HK1084160A - Reduction of surface oxidation during electroplating - Google Patents

Description

Reduction of surface oxidation during electroplating

Background

The present invention relates to solutions and methods for reducing or minimizing surface oxidation of metal deposits provided by plating processes, such as electroplating. The solution and method also provide improved deposit properties including appearance and solderability.

Electroplated tin and tin alloy coatings are used in electronic and other applications such as wire and continue to be used in steel strip for many years. In electronics, they are used as solderable and corrosion-resistant surface finishes (finish) for contacts and connectors. They are also used in lead finishes for integrated circuit ("IC") fabrication. In addition, thin layers of tin or tin solder are used as the final step in passivating components such as capacitors and transistors.

Although the application will vary, there is some commonality as to the requirements for this final surface finish. One problem is long-term solderability, which is defined as the ability of the surface finish to melt and make a good solder connection with other components without compromising electrical or mechanical connection.

There are many factors that determine good solderability, the three most important factors being the extent of surface oxide formation, the content of co-deposited carbon, and the extent of intermetallic compound formation. The formation of surface oxides is a naturally occurring process because it is thermodynamically favorable. The rate of surface oxide formation depends on temperature and time. In other words, the higher the temperature and the longer the time, the thicker the surface oxide formed. In the case of electroplated tin or tin alloy coatings or deposits, the formation of surface oxides also depends on the surface morphology of the coating or deposit. When comparing pure tin to tin alloy coatings, for example, tin alloys typically form less or thinner surface oxides when all other conditions are the same.

The co-deposited carbon is determined by the plating chemistry one chooses to use. The glossy finish contains a higher carbon content than the matte finish. Matte finishes are generally rougher than bright finishes and provide increased surface area that results in the formation of more surface oxides than are typically formed with bright finishes. Thus plating equipment presents a compromise between the potential amount of surface oxide and the surface finish.

The formation of intermetallic compounds is a chemical reaction between the tin or tin alloy coating and the substrate. The rate of formation depends on temperature as well as time. Higher temperatures and longer times result in thicker intermetallic layers.

To improve or ensure the highest degree of solderability, it is important to 1) use a non-bright tin or tin alloy plating solution, 2) deposit sufficient layers of tin or tin alloys so that the formation of surface oxides or intermetallics does not consume the entire layer, and 3) prevent or minimize exposure of the tin plated surface to high temperatures for extended periods of time.

1) and 2) are relatively easy to achieve, but 3) is very difficult to achieve. The temperature and time of subsequent part processing after plating of the tin or tin alloy deposit is generally determined by the assembly specifications and existing manufacturing schemes and practices. For example, in the "two tone" leadframe (leadframe) technology, after tin or tin alloy plating, the entire package must undergo numerous process steps (i.e., such processing occurs over an extended period of time) that require multiple thermal drifts at temperatures as high as 175 ℃. More and/or thicker surface oxides will necessarily be formed and this in turn reduces the solderability of the tin or tin alloy deposit. In current processes, it is not possible to omit these additional steps, as the final assembly or assembly will not be complete.

It is therefore highly desirable to find ways to prevent or minimize the formation of surface oxides on such components. One known way is to introduce a conformal coating on the surface of a tin or tin alloy deposit. This technique can be generalized to two general types: one to apply a noble metal coating and the other to apply an organic coating. The first is not ideal for protecting tin or tin alloy deposits because it introduces an expensive, additional process step. The second is also undesirable because it necessarily introduces impurities onto other critical areas of the leadframe or electronic assembly due to the non-selective nature of the deposited organic coating. These impurities have proven to be detrimental to subsequent leadframe and I C assembly processes.

Thus, further methods to address this problem are needed, and the present invention now provides such methods.

Summary of The Invention

The present invention relates generally to methods of providing improved metal coatings or metal deposits on substrates and to articles of manufacture involving metal coated substrates.

The present invention relates to a method for improving the solderability of a metal coating on a substrate, the method comprising incorporating trace amounts of phosphorus in the metal coating to reduce the formation of surface oxides during subsequent heating and thereby improve the long term solderability of the metal coating. The phosphorus is advantageously provided within the metal coating by incorporating a phosphorus source into the solution used to provide the metal coating on the substrate so as to provide phosphorus from the solution with the metal coating on the substrate.

Preferably, the metal coating is a metal deposit provided by electroplating and a source of phosphorus is added to the solution of metal ions so that phosphorus can co-deposit with the metal during electroplating. The phosphorus source is typically a phosphorus compound that is soluble in the solution and provides ppm levels of phosphorus in the metal deposit. Typically, the metal deposit is formed by electroplating at a current density of no greater than about 2000 ASF.

Another embodiment of the invention relates to an electroplating solution for use in providing a metal deposit on a substrate. The solution incorporates a phosphorus source therein in an amount to provide trace amounts of phosphorus in the metal deposit to reduce surface oxide formation and thereby enhance long term solderability of the metal deposit. Phosphorus is typically present in the resulting metal deposit at detectable levels, but less than about 200 ppm. In some metal deposits it can also be much lower than this.

The invention also relates to articles comprising a metal coating on a substrate, wherein the metal coating includes trace amounts of phosphorus therein to reduce surface oxide formation and thereby improve long term weldability of the metal deposit. Preferably, the article is produced by electroplating.

The metal in the metal coating, metal deposit or article of the invention preferably comprises tin or a tin alloy, as these are typically used when soldering of the article is required for further manufacture. Deposits of nickel, cobalt, copper or alloys thereof are also desirable.

Detailed description of the preferred embodiments

The present invention achieves the importance of incorporating trace or ppm levels of phosphorus in metal or metal alloy deposits or plating coatings. This element significantly reduces surface oxidation of such coatings or deposits, thus improving long-term solderability. Since phosphorus can preferably be added to the metal coating or deposit by the same manufacturing steps used to deposit the metal, no further processing steps are required, nor are impurities introduced into the overall package.

The term "trace amount" is used to refer to the detectable content of an element, such as phosphorus, that is present in the metal deposit and that is used in an amount that provides a measurable improvement in the long-term solderability of the metal deposit.

The term "ppm level" means the amount of an element, such as phosphorus, present in the metal deposit in parts per million to provide a measurable improvement in the long term solderability of the metal deposit.

The trace or ppm level may vary over a wide range depending on the particular metal deposit. For example, in nickel deposits, amounts on the order of 200ppm or less are used, while for tin and tin alloys, amounts on the order of 50ppm or less are used.

The additive may be used in any metal deposit to be welded. This includes, in particular, tin, nickel, copper, cobalt, tungsten, zinc or one of their alloys. Welding is basically a joining process which typically includes three materials: (1) a substrate; (2) components or other devices that are desired to be attached to a substrate; and (3) the solder material itself. The solder material itself is typically tin or a tin alloy, but the substrate or component/device may be made of other materials. In the present invention, phosphorus is added to the metal deposit to improve the solderability properties of the substrate containing such deposit and/or the component/device to be attached thereto. The substrate or component/device material comprises an electroplatable material such as copper, steel or stainless steel. The present invention reduces surface oxidation of the substrate and/or device, which improves its ability to be soldered to a solder material. It also reduces the formation of intermetallic compounds used for this purpose. Tin and tin alloy deposits are preferred as metallic deposits because they independently function as a flux or can undergo reflow when heated above their relatively low melting temperature. However, the reduction in surface oxidation is useful for other metals cited because it is easier for the flux to adhere to those metals due to reduced interference with the oxidized surface. For example, when phosphorus is present in the nickel deposit, it may eliminate the need for further coating of tin, tin alloys, or precious metals.

Tin and tin alloys are known to have various plating chemistries that can produce various characteristics within the resulting plated deposit. These include matte, bright, and other (e.g., satin) appearances. These can be achieved by many known chemistries based on sulfonates, mixed acids, sulfates, halogens, fluoroborates, gluconates, citrates, and the like. Sulfonic acids, such as alkyl or hydroxyalkyl sulfonic acids (e.g., methanesulfonic acid), are preferred for environmental reasons. In addition, the skilled artisan will appreciate that these baths may contain various additives to promote or enhance plating performance. Examples of preferred chemistries include U.S. patents 6,251,253, 6,248,228, 6,183,619 and 6,179,185; the contents of each are expressly incorporated herein by reference. These patents also disclose plating solutions and methods for metals other than tin.

According to the present invention, the plating solution can be modified by adding a small amount of a phosphorus source. The phosphorus source can be an organic or inorganic phosphorus compound that is at least partially and preferably highly or fully soluble in the plating solution. Various alkali or alkaline earth metal phosphites or phosphates may be used, with hypophosphites being preferred. Hypophosphorous acid and pyrophosphite (pyrophosphide) may be used as necessary. These compounds can be used in a wide range of concentrations, and the skilled artisan can perform routine experimentation to determine the optimum concentration for any particular bath formulation. It has been found that phosphorus compounds of between 0.5 and 15g/l and preferably about 1 to 10g/l are suitable for most conventional baths. The examples show that for some compounds in the tin or tin alloy bath, the preferred concentration range is between 1 and 5 g/l.

It has been found that phosphorus can be deposited under a wide range of electroplating conditions, depending on the particular metal to be plated. Generally, a current density of less than about 2000ASF is used. Depending on the particular plating equipment, current densities of less than 1000ASF, less than 500ASF, or even between 25 and 150ASF may be used. At higher current densities, the metal deposit deposits more rapidly, with the result that lower amounts of phosphorus are found in the deposit. The bath formulator should add a sufficient amount of the phosphorus source so that the amount of phosphorus in the deposit is detectable. One way is to increase the level of phosphorus source in the bath, but this is not preferred as it can affect bath stability for other performance criteria. In contrast, it is easier to control the current density to the desired range mentioned above, since a small amount of phosphorus source can be used without affecting or significantly impacting the overall bath chemistry.

The substrate to be plated can vary within wide limits. Of course, common metal substrates such as copper, steel or stainless steel are typically used, but the invention is also operable on composite substrates comprising conductive and non-conductive or electroplatable and non-electroplatable portions. This provides the plater with many manufacturing options with different types of parts or articles of the phosphorus-containing deposits of the present invention.

The resulting plated products can be used in many different applications in electronics, wire coating, steel plating, tin plates, and other areas where improved solderability of reflow properties is desired. It has been found that the incorporation of phosphorus in the deposit assists in significantly reducing surface oxidation in deposits having a matte or glossy finish. As mentioned, this results in improved weldability properties.

Examples

The following examples are used to illustrate preferred embodiments and methods of the present invention.

Example 1

The following electroplating solutions were prepared for obtaining satin/matte tin deposits:

45g/l tin as stannous sulfate (stannous sulfate)

80g/l sulfuric acid

15g/l sodium thiosulfate (isothionate)

5g/l surfactant

20ppm grain refiner

A phosphorus source: NaH₂PO₂

The balance of water

Example 2

The following electroplating solutions were prepared for obtaining satin/matte tin-lead deposits:

63g/l tin as stannous sulfate

7g/l lead as lead methanesulfonate

100g/l methanesulfonic acid

15g/l sodium thiosulfate

5g/l surfactant

20ppm grain refiner

A phosphorus source: NaH₂PO₂

The balance of water

Example 3

The following electroplating solutions were prepared for obtaining bright tin deposits:

50g/l tin as stannous sulfate

80g/l sulfuric acid

15g/l sodium thiosulfate

3g/l surfactant

5g/l brightener

A phosphorus source: NaH₂PO₂

The balance of water

Example 4

The following electroplating solutions were prepared for obtaining bright tin-lead deposits:

50g/l tin as stannous sulfate

5g/l lead as lead methanesulfonate

100g/l methanesulfonic acid

15g/l sodium thiosulfate

3.5% surfactant

1.5% brightener

A phosphorus source: NaH₂PO₂

The balance of water

Example 5

The solutions of examples 1-4 were plated on a Hull cell plate under the following plating conditions.

Hull cell plating: copper and steel Hull cell panels with paddle agitation for 1 minute at 110 ℃ F

Lead frame plating: 75 ASF: copper alloys and stainless steel substrates. Two sets of samples were plated: control and P-containing samples. Control samples were obtained without addition of a phosphorus source (NaH)₂PO₂) Each bath of (1). In these examples, found advantageous NaH₂PO₂The concentration is between 1 and 5 g/l.

And (3) measuring the content of P: a wet process was used in which the deposit was dissolved in nitric acid and the phosphorus content was measured using ICP detection techniques. The results show that the phosphorus content in each sample ranged from 1 to 7 ppm. In addition, reduced surface oxidation is encountered.

Weldability: the measure of weldability was determined according to IPC/JEDEC Industry Standard J-STD-002A using the Dip and Look, welding Balance and Surface Mount Solderabilitytest Method.

Examples 6 to 9

The following tests were conducted and show that ppm levels of phosphorus in the metal deposits of examples 1-4 provide unexpectedly improved results in terms of improved solderability, reduced surface oxidation.

The deposits provided by the baths of examples 1 to 4 were baked at 175 ℃ for 7 hours. Strips of stainless steel and copper Hull cell panels were placed in an oven maintained at this temperature and periodic checks were made to see if any surface discoloration occurred. The presence of yellow surface deterioration would indicate surface oxidation.

Example 6

For tin deposits produced by the bath of example 1, the stainless steel and copper plates, the control strip, i.e., the strip with the deposit having no added phosphorus, showed discoloration after 5 hours, and was even worse when the plating current density was below 100 ASF.

Stainless steel strips with deposits containing phosphorus did not discolor throughout the Hull cell panel under the same conditions. Furthermore, these strips did not change color after 7 hours. The copper Hull cell plates with the phosphorus-containing deposits showed a somewhat yellowish color below 100ASF, but they appeared significantly better than the control.

The solderability test was performed after 7 hours of baking, and the results were as follows:

control substance: samples plated at 50, 100 and 150 failed

Samples with phosphorus-containing deposits: all are qualified

Example 7

For the tin-lead deposit of example 2, both the control and the phosphorus-containing deposit showed no discoloration after baking, indicating that surface oxidation can be further reduced with the tin alloy deposit.

All samples passed the solderability test, but the samples containing the phosphorous deposit showed an improvement over the control.

Example 8

For the bright tin deposits of example 3, all samples (both controls and those with phosphorus-containing deposits) did not discolor after 7 hours of baking. These deposits were subjected to reflow conditions and the results indicated that the control color changed to yellow after reflow, while the samples with the phosphorus-containing deposits did not show any difference.

Example 9

For the bright tin-lead deposits of example 4, all samples (both controls and those with phosphorus-containing deposits) did not discolor after 7 hours of baking. These deposits were subjected to reflow conditions and the results indicated that the control color changed to yellow after reflow, while the samples with the phosphorus-containing deposits did not show any difference.

Claims (20)

1. A method of improving the solderability of a metal coating on a substrate, the method comprising incorporating trace amounts of phosphorus in the metal coating to reduce the formation of surface oxides during subsequent heating and thereby improve the long term solderability of the metal coating.

2. The method of claim 1, wherein the metal deposit comprises one of nickel, cobalt, copper, tungsten, zinc, tin, or alloys thereof.

3. The method of claim 1 wherein phosphorus is present in the metal deposit in an amount detectable but less than about 200 ppm.

4. The method of claim 1, wherein the phosphorus is provided in the metal coating by incorporating a phosphorus source into a solution used to provide the metal coating on the substrate, and providing the metal coating from the solution on the substrate.

5. The method of claim 4, wherein the phosphorus source is a phosphorus compound that is soluble in solution and provides ppm levels of phosphorus in the metal deposit.

6. A method according to claim 5, wherein the metal coating is a metal deposit provided by electroplating, and the source of phosphorus is added to the solution of metal ions so that phosphorus can co-deposit with the metal during electroplating.

7. The method of claim 6 wherein the metal deposit is produced by electroplating at a current density of no greater than about 2000 ASF.

8. In a plating solution for use in providing a metal deposit on a substrate, the improvement which comprises incorporating a source of phosphorus in the solution in an amount to provide trace amounts of phosphorus in the metal deposit to reduce the formation of surface oxides during subsequent heating of the deposit to enhance the long term solderability of the metal deposit.

9. The solution of claim 8 wherein the phosphorus source is a phosphorus compound that is soluble in the solution and provides ppm levels of phosphorus in the metal deposit.

10. The solution of claim 8 wherein phosphorus is present in the metal deposit in an amount detectable but less than about 200 ppm.

11. The solution of claim 8 wherein the metal deposit comprises one of nickel, cobalt, copper, tungsten, zinc, tin, or alloys thereof.

12. The solution of claim 8 wherein the metal deposit is produced by electroplating at a current density of no greater than about 2000 ASF.

13. An article comprising a metal coating on a substrate, wherein the metal coating comprises trace amounts of phosphorus therein and is produced by the method of claim 1.

14. An article comprising a metal coating on a substrate, wherein the metal coating comprises trace amounts of phosphorus therein and is produced by the method of claim 6.

15. An article comprising a metal coating on a substrate, wherein the metal coating includes trace amounts of phosphorus therein to reduce the formation of surface oxides and thereby improve the long-term solderability of the metal deposit.

16. The article of claim 15, wherein the substrate comprises a metal and the metal coating comprises one of nickel, cobalt, copper, tungsten, zinc, tin, or an alloy thereof.

17. An electroplated article comprising a metal deposit on a substrate, wherein the metal deposit comprises trace amounts of phosphorus therein and is produced by the method of claim 13.

18. The article of claim 17, wherein the substrate comprises a metal and the metal coating comprises tin or a tin alloy.

19. An electroplated article comprising a metal deposit on a substrate, wherein the metal deposit includes trace amounts of phosphorus therein to reduce the formation of surface oxides and thereby enhance the long-term solderability of the metal deposit.

20. The article of claim 19, wherein the substrate comprises a metal and the metal coating comprises tin or a tin alloy.