June 2017 Archives

#298 Tin Plating for Electronics

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Tin plating, or tinning, has long been known for hot-dipping of iron plates and has been used for various cans.
Recent improvement in tinning baths has achieved enhanced luster, solderability, and corrosion resistance and has widened the applications of tinning to electronic components. It is now widely used as a plating film with high long-term reliability to substitute for expensive gold plating.
Characteristics and applications of tin plating film are summarized in [Table 1].

[Table 1] Characteristics and applications of tin plating film
CharacteristicsApplications
Stability against organic acidsSteel sheet for cans, tableware, water heaters, etc.
Flexibility, lubricityBearings and sliding components for various machinery, prevention of steel nitriding, etc.
Solderability, electrical characteristicsElectronic components, semiconductor components, mechanical components, etc.

[Table 2] shows the types of tinning baths and hardness of the films.The hardness of plating film shown here is a reference, as it varies depending on brightening agent and plating conditions.
Bright plating has better solderability than matt plating.It also has better corrosion resistance. Bright tinning to a thickness of 5 um or more leaves almost no pinholes.
  [Table 2] Types of tin plating baths and hardness of the films
Types of plating bathsHardness of films (Hv)
Acid bright plating bath40-60
Acid matt plating bath5-8
Alkaline matt plating bath3-4
Neutral semi-bright plating bath10-15
Neutral bright plating bath30-50

Precautions for the use of tinning on electronics include the need for1. anti-discoloration treatment and 2. whisker mitigation measures.

*Whiskers are fibrous metal crystals that look like cat whiskers and that grow on untreated metal surfaces. These may cause an accident such as shorting of an electrical circuit for downsized and high-density electronic and communication equipment.One of the whisker mitigation techniques is reflow treatment (melting treatment) which co-precipitates5-10% of lead with tin (alloy plating).

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#297 Other Noble Metal Plating for Electronics

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(1)Rhodium plating Rh

Industrial rhodium plating has a high hardness of 800 to 1000 Hv, comparable to that of industrial chrome plating. It also has superior corrosion resistance. Therefore, it is used for those parts that are subject to intense abrasion and sliding and that require a long-term stable low contact resistance, such as for terminal plating of printed circuit boards, connectors, and switch contacts.

The following are the characteristics of rhodium plating:

 1. It is chemically stable and is not susceptible to oxidative discoloration at normal temperature.
 2. It has extremely high hardness and excellent wear resistance.
 3. It has superior heat resistance and does not oxidize in the air at 500℃ or lower.
 4. It has electrical resistance of 490 uΩ/m, which is twice that of gold but is the lowest among the platinum group.
 5. It is used for discoloration prevention of ornaments and silverware for its delicate silver gray luster with a high reflectivity of 80%.
 6. Contact-related plating thickness can be roughly categorized as follows:
・For discoloration prevention: 0.5 um or less
・For wear resistance: 0.5 to 2.0 um
・For high wear resistance: 2.5 to 25 um

(2)Platinum plating Pt

Platinum is more commonly known than rhodium but is less commonly used for plating.Although hardness of platinum plating is lower than that of rhodium plating, platinum plating containing 5 to 10% rhodium is said to have high hardness.It is used for contacts of electronic equipment, as in the case of rhodium plating, due to its excellent resistance to oxidation, corrosion, and heat.
For salt electrolysis, platinum-plated titanium is used as an insoluble anode.It has outstanding corrosion resistance.

(3)Palladium plating Pd

Compared to rhodium plating, palladium plating is less expensive but has inferior wear resistance and corrosion resistance.Its shortcomings include formation of polymer by binding to organic matters in the air when used as an electrical contact.Nonetheless, it is used as a low contact resistance plating to substitute for expensive rhodium plating.

(4)Ruthenium plating Ru

Ruthenium plating has similar low contact resistance, hardness, wear resistance, and corrosion resistance to rhodium plating but is 1/2 as expensive. Therefore, it is a promising substitute for rhodium plating with a well-anticipated future development to address the difficulty in plating a thick coat of 3 um or more without forming cracks caused by internal stress.

#296 Silver Plating for Electronics

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Similarly to industrial gold plating, industrial silver plating is also essential to electronics-related components. Among metals, silver, which is less expensive than gold, has the best electrical conductivity. It also has an excellent thermal conductivity, lubricity, and heat resistance, and therefore is used widely, not only in the electronics industry, but also in the electrical engineering and aircraft industries.

Applications and purposes of industrial silver plating are summarized in the Table.

[Table] Applications and purposes of industrial silver plating
ApplicationsParts to be silver-platedPurposes
Electrical
engineering
Electrodes on breakers, insulators, terminals, transformer terminals, busbars for distribution boards and switchboards, welding arcs, flash-butt electrodes, etc.Electrical conductivity, low contact resistance, solderability
ElectronicsVarious switches, contacts, connectors, lead frames, stems, etc.Electrical conductivity, low contact resistance, solderability
AircraftBearings, joints, bolts and nuts for high-temperature environmentLubricity, seizure prevention, heat resistance
Microwave
circuit
WaveguidesSmoothness, high-frequency characteristics

Silver plating, like gold plating, also uses both soft and hard silver. Soft silver plating is used as a substitute to gold plating for parts that require high electrical conductivity, such as lead frames and stems. Hard silver plating is used for parts with an emphasis on wear resistance, such as connectors, terminals, and switch contacts.

Silver plating is susceptible to sulphide staining caused by sulfur in the environment. Therefore, anti-tarnish treatment, such as chromating and oil coating, is carried out after silver plating.

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#295 Gold Plating for Electronics

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Industrial gold plating is indispensable for electronic and/or semiconductor components.Purposes and applications of gold plating are summarized in the [Table].

ApplicationsGold-plated partsPurposes
Electronic components, semiconductor components, components in light electrical appliancesVarious contacts and terminalsElectrical conductivity, low contact resistance, corrosion resistance, solderability, wear resistance
Microwave circuitWaveguidesSmoothness, high-frequency characteristics
Copying machineReflecting mirrorLight reflectivity

Although, for some industrial gold plating, such as on IC headers, stems, and lead frames, high-purity gold (99.7% or higher) may be used, for most cases, alloy plating is applied by combining other metals according to the required hardness and wear resistance, for example, of the plating film.
Metal, such as silver, copper, nickel, cobalt or indium, is added in an amount of about0.1 to 0.5% as an alloying element. Such gold alloy plating is said to be twice as hard as pure gold plating and to have three times the abrasion-resistance.

Gold-plated connectors

Gold-plated terminals

With the increase in its information processing capacity, a printed wiring board, which is indispensable for electronic and/or communication equipment, is required to achieve high-density mounting of electronic components. As a result, multilayer boards with multi-layered (three-dimensional) circuits created by laminating multiple printed wiring boards are now frequently used. The issue that has surfaced is how to dissipate the heat generated from the LSI.
Given the inherently high electrical and heat conductivity of copper, creating an electric circuit with copper plating film on a printed circuit board made of plastic with poor thermal conductivity is an excellent solution for this problem.
In addition, electroplated copper film also serves to suppress the deformation of plastic substrates that expand with heat. Furthermore, all substrates of a multilayer printed circuit board need to be electrically connected. Therefore, through-hole plating (i.e. creating a circuit by plating the interiors of tiny holes) is generally carried out.
In this case, the through-hole plating must meet the requirements for reliability, as it affects the nature of a printed circuit board.
Requirements for copper plating film of plated through-holes are as follows:

1. Excellent throwing power (uniform plating thickness)
2. Excellent elongation and tensile strength of the plated layer
3. Excellent preservability (in case where no other plating is applied on top of the copper)
4. Excellent solderability
5. Excellent excoriation resistance

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