Patent Grant
7250102
The invention relates generally to the field of electroplating and more specifically to aluminum and magnesium/aluminum electroplating.
The science of electroplating has been developed over a number of years, beginning perhaps with Ziegler et al., U.S. Pat. No. 2,849,349. This patent is hereby incorporated in its entirety by reference herein. Lehmkuhl et al., U.S. Pat. Nos. 5,007,991 and 5,091,063, and Birkle et al., U.S. Pat. No. 4,417,954 likewise are incorporated in their entirety by reference herein.
While electroplating is not a new art, possible advances remain. Areas of possible improvement include, for example, throwing power and current density. Throwing power refers to the ability of an electroplating solution to deposit metal uniformly on an irregularly shaped object. Current density refers to the electrical current (amp/dm2) that can be applied across the anode and the cathode during the electroplating process.
Aluminum electroplating electrolyte compositions include CA•(nAl(C3H7)3(2−n)AlR3) where n is greater than 0 and less than or equal to 2; C is Li, Na, K, Rb, Cs, NR′4, or mixtures thereof wherein R′ is H, C1-C8 alkyl, e.g., CH3, C2H5, C3H7, C4H9, C5H11, C6H13, C7H15, C8H17, or mixtures thereof; A is H, F, Cl, Br, or mixtures thereof; R is H, C1-C8 alkyl, e.g., CH3, C2H5, C3H7, C4H9, C5H11, C6H13, C7H15, C8H17, or mixtures thereof; and an aromatic hydrocarbon, aliphatic hydrocarbon, or mixtures thereof.
The above aluminum electrolyte composition of the above-identified formula also includes embodiments where n is from 0 to 2 when such aluminum electrolyte compositions are in admixture with other aluminum alkyls, ethers, alkoxy-aluminum alkyls, aluminoxanes, or mixtures thereof.
Aluminum and magnesium/aluminum electroplating compositions include C•(nAl(C3H7)4(1−n)AlR4) where n is from 0 to 1. C is a cation such as, Li, Na, K, Rb, Cs, or mixtures thereof. R is H or an alkyl such as, CH3, C2H5, C3H7, C4H9, C5H11, C6H13, C7H15, C8H17, or mixtures thereof. The aluminum magnesium/aluminum electroplating compositions formulation can include a solvent such as, an aromatic hydrocarbon, aliphatic hydrocarbon, or mixtures thereof.
The term “about” is presumed to modify all numeric values, whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the term “about” may include numbers that are rounded to the nearest significant figure.
The term “M” is molar amount commonly used in chemistry. The formulation examples and claims report values for concentrations in mol per mol salt or mol per mol cation.
Pre-treatment
Pre-treatment provides a clean surface on the piece or base metal to be coated. To remove fat, oxides, and other impurities from the surface, degreasing, etching, and/or descaling operations are carried out. Each treatment step is followed by one or more rinsing steps and conducted in such a manner as to minimize loss and to recycle valuable material.
The rinsed and degreased base metal proceeds to an electrolytic degreasing 210 to further degrease the base metal. The further degreased base metal proceeds to a water rinse 212. The further degreased and rinsed base metal proceeds to an acid etch 215 to remove oxides from the base metal. The acid may be any acid capable of removing oxides from the base metal. The acid etched base metal proceeds to a water rinse 220 to remove any remaining acid from the base metal.
The etched base metal then can be nickel plated 225. The nickel plating thickness can be 2 micrometers. The nickel plated base metal can be water rinsed 230 and dried 235.
The dried nickel-plated base metal enters an air lock 240 and vacuum environment. The plated base metal proceeds to an activation bath 245. The activation bath 245 can include an aqueous inorganic acid in aliphatic mono- or di- or tri-hydric alcohol. The inorganic acid may be hydrofluoric acid and the alcohol can be ethylene glycol, for example. The composition of the activation bath 245 must be compatible with the base metal.
The activated base metal proceeds to an intermediate rinse 250. The intermediate rinse 250 includes a material that is soluble with the later electrolyte solvent. The intermediate rinse 250 can be an aliphatic alcohol such as, for example, diethylene glycol monomethyl ether or a mixture of the electrolyte solvent with an aliphatic alcohol such as, for example, toluene and di-ethylene glycol monomethyl ether.
A first electrolyte solvent rinse 255 removes remaining intermediate rinse material. The first electrolyte solvent rinse 255 can include up to 10% of the aliphatic alcohol used in the intermediate rinse as impurity. A second electrolyte solvent rinse 260 further removes intermediate rinse material. The second electrolyte solvent rinse 260 can include up to 1% of the aliphatic alcohol used in the intermediate rinse as impurity. A third electrolyte solvent rinse 265 further removes intermediate rinse material. The third electrolyte solvent rinse 265 can include up to 0.1% of the aliphatic alcohol used in the intermediate rinse as impurity.
The base metal rinsed with the electrolyte solvent can proceed to an electroplating process 270.
Plasma Etch Pre-Treatment
The base metal enters the pre-treatment process 301 at degreasing 300. Degreasing 300 removes grease and the like from the base metal. Degreasing solutions can include surfactants or other degreasing materials. The degreased base metal proceeds to a water rinse 305. The water rinse 305 removes the degreasing solution from the degreased base metal.
The rinsed and degreased base metal proceeds to an electrolytic degreasing 310 to further degrease the base metal. The further degreased base metal proceeds to a water rinse 312. The further degreased and rinsed base metal proceeds to an acid etch 315 to remove oxides from the base metal. The acid may be any acid capable of removing oxides from the base metal. The acid etched base metal proceeds to a water rinse 320 to remove any remaining acid from the base metal and dried 335.
The dried acid etched base metal enters an air lock 340 and vacuum environment. In the vacuum environment, the acid etched base metal can be plasma etched 342 to remove further impurities from the base metal. The plasma etch 342 is an aprotic robust process capable of processing a wide variety of materials. The plasma etch 342 bombards the base metal with charged particles. The charged particles strike the base metal and “knock off” organic and inorganic impurities from the base metal surface. The plasma etch pre-treatment process 301 could eliminate multiple steps, and associated chemical, recycling and waste treatment costs, from the normal pre-treatment process 201.
A solvent rinse 365 follows the plasma etch 342 to remove dust from the metal surface to be plated. The solvent rinsed base metal can proceed to an electroplating process 370. If the base metal is only slightly greasy and slightly oxidized, it is possible to use the pretreatment process 301 shown in
Electrolyte Formulations
The electrolyte formulations described herein are useful for aluminum, and magnesium/aluminum electroplating. Several factors impact electrolyte development, such as, for example, electrolyte cost, technical and safety concerns, and plating performance.
Cost concerns include, for example, costs of chemicals required during electrolyte lifetime, costs for electrolyte recycling, costs for waste disposal and the electrolyte lifetime.
Technical and safety concerns include, for example, low chemical toxicity, low chemical pyrophoricity, low vapor pressure of the electrolyte solvent at plating temperature and minimal crystallization disturbance.
Plating performance concerns include, for example, high throwing power and covering power, high maximum current density on a part, 100% anodic current efficiency, 100% cathodic current efficiency (forming a pure aluminum layer) and forming a porefree, dense aluminum layer with an appealing visual appearance.
The inventive electrolyte formulations provide improved throwing power and improved current density. In particular, the electrolyte formulations are useful in electroplating base materials with either aluminum or a combination of aluminum and magnesium and the like.
Processing
Electroplating may be accomplished with direct current. Direct current provides a current density that is limited since as the current density increases the alkali metal (i.e. potassium, etc.) precipitates together with aluminum on the base metal, which decreases the life of the electrolyte and changes the corrosion resistance of the plating layer on the base metal.
Electroplating may be accomplished at least in part by pulse reverse plating. Pulse reverse plating is a method of electroplating where the electroplating current is periodically reversed. Forward current pulse time can be 30 to 150% of the time required to put one layer of aluminum atoms onto the base metal. Reverse current pulse time can be 1.5 to 5% of the time of the forward current pulse time. Peak reverse current can be 50 to 200% of peak forward current. Periodic pulse reverse current can increase the effective current forward (effective current density) if the periodic pulse reverse current is optimized. If an electrolyte has 1 A/dm2 maximum current density with direct current, an optimized periodic pulse reverse current can have the following properties:
The optimized periodic pulse reverse plating produces plated base metal with equal to or better physical properties for micro hardness, purity of plated layer, evenness of the plating (throwing power), roughness and visual appearance than the optimized direct current plating.
Aluminum Electroplating Formulations
The inventive electrolyte formulations can include a solvent, a salt, an aluminum alkyl and optional enhancers or additions. The mole ratio of aluminum alkyl to salt may be 2:1.
Solvent
The solvent can be any aromatic or aliphatic hydrocarbon such as, for example, benzene, toluene, xylene, meta-xylene, cumene, diphenylmethane, para-isopropyl-methylbenzene, tetralin, ethylbenzene, anisole, dipropylether, diisoproplyether, dibutylether, tetrahydrofuran, and the like.
Salt for 2:1 Complex
The salt is formed by a cation and an anion. The cation may be an alkali metal, such as, for example, lithium, sodium, potassium, rubidium, caesium and the like or the cation may be a tetrammonium, and the like. The anion may be a halogen, such as, fluoride, chloride, bromide and the like or the anion may be an straight or branched, substituted or unsubstituted alkyl, such as, for example, methyl, ethyl, propyl, butyl, and the like. The anion may also be a hydride.
Al-alkyls for 2:1 Complex
Aluminum alkyls can be illustrated as R1AlR2 where R is hydrogen, halogen or a 1-8 carbon straight or branched chain alkyl, such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, pentyl, hexyl, heptyl, octyl, and the like and where R1 is a 1-8 carbon straight or branched chain alkyl, such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, pentyl, hexyl, heptyl, octyl, and the like.
Tri-propyl-aluminum (TPA) has a thermal stability that is higher than most other aluminum alkyls. The 2:1 TPA complexes have a high solubility and a high decomposition voltage, which makes TPA a useful component in electrolytes for aluminum plating. High plating temperatures, high concentrations and high maximum current densities can be used with formulations that include TPA.
TPA formulations can be expressed as:
CA•(nAl(C3H7)3(2−n)AlR3)
where n is greater than 0 and less than or equal to 2. C is a cation such as, Li, Na, K, Rb, Cs, NR′4, or mixtures thereof where R′ is a C1-C8 alkyl, e.g., CH3, C2H5, C3H7, C4H9, C5H11, C6H13, C7H15, C8H17, or mixtures thereof. A is an anion such as, H, F, Cl, Br, or mixtures thereof. R is H, halogen, or an alkyl such as, CH3, C2H5, C3H7, C4H9, C5H11, C6H13, C7H15, C8H17, or mixtures thereof. The TPA formulation can include a solvent such as, an aromatic hydrocarbon, aliphatic hydrocarbon, or mixtures thereof. TPA complex formulations can include solvent and optionally an addition.
When n is equal to 0 (no TPA present in complex), the electrolyte includes the 2:1 complex, a solvent and an addition. Additions are described below.
Aluminum and Magnesium/Aluminum Electroplating Formulations
The inventive electrolyte formulations can include a solvent, a salt, an aluminum alkyl and optional enhancers or additions. The mole ratio of aluminum alkyl to salt may be 1:1.
Solvent
The solvent can be any aromatic or aliphatic hydrocarbon such as, for example, benzene, toluene, xylene, meta-xylene, cumene, diphenylmethane, para-isopropyl-methylbenzene, tetralin, ethylbenzene, anisole, dipropylether, diisoproplyether, dibutylether, tetrahydrofuran, and the like.
Cation for 1:1 Complex
The cation may be an alkali metal, such as, for example, lithium, sodium, potassium, rubidium, caesium, magnesium and the like.
Anion for 1:1 Complex
The anion may be an aluminum alkyl. Aluminum alkyl anions can be illustrated as AlR64 where R6 is hydrogen or a 1-8 carbon straight or branched chain alkyl, such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, pentyl, hexyl, heptyl, octyl and the like.
Quad-propyl-aluminum (QPA) has a thermal stability that is higher than most other aluminum alkyls. The 1:1 QPA complexes have a high solubility and a high decomposition voltage, which makes QPA a useful component in electrolytes for aluminum and magnesium/aluminum plating. High plating temperatures, high concentrations and high maximum current densities can be used with formulations that include QPA.
QPA formulations can be expressed as:
C1•(nAl(C3H7)4(1−n)AlR74)
where n is from 0 to 1. C1 is a cation such as, Li, Na, K, Rb, Cs, or mixtures thereof. R7 is H or an alkyl such as, CH3, C2H5, C3H7, C4H5, C5H9, C6H11, C7H13, C8H17, or mixtures thereof. The QPA formulation can include a solvent such as, an aromatic hydrocarbon, aliphatic hydrocarbon, or mixtures thereof. QPA complex formulations can include solvent and optionally an addition.
QPA formulations can include the following:
C1•(AlR74) and
C1•(AlR73—H—AlR73)
or mixtures thereof. C1 is a cation such as, Li, Na, K, Rb, Cs, or mixtures thereof. R7 is H, or an alkyl such as, CH3, C2H5, C3H7, C4H9, C5H11, C6H13, C7H15, C8H17, or mixtures thereof.
When magnesium/aluminum formulations are used, these electrolytes plate aluminum/magnesium alloys onto conductive substrates or base metals using magnesium and aluminum anodes or anodes made out of magnesium/aluminum alloy with the same or similar composition as the desired plating material. Also, these magnesium/aluminum formulations are used to dummy plate with aluminum/magnesium alloy anodes or magnesium anodes to reach the concentration of magnesium alkyls in the electrolyte required for the magnesium/aluminum alloy plating.
Additions
Aluminum alkyls can be added to the aluminum and magnesium/aluminum formulations defined above including embodiments where n is 0 in an amount in excess of the 2 moles of aluminum alkyl to one mole of salt forming the 2:1 complex. Adding aluminum alkyls in excess can enhance the current density physical property of the formulation. Aluminum alkyls can be illustrated as R1AlR2 where R2 is hydrogen, halogen, or a 1-8 carbon straight or branched chain alkyl, such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, pentyl, hexyl, heptyl, octyl, and the like and R1 is a 1-8 carbon straight or branched chain alkyl, such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, pentyl, hexyl, heptyl, octyl, and the like.
Ethers may be added to the formulations, as with aluminum alkyls, to improve the throwing power physical property of the formulation. The ether can be aliphatic or aromatic. Aliphatic ether may include straight chain or cyclic ethers, for example, dimethylether, ethylene glycol diether, dioxane, tetrahydrofuran, and the like. Aromatic ethers can include, for example, anisole and the like.
Aluminum alkyls may react with trace amounts of oxygen and/or water to form aluminoxanes and/or alkoxy-compounds. These compounds may be provided in the formulation via electrolyte recycle streams. These compounds can enhance the physical properties of the formulation. Alkoxy-compounds can be illustrated as R5pAl(OR5)3-p where p is 0, 1 or 2 and R5 is a 1-8 carbon straight or branched chain alkyl, such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, pentyl, hexyl, heptyl, octyl and the like. Alkoxy-aluminum-alkyls may include R25Al—O—(C2H4—O—)q—Al—R25 or R25Al—O—(C2H4—O—)q—R5 or the like where q is 0, 1, 2, 3, or 4. Aluminoxanes can be illustrated as R42Al—(O—AlR42)m—OAlR42 or (R4AlO)n where m is an integer from 1-8, n is an integer from 3 to 8, and R4 is a 1-8 carbon straight or branched chain alkyl, such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, pentyl, hexyl, heptyl, octyl and the like. The compound of the formula (R4AlO)n is a ring structured compound; for example, when n is 3 the compound has the structure
Particular electrolyte formulations can provide significant advances in current density and throwing power. Some formulations are directed to enhancing throwing power, while others are directed to improving current density or solubility of the complex. Some formulations provide an attractive balance between throwing power and current density. Examples 1-4 illustrate useful formulations.
Electrolyte Recycling
Several improved electrolyte recycling procedures are outlined below. In the first procedure,
The blower system 401 removes volatile electrolyte impurities such as, for example, AlR2OC2H5 where R is previously defined. The blower system 401 provides electrolyte bath agitation for the plating process. The blower system 401 provides cooling to the electroplating bath to prevent overheating of the electrolyte during plating. The blower system 401 provides continuous recycling of rinse solvents such as toluene since the condensate may contain higher concentrations of rinse solvent versus the used outlet rinse bath solution. The outlet rinse bath is the rinse bath used after plating to rinse off electrolyte with the electrolyte solvent from the plated parts.
Humidity can be added to the blower system 401 at a humidity intake conduit 430. Humidity may be added to the system prior to opening up the system for maintenance. The liquid can be drained from the electroplating process 400, however pyrophoric material and combustible solvent may remain. Humidity forced through the blower system 401 can destroy this pyrophoric material and the solvent can be condensed and removed from the system 400 before air is admitted into the electroplating process during maintenance. Thus, the blower system 401 can provide an important safety function to the electroplating process 400.
Bulk Plating Device
Sufficient bath agitation results in a minimal diffusion layer around the cathode, which achieves the maximum local current density, for example of 1.3 A/dm2 and hence the maximum local plating rate of 0.64 mils per hour. To maximize plating rate, the bulk should be spread out to provide a large surface for aluminum nucleation. The bulk surface to bulk volume ratio should be high. The bulk surface should also maintain a uniform distance to the anode, so that the maximum local plating rate can be achieved over the largest possible area of the bulk surface, resulting in the highest average plating rate.
Bulk should be mixed carefully with the right tumbling technique to achieve low abrasion rates. If the parts spread out, only slight agitation is necessary to get evenly plated parts. Greater mixing results in better distribution of aluminum on the bulk or parts. Abrasion and mixing should be carefully balanced to get the required plating uniformity and bath agitation.
Bulk plating may be chosen over rack plating after consideration of several considerations:
Bulk plating can be accomplished with a shaker type bulk plating device 500 shown in
During plating operation, the container chain 510 deposits the bulk 550 into the shaker entrance 560 and collects the bulk 550 from the shaker exit 570 for return to the shaker entrance 560. The electrolyte level 565 may submerse the entire container chain 510 during the plating process. After the bulk 550 is plated, the container chain 510 operations change so that the container chain collects the bulk 550 from the shaker exit 570 and deposits the bulk 550 in the plated parts exit 580. Thus, the container chain 510 will not turn over at the shaker entrance 560 as shown in
The bulk shaker device 500 includes one or more cathodic contact areas 540 that may be a perforated phenolic sheet. The bulk shaker device 500 includes one or more anodes 530. The anodes 530 can be parallel with the cathodic contact areas 540 and may be a sheet. The cathodic contact areas 540 can be between the anodes 530. Screening 520 can be placed around the cathodic contact areas 540 and the anodes 530. The cathodic contact areas 540 and the anodes 530 can be mounted into a frame on wheels that is moving/shaking back and forth with an amplitude shown with arrow 545. The bulk 550 enters the shaker entrance 560 and “shakes” down the cathodic contact areas 540 that may be a perforated phenolic sheet; the bulk 550 may move from a first cathodic contact area 540a to a second cathodic contact areas 540b to a third cathodic contact areas 540c and so on to the shaker exit 570. While on the cathodic contact areas 540 the bulk is plated.
The plating speed of the bulk shaker device 500 may be three times higher than conventional barrel plating systems. The plating time can be 2.25 hours for an average aluminum plate thickness of 0.4 mils for a specific electrolyte with a maximum current density of 1.3 A/dm2. The amount of bulk can be 1000 lbs or more.
Any of the electrolytes described herein are made using known techniques. The first step is to mix the dry toluene and dry salt with the solvent. Stir and slowly add (under inert gas) the aluminum alkyls. These should be added in order of their reaction heat, with the compound having the lowest reaction heat being added first. For example, add the old electrolyte first, then the dialkylaluminum halogenide, then TPA, then TEA and then TMA. Heat the mixture to 100° C. and stir for 5 hours to finish the reaction. After that, the electrolyte is ready for either use or for storage. Note that if impure compounds are used, it may be necessary to perform some dummy plating prior to production in order to remove any impurities present in the electrolyte.
Table 1 shows electrolyte formulations (A through L) that provide a good balance between throwing power and current density. These formulations are useful for rack plating.
Table 2 shows electrolyte formulations (M through T) that provides good throwing power. These formulations are useful for barrel or bulk plating.
Table 3 shows electrolyte formulations (U through CC) that provides high current density. These formulations provide fast plating via high current density and are useful for continuous plating and the purification of aluminum.
In these formulations, an excess of aluminum alkyl provides higher current densities but also provides for a rougher precipitation (Al-layer) and lower throwing power. Anisol or other ethers; alkoxy compounds such as, for example; R25Al—O—(C2H4—O—)q—Al—R25 or R25Al—O—(C2H4—O—)q—R5 where q and R5 is previously defined; aluminoxanes; dialkylaluminum halogenide compounds can be used to combat these negative effects. These formulations result in a maximum current density that is increased by more than 100% compared with the values achieved with a specific electrolyte with a maximum current density of 1.3 A/dm2.
The next set of formulations describe a magnesium/aluminum electroplating electrolyte that provides high current density. It has been found that an improved electrolyte can be developed by replacing QEA as the main aluminum alkyl with another aluminum alkyl like QPA, QMA, QiBA or mixtures thereof. Preferred formulations (DD through MM) are provided in Table 4 below.
These formulations employ TPA, TIBA and/or TMA as the main compound, rather than TEA. We have found that this results in a higher throwing power and a higher current density.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US02/13832 | 4/30/2002 | WO | 00 | 10/28/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/088434 | 11/7/2002 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 2849349 | Ziegler et al. | Aug 1958 | A |
| 3234114 | Ziegler et al. | Feb 1966 | A |
| 4417954 | Birkle et al. | Nov 1983 | A |
| 4948475 | Doetzer et al. | Aug 1990 | A |
| 5007991 | Lehmkuhl et al. | Apr 1991 | A |
| 5091063 | Lehmkuhl et al. | Feb 1992 | A |
| 6207036 | de Vries | Mar 2001 | B1 |
| Number | Date | Country |
|---|---|---|
| 19855666 | Aug 2000 | DE |
| 0505886 | Sep 1992 | EP |
| WO 9823795 | Apr 1998 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20040140220 A1 | Jul 2004 | US |
