EP0384227B1 - Bath for the galvanic deposition of pure gold coatings - Google Patents

Abstract

Readily bondable pure gold coatings are obtained from electroplating baths which contain 2 to 100 g/l gold as alkali or ammonium gold (I)-cyanide, 0.1 mg to 5 g/l of a soluble bismuth compound, 0 to 250 g/l of a phosphonic acid, its salts, esters or simple substitution products and/or 0 to 250 g/l phosphoric acid or its salts, but at least 5 g/l of said compound, and have a pH of 6 to 10.

Description

The invention relates to a bath for the electrodeposition of bondable fine gold coatings with a gold content of more than 99.9% by weight, containing gold as alkali or ammonium gold (I) cyanide as well as phosphonic acids and / or phosphoric acid or its salts or derivatives and a water-soluble bismuth compound .

Galvanic gold baths are used for many different purposes. In most cases they contain alkali or ammonium gold (I) cyanide, more rarely gold (III) cyanide or alkali gold sulfites.

Gold baths for decorative applications almost always deposit gold alloys with considerable proportions of alloy metal in order to achieve the desired color effects.

Gold baths for the deposition of gold with good electrical and mechanical properties are widely used as a material for electrical low-current contacts. These baths work in the weakly acidic pH range (pH 3-5). In addition to gold, the coating usually contains small amounts (0.1-1%) of nickel, cobalt or iron.

In addition, galvanic gold baths are also required for the deposition of fine gold layers with at least 99.9% gold content, for example as bondable coatings in semiconductor technology.

Bonding techniques are those processes in which in microelectronics system carriers with electronic components - e.g. Chips - to be conductively connected using fine wires (usually made of gold or aluminum). The wire is connected to a gold-plated pad by pressure, elevated temperature and often supported by ultrasonic energy.

This bond can only be achieved in a flawless manner if the gold plating on the connection surface is very pure (fine gold with at least 99.9% gold content), soft (hardness maximum 120 HV) and satin matt. Hard, high-gloss coatings are unsuitable. Further demands on the bondable gold layers result from the stress caused by thermal stress tests that are carried out to ensure good bondability. Such heat stress tests are carried out, for example, on the unbonded specimen, for example after a 5-minute exposure at 500 ° C in air, no discoloration of the gold layer may occur. In other tests, the bonded specimen is exposed to temperature cycling tests or to air at a temperature of 150 - 180 ° C for many hours. These Heat stress tests cause diffusion processes between gold and carrier material in the gold layer. The effect of such diffusion processes on the bond behavior depends on the layer thickness of the gold and its structure. Since the smallest possible gold layer thickness is required for economic reasons, the structure of the coating by means of bath additives and the choice of suitable deposition conditions must be optimized so that a minimum gold layer thickness ensures perfect bondability. Such bath additives have hitherto been known in the form of arsenic, thallium or lead.

Another criterion for the cost-effectiveness of an electroplating process is the deposition rate of the bath while observing the functional requirements of the coating. The deposition rate results from the applicable current density, the upper limit of which should be as high as possible without sacrificing the desired properties of the gold layers.

Simple ways of achieving high permissible current densities are increasing the gold content in the bath or rapid electrolyte movement. Because of the high gold price, the cost-effectiveness of the process suffers if the gold content is too high.

The use of equipment (special electroplating cells) with a sometimes very high electrolyte movement due to the flow or spraying of the electroplating material often ensures the selective limitation of the gold plating to the functionally important surface elements of the goods by using masks. The decisive factor for the speed of the deposition (maximum permissible current density) is the strong inflow through the electrolyte.

Despite the increase in the deposition rate that can be achieved with these measures, a further increase in the current density and deposition rate by improving the gold baths is very desirable, with increased demands on the high current density (cathodic and especially anodic) and the strong turbulence of the baths in flow systems Stability of the electrolytes result.

EP-OS 0 126 921 describes a bath for the electrodeposition of gold alloys which, in addition to alkali gold (I) cyanide and phosphonic acids, also contains between 10 mg and 100 g / l bismuth in the form of a water-soluble complex compound. This bath, which operates in a pH range from 6 to 13, deposits pink to violet-colored alloy coatings for decorative purposes with gold proportions of 65 to 85% by weight, which are completely unsuitable for bonding applications.

A gold bath for semiconductor applications according to JP-OS 56 084 495 contains 2 to 16 g gold as cyanoaurate, 5 to 200 g / l citric acid, 5 to 100 g / l phosphoric acid,
10 to 150 g / l ammonium sulfate and 0.01 to 40 mg / l titanium, cerium, lead, antimony, arsenic, bismuth and / or tellurium at pH values from 4 to 7. These baths are therefore operated in the weakly acidic range and have the disadvantage that the citric acid acts on nickel and copper as base materials and intermediate layers, which leads to contamination of the bath. These baths are therefore not ideal for bond connections.

Special gold layers are obtained from electroplating baths which, in addition to alkali gold (I) cyanide, contain hydrogen phosphates, phosphonic acids and nitrogen-containing carboxylic acids (DE-OS 35 37 283). Current densities of up to 15 A / dm² can be achieved in flow systems. With normal bath movement, however, you get matt brown coatings at current densities above 1 A / dm², which are unusable for bond applications.

US Pat. No. 3,879,269 describes gold baths for the deposition of bondable fine gold coatings in high-speed cells. In addition to 24 - 40 g / l gold, phosphates and carboxylic acids, these baths contain a critical amount of 2 - 12 mg / l trivalent arsenic ions. Despite being used in a high-speed cell and despite a very high gold concentration, usable coatings can only be achieved up to a current density of approx. 4 A / dm².

Galvanic gold baths with trivalent arsenic as a grain-refining additive all suffer from the well-known phenomenon that the trivalent arsenic is oxidized to the pentavalent one, this process already taking place when the bath is not in use, but with a particularly high reaction rate under the oxidative influences (high anodic current density, strong swirling of air) of high-speed electrolysis in flow or spray cells. The five-valued ace shows none grain-refining or gloss-forming effect. On the other hand, the required and permissible concentration of trivalent arsenic is in the range of a few mg / l. As there is no sufficiently precise analysis method for this effective component, safe bath management cannot be guaranteed. The instability of the additive leads to strong quality fluctuations, which can only be limited in an unsafe manner by frequent function tests and corrections.

DE-OS 3 3 41 233 contains an acidic fine gold bath with an addition of 5 to 50 ppm (mg / l) lead, which can be operated in the current density range 0.5-2 A / dm², the optimum current density being 0.6 A / dm².

The use of thallium as a grain refiner in concentrations of 1 - 140 mg / l in gold baths with pH values in the range 7 - 13 is e.g. described in DE-OS 2 1 31 815. The applicable current density range is 0.1 - 20 A / dm².

Galvanic gold baths containing lead or thallium as grain-refining additives are known to have gold coatings deposited from them that are perfectly bondable in the freshly deposited state, but they largely lose their tear resistance in the subsequent heat tests of, for example, 150 ° C / 24 hours that there is an accumulation of bond breaks.

It was therefore an object of the present invention to develop a bath for the electrodeposition of bondable fine gold coatings with a gold content of more than 99.9% by weight, containing gold as alkali or ammonium gold (I) cyanide, and also phosphonic acids and / or phosphoric acids or their Salts or derivatives and a water-soluble bismuth compound, which ensures a high deposition rate through a high applicable current density with perfect stability of the bath and with which a good bondability and heat resistance of the coatings can be achieved.

This object is achieved in that it contains 2 to 100 g / l gold, 0.1 mg / l to 5 g / l bismuth, 0 to 250 g / l phosphonic acid or its salts, esters or simple substitution products and / or 0 contains up to 250 g / l of phosphoric acid or its salts, the sum of the contents of phosphonic acid compounds and phosphates having to be at least 5 g / l and having a pH of 7.5 to 10.

The baths preferably contain phosphonic acids or their salts or esters, which have at least two groups of the form -PO (OH) ₂, which are links in an aliphatic simple or branched hydrocarbon chain which can also be interrupted by one or more nitrogen atoms.

Advantageously, the galvanic gold baths contain 2 to 50 g / l gold as alkali or ammonium gold (I) cyanide, 0 to 250 g / l alkali phosphates, alkali hydrogen phosphates or the corresponding ammonium salts, 5 to 250 g / l of a phosphonic acid in the form of 1- Hydroxyethane-1,1-diphosphonic acid, amino-tri-methylenephosphonic acid. Ethylenediamine-tetra-methylenephosphonic acid or its hexasodium salt and 0.5 mg to 4 g / l bismuth in the form of ammonium bismuth citrate,

Gold baths containing 8 to 24 g / l gold, 40 to 150 g / l phosphate, 20 to 120 g / l phosphonic acid and 0.5 to 4000 mg / l bismuth have proven particularly useful.

In addition, the baths may also contain polyphosphoric acids, superphosphates, amidopolyphosphates, pyrophosphates and conductive salts, buffers, complexing agents or wetting agents customary in electroplating.

With the galvanic gold baths according to the invention, deposition of bondable fine gold coatings is achieved with a high current density and deposition speed. With normal goods and bath movements, a current density of up to 4 A / dm² can be used, the deposition speed being approximately up to 2.50 μm / min. On high-speed cells (jet-plating cells) can the permissible current density is up to 25 A / dm², whereby deposition speeds of up to 14.5 µm / min can be achieved.

The gold coatings produced in this way are very pure (purity 99.99%), soft (hardness 70 - 120 HV), semi-glossy and excellent bondable. The bismuth content in the gold plating is below 50 ppm. The fine gold coatings show excellent stability against thermal loads, both before and after bonding. Gold plating with a thickness of 0.4 µm on a nickel base survives the heat test of 5 minutes aging at 500 ° C without discoloration. Bonded samples show no decrease in tensile strength after aging for 48 hours at 150 ° C. The weakening of the bond strength of the gold plating observed with thallium and lead additives in the bath does not occur.

The baths are completely stable regardless of the storage time (without use) and the duration of use.

The phosphonic acids used are preferably those in the galvanic gold baths according to the invention which contain at least 2 groups of the form -PO (OH) ₂, which are links in an aliphatic simple or branched hydrocarbon chain, which hydrocarbon chain can be interrupted by one or more nitrogen atoms. The members of the hydrocarbon chain can be substituted by NH₂ or OH groups or other simple substituents.

Beispiel für eine stickstoffhaltige Phosphonsäure mit 3 Säuregruppen ist die Amino-tri-(Methylen-Phosphonsäure)

Beispiel für eine stickstoffhaltige Phosphonsäure mit 4 Säuregruppen ist die Ethylendiamin-tetra-Methylenphosphonsäure

Beispiel für eine stickstoffhaltige Phosphonsäure mit 5 Säuregruppen ist die Diethylen-Triamin-penta-Methylenphosphonsäure

Wismut kann z.B. als Ammoniumwismutcitrat verwendet werden, doch sind auch alle sonstigen wasserlöslichen Wismutverbindungen einsetzbar.An example of a simple, nitrogen-free phosphonic acid of the type mentioned with 2 acid groups is 1-hydroxyethane-1,1-di-phosphonic acid

An example of a nitrogen-containing phosphonic acid with 3 acid groups is the amino-tri- (methylene-phosphonic acid)

An example of a nitrogen-containing phosphonic acid with 4 acid groups is ethylenediamine-tetra-methylenephosphonic acid

An example of a nitrogen-containing phosphonic acid with 5 acid groups is diethylene-triamine-penta-methylenephosphonic acid

Bismuth can be used, for example, as ammonium bismuth citrate, but all other water-soluble bismuth compounds can also be used.

The optimum concentration of bismuth depends on the phosphonic acid bath used or, if a pure phosphate bath is used, on this. It can be determined by test trials. The required amount of bismuth increases with the number of -PO₃H₂ groups on the phosphonic acid.

• 1. Es wird ein Goldelektrolyt in Form einer wässrigen Lösung aus folgenden Komponenten hergestellt:
  18 g/l KAu (CN)₂ (= 12 g/l Au)
  100 g/l K₂HPO₄
  6 mg/l Ammoniumwismutcitrat (= 3 mg/l Bi)
  Nach Einstellung des pH-Wertes auf 8,0 und der Badtemperatur auf 70° C werden mit Gleichstrom bei einer Stromdichte von 0,5 - 3 A/dm² und einer Warenbewegung von 5 cm/sec gelbe, seidenmatte Überzüge von guter Bondbarkeit erhalten.
  Das gleiche Bad liefert ebensolche Überzüge auf einer Spritzzelle (Jet-Plating) im Stromdichtebereich von 0,5 - 15 A/dm².
  Die so erzeugten Goldüberzüge sind für die vorgesehenen Bondanwendungen gut brauchbar, sie wirken jedoch in ihrer Oberflächenbeschaffenheit etwas uneinheitlich und leicht fleckig.
  Wird auf den Wismutzusatz im Bad verzichtet, so erhält man bei einer Warenbewegung von 5 cm/sec und Stromdichten über 0,5 A/dm² und in einer Spritzzelle bei Stromdichten über 2 A/dm² nur braune, matte, zum Bonden unbrauchbare Überzüge.
• 2. Es wird ein Goldelektrolyt in Form einer wässrigen Lösung aus folgenden Komponenten hergestellt:
  18 g/l KAu (CN)₂ (=12 g/lAu)
  5 g/l 1-Hydroxyethan-1,1-di-Phosphonsäure
  KOH bis zur Einstellung des pH-Wertes 7,5
  2 mg/l Ammoniumwismutcitrat (1 mg/l Bi)
  Bei einer Warenbewegung von 5 cm/sec und einer Badtemperatur von 70° C werden gelbe, seidenmatte Überzüge mit guter Bondbarkeit bei Anwendung von Gleichstrom mit Stromdichten von 0,5 - 3 A/dm² erhalten.
  Das gleiche Bad liefert nach Anhebung des Goldgehaltes auf 16 g/l auf einer Spritzzelle (Jet Plating) ebensolche Überzüge im Stromdichtebereich 0,5 - 20 A/dm².
• 3. Es wird ein Goldelektrolyt in Form einer wässrigen Lösung aus folgenden Komponenten hergestellt:
  24 g/l KAu (CN)₂ (= 16 g/l Au)
  100 g/l Ethylendiamin-tetra-Methylenphosphonsäure
  KOH zur Einstellung des pH-Wertes 8,0
  und 6 g/l Ammoniumwismutcitrat (3 g/l Bi)
  Die mit diesem Bad unter den gleichen Arbeitsbedingungen wie bei Beispiel 2 erzielten Überzüge entsprechen in Aussehen und Verhalten denen von Beispiel 2.
• 4. Einem Grundelektrolyten mit
  18 g/l KAu(CN)₂ (= 12 g/l Au) und
  100 g/l K₂HPO₄
  wird zugegeben entweder
  60 g/l 1-Hydroxyethan-1,1-di-Phosphonsäure und
  10 mg/l Ammoniumwismutcitrat (5 mg/l Bi) oder
  60 g/l Amino-tri-Methylenphosphonsäure und
  15 mg/l Ammoniumwismutcitrat (7,5 mg/l Bi) oder
  60 g/l Hexanatriumsalz der Ethylendiamin-tetra-Methylenphosphonsäure und
  1,5 g/l Ammoniumwismutcitrat (0,75 g/l Bi) oder
  100 g/l Ethylendiamin-tetra-Methylenphosphonsäure und 6 g/l Ammoniumwismutcitrat (3 g/l Bi).
  Anschließend wird der pH-Wert dieser Bäder mit KOH auf 8,0 und die Temperatur auf 70° C eingestellt.
  Alle Bäder liefern unter konventionellen Galvanisierbedingungen, d.h. mit geringer Elektrolytbewegung und einer Warenbewegung von ca. 5 cm/sec., mit ungepulstem Gleichstrom im Stromdichtebereich von 0,5 - 3 A/dm² oder auf Spritzzellen im Stromdichtebereich von 0,5 - 25 A/dm² gelbe, seidenmatte und in ihrer Oberflächenbeschaffenheit sehr gleichmäßige Überzüge.
  Die aus diesen Bädern hergestellten Überzüge besaßen eine Härte 70 - 110 HV (Vickers). Der Wismutgehalt war geringer als 0,005 % (50 ppm). 0,5 um dicke Goldüberzüge auf einer Nickelunterlage zeigten nach einer Auslagerung von 5 Minuten bei 500° C an Luft keinerlei Verfärbung. Die Bondbarkeit im Ultraschallverfahren mit Aluminiumdraht von 300 um Dicke war einwandfrei, wobei im Zugversuch bis über 500 cm keine Drahtabrisse beobachtet wurden. Nach einer Auslagerung der gebondeten Proben während 48 Stunden bei 150° C an Luft wurde gefunden, daß die Bondfestigkeit nicht die geringste Abnahme zeigte.

The following examples are intended to explain the baths according to the invention in more detail:

• 1. A gold electrolyte is produced in the form of an aqueous solution from the following components:
  18 g / l KAu (CN) ₂ (= 12 g / l Au)
  100 g / l K₂HPO₄
  6 mg / l ammonium bismuth citrate (= 3 mg / l Bi)
  After adjusting the pH to 8.0 and the bath temperature to 70 ° C, yellow, silk-matt coatings with good bondability are obtained with direct current at a current density of 0.5-3 A / dm² and a movement of 5 cm / sec.
  The same bath provides the same coatings on a spray cell (jet plating) in the current density range of 0.5 - 15 A / dm².
  The gold coatings produced in this way can be used well for the intended bond applications, but their surface texture is somewhat inconsistent and slightly stained.
  If the bismuth additive in the bathroom is dispensed with, then with a movement of 5 cm / sec and current densities above 0.5 A / dm² and in a spray cell at current densities above 2 A / dm², only brown, matt coatings that are unusable for bonding are obtained.
• 2. A gold electrolyte is produced in the form of an aqueous solution from the following components:
  18 g / l KAu (CN) ₂ (= 12 g / lAu)
  5 g / l 1-hydroxyethane-1,1-di-phosphonic acid
  KOH until the pH is adjusted to 7.5
  2 mg / l ammonium bismuth citrate (1 mg / l Bi)
  With a movement of 5 cm / sec and a bath temperature of 70 ° C, yellow, silk-matt coatings with good bondability are obtained when using direct current with current densities of 0.5 - 3 A / dm².
  After raising the gold content to 16 g / l on a spray cell (jet plating), the same bath provides the same coatings in the current density range 0.5 - 20 A / dm².
• 3. A gold electrolyte is produced in the form of an aqueous solution from the following components:
  24 g / l KAu (CN) ₂ (= 16 g / l Au)
  100 g / l ethylenediamine-tetra-methylenephosphonic acid
  KOH for pH 8.0 adjustment
  and 6 g / l ammonium bismuth citrate (3 g / l Bi)
  The coatings obtained with this bath under the same working conditions as in Example 2 correspond in appearance and behavior to those of Example 2.
• 4. A base electrolyte with
  18 g / l KAu (CN) ₂ (= 12 g / l Au) and
  100 g / l K₂HPO₄
  is added either
  60 g / l 1-hydroxyethane-1,1-di-phosphonic acid and
  10 mg / l ammonium bismuth citrate (5 mg / l Bi) or
  60 g / l amino-tri-methylenephosphonic acid and
  15 mg / l ammonium bismuth citrate (7.5 mg / l Bi) or
  60 g / l hexasodium salt of ethylenediamine-tetra-methylenephosphonic acid and
  1.5 g / l ammonium bismuth citrate (0.75 g / l Bi) or
  100 g / l ethylenediamine tetra-methylenephosphonic acid and 6 g / l ammonium bismuth citrate (3 g / l Bi).
  The pH of these baths is then adjusted to 8.0 with KOH and the temperature to 70 ° C.
  All baths deliver under conventional electroplating conditions, ie with low electrolyte movement and a goods movement of approx. 5 cm / sec., With unpulsed direct current in the current density range of 0.5 - 3 A / dm² or on spray cells in the current density range of 0.5 - 25 A / dm² of yellow, silk matt and very uniform surface finishes.
  The coatings produced from these baths had a hardness of 70-110 HV (Vickers). The bismuth content was less than 0.005% (50 ppm). Gold plating 0.5 µm thick on a nickel base showed no discoloration after exposure to air at 500 ° C for 5 minutes. The bondability in the ultrasound process with aluminum wire of 300 μm thickness was perfect, with no wire breaks being observed in the tensile test up to over 500 cm. After exposure of the bonded samples to air for 48 hours at 150 ° C., it was found that the bond strength did not show the slightest decrease.

Claims (2)

1. A bath for the electrolytic deposition of bondable fine gold coatings having a gold content of more than 99.9% by weight, containing gold as an alkali metal or ammonium gold (I) cyanide and phosphonic acids and/or phosphoric acid or salts or derivatives thereof and a water-soluble bismuth compound, characterised in that it contains 2 to 100 g/l of gold, 0.1 mg/l to 5 g/l of bismuth, 0 to 250 g/l of phosphonic acid, salts, esters or simple substitution products thereof and/or 0 to 250 g/l of phosphoric acid or salts thereof, whereby the sum of the amounts of phosphonic acid compounds and phosphates must be at least 5 g/l, and it has a pH of 7.5 to 10.
2. A bath according to Claim 1, characterised in that it contains phosphonic acids or salts or esters thereof, which have at least two groups of the form -PO(OH)₂, which are members of an aliphatic single or branched hydrocarbon chain which may also be interrupted by one or more nitrogen atoms.