US20070007144A1

US20070007144A1 - Tin electrodeposits having properties or characteristics that minimize tin whisker growth

Info

Publication number: US20070007144A1
Application number: US11/473,413
Authority: US
Inventors: Robert Schetty
Original assignee: Technic Inc
Current assignee: Technic Inc
Priority date: 2005-07-11
Filing date: 2006-06-23
Publication date: 2007-01-11
Also published as: CN101243210A; JP2009500527A; WO2007008369A1; EP1904669A1; TW200712266A; KR20080024525A

Abstract

A method of reducing tin whisker formation by creating a tin deposit which is inherently less prone to tin whisker formation or growth facilitated by oxide presence or corrosion reactions on the tin deposit surface. This is obtained by one or more of: (i) the deposition of a fine-grained tin deposit having an average grain diameter in the range of 0.05 to 5 microns; (ii) a phosphorous compound in the solution that is used to electroplate the tin deposit so that that the deposit incorporates trace amounts of phosphorous which in turn reduces tin whisker formation by preventing surface oxides even when exposed to heat or humidity; or (iii) a phosphorous compound, mercaptan, or organic or organo-metallic compound in a solution that applies a protective coating to the surface of a previously electroplated tin deposit, wherein the protective coating acts to minimize or prevent oxide formation or corrosion of the tin deposit during exposure to heat or humidity. Such tin deposits containing 80% to 100% by weight of tin exhibit minimal to no tin whisker growth.

Description

This application claims the benefit of U.S. provisional application No. 60/698,550 filed Jul. 11, 2005, the entire content of which is expressly incorporated herein by reference thereto.

FIELD OF INVENTION

The present invention relates to a method for depositing tin in a manner to reduce, minimize or prevent tin whisker growth from such deposits, as well as to electroplated components formed by such a method. More particularly, the invention relates to a modification or treatment of the deposit to render it less prone to whisker growth. In particular, prevention of oxide formation and/or corrosion reactions on the tin deposit surface or post treatment of the deposit surface has been found to be effective for this purpose.

BACKGROUND OF THE INVENTION

The use of a tin or tin alloy electroplated deposit has become increasingly important in fabricating electronic circuits, electronic devices and electrical connectors because of the benefits that such deposits provide. For example, tin and tin alloy deposits protect the components from corrosion, provide a chemically stable surface for soldering and maintain good surface electrical contact. There are many patents that disclose how to apply tin or tin alloy deposits using a variety of plating solutions and methods. Such deposits are typically produced by electroless plating or electroplating.
Regardless of the deposition process employed, it is desirable to form smooth and level deposits of tin on the substrate in order to minimize porosity. It is also desirable to form a coating having a relatively constant thickness in order to facilitate downstream component assembly operations. Furthermore, other problems must be avoided in order to obtain an acceptable deposit. When pure tin is used and is applied to a copper or copper alloy substrate, the resulting deposit suffers from interdiffusion of base material copper into the tin deposit and subsequent formation of copper-tin intermetallic compounds. While these copper-tin compounds can be brittle and may impair the usefulness of the tin coated component, their presence also results in compressive stress formation in the tin deposit. Subsequently, the generation of metal filaments known as tin whiskers sometimes grow spontaneously from these tin deposits. These whiskers are hair-like projections extending from the surface and may be either straight or curled or bent. Tin whiskers typically have a diameter of about 6 nanometers to 6 microns. The presence of such whiskers is undesirable due to the very fine line definition required for modern circuitry, since these whiskers can form both electrical shorts and electrical bridges across insulation spaces between conductors. The whiskers may create shorts or introduce failures into electronic circuitry.
The mechanism of tin whisker growth is not fully understood. The whiskers can begin to grow within days of the application of the coating or even several years thereafter. There is speculation in the literature that the whiskers grow from compressive stress concentration sites, such as those created through many electrodeposition techniques and/or storage conditions. There is evidence that elevated temperature and humidity storage conditions enhance whisker growth. The article “Simultaneous Growth of Whiskers on Tin Coatings: 20 Years of Observation,” by S. C. Britton, Transactions of the Institute of Metal Finishing, Volume 52, 1974, pp. 95-102 discusses the tin whisker growth problem and offers several recommendations for reducing the risk of whisker formation.
One approach for addressing the tin whisker problem has been to specify short storage times for tin plated materials. However, this approach does not fully address or necessarily avoid the problem. Another approach has been to mildly strengthen the tin matrix to prevent extrusion of the whiskers. The formation of an intermetallic compound and diffusion of copper into the tin deposit have served this purpose but at prohibitive performance cost in the final product.
Another approach is to treat the surface of the substrate before applying the tin deposit. Ultrasonic agitation of the plating solution and/or alternating the polarity of the electrodes during plating have been suggested to reduce the amount of hydrogen absorbed or occluded in the structure of the plating metal.
Additional approaches for dealing with this problem have generally involved a whisker inhibiting element addition to the tin plating solution. In order to avoid the high cost of precious metals, the most common approach has been to deposit an alloy of tin and lead. This alloy is also compatible with the solders that are later used to make electrical connections to wires or other electrical components. Unfortunately, lead and a number of other alloying elements are undesirable due to their toxicity and related environmental issues. Thus, pure tin or very high tin content deposits are now used, and these are subject to whiskering under certain conditions. This is particularly significant problem for small electronic parts that are provided with a tin deposit, as short circuits can result.
B.-Z. Lee and D. N. Lee in their article “Spontaneous Growth Mechanism of Tin Whiskers” appearing in Acta mater. Vol. 46, No. 10, 1998, pages 3701-3714, first elucidated the concept that the driving force for tin whisker growth is compressive stress formed by intermetallic compound formation at the tin deposit/copper substrate interface. It has subsequently been theorized that this increase in compressive stress is due to diffusion of copper from the base material into the tin deposit and the subsequent formation of copper-tin intermetallic compounds; the accompanying volume transformation which occurs in turn generates the compressive stress that results in tin whisker formation. Several methods have been developed to address tin whisker growth occurring from this origin. Schetty et al. in U.S. Pat. No. 6,860,981 describe a method to deposit tin of a preferred crystal orientation which matches the substrate which in turn minimizes tin whisker growth. Schetty in a U.S. non-provisional Patent Application No. 11/***,*** filed Jun. 23, 2006 (claiming the benefit of provisional Patent Application No. 60/693,701 filed Jun. 24, 2005) describes a method using silver as a barrier layer to minimize copper-tin intermetallic compound formation, while Egli et al. in U.S. patent application Ser. No. 2002/0187364 A1 describe a method using thin barrier layer of nickel or cobalt to minimize tin whisker growth.
In May 2005, the electronics industry standard setting organization JEDEC published JEDEC STANDARD JESD22A121 “Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes”, which describes three test methods for testing tin whisker growth, two of which use elevated heat and/or humidity. It has recently been determined that another driving force for tin whisker growth occurs when certain corrosion reactions and/or tin oxide formation takes place on the surface of electroplated tin deposits which are subjected to elevated heat and/or humidity conditions over extended time periods.
For example, three papers were presented on this subject at the iNEMI Tin Whisker Workshop Jun. 1, 2005 in Orlando, Fla., USA. “Humidity Effects on Sn Whisker Formation” by Marc Dittes et al. of Infineon; “A Statistical Study of Sn Whisker Population and Growth during Elevated Temperature/Humidity Storage Test” by Dr. Peng Su of Freescale Semiconductor; and “Sn Corrosion and its Influence on Whiskers” by J. Osenbach of Agere all describe the mechanism for this recently discovered new mechanism of tin whisker growth. The following is a summary of these papers.
Specifically, tin when exposed to heat & humidity converts to tin oxide which in turn can result in localized compressive stress (due to localized volume increase) and/or a corrosion reaction due to water condensation in association with exposed base material forming a galvanic couple which induces compressive stress which becomes the driving force for tin whisker growth, as shown in the diagrams of the Dittes article.
It would be highly desirable to identify a method or methods to counter-act the series of events described in the aforementioned paragraph. This problem is currently un-resolved in the industry today since most companies dealing in tin whisker growth phenomenon have not considered this newly discovered driving force for tin whisker growth or how to combat it. It would also be beneficial to identify a method to minimize and/or prevent tin oxidation and/or corrosion reactions from occurring on the tin deposit during exposure to high heat and humidity conditions. The present invention now provides such methods.

SUMMARY OF THE INVENTION

The invention relates to a number of methods for reducing tin whisker formation or growth in tin deposits on a substrate. Generally, at least one physical property or characteristic of a tin deposit is modified during deposition of the same on a substrate or immediately thereafter. Thus, the tin deposit is provided on the substrate with at least one physical property or characteristic that renders it less prone to tin whisker formation or growth so that tin whiskering is substantially reduced or is even prevented.
In one embodiment, the tin deposit can be provided on the substrate as a fine grain structure which minimizes tin whiskering, where the tin deposit preferably has an average grain size of about 0.05 to 5 and preferably less than about 2 to 3 microns.
In another embodiment, the tin deposit can be treated during deposition to render it less prone to tin whisker formation or growth. This can be done by rendering the tin deposit essentially free of surface oxides to minimize tin whiskering thereon. The prevention of surface oxide formation is conveniently achieved by providing the tin deposit from a solution containing a phosphorous compound, thereby incorporating trace or small amounts of phosphorous in the tin deposit. Preferably, the tin plating solution contains sufficient phosphorous compound to provide an amount of about 0.1 ppm to 30% by weight phosphorus in the tin deposit.
In another embodiment of the method, the tin deposit is treated after deposition to render it less prone to tin whisker formation or growth. One way to achieve this is to apply a protective coating upon the tin deposit (i.e., after the tin plating is completed). To do this, the tin deposit can be immersed in or contacted by a post-treatment solution containing one or more of a phosphorous compound, an organic compound, a mercaptan, or an organo-metallic compound. Conveniently, the protective coating is applied at a thickness of about 0.1 angstroms to 1 micron.
The invention also relates to an improvement in a tin plated electronic component that includes a copper surface upon which a fine grained tin deposit is present. The improvement comprises minimizing or preventing tin whisker formation or growth by providing the tin deposit with one or more of (a) a fine grained structure so that the tin deposit is less prone to tin whisker formation or growth compared to tin deposits having larger grain structures; (b) trace or small amounts of phosphorous so that the tin deposit is essentially free of surface oxidation and is less prone to tin whisker formation or growth compared to tin deposits having surface oxidation; or (c) a protective layer so that it is less prone to tin whisker formation or growth compared to tin deposits having no protective layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to creating a tin deposit which is inherently less prone to oxide formation and/or corrosion reactions on the tin deposit surface through use of one or more methods for reducing whisker formation in tin deposits on a substrate. One method comprises creating a tin deposit which is inherently less prone to oxide formation and/or corrosion reactions on the tin deposit surface through use of: (i) the deposition of a “fine-grained” tin deposit with an average grain diameter in the range of 0.05 to 5 microns, such a grain diameter resulting in a tin deposit which is inherently less prone to surface oxidation; (ii) the use of a phosphorous compound in a solution that is used to deposit the tin deposit on a substrate which thereby incorporates trace amounts of phosphorous in the tin deposit which in turn reduces tin oxide formation on the surface during exposure to heat and/or humidity; and/or (iii) the use of a phosphorous compound, organic, mercaptan, and/or organo-metallic compound(s) in a solution(s) that is(are) used to apply a protective coating to the surface of a previously electroplated tin deposit, said protective coating acting to minimize or prevent oxide formation and/or corrosion of the tin deposit during exposure to heat and/or humidity; all of which alone or in combination in turn minimize tin whisker growth of the tin deposit.
The invention also relates to a plated substrate that includes a tin deposit with reduced surface oxidation and/or corrosion characteristics in the layer comprising tin. The layer comprising tin preferably includes no less than 80% tin by weight and preferably much higher amounts of tin without experiencing significant whiskering. Even deposits of essentially pure tin (i.e., tin with no intentionally added alloying elements and which contains no other elements except for incidental impurities) can be deposited without experiencing significant whiskering problems. Thus, the substrate can be an electronic component that includes non-electroplatable portions and electroplatable portions which are to be plated. In a preferred embodiment, the tin content of the deposit is greater than 80% to 100% by weight of the deposit. If desired, alloying elements can be added to the solution used to deposit tin to provide a deposit of a tin alloy.
The tin plating solutions that are useful in the present invention include, but are not limited to those described below:
FLUOBORATE SOLUTIONS: Tin fluoborate plating baths are widely used for plating all types of metal substrates including both copper and iron. See for example, U.S. Pat. Nos. 5,431,805, 4,029,556 and 3,770,599. These baths are preferred where plating speed is important and the fluoborate salts are very soluble.
HALIDE SOLUTIONS: Tin plating baths with the main electrolyte being a halide ion (Br, Cl, F, I) have been used for many decades. See for example, U.S. Pat. Nos. 5,628,893 and 5,538,617. The primary halide ions in these baths have been chloride and fluoride.
SULFATE SOLUTIONS: Tin and tin alloys are commercially plated from solutions with sulfate as the primary anion. See for example U.S. Pat. Nos. 4,347,107, 4,331,518 and 3,616,306. For example the steel industry has been tin plating steel for many years from sulfuric acid/tin sulfate baths where phenol sulfonic acid is used as a special electrolyte additive which improves both the oxidative stability of the tin as well as increasing its current density range. This process, known as the ferrostan process, is usable in the present invention but is not preferred because of environmental problems with phenol derivatives. Other sulfate baths based on sulfuric acid but without environmentally undesirable additives are preferred.
SULFONIC ACID SOLUTIONS: In the last two decades the commercial use of sulfonic acid metal plating baths has increased considerably because of a number of performance advantages. Tin has been electroplated from sulfonic acid (see for example U.S. Pat. Nos. 6,132,348, 4,701,244 and 4,459,185. The cost of the alkyl sulfonic acid is relatively high, so that the preferred sulfonic acid used has been methane sulfonic acid (MSA) although the prior art includes examples of other alkyl and alkanol sulfonic acids. The performance advantages of alkyl sulfonic acid baths include low corrosivity, high solubility of salts, good conductivity, good oxidative stability of tin salts and complete biodegradability.
These solutions can be used alone or in various mixtures. One of ordinary skill in the art can best select the most preferred acid or acid mixture for any particular plating application.
The amount of tin (as tin metal) in the plating solutions of the present invention may be varied over a wide range such as from about 1 to about 120 grams of metal per liter of solution (g/l), or up to the solubility limit of the particular tin salt in the particular solution. It should be understood that the foregoing quantities of tin in the plating solution are disclosed as metallic tin, but that the tin may be added to the solutions in the form of tin compounds. Such compounds may include, for example, tin oxide, tin salts, or other soluble tin compounds, including formates, acetates, sulfates, alkane sulfonates, hydrochlorides and other halides, carbonates and the like.
Various surfactant and wetting agents can be used as desired. The surfactants are selected to achieve the desired deposit qualities and characteristics. The examples illustrate commercially available baths that include preferred surfactants and/or wetting agents to provide the desired tin deposits according to the invention. Any of the surfactants mentioned in U.S. Pat. No. 6,860,981 can be used, so that patent is incorporated herein by reference thereto for its disclosure of such components. Additionally, any of the generally tin or tin-lead surfactants known in the prior art (e.g., block copolymers, alkylene oxides, polyalkylene glycols, ENSA, nonyl-phenol ethoxlyate 8-14 moles ethylene oxide, etc.) can all be used but when a fine grained deposit is desired, a secondary grain refiner is generally used in combination with the surfactant(s).
Any one of a number of alloying elements can be added to the tin plating solution. These are primarily added in an amount such that less than 5% of the alloying element is present in the deposit. Preferred alloying elements include silver (up to 3.5% of the deposit), bismuth (up to 3% of the deposit), copper (up to 3% of the deposit) and zinc (up to 2% of the deposit). While other alloying elements can be used, it is generally not preferred to use those that may have an adverse effect on the environment, i.e., antimony, cadmium, and particularly lead. Preferably, as noted above, alloying elements are optional, and the tin content of the deposit is as high as possible, usually on the order of as high as 99% by weight or more with the balance being unavoidable impurities.
In the first method of the invention, tin whisker growth is prevented or at least substantially minimized by depositing a fine-grained tin deposit with an average grain diameter of 0.05 to 5 microns. A more preferred grain diameter is 1 to 3 microns. Any tin electroplating solution which can create an average grain diameter in the aforementioned range can be used but as noted above those described in U.S. Pat. No. 6,860,981 are preferred. As noted above, a secondary grain refiner is used to assure that a fine grain structure is obtained in the deposit. The additives biquinoline and dimethyl-phenanthroline in particular are preferred secondary grain refiners which reduce the grain size although other materials known by those skilled in the art could also be used to achieve the same result.
In order to confirm that the desired grain size is achieved, it is conventionally measured with a scanning electron microscope (SEM) at 2000× and 5000× magnification, with a photo taken that has a scale on it to measure the grain diameter of multiple grains of the tin deposit. An average of the measurements is taken to determine grain size. What differentiates the present invention from the prior art is the fact that SEM measurement is used to confirm that the grain size is within the desired range. While it may be possible that this type of fine grain structure can occur in some of the prior art baths, that would occur purely by coincidence and in an unintentional manner. In this invention, the presence of the fine grained structure is confirmed to obtain the desired reduction or elimination of tin whiskering.
Tin deposits of varying grain sizes were exposed to high heat & humidity conditions to purposely enhance tin oxide formation and these were subsequently analyzed by Surface Electrochemical Reduction Analysis (“SERA”). The fine-grained tin deposits (average grain diameter 0.05 to 3 microns) consistently produced lower thickness of tin oxide on the surface compared to the conventional, larger grain diameter tin deposits (average grain diameter of above 3 to 8 microns). It is theorized that this phenomenon is due to the fact that the fine-grained tin deposits have a generally smoother surface structure and therefore less exposed microscopic “peaks” and “valleys” compared to the large generally coarse-grained tin deposits and this leads to a smaller exposed surface area for oxidation/corrosion reactions to occur on the fine grained tin deposits compared to the large grained tin deposits. It is also believed that the multiple grain boundaries of the fine grained structure act as a stress distribution or stress relieving network to prevent compressive stress on the deposit from concentrating and then generating tin whiskers.
In another method of the invention, the use of a phosphorous compound in a solution that is used to deposit the tin deposit on a substrate which thereby incorporates trace amounts of phosphorous in the tin deposit in turn reduces tin oxide formation on the surface during exposure to heat and/or humidity. Preferred solutions and amounts of phosphorus to add are disclosed by Zhang et al. in U.S. Pat. No. 6,982,030, a patent which discusses the use of such additives in the context of improving deposit solderability. With the new knowledge recently gained in the industry about the effects of tin oxide formation on tin whisker growth after exposure to high heat and humidity conditions, it has now been found that the method originally disclosed in that patent can be used to provide benefits in minimizing tin whisker growth as well as in the reduction of surface oxidation when deposits are provided under the conditions previously described in that patent.
In yet another method of this invention, the use of one or more of a phosphorous compound, organic, and/or organo-metallic compound in a solution is used to apply a protective coating to the surface of a previously electroplated tin deposit, with the protective coating acting to minimize or prevent oxide formation and/or corrosion of the tin deposit during exposure to heat and/or humidity. The products Tarniban, Tarniban 51, Tarniban E260 all available from Technic, Inc., Cranston, R.I., are examples of solutions that contain such additives; additionally the examples section that follows identifies other useful specific compounds for this purpose. This “post-treatment” process applies a thin film (0.5 angstroms to 0.5 microns thickness) to the tin deposit surface which effectively blocks and thereby minimizes negative reactions from occurring such as oxidation and/or corrosion, both of which are now known to be the driving force for tin whiskers during high heat and humidity exposure.

EXAMPLES

The following examples illustrate the most preferred embodiments of the invention.

Example 1 (Comparative)

Tin was electroplated from an MSA electrolyte (“Solderon ST300” from Rohm & Haas) onto a Cu alloy substrate (Cu99.85%, Sn0.15%) at a current density of 150 A/ft²for a period of time sufficient to obtain an average of 10 μm tin deposit thickness. The grain size of the tin deposit was measured and found to be 5-8 microns average grain diameter. The deposit was subjected to high temperature and humidity (HTH) testing of 155 deg C. for 16 hrs (in an uncontrolled humidity environment) followed by 97 deg C./99% relative humidity (RH) conditions for 8 hrs. This deposit was measured by SERA and the tin oxide thickness on the surface was found to be 122 angstroms. This deposit was subjected to the high temperature & humidity whisker test condition specified by JEDEC STANDARD JESD22A121 “Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes”, specifically: high temperature/humidity storage of 60° C./90% RH for 3000 hrs. Upon completion of the whisker test method, the maximum whisker length was measured and was determined to be 112 μm.

Example 2

Tin was electroplated from a mixed acid sulfate electrolyte (“Technistan EP” from Technic Inc) onto a Cu alloy substrate (Cu99.85%, Sn0.15%) at a current density of 150 A/ft²for a period of time sufficient to obtain an average of 10 μm tin deposit thickness. The grain size of the tin deposit was measured and found to be 1-2 microns avg. grain diameter. The deposit was subjected to high temperature & humidity (HTH) testing of 155 deg C. for 16 hrs (in an uncontrolled humidity environment) followed by 97 deg C./99% relative humidity (RH) conditions for 8 hrs. This deposit was measured by SERA and the tin oxide thickness on the surface was found to be 68 angstroms. This deposit was subjected to the high heat & humidity whisker test condition specified by JEDEC STANDARD JESD22A121 “Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes”, specifically: high temperature/humidity storage of 60° C./90% RH for 3000 hrs. Upon completion of the whisker test method, the maximum whisker length was measured and was determined to be 55 μm.

Example 3

Tin was electroplated from a mixed acid sulfate electrolyte (“Technistan EP” from Technic Inc. which also contained a phosphorous compound at a concentration of 4 g/l in the plating solution as described in U.S. patent application Ser. No. 2004/0099340 A1) onto a Cu alloy substrate (Cu99.85%, Sn0.15%) at a current density of 150 A/ft²for a period of time sufficient to obtain an average of 10 μm tin deposit thickness. The grain size of the tin deposit was measured and found to be 1-2 microns avg. grain diameter. The deposit was subjected to high temperature & humidity (HTH) testing of 155 deg C. for 16 hrs (in an uncontrolled humidity environment) followed by 97 deg C./99% relative humidity (RH) conditions for 8 hrs. This deposit was measured by SERA and the tin oxide thickness on the surface was found to be 43 angstroms. This deposit was subjected to the high heat & humidity whisker test condition specified by JEDEC STANDARD JESD22A121 “Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes”, specifically: high temperature/humidity storage of 60° C./90% RH for 3000 hrs. Upon completion of the whisker test method, the maximum whisker length was measured and was determined to be 43 μm.

Example 4

Tin was electroplated from a mixed acid sulfate electrolyte (“Technistan EP” from Technic Inc. which also contained a phosphorous compound at a concentration of 4 g/l in the plating solution as described in U.S. patent application Ser. No. 2004/0099340 A1) onto a Cu alloy substrate (Cu99.85%, Sn0.15%) at a current density of 150 A/ft²for a period of time sufficient to obtain an average of 10 μm tin deposit thickness. After tin plating the substrate was placed into a solution containing a phosphorous compound (phosphoric acid@70 ml/l)+sodium gluconate at 50 g/l. The grain size of the tin deposit was measured and found to be 1-2 microns avg. grain diameter. The deposit was subjected to high temperature & humidity (HTH) testing of 155 deg C. for 16 hrs (in an uncontrolled humidity environment) followed by 97 deg C./99% relative humidity (RH) conditions for 8 hrs. This deposit was measured by SERA and the tin oxide thickness on the surface was found to be 35 angstroms. This deposit was subjected to the high heat & humidity whisker test condition specified by JEDEC STANDARD JESD22A121 “Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes”, specifically: high temperature/humidity storage of 60° C./90% RH for 3000 hrs. Upon completion of the whisker test method, the maximum whisker length was measured and was determined to be 38 μm.

Example 5

Tin was electroplated from a mixed acid sulfate electrolyte (“Technistan EP” from Technic Inc.) onto a Cu alloy substrate (Cu99.85%, Sn0.15%) at a current density of 150 A/ft²for a period of time sufficient to obtain an average of 10 μm tin deposit thickness. After tin plating the substrate was placed into a solution containing 10 ml/l solvent (butyl cellosolve), 10 ml/l surfactant (Jeffox WL 4000), and 4 g/l mercaptopropionic acid at 40 deg C. for 30 sec. The grain size of the tin deposit was measured and found to be 1-2 microns avg. grain diameter. This deposit was subjected to the high heat & humidity whisker test condition specified by JEDEC STANDARD JESD22A121 “Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes”, specifically: high temperature/humidity storage of 60° C./90% RH for 3000 hrs. Upon completion of the whisker test method, the maximum whisker length was measured and was determined to be 33 μm.
These examples illustrate the following advantages of the invention. Example 1 shows the results from a standard large-grained tin deposit which is commonly used in the industry and it demonstrates that (i) tin oxide formation is very high after heat & humidity exposure at 122 angstroms and (ii) the corresponding tin whisker growth following the JEDEC procedures is excessive at 122 microns; although no industry standard exists yet with regard to maximum acceptable whisker length it is commonly considered to be 50 microns. Example 2 shows the results from the current invention comprising a fine-grained tin deposit which demonstrates that (i) tin oxide formation is significantly reduced vs. the conventional large grained deposit from example 1 above after heat & humidity exposure at 68 angstroms and (ii) the corresponding tin whisker growth following the JEDEC procedures is significantly reduced vs. the conventional large grained deposit from example 1 at 55 microns, evidence that the corrosion/oxidation protection enhancement effect of the current invention is extremely effective in its ability to minimize tin whisker growth.
Example 3 shows the results from the current invention comprising a fine-grained tin deposit combined with the phosphorous-containing additive in the tin plating solution which demonstrates that (i) tin oxide formation is significantly reduced vs. the conventional large grained deposit from example 1 and further reduced vs. the deposit in example 2 above after heat & humidity exposure at 43 angstroms and (ii) the corresponding tin whisker growth following the JEDEC procedures is significantly reduced at 43 microns, evidence that the corrosion/oxidation protection enhancement effect of the current invention is extremely effective in its ability to minimize tin whisker growth.
Example 4 shows the results from the current invention comprising a fine-grained tin deposit combined with the phosphorous-containing post-treatment solution which demonstrates that (i) tin oxide formation is significantly reduced vs. the conventional large grained deposit from example 1 and further reduced vs. the deposit in Examples 2 & 3 above after heat & humidity exposure at 35 angstroms and (ii) the corresponding tin whisker growth following the JEDEC procedures is significantly reduced at 38 microns, evidence that the corrosion/oxidation protection enhancement effect of the current invention is extremely effective in its ability to minimize tin whisker growth.
Example 5 shows the results from the current invention comprising a fine-grained tin deposit combined with the organic-containing post-treatment solution which demonstrates that the corresponding tin whisker growth following the JEDEC procedures is significantly reduced at 33 microns, evidence that the corrosion/oxidation protection enhancement effect of the current invention is extremely effective in its ability to minimize tin whisker growth.

Claims

1. A method of reducing tin whisker formation in a tin deposit on a substrate, which comprises electroplating a tin deposit on an electroplatable substrate, and modifying at least one physical property or characteristic of the tin deposit sufficiently to the render the deposit less prone to tin whisker growth over time or to prevent tin whisker formation, wherein the modifying occurs during or after deposition of the tin deposit on the substrate.

2. The method of claim 1, wherein the physical property or characteristic is modified when the tin deposit is electroplated on the substrate to render it less prone to or to prevent tin whisker formation or growth in the deposit.

3. The method of claim 2, wherein the physical property or characteristic that is modified is its grain structure, wherein the tin deposit is provided with a fine grain structure.

4. The method of claim 3, wherein the tin deposit is provided with an average grain size of about 0.05 to 5 microns.

5. The method of claim 2, wherein the physical property or characteristic that is modified is its surface oxidation, wherein the tin deposit is essentially free of surface oxides when deposited.

6. The method of claim 5, wherein surface oxide formation on the tin deposit is reduced or prevented by depositing the tin deposit from a solution containing a phosphorous compound, thereby incorporating trace or small amounts of phosphorous in the tin deposit which resist surface oxide formation even when the deposit is exposed to heat or humidity.

7. The method of claim 6, wherein the solution contains sufficient phosphorous compound to provide an amount of about 0.1 ppm to 30% by weight phosphorus in the tin deposit.

8. The method of claim 1, wherein the tin deposit is treated after deposition to render it less prone to tin whisker growth.

9. The method of claim 8, wherein the tin deposit is treated by applying a protective coating thereon.

10. The method of claim 9, wherein the tin deposit is immersed in or contacted by a post-treatment solution containing one or more of a phosphorous compound, a mercaptan compound, or an organic or organo-metallic compound to apply the protective coating thereon.

11. The method of claim 1, wherein the protective coating is applied at a thickness of about 0.1 angstroms to 1 micron.

12. The method of claim 1, wherein the tin deposit includes at least 80% to 100% tin by weight and the substrate is an electrical component.

13. The method of claim 12, wherein the substrate is an electronic component that include electroplatable portions and non-electroplatable portions and the tin deposit is provided upon the electroplatable portions.

14. The method of claim 1, wherein the substrate comprises copper.

15. In a tin plated electronic component that includes a copper surface upon which a fine grained tin deposit is present, the improvement which comprises minimizing or preventing tin whisker formation or growth by providing the tin deposit with one or more of:

(a) a fine grained structure so that the tin deposit is less prone to tin whisker formation or growth compared to tin deposits having larger grain structures;

(b) trace or small amounts of phosphorous so that the tin deposit is essentially free of surface oxidation and is less prone to tin whisker formation or growth compared to tin deposits having surface oxidation; or

(c) a protective layer so that it is less prone to tin whisker formation or growth compared to tin deposits having no protective layer.

16. The tin plated electronic component of claim 15, wherein the tin deposit is provided with a fine grained structure having an average grain size of about 0.05 to 5 microns so that the tin deposit is less prone to tin whisker formation or growth compared to tin deposits having larger grain structures.

17. The tin plated electronic component of claim 15, wherein the tin deposit is provided with trace or small amounts of phosphorous so that the tin deposit is essentially free of surface oxidation even when the deposit is exposed to heat or humidity and is less prone to tin whisker formation or growth than tin deposits having surface oxidation.

18. The tin plated electrical component of claim 17, wherein the phosphorus is present in the tin deposit in an amount of about 0.1 ppm to 30% by weight.

19. The tin plated electronic component of claim 15, wherein the tin deposit is provided with a protective layer so that it is less prone to tin whisker formation or growth compared to tin deposits having no protective layer, wherein the protective layer is provided by immersion in or contact by a post-treatment solution containing one or more of a phosphorous compound, a mercaptan compound, or an organic or organo-metallic compound.

20. The tin plated electronic component of claim 19, wherein the protective coating is present at a thickness of about 0.1 angstroms to 1 micron.