Sterling Silver is an alloy comprising 92.5% pure Silver with 7.5% other metals, usually copper

How to repair Silver Plating

Repairing Silver Plated Items

If you have items that are silver plated where the silver plating has worn off there are a number of fixes to the problem. (of course this problem doesn't exist with Sterling Silver)

Firstly most jewellers (bench jeweler) can repair silver plated items such as silver service items like trays, jugs, bowls, pichers etc as well as gold and sterling silver jewellery. Silver plating will invariably wear off throgh continual use on silverware and even on silver jewellery as it rubs continually on the skin. The silver plating will also tarnish and vigorous and frequent polishing to remove the tarnish will eventually cause the very thin layer of silver plating to wear off.

If you have something that you treasure, maybe a wedding gift or family heirloom or keepsake, which has areas where the silver plating has worn off then it can be a little distressing that the item is 'spoilt' and this is more so if the item is usually on display.

A jeweller can use typically one of two methods to restore the worn silver plating, both of which are not complicated and one in particular can be done by anyone at home.

Silver Plating: method 1 - electroplating

The first would be to use a small machine that produces a suitable D.C. electric current of about only 1.7volt to a handheld wand - the pad on the tip of the wand is dipped in a silver plating solution and then the wand is moved in a rapid circular motion with a little pressure on the item or area of an item to be silver plated. The process is very rapid and repeating the process can increase the thickness of the silver plating layer marginally. The finish is dull however a quick polish with a polishing cloth with result in a brilliant shine. If doing a worn area it is best to apply the wand slightly past the edge of the worn area onto the good section to 'fair in' the new silver plating to the existing.

This method and the equipment used is very safe as it uses D.C. (Direct Current) at very low voltage so there is no risk of electrocution.

Silver Plating: method 2 - rubbing

This second method is very suitable indeed to use by anyone at home. It is simply 'rubbing on a silver plating'. Yes it is that simple and uncomplicated. With one product a solution of almost pure silver with a small amount of a catalyst chemical is dipped onto a dry soft cloth and then applied to the area that requires silver plating with vigorous rubbing. Another product. Another product uses a powder that is applied to a wet cloth and used in the same manner as the first product.

The combination of the catalyst chemical and the 'energy' created with the vigorous rubbing creates a Eutectic Bonding of the silver to the material. Within seconds the area being worked on becomes silver plated. Whilst the integrity of the silver plating may not be as good as that as a result of electroplating it is very acceptable and can be repeated at intervals as required over a period of time with absolute ease. A small 120ml bottle of the liquid solution will cost about $85 including delivery if purchased on the internet or it can be purchased from some jewellery supply companies.

As simple as this method (and product(s)) sounds it really does work. Manufacturers specifications amd recommended applications should be checked as some available products will differ in their application methods.

Check out the videos below to see just how simple the second method is;

Both of the above methods are suitable for just about anything that is already plated or requires silver plating such as jewellery, electrical contacts and slip rings, silverware, flatware and even the reflector inside headlights on vehicles. 

The article below explains the Eutectic Bonding process.

Eutectic bonding

From Wikipedia, the free encyclopedia, Text is available under the Creative Commons Attribution-ShareAlike License

Eutectic bonding, also referred to as eutectic soldering, describes a wafer bonding technique with an intermediate metal layer that can produce a eutectic system. Those eutectic metals are alloys that transform directly from solid to liquid state, or vice versa from liquid to solid state, at a specific composition and temperature without passing a two-phase equilibrium, i.e. liquid and solid state. The fact that the eutectic temperature can be much lower than the melting temperature of the two or more pure elements can be important in eutectic bonding.

Eutectic alloys are deposited by sputtering, dual source evaporation or electroplating. It also can be formed by diffusion reactions of pure materials and subsequently melting of the eutectic composition.[2]

Eutectic bonding is able to produce hermetically sealed packages and electrical interconnection within a single process (compare ultrasonic images). In addition this procedure is conducting at low processing temperatures, low resultant stress induced in final assembly, high bonding strength, large fabrication yield and a good reliability. Those attributes are dependent on the coefficient of thermal expansion between the substrates.[1]

The most important parameters for eutectic bonding are:

  • bonding temperature
  • bonding duration
  • tool pressure
Ultrasonic image of a blank Au-Si eutectic bonded wafer [1 ]
Ultrasonic image of a patterned Au-Si eutectic bonded wafer [1 ]

 

 

 

Overview

Eutectic bonding is based on the ability of silicon (Si) to alloy with numerous metals and form a eutectic system. The most established eutectic formations are Si with gold (Au) or with aluminium (Al).[3] This bonding procedure is most commonly used for Si or glass wafers that are coated with an Au/Al film and partly with adhesive layer (compare with following image).

Bonding of Si wafer to (l) glass or (r) silicon wafer coated with Au or Al layer.[3 ]

The Si-Au couple has the advantages of an exceptionally low eutectic temperature, an already widespread use in die bonding and the compatibility with Al interconnects.[4] Additionally, often used eutectic alloys for wafer bonding in semiconductor fabrication are shown in the table. Choosing the correct alloy is determined by the processing temperature and compatibility of the materials used.[5]

Commonly used eutectic alloys
Eutectic alloy Eutectic composition Eutectic temperature
Au-In 0.6 / 99.4 wt-% 156 °C
Cu-Sn 5 / 95 wt-% 231 °C
Au-Sn[6 ] 80 / 20 wt-% 280 °C
Au-Ge 28 / 72 wt-% 361 °C
Au-Si 97.15 / 2.85 wt-% 370 °C
Al-Ge[7 ] 49 / 51 wt-% 419 °C
Al-Si 87.5 / 12.5 wt-% 580 °C
Si-Au phase diagram.[1 ]

Further, the bonding has less restrictions, concerning substrate roughness and planarity than direct bonding. Compared to anodic bonding, no high voltages are required that can be detrimental to electrostatic MEMS. Additionally, the eutectic bonding procedure promotes a better out-gassing and hermeticity than bonding with organic intermediate layers.[8] Compared to glass frit bonding, the advantage sticks out that the reduction of seal ring geometries, an increase of the hermeticity levels and a shrinking of device size is possible. The geometry of eutectic seals is characterized by a thickness of 1 - 5 µm and a wideness of > 50 µm. The use of eutectic alloy brings the advantage of providing electrical conduction and interfacing with redistribution layers.

The temperature of the eutectic bonding procedure is dependent on the used material. The bonding happens at a specific weight-% and temperature, e.g. 370 °C at 2.85 wt-% Si for Au intermediate layer (compare to phase diagram).[3]

The procedure of eutectic bonding is divided into following steps:[9]

  1. Substrate processing
  2. Conditioning prior to bonding (e.g. oxide removal)
  3. Bonding process (Temperature, Mechanical Pressure for a few minutes)
  4. Cooling process

Procedural steps

Pre-treatment

The surface preparation is the most important step to accomplish a successful eutectic bonding. This bonding procedure is due to oxide presence on the silicon substrates very limited based on the poor wettability of Au on the oxide layer. This leads to a poor adhesion of the eutectic bond. The oxide on the silicon surface acts as a diffusion barrier.[4] The surface preparation's main task is to facilitate the deposition of the eutectic metal by oxide removal or adhesion layer deposition.[10]

To remove existing native oxide layers wet chemical etching (HF clean), dry chemical etching or chemical vapor deposition (CVD) with different types of crystals can be used. Also some applications require a surface pre-treatment using dry oxide removal processes, e.g. H2 plasma and CF4 plasma.[1]

An additional method for the removal of unwanted surface films, i.e. oxide, is applying ultrasound during attachment process.[11] As soon the tool is lowered a relative vibration between the wafer and the substrate is applied. Commonly, industrial bonders use ultrasonic with 60 Hz vibration frequencies and 100 µm vibration amplitude.[12] A successful oxide removal results in a solid, hermetically tight connection.[3]

Scheme of typical wafer composition including an optional layer of Ni/Pt.

A Second method to ensure the eutectic metal adheres on the Si wafer is by using an adhesion layer. This thin intermediate metal layer adheres well to the oxide and the eutectic metal. Well suitable metals for an Au-Si compound are titanium (Ti) and chromium (Cr) resulting in, e.g. Si-SiO2-Ti-Au or Si-SiO2-Cr-Au. The adhesion layer is used to break up the oxide by diffusion of silicon into the used material. A typical wafer is composed of a silicon wafer with oxide, 30 - 200 nm Ti or Cr layer and Au layer of > 500 nm thickness.

In the wafer fabrication a nickel (Ni) or a platinum (Pt) layer is added between the gold and the substrate wafer as diffusion barrier.[8] The diffusion barrier avoids interaction between Au and Ti/Cr and requires higher temperatures to form a reliable and uniform bond. Further, the very limited solubility of silicon in titanium and chromium can prevent the developing of Au-Si eutectic composition based on the diffusion of silicon through titanium into gold.[4]

The eutectic materials and optional adhesion layers are usually approached by deposition as alloy in one layer by dual component electroplating, dual-source evaporation (physical vapor deposition) or composite alloy sputtering.[10]

The removal of contamination, on the for silicon most established Au layer, is usually realized with water flushing and wafer heating.[1]

Bonding process

The contacting of the substrates is applied directly after the pre-treatment of the surfaces to avoid oxide regeneration. The bonding procedure for oxidizing metals (not Au) generally takes place in a reduced atmosphere of 4% hydrogen and an inert carrier gas flow, e.g. nitrogen. The requirements for the bonding equipment lies in the thermal and pressure uniformity across the wafer. This enables uniformly compressed seal lines.[2]

The substrate is aligned and fixed on a heated stage and the silicon wafer in a heated tool. The substrates inserted in the bonding chamber are contacted maintaining the alignment. As soon the layers are in atomic contact the reaction between those starts. To support the reaction mechanical pressure is applied and heating above the eutectic temperature is carried out.[1]

The diffusivity and solubility of gold into silicon substrate increases with rising bonding temperatures. A higher temperature than the eutectic temperature is usually preferred for the bonding procedure. This may result in the formation of a thicker Au-Si alloy layer and further a stronger eutectic bond.[13]

The diffusion starts as soon as the layers are in atomic contact at elevated temperatures.[1] The contacted surface layer containing the eutectic composites melts, forming a liquid phase alloy, accelerating further mixing processes and diffusion until the saturation composition is reached.[14][15]

Other common eutectic bonding alloys commonly used for wafer bonding include Au-Sn, Al-Ge, Au-Ge, Au-In and Cu-Sn.[7]

The chosen bonding temperature usually is some degrees higher than the eutectic temperature so the melt becomes less viscous and readily flows due to higher roughness to surface areas that are not in atomic contact.[10] To prevent the melt pressed outside the bonding interface the optimization of the bonding parameter control is necessary, e.g. low force on the wafers. Otherwise, it may lead to short circuits or device malfunctions of the used components (electrical and mechanical).[1] The heating of the wafers leads to a change in the surface texture due to formation of fine silicon micro structures on top of the gold surface.[15]

Cooling process

Cross-section SEM image of the bonding interface between Si and Au with 80.3 Si atom percentage.[1 ]

The material mix is solidified when the temperature decreases below eutectic point or the concentration ratio changes (for Si-Au: T < 370 °C).[1] The solidification leads to epitaxial growth of silicon and gold on top of the silicon substrate resulting in numerous small silicon islands protruding from a polycrystalline gold alloy (compare to cross-section image of the bonding interface).[4] This can result in bonding strengths around 70 MPa.

The importance lies in the appropriate process parameters, i.e. sufficient bonding temperature control.[15] Otherwise the bond cracks due to stress caused by a mismatch of the thermal expansion coefficient. This stress is able to relax over time.[4]


Examples

Based on the high bonding strength this procedure is special applicable for pressure sensors or fluidics. Also smart micro mechanical sensors and actuators with electronic and/or micro mechanical functions over multiple wafers can be fabricated.[15]

Technical specifications

Materials

Substrate:

  • Si-Si
  • Si-glass w. adhesive layer

Intermediate layer:

  • Au
  • Ag
Temperature
  • Au: 370 °C
  • Al: 580 °C
Advantages
  • low resultant stress induced in final assembly
  • good reliability
  • large fabrication yield
  • low demands on surface topography and roughness
  • simple process technology
  • high bonding strength
  • relatively small seal ring geometries
  • small device feature size possible
Drawbacks
  • different CTE of intermediate layer and wafer material
  • restrictions of widespread connections caused by mechanical stress
  • further technological procedures to prevent oxidation of silicon surface
Researches
  • low melting alloys
  • SLID bonding

References

  1. Lin, Y.-C. and Baum, M. and Haubold, M. and Fromel, J. and Wiemer, M. and Gessner, T. and Esashi, M. (2009). "Development and evaluation of AuSi eutectic wafer bonding". "Solid-State Sensors, Actuators and Microsystems Conference, 2009. TRANSDUCERS 2009. International". pp. 244–247. doi:10.1109/SENSOR.2009.5285519.
  2. Farrens, S. and Sood, S. (2008). "Wafer Level Packaging: Balancing Device Requirements and Materials Properties". "IMAPS". International Microelectronics and Packaging Society url = http://www.suss.com/service/technical-publications/wafer-level-packaging/wafer-level-packaging-balancing-device-requirements-and-materials-properties.html. SUSS MicroTec.
  3. G. Gerlach and W. Dötzel (2008). Ronald Pething, ed. Introduction to Microsystem Technology: A Guide for Students (Wiley Microsystem and Nanotechnology). Wiley Publishing. ISBN 978-0-470-05861-9.
  4. R. F. Wolffenbuttel (1997). "Low-temperature intermediate Au-Si wafer bonding; eutectic or silicide bond". Sensors and Actuators A: Physical 62 (1-3). pp. 680–686.
  5. Farrens, S. (2008). Latest Metal Technologies for 3D Integration and MEMS Wafer Level Bonding (Report).
  6. Matijasevic, G.S. and Lee, C.C. and Wang, C.Y. (1993). "Au-Sn alloy phase diagram and properties related to its use as a bonding medium". Thin Solid Films 223. pp. 276–287.
  7. Sood, S. and Farrens, S. and Pinker, R. and Xie, J. and Cataby, W. (2010). "Al-Ge Eutectic Wafer Bonding and Bond Characterization for CMOS Compatible Wafer Packaging". ECS Transactions 33. pp. 93–101.
  8. Lani, S. and Bosseboeuf, A. and Belier, B. and Clerc, C. and Gousset, C. and Aubert, J. (2006). "Gold metallizations for eutectic bonding of silicon wafers". Microsystem Technologies 12. pp. 1021–1025. doi:10.1007/s00542-006-0228-6.
  9. M. Wiemer and J. Frömel and T. Gessner (2003). "Trends der Technologieentwicklung im Bereich Waferbonden". In W. Dötzel. "6. Chemnitzer Fachtagung Mikromechanik & Mikroelektronik" 6. Technische Universität Chemnitz. Technische Universität Chemnitz. pp. 178–188.
  10. Farrens, S. (2008). "Wafer-Bonding Technologies and Strategies for 3D ICs". In Tan, C. S. and Gutmann, R. J. and Reif, L. R. "Wafer Level 3-D ICs Process Technology". Integrated Circuits and Systems. Springer US. pp. 49–85. doi:10.1007/978-0-387-76534-1.
  11. Schneider, A. and Rank, H. and Müller-Fiedler, R. and Wittler, O. and Reichl, H. (2009). "Stabilitätsbewertung eutektisch gebondeter Sensorstrukturen auf Waferlevel". In Hermann, G. "9. Chemnitzer Fachtagung Mikromechanik & Mikroelektronik". pp. 51–56.
  12. Yost, F. (1974). "Ultimate strength and morphological structure of eutectic bonds". Journal of Electronic Materials 3 (2). pp. 353–369. doi:10.1007/BF02652947.
  13. Cheng, Y.T. and Lin, L. and Najafi, K. (2000). "Localized silicon fusion and eutectic bonding for MEMS fabrication and packaging". Journal of Microelectromechanical Systems 9 (1). pp. 3–8. doi:10.1109/84.825770.
  14. Kim, J. and Cheng, Y.-T. and Chiao, M. and Lin, L. (2007). "Packaging and Reliability Issues in Micro/Nano Systems". In Bhushan, B. "Springer Handbook of Nanotechnology". Springer Berlin Heidelberg. pp. 1777–1806. doi:10.1007/978-3-540-29857-1.
  15. R. F. Wolffenbuttel and K. D. Wise (1994). "Low-temperature silicon wafer-to-wafer bonding using gold at eutectic temperature". Sensors and Actuators A: Physical 43 (1-3). pp. 223–229.

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