iii: electrical contacts processing...

III: Electrical Contacts Processing Techniques

Introduction to electrical contacts

An electrical contact is defined as the interface between the current-carrying members of electrical/electronic devices that assure the continuity of electric circuit, and the unit containing the interface. The current-carrying members in contact, often made of solids, are called contact members or contact parts. The contact members connected to the positive and negative circuit clamps are called the anode and cathode, respectively. The primary purpose of an electrical connection is to allow the uninterrupted passage of electrical current across the contact interface.

Electrical Contacts Processing Techniques

Deposition processes

Pattering and scribing

processes

Cutting/Drilling

Welding processes

Sintering processes

1. Deposition processes

One of the basic building blocks in Microelectromechanical Systems (MEMS) processing is the ability to deposit thin films of material. Deposition is any process that deposits a thin film of material onto an object. In this course we assume a thin film to have a thickness anywhere between a few nanometer to about 100 micrometer. The film can subsequently be locally etched using processes like Lithography and Etching.

1. Deposition processes a. Electrodeposition

Electroplating Electroless plating

Electrodeposition is typically restricted to electrically conductive materials. It is well suited to make films of metals such as copper, gold and nickel. The films can be made in any thickness from ~1µm to >100µm. The deposition is best controlled when used with an external electrical potential, however, it requires electrical contact to the substrate when immersed in the liquid bath. In any process, the surface of the substrate must have an electrically conducting coating before the deposition can be done.


Electroplating

The electroplating method is widely used in different industries, including electrical contact manufacturing. Electroplating is the deposition of metals (or alloys) on top of an electronically conductive surface from an electrolyte contains the metals ions. The process is usually done form aqueous electrolytes at room temperature. A unique feature of the electrolyticprocess is the possibility of controlling the deposition rate of the coating and its basic characteristics by changing the current density.

Electrical Contacts: Fundamentals, Applications and Technology, CRC Press 2006


Electroplating The coatings produced by this method possess a higher resistance to electrical erosion and hardness than corresponding bulk materials and coatings of the same composition deposited by other techniques. Electroplated rhodium and platinum show an especially high hardness (the Brinell hardness is up to 700 and 500, respectively). The hardness of palladium reaches 250, silver reaches 100, and gold reaches 70. In general, coatings with a higher hardness have higher wear resistance, providing the coating with no tendency toward brittle fracture. By varying the deposition regime, coating of different thicknesses can be obtained with characteristics otherwise not achievable by other methods.



Electroplating •  The advantages of electroplating are high deposition rate, a nonuniform thickness due to ohmic effects, a required conductive surface, a wide variety of materials that can be electroplated, low cost, and ease of control. •  One of the most serious disadvantages of electroplating is the presence of high internal stresses that can lead to cracking generated in the coatings. Hence, to obtain a strong adhesive strength for the coating, careful surface preparation prior to electroplating is required. Thick coatings without internal stresses can be obtained by dipping a part to be plated into a molten metal bath rather than using a salt solution bath.



Electroplating

Another disadvantage of this method is the limited variations in the composition. Due to the instability of metals in different salt solutions, electrolytes for all combinations of metals are not possible. Even if the solution of two salts is stable, such as for Au and Co, the ratio of the composition cannot be varied over a wide range.



Electroless plating

This is a chemical reduction process whereby any catalytic surface in contact with the plating solution is coated uniformly regardless of part geometry. In this process the electrons still flow between the couples in the bath event in the absence of the electrodes. Several metals can be electroless plated, including palladium, copper, nickel, and silver. However, the most common are nickel and copper.



Electroless plating •  Ni Electroless Plating. Nickel is usually plated from a hypophophite bath on activated surfaces. The deposition is an alloy of nickel and phosphorous (6–18% P). The surface is activated by a colloidal mixture of tin chloride and palladium. Electroless nickel can be applied with excellent adhesion to many different substrates, including steels, aluminum; copper, bronze and brass, as well as nonconductors (ceramics, plastics), powdered or sintered metals, and magnesium, beryllium and titanium. •  Cu Electroless Plating. Copper plating is limited for certain applications, such as in the semiconductor industry. The reducing agent in Cu electroless plating is formaldehyde, which is a carcinogenic material. This process can be costly due to environmental restrictions.


1. Deposition processes b. Chemical deposition

Chemical deposition of metallic coatings involves the reduction of metal ions present in the solution on the active surface. The solution for chemical coating deposition includes salts of the metal being deposited, a reducing agent, a stabilizer, buffer compounds, brighteners, ligand donors, and other components. With the best developed processes of chemical silvering and gilding, coatings of up to 3–5 mm thick can be obtained. Solutions for the chemical deposition of palladium, platinum, and rhodium are available, but the deposition rate of these metals and the coating thickness obtained are low. The hardness of chemically deposited coatings exceeds that of corresponding annealed metals, yet it is by 30–50% less than that of electroplated coatings. For this reason the wear resistance of these coatings is low. Disadvantages of the chemical deposition method lie in the instability of solutions, which require constant renewal, as well as limitations of the substrate materials. Due to these shortcomings, this method serves mainly as the first stage of the metallization of insulating materials, followed by the deposition of an electroplated coating.


1. Deposition processes c. Physical Vapor Deposition Technology

Although electroplating is used for the mass production of electrical contacts, vapor deposition has been suggested as an alternative method of coating. The sputtering process, used in layer production, is the physical vapor deposition (PVD) technology, which is based on the processes resulting from glow discharge. Metal atoms are sputtered from the target by bombardment with inert gas ions (argon) and deposited on the substrate placed in the chamber. The mechanical, electrical, and chemical properties of the growing layer can be varied by changing deposition parameters, such as the residual gas pressure and corn position, the sputtering rate, the substrate temperature, and the substrate current.



The PVD technology has important advantages for the development of alloys because of the possibility of co-sputtering (several targets of different materials are sputtered simultaneously). The composition of the alloy can thus be varied in a wide range simply by changing the individual sputtering rates, i.e., the Ar+ flux bombarding the target. The electron-beam evaporation process is not flexible especially for materials with a high melting point. Additionally, the adhesion of the layers and their structure are more appropriate in the case of sputter deposition because of a higher kinetic energy of the atoms impinging on the substrate surface.


c. Physical Vapor Deposition Technology


The advantages of vapor deposition over electroplating are numerous. For example, vapor deposition can be used to create coating structures and compositions which are often either difficult or impossible to obtain by electroplating, including multilayers, composites, and amorphous alloys of metals and ceramics. From a processing point of view, vapor deposition is also an agile manufacturing technique; as the same vapor deposition system can be used to deposit almost any coating material onto any substrate. Furthermore, vapor deposition is an environmentally friendly manufacturing process which does not generate hazardous byproducts, such as the chemicals used in some plating baths.


c. Physical Vapor Deposition Technology

2. Pattering and scribing processes

Pulsed and CW lasers can be used to scribe fine features in any material. This can be for surface patterning, dicing or as the first step in a “scribe and break” process. Fine slots can be cut in almost any material including: ceramics, silicon,sapphire, metals and glass. These very fine slots can be used directly in photovoltaic cell production.

a. Laser patterning and scribing

2. Pattering and scribing processes

b. Photolithography

Photolithography is the transfer of geometric shapes on a mask to a smooth surface. In modern semiconductor manufacturing, photolithography uses optical radiation to image the mask on a silicon wafer using photoresist layers. Other similar methods are electron beam, scanning probe, X-ray and XUV lithography.

Steps Used in Photolithography •  Surface cleaning •  Barrier layer formation (Oxidation) •  Spin coating with photoresist •  Soft baking •  Mask alignment •  Exposure •  Development •  Hard baking •  Post process cleaning

3. Laser cutting/drilling

Laser cutting/drilling: a technique that utilizes a focused laser beam at power densities sufficient to melt and vaporize the material. Main features: ü  Non contact and therefore wear-free cutting/drilling ü  High flexibility ü  Possibility for automatization ü  High speed ü  Small achievable hole diameters (less than 1µm)

Usability: ü  Steels and different metallic alloys ü  High strength materials, semiconductors, ceramics, carbon compounds, composites

3. Laser cutting/drilling

Pulsed lasers which provide a high-power burst of energy for a short period are very effective in some laser cutting processes, particularly for piercing, or when very small holes or very low cutting speeds are required, since if a constant laser beam were used, the heat could reach the point of melting the whole piece being cut. Most industrial lasers have the ability to pulse or cut CW (Continuous Wave) under NC (numerical control) program control. Double pulse lasers use a series of pulse pairs to improve material removal rate and hole quality. Essentially, the first pulse removes material from the surface and the second prevents the ejecta from adhering to the side of the hole or cut.

4. Welding processes

Welding is a process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint. Some of the best known welding methods include: Ø  Electro-Spark Deposition (ESD) Ø  Cold welding Ø  Electron Beam Welding (EBW) Ø  Laser Beam Welding (LBW)


Electro-Spark Deposition (ESD) ESD is a capacitor discharge, micro-arc welding process that utilizes short duration electrical pulses, discharged at controlled energy levels, to create a metallurgically bonded surface modification. Electric sparks are generated at 10–1000 per second for 10-6 – 10-5 seconds per spark. Direct current from the power supply will heat the electrode to 8000–25,000 oC only at the contact areas and transfers a small quantity of the electrode to the work piece under an ionized state to produce a strong metallurgical bonding.


Electro-Spark Deposition (ESD) In this process very high densities of energy flows are achieved without significant heating of the specimen under treatment. Since the process is extremely nonequilibrium, principally new materials can be obtained which under common equilibrium conditions could not be produced. Moreover, in spark discharge the distance between the electrodes and substrate can be made extremely short, down to a few tens of microns. This eliminates the effect of the environment on the deposition process and excludes the necessity of a vacuum and thus significantly widens the application range of the coatings.


Cold welding Cold welding is a hermetic sealing process widely used in the crystal, transistor and high powered solid state electronic switching industries. The cold welding process bonds ductile metals together, by the use of pressure alone, acting through a specific weld tool design. A general flow of metal takes place between the die surfaces at room temperatures, stretching the mating surfaces of the metals. A true homogeneous weld is formed with no introduction of a bonding agent. While most ductile metals can be welded into similar or dissimilar metal joints, some of them are more readily joined together. Aluminium, copper and ferrous metals clad with aluminium or copper flow together with relative ease. Copper or copper clad material must be electro less nickel plated to provide optimum weldability.


Electron beam welding (EBW) Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. The workpieces melt and flow together as the kinetic energy of the electrons is transformed into heat upon impact. EBW is often performed under vacuum conditions to prevent dissipation of the electron beam.


Electron beam welding (EBW) Free electrons in vacuum can be accelerated, with their orbits controlled by electric and magnetic fields. In this way narrow beams of electrons carrying high kinetic energy can be formed, which upon collision with atoms in solids transform their kinetic energy into heat. Electron beam welding provides excellent welding conditions because it involves: Ø  Strong electric fields, which can accelerate electrons to a very high speed. Thus, the electron beam can carry high power, equal to the product of beam current and accelerating voltage. By increasing the beam current and the accelerating voltage, the beam power can be increased to practically any desired value. Ø  Using magnetic lenses, by which the beam can be shaped into a narrow cone and focused to a very small diameter. This allows for a very high surface power density on the surface to be welded. Values of power density in the crossover (focus) of the beam can be as high as 104 – 106 W/mm2. Ø  Shallow penetration depths in the order of hundredths of a millimeter. This allows for a very high volumetric power density, which can reach values of the order 105 – 107 W/mm3. Consequently, the temperature in this volume increases extremely rapidly, 108 – 1010 K/s.


Laser beam welding (LBW)


Laser beam welding (LBW) Laser beam welding is a technique used to join multiple pieces of metal through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. Laser beam welding is a non-contact process which requires access to the weld zone from only one side of the parts being welded. The laser weld is formed as the intense laser light rapidly heats the material - typically calculated in milliseconds. LBW has high power density (on the order of 1 MW/cm2) resulting in small heat-affected zones and high heating and cooling rates. The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece. A continuous or pulsed laser beam may be used depending upon the application. Millisecond-long pulses are used to weld thin materials such as razor blades while continuous laser systems are employed for deep welds.


Laser beam welding (LBW) LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Due to high cooling rates, cracking is a concern when welding high-carbon steels. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces. Three type of laser welds can be achieved with LBW: conduction, conduction/penetration and penetration or ‘keyhole’. Ø  Conduction welds are performed at low energy, resulting in wide, shallow weld nuggets. Ø  Conduction/penetration welds utilize a medium energy density and result in a deeper weld nugget. Ø  Penetration or keyhole welds are resultant of direct energy delivery into the material being welded resulting in deep, narrow nuggets.


Laser beam welding (LBW) Some of the advantages of LBW in comparison to EBW are as follows: Ø  the laser beam can be transmitted through air rather than requiring a vacuum Ø  the process is easily automated with robotic machinery Ø  x-rays are not generated Ø  LBW results in higher quality welds

5. Sintering processes

Pulse Electric Current Sintering (PECS)


Pulse Electric Current Sintering (PECS) The main characteristic of PECS is that pulsed DC current directly passes through the powder compact. Joule heating has been found to play a dominant role in the densification of powder compacts, which results in achieving near theoretical density at lower sintering temperature compared to conventional sintering techniques. The heat generation is internal, in contrast to the conventional hot pressing, where the heat is provided by external heating elements. This facilitates a very high heating or cooling rate (up to 1000 K/min), hence the sintering process generally is very fast. The general speed of the process ensures it has the potential of densifying powders with nanosize or nanostructure while avoiding coarsening which accompanies standard densification routes. Compared with conventional sintering, hot pressing and hot isostatic pressing, PECS can consolidate powders to near-full density at a relatively lower temperature and in a much shorter sintering duration, typically a few minutes.


Direct Metal Laser Sintering (DMLS) Direct metal laser sintering (DMLS) is an additive manufacturing technique that uses a laser as the power source to sinter powdered material (typically metal), aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. It is similar to selective laser sintering (SLS); the two are instantiations of the same concept but differ in technical details. Selective laser melting (SLM) uses a comparable concept, but in SLM the material is fully melted rather than sintered, allowing different properties (crystal structure, porosity, and so on). Any laser of sufficient power to heat the powder particles can be used for DMLS.

iii: electrical contacts processing...

Documents