The Development of Novel Metallization processes for Electronic Manufacturing
Printed circuit board is used in the electronic manufacturing for mechanical and electrical support. It is electronically connects the electric components using conductive traces, carved from copper covered onto a non-conductive material. Printed circuit board are usually include copper and copper mixture materials that are coated to provide good mechanical and good conductivity with other devices in the assembly. Printed circuits board are used in all electronic equipments such as computer and mobile phones and TV and communications equipment and satellite as well as in the control of gadgets in the factories, companies and other uses of the innumerable.
PCB are inexpensive, and can be highly reliable. They require much more design effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper and faster for high-volume production. Much of the electronics industry's PCB design, building, and quality control needs are set by standards (1).
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In 1885 before the appearance of electric circuit board and point to point production, plate of carton was used to connect the electric components with wires and it was heavy and has big volume.
Before printed circuits point-to-point production was used for primary sample or small production runs wire.
Circuit boards were produced in the mid-1930, by Austrian inventor Paul Eisler. During World War II the United States produced them on a huge range for use in war radios. During this period the invention remained use in the military part, and until the end of the war it became available for commercial use.
Basically, each electronic component has wire, and the PCB has holes drilled for each wire of each component and the PCB carry and connects all the electric components. Printed circuit boards have copper tracks connecting the holes where the components are placed. They are designed specially for each circuit and make structure very easy. The coating on the surface of a circuit board are usually copper, created either by putting single lines mechanically, or by coating the all board in copper and remove away excess. The method of assembly is called through-hole formation. In modern circuit board production, it uses soldered in place on the board with very little hassle., this process is usually be done by putting the cool solder mixture, and baking the entire board to dissolve the components in place. Soldering could be done automatically by passing the board over wave, of molten solder in a wave-soldering machine(1). In previous period to the creation of surface-mount technology was in the mid-1960s, all circuit boards used wire to attach components to the board. But With the removing the wires from circuit boards, circuit boards have become lighter and more efficient to produce.
Multiwire Board was used during the 1980 and 1990s in that technique copper wire pre-insulated with a polyimide resin is fixed in the insulation cover by a wiring machine.
Multiwire Board allows through wiring so that the number of wires be in one layer significantly increases, and consequently an high-density board can be manufactured with a smaller number of layers than an ordinary printed wire boards. In addition, as Multiwire Board uses copper wire of a uniform diameter, it is superior in various electric characteristics such as providing stable characteristic impedance.
Surface-mount technology appeared in the 1960s, and became famous in the early 1980s and became widely used by the mid 1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly on to the PCB surface. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much higher circuit densities. Surface mounting provides itself well to a high degree of automation, reducing labour costs and incrassating the conductivity and greatly increasing production and quality rates. Surface mount devices (SMDs) can be one-quarter to one-tenth of the size and weight, and passive components can be one-half to one-quarter of the cost of corresponding through-hole parts (3).
The advantages of Surface mount technology are:
- Smaller components. Smallest is currently 0.5 x 0.25 mm.
- Has higher number of components and more connections per component.
- Fewer holes should be drilled through abrasive boards.
- Easy automated assembly.
- Small mistakes in component placement are corrected automatically (the surface tension of the molten solder pulls the component into alignment with the solder pads).
- Components can be putted on both sides of the circuit board.
- Lower resistance at the connection.
- Good mechanical performance under shake and vibration conditions.
- SMT parts generally cost less than through-hole parts.
- Fewer unwanted RF signal effects in SMT parts when compared to leaded parts, yielding better predictability of component characteristics.
- Faster assembly. Some placement machines are capable of placing more than 50,000 components per hour.
And there are some Disadvantages
- Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted.
- Usually some type of error, either human or machine-generated, and includes the following steps.
- Melt solder and component removal
- Residual solder removal
- Printing of solder paste on PCB, direct component printing or dispensing
- Placement and reflow of new component.
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Over the past few year, electronic products, and especially those which fall within the category of Consumer Electronics have been significantly reduced in physical size and weight. Products such as cellular telephones, lap-top computers, pagers, camcorders, have been reduced by as much as3/4 of their original introductory size and weight. The most significant contributing factor to this reduction has been the inclusion of fine pitch, Surface Mount (SM) components. The larger, thicker and heavier leaded Through-Hole (TH) packages.
The Surface Mount (SM) was developed to give the customer with increased component density and performance over the larger Dual-Inline-Package (DIP). The SM also provides the same high consistency. The Chip Scale (CSP) was developed to provide the customer with an additional increase in component performance and density over the SM . The CSP also provides the same high reliability as the DIP and SM package
Components which are used in integrated circuits (chips), resistors, and capacitors can be soldered to the surface of the board or more commonly, attached by inserting their connecting pins or wires into holes drilled in the board. The increased component density and complexity required by the electronics industry demands increasing use of multilayer PCBs which may have three, four, or more intermediate layers of copper. Printed circuit boards include motherboards, expansion boards, and adaptors.
Epoxy polymers are regularly used for electric circuit board manufacturing purposes, especially for built up layers and micro-vias in modern printed circuit boards. The sticking together of the plated metal layers to this polymer surface is primary importance for the consistency of the internal connection. Chemical treatment of the polymer surface changes the chemical and physical nature of the polymer. These results in specific groups of the polymer chain present on the surface and changes the roughness of the polymer layer. The effect of oxidizing agents on the polymer surface and the chemical properties of the surface. .
Conducting layers are typically made of thin copper foil. Isolating layers are usually laminated together with epoxy resin. The board is usually coated with a solder cover that is green in color. Other colors that are normally available are blue, and red .
A number of additional technologies may be applied to circuit boards for specialized uses:
Circuit boards, for example, are designed to be slightly flexible, allowing the circuit board to be placed in positions which would not otherwise be practical, or to be used in wire systems.
- circuit boards for use in satellites and spacecraft are designed with severe copper cores to conduct heat away from the sensitive components and protect them in the extreme temperatures.
- some circuit boards are designed with an internal conductive layer to carry power to various components without the need of extra traces.
Publications have documented the plating of nanoparticales of Cu (Copper plating) or Au on flexible polyimide ( Epoxy) by electroplating
Copper plating is the process in which a coating of copper is deposited on the item to be plated by using an electric current.
Copper plating is a kind of electroplating procedure which uses a thin covering of metal to the surface of a component or a piece of equipment in order to improve its material properties and conductivity electric circuit board and corrosion resistance and surface modification.
Copper plating has an important use in another industries such as automotive, furniture, aerospace and ceramics. Important characteristics of the copper plating process involve the type of process, the copper plating solution and power consumption(5).
Some important parameters must be take during copper plating:
- Kind of copper plating
- How much necessary capacity of the copper plating system
- How much power will spending during the copper plating process.
- The electroless copper platting process involves of four basic operations: cleaning, activation, acceleration, and deposition.
Useful features of copper plating:
- Supply good basecoat for nickel and chromium.
- Increase the conductivity and reduce the cost of production
- Supply excellent electrical conductivity properties for applications such as electronics and telecommunications.
- Can be use as a mask in surface hardening procedures.
- Provide good lubrication in metal forming operations.
- Makes jewels look good.
- Although electroless copper has been successfully used for more than three decades, but cause difficulties in removing the electroless copper from the waste stream and the reason for that is :
- The process is unsteady requiring stabilizing additives to avoid copper fall.
- Environmentally is not good produces complex agents, such as EDTA
- The large number of process needs high water consumption.
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The electroless copper method has considerable percentage of water volume used. water use is high due to the essential rinsing required between nearly all of the process steps. Copper is found into the wastewater stream due to pull out from the cleaner conditioner, accelerator, and deposition baths process. Much of this copper is complexed with EDTA and needs special waste treatment considerations and that is not good for environmental. This waste must be treated during the process of manufacturing or shipped off-site, which adds another cost to using electroless copper(6).
Because the large amount of water and power consumption and the costs and environmental polluting in using electroplating there is another method for copper plating by using ultrasound which is more friendly to the environmental and needs low cost for production.
Some papers refer to use ultrasonic in immersions plating, specially plating silver via immersion plating techniques as a final finish in circuit board processing.
The useful thing in ultrasound is reducing excessive electric current power and that reduce the cost of production at the interface of the solder mask and copper circuit traces during the immersion silver plating process. Ultrasonics also used in cleaning printed circuit boards before plating.
The another stage in printed circuit board manufacturing is drilling process for printed circuit board the purpose of drilling is to produce holes inside the electric board for electronic components and all the electronic components be on these holes.
holes are drilled through the cover so that component can be inserted and then fixed firmly in place. There are generally two types of components that are attachable to the circuit board such as resistors, transistors, which are attached to the circuit board by putting each of the legs of components through a hole in the board. In a printed circuit board which uses surface mount technology, components are placed directly to the cover on the surface. Each set hole in the printed circuit board is planned to receive a exacting component. Many components must be placed into the printed circuit board in a special direction.
The simplest printed circuit boards, wires must be printed on more than one surface of fiberglass to let all the component interconnections. Each surface containing printed wires is called a layer or film. Simple printed circuit board which requires only two layers, only one piece of fiberglass is required because wires can be printed on each sides. Some printed circuit board has several layers, individual circuit boards are manufactured individually and then coated together to produce one multi layer circuit board. To connect wires on two or more layers small holes called vias are drilled through the wires and fiberglass board at the point where the wires on the different layers cross. The interior surface of these holes is coated with metal so that electric current can flow through the vias. Some more complex computer circuit boards have more than 20 layers.
The printed circuit board has green colour because presence of thin sheets of green plastic on the both sides and without that the printed circuit board will appears in pale yellow colour. Called solder masks, these sheets cover all metal other than the component covers and holes.
Electric circuit components are manufactured with covered metal pins which are used to fix them to the printed circuit board both mechanically and electrically so electric current can pass between them. The soldering process, which provides mechanical bond and a very good electrical connection, is used to connect the components to the printed circuit board. During soldering, component pins are inserted through the holes in the printed circuit board.
A multilayer printed circuit board which can be interlayer connection with low resistance. The multilayer printed circuit board have a conductive design on one face and without connection hole on the other face, for applying the conductive design to outside; a second substrate having a conductive design formed on a face opposed to the other face of first substrate and a conductive bump on the conductive design integrally. The first substrate and the second substrate are integrated by engaging the bump of the second substrate with the connection hole of the first substrate and by intervening a conductive cement between the bumps and the conductive pattern exposed to outside from the connection holes.
Some papers refer to use laser drilling to create holes during the manufacturing process for printed circuit board and that is also possible with controlled drilling by using computer program software or by pre-drilling the individual sheets of the printed circuit board before production, in order to produce holes which connect only some of the copper covers, rather than let them to go through the all board. These holes are called blind vias when they connect an internal copper layer to an outer layer