Surface Coating of Copper Alloy Strip for Electrical Connector Applications

Surface Coating of Copper Alloy Strip for Electrical Connector Applications In order for electrical contacts to function properly, it is necessary to keep the total resistance of the contact interface low. This requires smooth contact surfaces, adequate normal force, good wear resistance of the contact surfaces, good electrical conductivity, and absence of corrosion products. These factors can be adjusted coating the surfaces of the terminal. Each coating type will contribute some combination of these critical properties to the interface. The choice of a correct coating can mean the difference between a good electrical contact (signal transmission) and a failed connection.

COATING METHODS Hot dipping involves passing the metal strip through a There are several methods that are used to apply materials molten version of the desired coating material, enabling it to to connector interfaces. These methods include electro- adhere to the strip and solidify on it. and electroless plating, cladding, hot dipping, reflowing, and vapor deposition. For overlay cladding, a strip of the coating material is rolled onto the base metal strip, and the two are sintered In electroplating, the metal strip is immersed in an together. This creates a metallurgical bond between the electrolytic solution while connected to the negative end of two surfaces. In inlay cladding, a small section of the a DC terminal. The positive end is connected to pieces of base metal strip is skived out or milled out along its length. the desired plating metal immersed in the same solution. A strip of the coating material is then rolled into place in the The action of the electric current in this electrolytic cell newly exposed groove, and the two pieces are then rolled causes the contact metal to dissolve and plate out onto the together and sintered. Multiple layered clads can be added surface of the strip. to the base metal, and thick coatings can be created quickly and easily.

Electroplating

Voltage Source Skive Inlay Technique

- + 4. Annealing

Plating Strip Metal

Dissolved 1. Skiving

Metal 2. Bonding Electrolytic 3., 5. Rolling Solution Cathode Anode 6. Slitting

Electroless plating differs from electroplating in that it is not activated by an applied electric current. It employs a Solder print-on involves moving the strip over counter- solution containing dissolved ions of the plating metal. The rotating rollers partially immersed in liquid solder. This base metal of the strip catalyzes the reaction that plates the prints solder onto the strip in much the same way as a ions onto its surface. printing press placing ink on paper.

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Alternatively, a strip of solid solder may be laid on the base the contact, especially when the overplate is gold. metal (by plating, cladding, ect.) and then briefly melted and resolidified for adhesion to the surface. This is known as MULTI-LAYER COATINGS reflowing. Reflowing of a solder coating, particularly one There are some instances when it is desirable to use that is electroplated, will eliminate the residual stresses that coatings consisting of multiple layers. Capping involves lead to the formation of tin whiskers. placing a thinner layer of one coating on top of another one, as in gold over a nickel underplate. A flash coating is a In physical vapor deposition, or PVD the plating material very thin overcoat, generally used to improve the corrosion is evaporated in a low pressure environment by electrical resistance of the surface. Diffusion involves heating the arcing or by a focused electron beam (sputtering). The material, so atoms of the top coating diffuse down into the vapor condenses on the base metal and slowly coats it. lower layers.

CLEANING RESIDUAL AND INDUCED STRESSES All surfaces should be clean and free of grease and oxide Coatings applied by electroplating, vapor deposition, and before the surface can be coated. BeCu strip may be electroless plating will contain a certain amount of internal degreased with an alkaline cleaner. It may then be cleaned residual stress. This stress is created as the coating grows using one of the following solutions: sulfuric peroxide (20% in thickness. Additionally, thermal expansion differences

H2SO4, 3% H2O2) at 125 F, or Phosphoric/Nitric/Acetic between the coating and the substrate will create thermal

(38% H3PO4, 2% HNO3, 60% acetic acid) at 160°F. stresses, especially if the coating is deposited at high Alternatively, the strip may be pretreated with caustic (50- temperatures and then cooled. (The thermal stresses will 60% NaOH) at 265°F, then pickled in Nitric acid. For also cycle with temperature during normal service of the further information on cleaning and oxide removal, refer to part.) Residual tensile stresses may lead to premature the “Cleaning Copper Beryllium” Tech Brief. fatigue failure, while residual compressive stresses can improve resistance to this type of failure. Excessive tensile NICKEL UNDERPLATING stress in the coating may also cause cracking. In order for a coating to work properly, it must prevent the base metal from coming into contact with the outside PREPLATING VS. POSTPLATING environment. However, the base metal may slowly diffuse Mill hardened strip may be plated before or after stamping through the coating and combine with oxygen to form an and forming operations. Heat treatable strip must be plated oxide on the surface. For this reason, an undercoat of after heat treatment, to prevent diffusion of the base metal nickel is usually placed between the base metal and the into and through the plating material. outer coating. The nickel acts as a diffusion barrier that inhibits the migration of copper atoms to the contact Preplating or precladding will leave the base metal exposed surface. Additionally, it functions as a thermal barrier to at edges of the part after stamping. Corrosion can occur at prevent the heat from surface soldering from affecting the these bare edges. This can allow corrosion products to base metal. creep across the surface of the contact, disrupting the flow of current. Preplating will also produce stamping scrap that Without an underplate, pores in the plating may leave BeCu has been coated with the plating material. It can be difficult exposed to the outside air. This will allow copper oxide to recover plating metal from this scrap. Depending on the corrosion products to form in the pores and migrate to the thickness and ductility of the coating, the formability of the surface. By using an underplate, nickel oxide will form in part may be substantially reduced, since a less ductile the pores instead. Since the corrosion rate of NiO is very coating will fracture more easily than the base metal. slow under stable conditions, once the underlying Ni is covered, the corrosion products do not migrate and the Postplating requires that the part be stamped, and then pores are essentially passivated. This will also slow the coated selectively or overall. However, the edges of the part migration of corrosion products from bare edges of the will be coated. If the part is to be spot plated, it must be contact. blanked, spot plated, and then formed. This requires that the stamping and forming operations be done at separate A nickel underplate will also increase the wear resistance of times, with an intermediate coating operation.

It should also be noted that certain plating materials are tendency to form a brittle intermetallic phase when applied available only as preplated, and others only as postplated. directly on copper alloys, so a nickel underplate is Cladding is only done before stamping. recommended. Tin coated contacts should not be mated to gold. Materion Brush Performance Alloys supplies strip in both the bare and preplated conditions. Tin-Lead (Sn-PB) Solders are used on interfaces that need to be solderable. The lead content helps to prevent COATING MATERIALS the formation of tin whiskers. However, these alloys are no Gold (Au) is the most inert of all of the plating materials. It longer permitted under the European RoHS regulations. does not readily tarnish or stain. Soft (pure) gold is not durable, and is only suitable for light contact force. Hard A number of Lead-Free (Pb-Free) Solders are now gold (with added cobalt or nickel) provides added durability. available to replace tin-lead solders in electronic Gold is usually applied over a nickel undercoat. However, it applications. These are primarily made of various may also be applied as a flash coating over another, combinations of tin, silver, copper, bismuth, zinc, and cheaper contact material. Gold is not easily soldered, and indium. They do have higher reflowing temperatures than should not be mated against tin. tin-lead solders, so electronic components have to be carefully designed to ensure that they are not damaged by Silver (Ag) has very high electrical conductivity, but is the higher processing temperatures. prone to tarnishing in the presence of chlorine or sulfur. Silver is fairly resistant to arcing damage, and is often used READING THE ACCOMPANYING CHART in high current applications. The chart on the next two pages details some common coatings and their application methods. The common Platinum (Pt) and Palladium (Pd) are similar to gold in that reasons for use and drawbacks are listed. The surface they do not readily tarnish, and do require a nickel hardness values are tabulated, along with electrical underplate. They are harder and more durable than gold. conductivity in % IACS (Pure copper has 100% IACS.) The In the presence of organic compounds, they may form a typical current and voltage used with each coating is listed powdery film. Often, they will be topped by a gold flash to (except for the solders, which are used for permanent prevent this film from forming. bonding, and not for contact). Recommendations are given Nickel (Ni) is typically used as an underplate for the for minimum normal force requirements, and for the reasons outline above. A nickel underplate also prevents maximum operating temperature. Please note that this the formation of a brittle copper-tin intermetallic phase in chart serves as a general guide only. The normal force tin-based coatings. If used as a standalone coating, it requirements are also dictated by the severity of the requires very high normal force (greater than 200 grams) environment and smoothness of the surfaces. Some and wiping action during contact mating, with a minimum of applications may function properly at greater temperatures 12 Volts to frit through the nickel oxide layer on the surface. and voltages than those specified on the chart. Nickel platings can have high residual stresses, particularly MATERION TECHNICAL MATERIALS when brightening agents are present in the plating bath. Materion Technical Materials is a Materion company that These stresses may negatively affect formability and fatigue specializes in coating technology. They have the life. Post-plating heat treatments can be applied to harden capabilities to inlay and overlay clad metals in single or the plating or reduce the residual stresses. multiple layers. Many precious and nonprecious metals can Tin (Sn) is a very low cost, solderable material. It is prone be clad, including solders. TMI also has the capability to to fretting corrosion, so the interface should be lubricated electroplate overall or selective stripes of hard Au, Pd, and mated with sufficient normal force (typically greater PdNi, Ni, Sn, and SnPb solders. They are also able to spot than 100 grams) and wiping action during the connector plate hard and soft Au, Pd, PdNi, Ni, Sn, and SnPb solders mating process. Unless they are reflowed, tin platings have on previously stamped strip. Materion Technical Materials a tendency to produce tin whiskers, which may lead to short may be reached at 1-800-241-2523. circuits or other problems electrical problems. Tin has a

SAFE HANDLING OF COPPER BERYLLIUM Sheet (MSDS) before working with this material. For Please refer to the Materion Corporation publications additional information on safe handling practices or “Safety Facts 104 - Safety Practices for the Chemical technical data on copper beryllium, contact Materion Brush Processing of Small Copper Beryllium Alloy Parts”, and Performance Alloys, Technical Service Department at 1- “Safety Facts 105 - Processing Copper Beryllium Alloys.” 800-375-4205.

Handling copper beryllium in solid form poses no special FOR MORE INFORMATION health risk. Like many industrial materials, beryllium- For additional literature, further information, or technical containing materials may pose a health risk if assistance on alloy properties or processing of high recommended safe handling practices are not followed. performance alloy strip, contact Materion Brush Inhalation of airborne beryllium may cause a serious lung Performance Alloys Customer Technical Service disorder in susceptible individuals. The Occupational Department in Mayfield Heights, Ohio at 1-800-375-4205. Safety and Health Administration (OSHA) has set For more information on platings, claddings, and other mandatory limits on occupational respiratory exposures. methods of application, contact Materion Technical Read and follow the guidance in the Material Safety Data Materials at 1-800-241-2523.

IACS Electrical

% Conductivity

Service

Maximum Temperature

Normal Force Requirements Hardness

Methods

Application PVD electroless electrolress hot dipping reflowing cladding

corrosion Cuintermetallic with brittle PVD PVD Connection at higher reflow and Melt solders. temperatures leaded than on to warpage/cracking lead May PCB or components. thickness

Strip Surface Coatings for High Performance Copper Alloy

Noble, formsno filmsloads only cladding conductivity conductivity cost,can and density lower but coatings other cap to used be surface gold. Formsno oxides or sulfidesmay requireAu flash cladding force formability good surface burst through current to filmssulfides Forms or sulfides.no oxides costs more PdAg than of migration corrosion prevents forms sulfides,requires Au flash pores passivates products, intermetallics lifefatigue reduce may coatings electroless cladding to fretting vulnerable If alone, used plating alone used cladding on depending plating process and PVD high voltage, forused be high Can fretting corrosion applications current non-switching Thick strong bonds,solders allow fuses in melt readily resistant wear Not dipping hot RoHS-Compliant Not reflowing cladding cladding automotive applications SnPb

Alloy Uses Advantages and Susceptibilities Soft (Au) Gold Wire wires gold to bonding light Wears for good very quickly, electroplatinggrams 20 < HV 75 < 200°C 73 Hard GoldHard 4 Ni/Co)(96 Au Ag 25 75 Au soft resistance over gold wear forms Noble, films. no Increased higher gold, than Less expensive costhigh solderable, easily Not Forms sulfides electroplating g 25 > HV - 210 150 125°C cladding g 50 > 73 cladding HV - 110 50 125°C 17 69 Au 25 Ag 6 PtAg 6 25 69 Au Au, to hard Hardness equivalent form (Pd)Palladium small May amounts of film on claddingresistanceof wear Hardness and - forms Catalyst films, organic g 50 > electroplating HV - 140 80 g 50 > 300°C HV - 400 200 200°C 10 16 60 Pd 40 Ag 40 60 Pd normal little requires hard, Very - forms Catalyst films, organic cladding g 100 > HV - 195 175 200°C 4 Pd Ni / Pd CoPd Ni / (Ag)PdAg hardness Higher Silver than Fine allowsconductivity, Highest Silver Coin - forms Catalyst films, organic 10 Cu)(90 Ag Platinum (Pt) melting temperature,forms Low conductivity high Ag, fineresistancethan wear Better Nickel (Ni) electroplating resistance. hardness wear High and cladding- forms films, Catalyst organic melting temperature,forms Low g 50 > underplating, as used an Usually cladding cladding HV - 550 300 sulfides intermetallics oxides, and g 75 > stress Forms or high brittle oxides, electroplating 200°C HV - 200 75 g 75 > g g if 50 > 300 > 125°C HV - 700 150 HV - 100 65 HV - 400 200 8 125°C 200°C 125°C 103 18 21 86 Aluminum (Al) Wirewires Al to bonding forms forms oxides, Readily cladding PermanentHV - 90 70 125°C 60 Tin (Sn) solderable cost, easily Low Oxides, intermetallics, whiskers, electroplating grams 100 > HV 30 ~ for 150°C some 14 Tin Lead Solders cheap solderable, Easily Tin CopperSilver Pb-free, RoHS-compliant alternativeSACSnAgCu or coating/lubricant protective Solders solders lead tin to surfaces clean Soft, requires and fluxes for bonding electroplating Permanent HV 25 ~ dipping hot 60°C Connection electroplatingfor 11-12 Permanent HV 30 ~ 60°C dipping hot Connection 1300% eutectic