Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Reexamination Certificate
1999-02-08
2001-04-03
Mayekar, Kishor (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
C106S001230, C106S001250
Reexamination Certificate
active
06210556
ABSTRACT:
BACKGROUND OF INVENTION
Tin-lead alloy electroplating processes have been in use for many years, and eutectic tin-lead is used extensively in applications requiring attachment of electronic components to printed circuit boards by soldering or reflowing. During assembly, sufficient heat is applied to melt the eutectic tin-lead deposit, and upon cooling, a metallurgical bond between the component and circuit board is formed.
The temperature at which the tin-lead alloy melts is very important. Eutectic tin-lead contains 63% tin and 37% lead, and melts at a temperature of 183° C. which is very well matched to the prevalent materials of construction in use today for circuit boards. At liquidus temperatures much higher than this, dimensional instability of the circuit board laminate may result. At liquidus temperatures much lower than this, the alloy may melt prematurely during prior thermal operations of the assembly process.
The electronics industry are continuously looking for alternatives to lead, as the toxic properties of this material are well known and future use may become restricted. The challenge to the industry has been finding suitable replacements for tin-lead alloy solders that possess the same or similar properties. Once found, the challenge to the electrochemist is to develop an electroplating process capable of codepositing the alloying metals in just the right proportion to impart the necessary properties to the electrodeposit.
Solid alloys of eutectic tin-silver have been used as effective solders for years, most notably in the plumbing industry where the use of lead containing solders has been prohibited because of the potential for lead leaching into the drinking water supply. Eutectic tin-silver contains 96.5% tin and 3.5% silver, and becomes liquidus at a temperature of 221° C., however, as the silver content of the alloy increases slightly, the liquidus temperature increases dramatically. One hundred percent tin melts at 232° C. Increasing the silver content to 10% raises the liquidus temperature to 300° C., a temperature which is too high for common printed circuit board materials to withstand. Applying a temperature of less than 300° C. to a tin-silver alloy containing 10% silver would not provide sufficient heat to fully melt the alloy, resulting in incomplete and inadequate formation of the solder joint. In attempting to electroplate eutectic tin-silver alloys, critical control of the silver content of the deposit is therefore essential.
W. Flühmann, et. al., “PROPERTIES AND APPLICATION OF ELECTRODEPOSITED CU-Zn-SN ALLOYS, Am. Elect. Soc. 69
th
Ann. Tech. Conf. Proceedings, Vol. 2, 1982, describes a silver-tin deposit containing 90% silver and 10% tin produced from a pyrophosphate electrolyte for decorative applications, claiming a reduction in the tendency for silver to tarnish when alloyed with small amounts of tin. The wear resistance of such alloys is reportedly improved. H. Leidheiser, Jr., et. al., PULSE ELECTROPLATING OF SILVER-TIN ALLOYS AND THE FORMATION OF Ag
3
Sn, J. Electrochem Soc., April 1973, pp. 484-487 describes a silver-tin electrolyte using tin stannate in place of pyrophosphate with pulsed current to improve the deposit quality. A number of other references disclose electrolytes for silver-tin alloys, including U.S. Pat. No. 5,514,261 and DE patent application 4,330,068. Here silver is the primary ingredient and the tin is present in much smaller amounts.
The electrodeposition of tin rich alloys of tin-silver is difficult given the large difference in reduction potential between the two metals. Furthermore, the preferential reduction of tin is made more difficult by the fact that silver exists in solution as a monovalent ion, whereas tin is either divalent or tetravalent and thereby requires two or four times the amount of current for reduction to occur relative to silver. In addition, an appreciable amount of silver should be present in solution to allow for the practical operation of the electrolyte on a production scale.
SUMMARY OF INVENTION
The invention relates to an electrolyte for depositing tin-rich alloys of tin-silver upon a substrate. This electrolyte comprises a basis solution containing a solution soluble tin compound and a solution soluble silver compound; a tin chelating agent of a polyhydroxy compound in an amount sufficient to complex tin ions; and a silver chelating agent of a heterocyclic compound in an amount sufficient to complex silver ions. Generally, the tin compound is a tin salt, the silver compound is a silver salt and the salts are present in relative amounts to enable deposits containing 85 to 99% by weight tin and 0.5 to 15% by weight silver to be obtained. To achieve this deposit, the tin concentration is typically about 20 to 60 g/l and the silver concentration is typically about 4 to 8 g/l.
The tin chelating agent is an alcohol or an alkali metal or ammonium salt of an acid. The agent has the general formula
R—(CHOH)
x
—R
where each R is the same or different and each is —H, —(CH
2
)
y
—OZ, —(CH
2
)
y
—C(O)OZ, —(CH
2
)
y
—CHO, or —(CH
2
)
y
—CH
3
where x is 1 to 6, y is 0 to 4 and Z is —H, —NH
4
or an alkali or alkaline earth metal. Preferably, the tin chelating agent is a polyhydric alcohol, an aldonic or aldaric acid, or a salt of those acids, and is present in an amount between about 10 and 500 g/l.
The silver chelating agent is a heterocyclic ring compound containing 3 to 7 carbon atoms and 1 to 3 nitrogen atoms in the ring. Preferably, the silver chelating agent is an imide and is present in an amount of between about 5 and 150 g/l.
The electrolyte generally has a pH of about 6 to 11 and a temperature of about 75 to 160° F. during use. Also, a grain refining agent, such as an alkoxylate or gelatin, can be included in an amount sufficient to enhance the properties of the resulting deposit.
The invention also relates to a method for depositing a tin-rich tin-silver deposit on a substrate which comprises contacting the substrate with the electrolyte described above and passing a current through the electrolyte to deposit the desired alloy on the substrate.
DETAILED DESCRIPTION OF THE INVENTION
The present electrolyte is used for depositing tin rich tin-silver alloys upon a substrate. By “tin-rich” what is meant is more than 50% and preferably more than 80% by weight tin relative to the amount of silver in the resulting deposit.
The electrolyte comprises a basis solution containing a solution soluble tin compound, a solution soluble silver compound, a chelating or complexing agent for tin and a chelating or complexing agent for silver. Preferably, the electrolyte produces tin-silver deposits containing 0.5 to 15% by weight silver, more preferably 2 to 5% by weight silver and most preferably 3 to 5% by weight silver.
Any solution soluble tin compound can be used, with salts of halides or acids being typical. Preferred tin compounds include salts such as tin sulfate, tin chloride and tin methane sulfonate.
Any solution soluble silver compound can be used, with salts of halides or acids being typical. Preferred silver compounds include salts such as silver nitrate and silver methane sulfonate.
The tin concentration of the electrolyte typically varies from about 20 to 60 g/l, and preferably from about 30 to 55 g/l. The silver concentration of the electrolyte typically varies from about 3 to 9 g/l, and preferably about 4 to 8 g/l.
Suitable chelating agents for tin ions include polyhydroxy compounds such as polyhydroxy alcohols, polyhydric acids or the alkali or ammonium salts of those acids. Preferred chelating agents have the general formula:
R—(CHOH)
x
—R
where each R is the same or different and each is —H, —(CH
2
)
y
—OZ, —(CH
2
)
y
—C(O)OZ, —(CH
2
)
y
—CHO, or —(CH
2
)
y
—CH
3
where x is 1 to 6, y is 0 to 4 and Z is —H, —NH
4
or an alkali or alkaline earth metal. Advantageously, these agents have at least four carbon atoms and at least two hydroxy groups. Preferred agents include polyhydric alcohols such as threitol, xylitol, and sorbitol; aldonic acid salts such as gluconates, glucoheptonates,
Brown Neil D.
Doyle Colleen A.
Marcktell Daniel C.
Toben Michael P.
Lea-Ronal, Inc.
Mayekar Kishor
Pennie & Edmonds LLP
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