1.Flexibility and reliability of flexible circuits
At present, there are four kinds of flexible circuits: single side, double side, multi-layer flexible board and rigid flexible board.
The cable single - sided flexible board is the lowest cost when electrical performance requirements are not high board.When wiring on one side, one side flexible board should be chosen.It has a conductive pattern etched by chemistry. The conductive pattern layer on the flexible insulating substrate is calendered copper foil.The insulating base material can be polyimide, polyethylene terephthalate, aramid and polyvinyl chloride.
The double side flexible plate of the ring is a conductive pattern etched on each side of the insulating base film.The metallized hole connects the two sides of the insulation to form a conductive channel to meet the flexural design and use function.The covering film can protect single and double-sided conductors and indicate where the components are placed.
The embedded multi-layer flexible plate is made of three or more layers of single-side or double-side flexible circuits. Through drilling holes and electroplating, metallized holes are formed to form conductive pathways between different layers.In this way, no complex welding process is required.Multilayer circuits have significant functional differences in higher reliability, better thermal conductivity, and more convenient assembly performance.The interaction of assembly size, number of layers and flexibility should be taken into account when designing the layout.
The traditional rigid flexible panel consists of rigid and flexible substrates with selective stratum pressure.The structure is compact and electrically connected with a metallic hole.If a printed board has components on both sides, a rigid flexible board is a good choice.But if all the components are on one side, it would be more economical to use a double-sided flexible board and apply a layer of FR4 reinforcement on the back.
The flexible circuit of the mixed structure is a multilayer board, and the conductive layer is composed of different metals.An eight-layer board USES fr-4 as the inner medium and polyimide as the outer medium, extending the leads from three different directions of the main board, each of which is made of different metals.The copper alloy, copper and gold lead separately.This hybrid structure is mostly used in the relationship between electrical signal conversion and heat conversion, and is the only feasible solution in the case of low temperature where the electrical performance is relatively harsh.
The best performance/cost ratio can be achieved by evaluating the convenience and total cost of the interior design.
2.Economics of flexible circuits
If the circuit design is relatively simple, the overall volume is not large, and the space is appropriate, the traditional method of interconnection is much cheaper.Flexible circuits are a good design option if the circuit is complex, processes many signals or has special electrical or mechanical performance requirements.Flexible assembly is most economical when the size and performance of the application exceeds the ability of rigid circuits.A flexible circuit with 5mil through hole and 3mil lines and spacing can be made on a thin film.Therefore, it is more reliable to directly attach the chip on the film.Because it does not contain a flame retardant that might be a source of contamination for ion drills.These films may be protective and solidify at higher temperatures, resulting in higher glass temperatures.The cost savings of flexible materials over rigid materials are due to the exemption of connectors.
High cost raw material is the main reason of high price of flexible circuit.The price of raw materials varies greatly. The cost of raw materials for flexible polyester circuits with the lowest cost is 1.5 times;The high performance polyimide flexible circuit is as high as 4 times or higher.At the same time, the flexibility of materials makes it difficult to carry out automatic processing in the manufacturing process, which leads to the decline of production.Defects are easy to occur in the final assembly process, including peeling off flexible attachments and breaking of lines.This is more likely to happen when the design is not suitable for application.Under the high stress caused by bending or forming, reinforcement or reinforcement materials are often required.Despite the high cost of raw materials and the trouble of manufacturing, the collapsible, flexible and multi-layer plywood function will reduce the size of the whole component and the materials used will reduce the total cost of assembly.
The flexible circuit industry is in a small but rapid development.Polymer thick film method is an efficient and low-cost production process.The process is based on cheap flexible substrates, selective network printing conductive polymer ink.The typical flexible base material is PET.Polymer thick film conductors include silk-screen metal fillers or toner fillers.The polymer thick film method itself is clean, using lead-free SMT adhesive without etching.The polymer thick film circuit is 1/10 of the price of copper polyimide film circuit because of the addition process and low base cost.It's 1/2 to 1/3 of the price of rigid circuit boards.The polymer thick film method is especially suitable for the control panel of the equipment.On mobile phones and other portable products, the polymer thick film method is suitable for converting printed circuit board components, switches and lighting devices into polymer thick film circuit.Save cost and reduce energy consumption.
Generally speaking, flexible circuits do cost more and cost more than rigid ones.In many cases, the flexible panel has to face the fact that many parameters are beyond the tolerance range.The difficulty in making flexible circuits lies in the flexibility of the material.
3.The cost of flexible circuits
Despite these cost factors, the price of flexible assembly is falling, becoming close to that of conventional rigid circuits.The main reason is the introduction of updated materials, improved production process and changed the structure.The current structure makes the product more thermally stable, with few material mismatches.Some newer materials can produce more fine lines because the copper layer is thinner, making the components lighter and more suitable for loading into small Spaces.In the past, copper foil was attached to the adhesive by roller technology. Now, copper foil can be generated directly on the medium without adhesive.These techniques allow you to get copper layers that are a few microns thick, and you get 3m.An even narrower precision line.The flexible circuit after removing some adhesive has the flame retardant property.This can accelerate uL certification process and further reduce the cost.Flexible PCB solder mask and other surface coatings further reduce the cost of flexible assembly.
In the coming years, smaller, more complex and more expensive flexible circuits will require more novel methods of assembly and the addition of hybrid flexible circuits.The challenge for the flexible circuit industry is to use its technological advantages to keep pace with computers, telecommunications, consumer demand and active markets.In addition, flexible circuits will play an important role in lead-free operations.