive tape is glued and pulled at the same position for three times, the carbon film coating shall not fall off

(5) solvent resistance

when the samples are tested with active flux with chlorine content ≤ 0.2% and specified solvent respectively, the carbon film layer shall be free of abnormal phenomena such as blistering, delamination, dissolution and obvious color change

(6) wear resistance

after 500000 pressing, the change of resistance value of carbon film shall not be greater than 10% of the initial value

(7) flame retardancy

the flame retardancy of carbon film plate shall not be lower than the flame retardant grade of foil clad plate substrate

(8) adaptability to climate and environment

temperature change: carbon membrane plates shall be exposed to high temperature of 40 ℃ and low temperature of -10 ℃ for 3H and tested for 5 cycles. The change of square resistance shall not be greater than 10% of the initial value

(9) constant temperature and humidity

the insulation resistance of carbon film plate shall not be less than 1 after it is recovered at room temperature for 2h according to the test conditions in Table 2 × 108 Ω, the change of square resistance shall not be greater than 10%

3. manufacturing process

at present, the main varieties of carbon film printed boards include carbon film key printed boards and single-sided double-layer carbon film boards

(1) carbon film key printed board

carbon film key printed board is to overprint conductive carbon print key graphics on the key part of the original printed board to replace the electroplated nickel gold layer on the key part

its brief manufacturing process is as follows:

production of printed graphics → printing of solder resist printing materials → printing of carbon conductive printing materials for key parts

although the manufacturing process of carbon key printed boards is simple, its difficulty is that the accuracy of carbon film overprint is required to be high, especially in the overprint of thin wires/spacing by keyboard, the alignment between the carbon film graphics and the underlying copper foil key graphics is required to be accurate without offset, and all of them should be covered. If the position is offset and overprint is not correct, the spacing will become smaller, the insulation performance will be reduced and short circuit will occur; At the same time, the offset bottom copper foil is not completely covered by the carbon film and is exposed. It is very easy to oxidize the surface, resulting in increased resistance or poor contact

(2) single sided and double-layer printed boards

single sided and double-layer printed boards are characterized by forming an insulating layer on the surface of the copper conductor circuit of the single-sided printed board, and then printing a carbon film on the surface of the insulating layer to form a conductive pattern across a new graphene material that can be more than just a smart chip. The basic structure of single-sided and double-layer printed circuit board is shown in Figure 4

the manufacturing process is as follows:

production of printed circuit graphics → printing of solder resist → printing of insulating bottom layer → printing of conductive carbon film

conductive graphics, solder resist, base insulating layer and carbon film conductive layer of single-sided and double-layer printed boards can be printed with silk and must be heat cured or ultraviolet (UV) cured. Therefore, high dimensional stability The paper phenolic resin copper-clad laminate with low warpage and relatively low price is heat-resistant. The process control in the manufacturing process is very important for the quality reliability of the carbon film cross wire printed board

in terms of process control, when printing solder resist, it is necessary to prevent the solder resist from contaminating the solder pad used for welding and the solder pad to be printed with carbon film circuit, and to prevent the bottom copper circuit pattern edge from being coated with solder resist, resulting in the decrease of interlayer insulation resistance; When printing the base insulation layer, it is necessary to prevent the bare copper solder joint from being contaminated and the edge of the copper wire pattern from being coated. It is required to ensure that it reaches 25 ~ 40 μ M uniform insulation thickness. Therefore, the scraping direction shall not be the same when printing solder mask and insulating layer mask (cross printing shall be conducted), that is, when the moving direction of the scraper is parallel to a certain wire when printing solder mask, the insulating layer mask must be rotated by 90 ° and the moving direction of the scraper is vertical to this wire to prevent sand holes and skip printing

the adhesion between the above layers must be good, especially the adhesion between the base copper circuit welding plate and the carbon film conductive film determines the resistance change rate of the carbon film conductive film. In addition, it should be noted that the base insulation layer cannot affect the performance of the carbon film conductive layer circuit, and the insulation resistance between layers meets the technical index requirements

the conduction function of carbon film printed boards and the reliability of conductive circuits are not only related to the quality of carbon conductive printing materials. It is also related to the rationality and correctness of the circuit design in the manufacturing process and the operation of plate making and printing process. For example, the control of carbon conductive film thickness is related to the control of photosensitive film thickness. Too thin photosensitive emulsion film will affect the conductivity of carbon film printed boards. In addition, the square resistance value of printed materials that are not well kept or exceed the storage period will increase

when the carbon film printed board is hot-air flattened or wave soldering, in order to avoid the influence and pollution of the residual flux on the conductive carbon film, it is recommended to use strippable printing materials for safety protection

the printed board with carbon film cross wire is a single-sided double-layer printed board with a layer of carbon film conductive graphics on the surface of the single-sided printed board. By analogy, the double-sided printed board can also be simply made into a double-sided three-layer or double-sided four layer printed board. However, if the carbon film conductive circuit is formed on a single-sided and double-layer printed board, it will cause reliability problems, so it is not appropriate

source: Membrane Switch Technology Forum

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