HK1101602A - Metal duplex and method - Google Patents

Description

Metal diphase and preparation method thereof

Technical Field

The invention relates to a low-phosphorus metal binary (metal duplex) having a gold or gold alloy layer and to a method for producing said low-phosphorus metal binary having a gold or gold alloy layer. More particularly, the present invention relates to a low phosphorous content metal binary with a gold or gold alloy layer and a method of preparing the low phosphorous content metal binary with a gold or gold alloy layer to inhibit corrosion and improve solderability.

Background

In many industrial fields, preventing corrosion of gold and gold alloys is a challenging problem. Corrosion of gold and gold alloys is a particularly significant problem in the electronics industry where corrosion can cause electrical contact failures between components in electronic devices. For example, gold and gold alloy coatings have been used in electronics and other applications for many years. In electronics, they are used as solderable and corrosion resistant surfaces for contacts and connectors. They are also used for lead protection layers in Integrated Circuit (IC) fabrication. However, gold protective layers do not always prevent corrosion.

IC devices having IC units, lead frames, and passive components (e.g., capacitors and transistors) have wide application in products including consumer electronics, home appliances, electronic computers, automobiles, telecommunications products, robots, and military equipment. The IC unit includes an IC chip and a hybrid circuit junction including one or more IC chips and other electronic components on a plastic or ceramic base.

A lead frame or connector is a device used to electrically interconnect an IC unit with external circuitry. The lead frame is formed of a conductive material such as copper or a copper alloy or is formed by stamping or etching a metal blank into a plurality of leads defining a central region for mounting the IC unit. The lead frame may be connected to the IC units in the assembly by various connection techniques. These techniques include wire bonding, soldering, die attachment, and packaging. Soldering is typically used to bond the IC in the assembly. In all cases, the connection requires the leadframe surface to have special properties. Typically this means that the surface is not corroded and is susceptible to interaction with other components such as gold or aluminum wires, silver filled epoxy or solder.

One problem is long term solderability, which is defined as the ability of the surface protective layer to melt and form a good solder joint with other components (the solder joint is free of defects that would impair either the electrical or mechanical connection). Welding is generally a joining method involving three materials: (1) a substrate, (2) a component or other device that needs to be attached to the substrate, and (3) the solder material itself.

There are many factors that determine good solderability, the three most important of which are the degree of corrosion, the amount of co-deposited carbon and the degree of intermetallic formation. Corrosion is a thermodynamically favored process and therefore occurs naturally. The rate of corrosion depends on temperature and time. The higher the temperature and the longer the exposure time, the thicker the corrosion.

The carbon to be co-precipitated is determined by the plating chemistry selected. The carbon content of the bright protective layer is higher than that of the matte protective layer. Matte finishes are generally rougher than bright finishes and have a greater surface area and therefore are generally more corrosive than bright finishes.

The formation of intermetallic compounds is a chemical reaction between the metal coating and the underlying metal substrate. The rate of formation also depends on temperature and time. Higher temperatures and longer times result in thicker intermetallic layers.

Certain physical properties of gold, such as its relative porosity, present problems when depositing gold on a substrate. For example, the porosity of gold can create voids on the surface of the plating. These small spaces can promote corrosion or indeed accelerate corrosion by the galvanic coupling of the gold layer with the underlying base metal layer. This is believed to be due to the possibility that the base metal substrate and any accompanying underlying metal layer may be exposed to corrosive elements through the holes in the gold outer surface.

In addition, many applications include exposing the coated lead frame to heat. Under heat aging conditions, if the underlying metal diffuses into the precious metal surface layer, e.g., nickel diffuses into gold, diffusion of the metal between the layers may result in reduced surface properties.

At least three different approaches have been tried to overcome the problem of corrosion: 1) reducing porosity of the coating, 2) suppressing galvanic effects due to potential differences of different metals, and 3) sealing the pores in the electroplated layer. The method of reducing porosity has been widely studied. Pulsed gold plating and the use of various wetting agents/grain refiners in the gold plating bath affect the structure of the gold, two factors that can reduce the porosity of the gold. The combination of regular carbon bath treatment and good filtration and preventive maintenance procedures, usually in a series of electroplating baths or vessels, helps to maintain the metal deposition level of gold and correspondingly reduces surface porosity. But still have some porosity therein.

Hole sealing, and other corrosion inhibiting methods have been attempted, but the results achieved have been limited. The underlying mechanism of using organic precipitates with corrosion inhibiting effects is known in the art. Many of these compounds are generally soluble in organic solvents and therefore are not considered to provide long-term preservative protection. Other methods of sealing or closing the pores are based on the formation of insoluble compounds within the pores. These insoluble complexes and deposits, as will be apparent to those skilled in the art, are also potential candidates for blocking the pores.

In addition to the problem of forming pores, exposing gold to high temperatures, such as during thermal aging, also undesirably increases the contact resistance of gold. This increase in contact resistance reduces the performance of gold as a current conductor. Theoretically, workers believe that the problem is caused by the diffusion of organic materials that co-precipitate with gold on the contact surface. Various techniques have been tried to date to eliminate this problem, and these techniques typically involve electropolishing. However, none of these techniques has been entirely satisfactory for this purpose, and research continues.

Yeh et al, U.S.6,287,896, disclose the application of a thick coating of a nickel-cobalt alloy on a copper or copper alloy substrate. The patent states that the thick layer of nickel-cobalt is important to reduce the effects of porosity and to prevent copper diffusion from the substrate to the surface for soldering. The thick nickel-cobalt layer is coated with nickel or a nickel alloy to prevent it from cracking (which is an inherent problem with nickel-cobalt layers). A gold or gold alloy layer is coated on the nickel or nickel alloy layer. The gold or gold alloy layer is said to improve solderability.

While there have been some approaches to addressing the corrosion and solderability problems on metal deposits, there remains a need for improved methods to inhibit corrosion and improve solderability.

Disclosure of Invention

The invention relates to a method comprising depositing a nickel layer on a substrate, depositing a nickel phosphorous (nickel phosphorous) layer on the nickel layer, and depositing a gold or gold alloy layer on the nickel phosphorous layer, the nickel phosphorous layer comprising 0.1-10 wt% phosphorous. The combination of the nickel and nickel-phosphorous layers may inhibit or retard the effects of thermal aging, thereby reducing the formation of pores in the gold or gold alloy layer. The reduction of pores in the gold or gold alloy layer may reduce or prevent corrosion of the underlying substrate, thereby improving the solderability of the gold or gold alloy protective layer.

In another aspect, the invention relates to a method comprising depositing a 1-10 micron nickel layer on a substrate, depositing a 0.1-5 micron nickel phosphorous layer on the nickel layer, and then depositing a gold or gold alloy layer on the nickel phosphorous layer, said nickel phosphorous layer comprising 0.1-10 wt% phosphorous.

In another aspect, the present invention relates to an article comprising a nickel layer on a substrate, a nickel phosphorous layer on the nickel layer, and a gold or gold alloy layer on the nickel phosphorous layer, the nickel phosphorous layer comprising 0.1 to 10 weight percent phosphorous.

Brief description of the drawings

Fig. 1 is a 50 x magnified photograph of a gold layer with holes on a nickel underlayer;

fig. 2 is a 50 x magnified photograph of a gold layer with holes on a nickel-phosphorous underlayer.

Detailed Description

In this embodiment, unless the context clearly dictates otherwise, the following abbreviations have the following meanings: DEG C is centigrade; g is gram; mg ═ mg; l is liter; ml is equal to milliliter; Å Angstrom (1 × 10)^-4Micron size; ASD (ampere per decimeter)²(ii) a Weight percent is weight percent. In this specification, the terms "deposition" and "plating" may be used interchangeably. All numerical ranges are inclusive and combinable with each other in any order, unless it is logical that the numerical ranges are limited to the sum and 100%.

The method of the invention comprises depositing a nickel layer on a substrate, depositing a nickel-phosphorous layer on the nickel layer, and depositing gold or a gold alloy on the nickel-phosphorous layer, the phosphorous nickel layer comprising 0.1 to 10 wt.% phosphorous. The combination of the nickel and nickel-phosphorous layers may reduce the effects of thermal aging, thereby reducing the formation of pores in the gold and gold alloy layers. By reducing the formation of pores, corrosion of the underlying metal substrate may be reduced or eliminated, and the solderability of the gold or gold alloy layer may also be improved. In addition, the nickel and nickel-phosphorous layers may reduce or inhibit the migration of nickel to the gold or gold alloy layer.

The substrate may optionally be cleaned prior to metallization. Any suitable cleaning method acceptable in the metallization art may be used. The substrate is typically ultrasonically cleaned in a wash liquor. These washing liquids may comprise silicate compounds, alkali metal carbonates and other compounds, such as alkali metal hydroxides, glycol ethers and one or more chelating agents. The cleaning may be carried out at a temperature of 30-80 ℃.

The substrate may optionally be activated with a suitable acid, such as a mineral acid, after the cleaning step. Dilute mineral acids are used. An example of such an acid is sulfuric acid. However, other mineral acids, such as hydrochloric acid and nitric acid, may also be used. The concentrations at which these acids are used are conventional concentrations well known in the art. The activation is usually carried out at a temperature of from room temperature to 30 ℃.

The substrate is then plated by depositing nickel metal. The bath temperature is 30-70 deg.C, for example 40-60 deg.C.

Any suitable nickel plating bath may be used to deposit the nickel layer on the substrate. These nickel plating baths contain one or more sources of nickel ions. Sources of nickel ions include, but are not limited to, nickel halides such as nickel chloride, nickel sulfate, and nickel sulfamate. These sources of nickel ions may be present in the nickel bath in an amount of 50 mg/l to 500 mg/l, or for example 100 mg/l to 250 mg/l.

The nickel plating bath may also include one or more additives in addition to the one or more sources of nickel ions. These additives include, but are not limited to, brighteners, grain refiners, levelers, surfactants, anti-pin-holing agents, chelating agents, buffers, biocides, and other additives known to those skilled in the art to adjust the plating bath to achieve the desired bright or matte finish and throwing power.

Whitening agents include, but are not limited to, aromatic sulfonates, sulfonamides, sulfonimides, unsaturated sulfonates of aliphatic and aromatic-aliphatic alkenes or alkynes, sulfonamides, and sulfonimides. Examples of such brighteners are sodium o-sulfobenzoylimine, disodium 1, 5-naphthalenedisulfonate, trisodium 1, 3, 6-naphthalenetrisulfonate, sodium benzenemonosulfonate, diphenylsulfonamide, sodium allylsulfonate, sodium 3-chloro-2-butene-1-sulfonate, sodium β -styrenesulfonate, sodium propargylsulfonate, monoallylthioamide, diallylsulfonamide and allylsulfonamide. These brighteners may be used in conventional amounts, for example from 0.5 g/l to 10 g/l, or for example from 2 to 6 g/l.

Other brighteners include, but are not limited to, the reaction products of epoxides with alpha-hydroxy acetylenic alcohols (e.g., diethoxylated but-2-yne-1, 4-diol or dipropoxylated but-2-yne-1, 4-diol), N-heterocycles, other acetylenic compounds, reactive sulfur compounds, and dyes. Examples of such brighteners are 1, 4-bis- (. beta. -hydroxyethoxy) -2-butyne, 1, 4-bis- (. beta. -hydroxy-. gamma. -chloropropoxy) -2-butyne, 1, 4-bis- (. beta. -,. gamma. -glycidoxy) -2-butyne, 1, 4-bis- (. beta. -hydroxy-. gamma. -butenyloxy) -2-butyne, 1, 4-bis- (2 ' -hydroxy-4 ' -oxa-6 ' -heptenyloxy) -2-butyne, N- (2, 3-dichloro-2-propenyl) -pyridine chloride, 2, 4, 6-trimethyl-N-propargylpyridine bromide, n-allyl quinaldinium bromide, 2-butyne-1, 4-diol, propargyl alcohol, 2-methyl-3-butyn-2-ol, quinaldyl-N-propanesulfonic acid betaine, quinaldine dimethylsulfate, N-allylpyridine bromide, isoquinaldyl-N-propanesulfonic acid betaine, isoquinaldine dimethylsulfate, N-allylisoquinaldine bromide, disulfonated 1, 4-bis- (β -hydroxyethoxy) -2-butyne, 1- (β -hydroxyethoxy) -2-propyne, 1- (β -hydroxypropoxy) -2-propyne, sulfonated 1- (β -hydroxyethoxy) -2-propyne, saffron and fuchsin. These brighteners are used in conventional amounts, for example from 5 to 1000 mg/l, or for example from 20 to 500 mg/l.

Any suitable surfactant may be used. Such suitable surfactants include, but are not limited to, ionic surfactants such as cationic and anionic surfactants, nonionic surfactants, and amphoteric surfactants. The surfactant may be used in conventional amounts, for example from 0.05 to 30 mg/l, or for example from 1 to 20 mg/l, or for example from 5 to 10 mg/l.

An example of a suitable surfactant is one using 1-8 sulfonic acid groups (-SO)₃H) A sulfonated naphthalene. Examples of these surfactants are naphthalene-1, 3, 6-trisulfonic acid and naphthalene-1, 3, 7-trisulfonic acid. Alkali metal salts such as sodium and potassium salts may also be used. Examples of other suitable surfactants are alkyl hydrogen sulfates, such as sodium lauryl sulfate, sodium lauryl ether sulfate and sodium dialkyl sulfosuccinates. Examples of other useful surfactants are quaternary ammonium salts including perfluorinated quaternary amines, such as perfluorododecyltrimethylammonium fluoride.

Suitable chelating agents include, but are not limited to, aminocarboxylic acids, polycarboxylic acids, and polyphosphonic acids. These chelating agents may be used in conventional amounts, for example from 0.01 to 3 moles/liter, or for example from 0.1 to 0.5 moles/liter.

The pH of the nickel bath may be 1-10, or e.g. 3-8. The pH of the nickel bath may be maintained by various methods. Any desired acid or base may be used, and any inorganic acid, organic acid, inorganic base, or organic base may be used. In addition to acids such as sulfuric acid, hydrochloric acid or sulfamic acid, acids used as chelating agents such as acetic acid, glycine or ascorbic acid may be used. In addition to inorganic bases such as sodium hydroxide or potassium hydroxide and organic bases such as various amines, bases such as nickel carbonate may also be used. In addition, if the pH fluctuates due to operating conditions, a pH buffering component such as boric acid may be used. Buffers may be added in amounts necessary to maintain the desired pH.

Other conventional additives well known to those skilled in the art may also be added to the nickel metal plating bath. They can be used in conventional amounts to adjust the nickel layer to the desired matte, semi-bright or bright protective layer.

Nickel is deposited on the substrate until a 1-10 micron, or for example, 2-5 micron thick layer of nickel is formed on the substrate. A nickel-phosphorous layer is then plated on the nickel layer using a nickel-phosphorous bath.

The nickel phosphorous bath may comprise one or more of the above-described sources of nickel ions and may comprise one or more of the above-described additives. Additionally, the nickel phosphorous bath comprises one or more phosphorous sources. Any suitable phosphorous or phosphoric acid or salts and mixtures thereof may be used. The phosphorous acid and phosphoric acid and salts thereof may be present in the bath in an amount of 5 to 100 mg/l, or such as 10 to 80 mg/l, or such as 20 to 50 mg/l. Phosphorous acid of the general formula H₃PO₃Also known as ortho-phosphorous acid. Phosphoric acids include, but are not limited to, for example phosphoric acid (H)₃PO₄) (also referred to as orthophosphoric acid) and the like. Polyphosphoric acid may also be used. Inorganic phosphoric acid can be represented as: h_n+2P_nO_3n+1Wherein n is an integer equal to or greater than 1. When n is an integer of 2 or more, the formula represents polyphosphoric acid. When the inorganic phosphoric acid is a polyphosphoric acid, n is generally an integer such that the polyphosphoric acid has an average molecular weight of 110-1,500 atomic weight units. Phosphorous acid is generally used.

Phosphates such as alkali metal phosphates and ammonium phosphates may be used. The alkali metal phosphates include disodium hydrogen phosphate, trisodium phosphate, dipotassium hydrogen phosphate and tripotassium phosphate. Polyphosphates may also be used. Mixtures of inorganic phosphoric acids and their salts may also be used. These acids are commercially available or can be prepared according to literature descriptions.

A nickel phosphorous layer is deposited on the nickel layer at the same temperature as the nickel layer is deposited on the substrate. The deposition is continued until a nickel-phosphorous layer of 0.1 to 5 microns, or for example 0.2 to 1 micron, is deposited on the nickel layer to form a biphasic body.

Typically, the weight ratio of nickel to nickel phosphorus in the biphasic body is from 2: 1 to 8: 1. The nickel and nickel-phosphorous layers may be deposited using any suitable electrodeposition method known in the art. Conventional plating equipment can be used to deposit the nickel and nickel phosphorous layers. The current density may be 1ASD to 20ASD, or, for example, 5ASD to 15 ASD.

Typically, the nickel and nickel-phosphorus biphasic layer has a thickness of 2-3 microns. After depositing the nickel and nickel-phosphorous layers on the substrate, a gold or gold alloy surface protection layer is deposited on the nickel-phosphorous layer.

The article formed by the above method comprises a substrate plated with a nickel layer and a nickel phosphorous layer on the nickel layer forming a biphasic body. The phosphorus content of the nickel phosphorus layer may be 0.1 to 10 wt.%, or such as 0.5 to 10 wt.%, or such as 1 to 9 wt.%, or such as 2 to 8 wt.%, or such as 3 to 7 wt.%. A gold or gold alloy layer is deposited on the nickel-phosphorous layer to form the article. The two-phase layer formed of nickel and nickel-phosphorus layer suppresses the formation of pores in the gold or gold alloy layer and also suppresses the migration of nickel into the gold or gold alloy layer. The connector soldered to the gold or gold alloy coated article of the present invention is stronger and less likely to separate than a gold or gold alloy coated article that does not include the two-phase layer.

Any suitable source of gold and alloy metal ions may be used to deposit gold or gold alloy on the nickel-phosphorous layer. Sources of gold ions include, but are not limited to, sodium dicyanoaurate, ammonium dicyanoaurate and other salts of dicyanoaurate; potassium tetracyanoaur (III), sodium tetracyanoaur (III), ammonium tetracyanoaur (III) and other tetracyanoaur (III) acid salts; gold (I) cyanide, gold (III) cyanide; gold (I) dichloride salt; tetrachloroauric (III) acid, sodium tetrachloroauric (III) acid and other tetrachloroauric (III) acid compounds; gold ammonium sulfite, gold potassium sulfite, gold sodium sulfite and other gold sulfite salts; gold oxide, gold hydroxide and other alkali metal salts thereof; and a gold nitrosulfite complex. The amount of gold source is generally used in conventional amounts, for example from 0.1 mg/l to 10 g/l or for example from 1 to 5 mg/l.

Alloy metals include, but are not limited to, copper, nickel, zinc, cobalt, silver, platinum group metals, cadmium, lead, mercury, arsenic, tin, selenium, tellurium, manganese, magnesium, indium, antimony, iron, bismuth, and thallium. Typically the alloying metal is cobalt. These alloying metal sources are well known in the art. The source of the alloying metal in the bath is in a conventional amount and can vary over a wide range depending on the type of alloying metal used.

In addition to the gold source and the alloy metal source, the gold and gold alloy bath may also comprise one or more additives. These additives include, but are not limited to, surfactants, brighteners, levelers, complexing agents, chelating agents, buffers, and biocides. These additives are used in conventional amounts and are well known to those skilled in the art.

Gold or a gold alloy may be deposited on the nickel-phosphorous layer by any suitable method known in the art. These methods include, but are not limited to, electrolytic deposition and immersion deposition, typically gold or gold alloy is electrodeposited. The current density used for the electrolytic deposition of gold and gold alloys is in the range of 1-30ASD, or for example 5-20 ASD.

After the gold or gold alloy is deposited on the nickel-phosphorous layer, it is typically thermally aged. Heat aging can be carried out by any suitable method known in the art. At present, no standard heat aging requirement exists, and various methods are adopted for heat aging. These methods include, but are not limited to, steam aging and dry baking. The nickel and nickel-phosphorus two-phase body inhibits the surface of nickel from diffusing into the gold surface protective layer, thereby improving the weldability.

In addition to preventing diffusion of the non-noble metal into the gold and gold alloy, the two-phase body may also reduce or inhibit porosity of the gold and gold alloy layer, thereby reducing oxygen penetration into the metal layer. The reduction of porosity in gold and gold alloys may reduce or prevent undesirable oxidation of the underlying metal. This in turn may improve the solderability of the gold and gold alloy surface finish and reduce contact resistance.

These methods can be used to deposit a nickel and nickel phosphorous biphasic on any suitable substrate. Typically these substrates are metals or metal alloys. Suitable metals include, but are not limited to, copper, iron and alloys thereof, stainless steel, and precious metals such as gold, platinum, palladium, silver and alloys thereof. Typically the substrate is copper, copper alloys, iron and iron alloys. Suitable copper alloys include, but are not limited to, copper-tin, copper-silver, copper-gold, copper-silver-tin, copper-phosphorus-gold, copper-zinc, copper-silver-magnesium, copper-iron-zinc, copper-tin-nickel-silicon, copper-zirconium, copper-iron-phosphorus-zinc, and copper-nickel-silicon-magnesium. Suitable iron alloys include, but are not limited to, iron-copper and iron-nickel. These substrates include, but are not limited to, components of electrical devices such as printed circuit boards, connectors, die bumps, lead frames, and passive components such as resistors and capacitors for IC units.

The present invention is further illustrated by the following examples of the invention, which are not intended to limit its scope.

Examples

Example 1 (comparative example)

At 65 deg.C, in a container containing 100 g/l of RONAClean^TMThree (3) copper-tin alloy lead frames were ultrasonically cleaned for 30 seconds in a CP-100 (the reagent was a silicate-containing cleaning composition available from rohm and haas, philadelphia, pa).

After cleaning, each lead frame was immersed in 100 ml/l of technical-grade sulfuric acid for 10 seconds at room temperature. A 2 micron thick layer of nickel was then electroplated on each lead frame. The nickel bath used to plate each lead frame had the composition shown in table 1 below.

TABLE 1

Components	Concentration of
		Nickel sulfamate	125 g/l
Nickel chloride	8 g/l
		Boric acid	35 g/l
Sulfonated naphthalene compounds	0.5 g/l
		Benzosulfimide compounds	5 g/l
Aldehydes	0.5 g/l
		Water (W)	Balance of

The pH of the nickel bath was 4 and the bath temperature was 50 ℃. Each lead frame was then placed in a thin film battery (Hull cell) containing an electrolytic nickel bath. The lead frame serves as the cathode and the anode is a sulfur/nickel electrode. The nickel bath was stirred with a paddle during deposition. The device is connected to a conventional rectifier. The current density was 10 ASD. The nickel deposition was completed in 90 seconds.

Two (2) nickel-plated lead frames were then plated with a 1 micron thick layer of nickel phosphorous. The nickel phosphorous bath used to plate the two lead frames had the composition shown in table 2 below.

TABLE 2

Components	Concentration of
		Nickel sulfamate	125 g/l
Phosphorous acid	50 g/l
		Boric acid	35 g/l
Diglyceryl ethers	30 mg/l
		Alkyl hydrogen sulfate	1 g/l
Water (W)	Balance of

The pH of the nickel phosphorus bath was 1 and the temperature during nickel phosphorus deposition was 60 ℃. The nickel phosphorus deposition was carried out in a thin film battery using a paddle for agitation during deposition, with a current density of 10ASD for plating the first lead frame and 5ASD for plating the second lead frame. The plating of the first lead frame was completed in 60 seconds (10ASD), and the plating of the second lead frame was completed in 30 seconds (5 ASD). The phosphorus content of the plated nickel-phosphorus layer was 6 wt% at a current density of 10ASD and 9 wt% at a current density of 5 ASD. The phosphorus content is determined by the usual conventional energy dispersive X-ray spectroscopy. The area of 1 × 1 mm was measured.

A gold layer of 0.2-0.5 microns thickness was then plated on each of the three lead frames. The gold electrolyte included 12 grams/liter of gold ions derived from sodium gold sulfite and conventional gold electrolyte additives. The gold plating was performed at 35 ℃ for 4 seconds under a current density of 3 ASD.

Eutectic tin/lead solders were then soldered onto the respective lead frames at 235 ℃. The soldered lead frames were then steam aged according to military specification 883C, method 2003, which included steam aging at 95℃ and 95% relative humidity for 8 hours. This simulates a shelf life of at least 6 months.

After steam aging, the zero crossing time of each lead frame was measured to determine its solderability. The zero crossing time was determined using a Menisco ST-50 weldability tester. The zero crossing time of the lead frame excluding the nickel-phosphorus layer was 3-2 seconds, the zero crossing time of the lead frame having a phosphorus content of 6 wt% was 1.8 seconds, and the zero crossing time of the lead frame having a phosphorus content of 9 wt% was 0.9 seconds. Thus, the solderability of the nickel and nickel phosphorous layers is superior to lead frames that do not include nickel and nickel phosphorous layers.

Example 2 (comparative example)

At 60 ℃ in RONAClean^TMTwo (2) brass leadframes were ultrasonically cleaned in CP-100 cleaning solution for 30 seconds. The lead frame was then immersed in a 100 ml/l technical grade sulfuric acid solution for 10 seconds at room temperature. A nickel bath having the composition shown in table 3 below was then used to plate a 2 micron thick layer of nickel on each lead frame.

TABLE 3

Components	Concentration of
		Nickel sulfamate	125 g/l
Nickel chloride	8 g/l
		Amino acetic acid	30 g/l
Sulfonated naphthalene compounds	1 g/l
		Benzothioimides	5 g/l
Aldehydes	0.5 g/l
		Water (W)	Balance of

The pH of the nickel bath was 3 and the bath temperature was 50 ℃. Each lead frame was placed in a thin film battery containing the electrolytic nickel bath. Nickel was deposited on the lead frame at a current density of 10 ASD. The nickel deposition was complete in 80 seconds. The plating bath was stirred with a paddle during the plating process.

A 1 micron layer of nickel phosphorous was plated on one of the lead frames using a plating bath having the composition shown in table 4 below.

TABLE 4

Components	Concentration of
		Nickel sulfamate	125 g/l
Phosphorous acid	50 g/l
		Boric acid	30 g/l
Amino acetic acid	25 g/l
		Sulfonated naphthalene compounds	0.5 g/l
Diglyceryl ethers	35 mg/l
		Water (W)	Balance of

The phosphorus content of the nickel-phosphorus layer was 9 wt.%. The pH of the bath was 1 and the plating was completed at 60 ℃. Nickel phosphorus deposition is carried out in the thin film battery, and stirring is carried out by a paddle in the deposition process. The current density was maintained at 5ASD during the plating and was completed in 20 seconds.

A gold layer of 0.2-0.5 microns thickness is then plated on each lead frame. The gold bath contained 12 grams/liter of gold ions from sodium gold sulfite, and conventional gold electrolyte additives. The gold plating was completed in 4 seconds under the conditions of 30 ℃ and current density 3 ASD.

The individual lead frames are then placed on plastic supports in a desiccator. Fuming nitric acid vapor at a concentration of greater than 65% was then added to the dryer at room temperature for 1 hour. The nitric acid is then removed by rinsing with deionized water. The gold surface of each lead frame was then placed under a Nomarski microscope at 50 x magnification to examine whether the gold layer formed holes. Fig. 1 shows the gold surface of the lead frame without the nickel phosphorous layer plated. A large number of holes are formed in the gold layer. In contrast, fig. 2 shows a gold layer on a lead frame that includes a nickel-phosphorous layer. The pores formed are sparse. Thus, the nickel and nickel-phosphorous layers under the gold layer can reduce the formation of pores in the gold.

Example 3 (comparative example)

At 65 deg.C, 100 g/l ROANACLEAN is used^TMThree (3) brass leadframes were ultrasonically cleaned in CP-100 cleaning solution for 30 seconds. After cleaning, each lead frame was immersed in 100 ml/l of a technical-grade sulfuric acid solution at room temperature for 10 seconds. A 2 micron thick layer of nickel was then electroplated on each lead frame using the nickel electrolyte shown in table 1 above. Nickel deposition was carried out in a thin film battery at a pH of 4 and a temperature of 50 ℃. The current density was 10ASD and the deposition was complete in 80 seconds.

A 1 micron thick layer of nickel phosphorous was then plated on both (2) lead frames. The nickel-phosphorus electrolyte used was the same as described in table 2 above. The pH of the electrolyte was 2 and the nickel phosphorous deposition was carried out at a temperature of 50 ℃. One of the baths had a current density of 10ASD and the other was 5 ASD. The nickel phosphorous deposition for each plating bath was completed in 20 seconds.

All three lead frames were then electroplated with gold for 4 seconds in a thin film battery under current density 3ASD conditions. The gold electrolyte contained 12 grams/liter of gold ions derived from gold sulfite and conventional gold electrolyte additives. The temperature of the plating bath was 30 ℃.

Each of the gold plated lead frames was then vapor aged according to military specification 883C, method 2003, which included vapor aging at 95℃ and 95% relative humidity for 8 hours. The lead frame was then tested for oxygen penetration into the nickel layer, and migration of the nickel to the gold layer.

Table 4 below reveals the test results. The atomic% of the various elements at different sputtering depths was determined in an ultra-high vacuum chamber using a conventional X-ray photoelectron spectrometer.

TABLE 4

	Ni/Au				Ni/Ni-P/Au (7 wt% P)					Ni/Ni-P/Au (10 wt% P)
														Depth (Å)	Au		Ni	O	Au	Ni		O	P	Au	Ni	O	P
0	8.9		32.3	58.8	23.2	19.9		56.9	-	8.1	36.3	55.6	-
														100	15.1		50.6	34.3	97.8	5.4		0.4	-	77.1	17.7	5.2	-
200	14.1		44.4	41.5	89.6	2.2		-	-	88.7	7.3	2.7	1.3
														300	6.2		54.1	39.7	89.6	9.3		-	1.1	91.9	7.1	1.0	-
330	5.4		54.4	40.42	58.1	37.5		-	4.4	85.0	13.1	-	1.9
														360	4.9		55.4	39.7	27.6	66.0		-	6.4	63.4	29.4	2.1	5.1
390	4.5		56.1	39.4	13.2	82.2		-	4.6	42.6	48.0	1.0	8.4
														420	4.5		56.2	39.3	7.4	88.1		-	4.5	26.5	63.4	1.9	8.2
450	4.2		56.7	39.1	5.2	91.1		-	3.7	16.5	75.6	2.4	5.5
														480	4.3		56.3	39.4	4.3	91.7		-	4.0	11.0	82.4	1.9	4.7
510	4.2		56.9	38.9	3.7	92.8		-	3.5	7.5	88.5	1.8	2.2
														540	4.0		56.7	39.3	3.5	92.5		0.7	3.3	5.6	91.6	1.8	1.0
570	4.0		56.9	39.1	3.3	92.0		0.6	4.1	4.3	93.6	1.8	0.3
														600	3.9		57.1	39.0	3.1	92.5		0.4	4.0	3.4	94.9	1.7	-
630	3.9		57.1	39.0	2.9	92.7		0.7	3.7	2.6	95.6	1.8	-
														660	3.9		57.5	38.6	2.9	92.7		0.4	4.0	2.3	96.0	1.7	-
690	3.9		57.4	38.7	2.8	93.3		0.4	3.5	2.1	96.2	1.7	-
														720	3.9		57.4	38.7	2.8	93.5		0.3	3.4	2.1	96.2	1.7	-
750	3.8		57.8	38.4	2.7	93.0		0.7	3.6	1.6	96.8	1.6	-
														780	3.9		57.4	38.7	2.6	93.1		0.4	3.9	1.5	96.7	1.8	-
810	3.8		58.0	38.2	2.5	93.5		0.4	3.6	1.4	96.7	1.9	-
														840	3.7		58.0	38.3	2.6	93.3		0.4	3.7	1.3	97.0	1.7	-
870	3.8		58.7	37.5	2.4	93.5		0.5	3.6	1.2	97.2	1.6	-
														900	3.6		58.1	38.3	2.5	93.0		0.3	4.2	1.2	96.9	1.9	-

The results in the above table illustrate that the oxygen content in the metal layer of the nickel phosphorus plated lead frame is much lower than the oxygen content of the lead frame with only the nickel layer. The atomic percentage of oxygen in the metal layer of the lead frame having a nickel phosphorous layer with a phosphorous content of 10 wt.% is: a high oxygen content of 5.2 atomic% at a depth of 100 Å and a low oxygen content of 0 atomic% at 330 Å. The average oxygen atom% of the entire depth was 4.1. A lead frame with a phosphorus content of 7 wt.% has a high oxygen content of 0.7 at.% at 540 Å and a low oxygen content of 0 at.% at 200-510 Å. The average oxygen content at all depths was 2.6 atomic%. In contrast, the lead frame without the nickel-phosphorous layer had a high oxygen content of 41.5 atomic% at 200 Å and a low oxygen content of 34.3 atomic% at 100 Å. The average oxygen content at all depths was 36 atomic%. Thus, the nickel phosphorous layer reduces oxygen penetration.

Example 4

Example 2 was repeated except that an aqueous gold-cobalt alloy electroplating bath was used to plate a gold-cobalt layer on the nickel or nickel-phosphorous layer. The gold-cobalt alloy bath contained 4 g/l gold potassium cyanide, 1 g/l cobalt sulfate (0.25 g/l cobalt ions) and 150 g/l methylenephosphonic acid. The aqueous gold-cobalt plating bath had a pH of 4. The pH of the plating bath was maintained using potassium hydroxide. The bath was maintained at 40 ℃ and the current density was 1 ASD. The plating was continued until a 0.2-0.5 micron layer of gold-cobalt was formed on the nickel-phosphorous layer of each lead frame.

Each lead frame was then placed on a plastic support in a desiccator and exposed to fuming nitric acid as described in example 2. It is expected that the number of holes in the gold-cobalt layer on a lead frame with a two-phase body will be less than the number of holes in the gold-cobalt layer on a lead frame with only a nickel layer.

Example 5

Example 4 was repeated except that the aqueous gold-cobalt alloy electroplating bath contained 4 g/l of gold potassium cyanide, 1 g/l of cobalt in the form of the tetrapotassium salt of cobalt methylenephosphonate, 124 g/l of tripotassium citrate, 29 g/l of citric acid and 45 g/l of potassium dihydrogen phosphate. The pH of the bath was maintained at 5 and the temperature of the bath was maintained at 35 ℃. A gold-cobalt layer of 0.2 to 0.5 μm was formed on each lead frame by plating with a current density of 1 ASD.

After treating each lead frame with fuming nitric acid, the number of holes in the gold-cobalt layer on the lead frame having the two-phase layer is expected to be smaller than the number of holes in the gold-cobalt layer on the lead frame not including the two-phase layer.

Claims (8)

1. A method comprising depositing a nickel layer on a substrate, depositing a nickel-phosphorous layer on said nickel layer and depositing a gold or gold alloy layer on said nickel-phosphorous, said nickel-phosphorous layer comprising 0.1 to 10 wt.% phosphorous.

2. The method of claim 1, wherein the nickel-phosphorous layer comprises 1 to 9 wt% phosphorous.

3. A method comprising depositing a 1-10 micron nickel layer on a substrate, depositing a 0.1-5 micron nickel phosphorous layer on the nickel layer, and depositing a gold or gold alloy layer on the nickel phosphorous layer, the nickel phosphorous layer comprising 1-10 wt.% phosphorous.

4. The method of claim 3, further comprising the step of heat aging the gold or gold alloy layer.

5. An article comprising a nickel layer on a substrate and a nickel-phosphorous layer on the nickel layer, the nickel-phosphorous layer comprising 0.1 to 10 weight percent phosphorous, and a gold or gold alloy layer on the nickel-phosphorous layer.

6. The article of claim 5, wherein the nickel-phosphorous layer comprises 1 to 9 wt% phosphorous.

7. The article of claim 5, wherein the nickel layer has a thickness of 1 to 10 microns and the nickel-phosphorous layer has a thickness of 0.1 to 5 microns.

8. The article of claim 5, wherein the article is a printed circuit board, a lead frame, a connector, a die bump, or a passive component.