Lead-free tin-silver-copper alloy solder composition

A method for forming an electrical structure, comprising:

• providing a first substrate and a first solder ball attached to a first electrically conductive pad that is coupled to the first substrate, wherein the first solder ball comprises a solder alloy, wherein the alloy is substantially free of lead, wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy of at least about 90%, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%;
• providing a second substrate and a second electrically conductive pad coupled to the second substrate;
• coupling the first solder ball to the second pad;
• melting the first solder ball by heating the first solder ball to form a modified solder ball;
• and
• solidifying the modified solder ball by cooling the modified solder ball to a lower temperature at which the solid Sn phase is nucleated, and wherein the lower temperature corresponds to an undercooling δT relative to the eutectic melting temperature of the alloy, wherein the solidified modified solder ball is a solder joint that couples the first substrate to the second substrate, and wherein a silver weight percent concentration X2 in the modified solder ball is sufficiently small that formation of Ag3Sn plates is substantially suppressed during said cooling, and wherein said cooling the modified solder ball is at a cooling range of about 1.2° C./sec to about 3.0° C./sec.

A method for forming a solder composition, comprising:

• providing a solder alloy, wherein the alloy is substantially free of lead, wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy of at least about 90%, wherein the silver has a weight percent concentration in the alloy not exceeding about 4.0%, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%;
• melting the alloy by heating the alloy;
• and
• solidifying the melted alloy by cooling the melted alloy at a cooling rate in a range of about 1.2° C./sec to about 3.0° C./sec.

A method for forming an electrical structure, comprising:

• providing a first substrate and a first solder ball attached to a first electrically conductive pad that is coupled to the first substrate, wherein the first solder ball comprises a solder alloy, wherein the alloy is substantially free of lead, wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy of at least about 90%, wherein the silver has a weight percent concentration in the alloy not exceeding about 4.0%, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%;
• providing a second substrate and a second electrically conductive pad coupled to the second substrate;
• coupling the first solder ball to the second pad;
• melting the first solder ball by heating the first solder ball to form a modified solder ball;
• and
• solidifying the modified solder ball by cooling the modified solder ball at a cooling rate in a range or about 1.2° C./sec to about 3.0° C./sec.

A solder composition, comprising a solder alloy,

• wherein the alloy is substantially free of lead,
• wherein the alloy includes tin (Sn), silver (Ag), copper (Cu), and bismuth (Bi),
• wherein the tin has a weight percent concentration in the alloy of at least about 90%,
• wherein the silver has a weight percent concentration of X in the alloy,
• wherein X is sufficiently small that formation of Ag3Sn plates is substantially suppressed when the alloy in a liquefied state is being solidified by being cooled at to a lower temperature at which the solid Sn phase is nucleated,
• wherein the lower temperature corresponds to an undercooling δT relative to the eutectic melting temperature of the alloy,
• wherein the bismuth has a weight percent concentration in the alloy from 0.1% to about 0.2%, and
• wherein the copper has a weight percent concentration in the alloy not exceeding about 0.9%.

A method for forming a solder composition, comprising:

• providing a solder alloy, wherein the alloy is substantially free of lead, herein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy in a range of 90% to 95%, wherein the silver has a weight percent concentration in the alloy not exceeding about 4.0%, and wherein the copper has a weight percent concentration in the alloy not exceeding about 1.5%;
• melting the alloy by heating the alloy;
• and
• solidifying the melted alloy by cooling the melted alloy at a cooling rate that is high enough to substantially suppress Ag3Sn plate formation in the alloy during said cooling, wherein the cooling rate is in a range of about 1.2° C./sec to about 3.0° C./sec.

A method for forming a solder composition, comprising:

• providing a solder alloy, wherein the alloy is substantially free of lead wherein the alloy includes tin (Sn), silver (Ag), and copper (Cu), wherein the tin has a weight percent concentration in the alloy in a range 90% to 95%, wherein the silver has a weight percent concentration in the alloy not exceeding about 2.8%, and wherein the copper as a weight percent concentration in the alloy not exceeding about 1.5%;
• melting the alloy by heating the alloy;
• and
• solidifying the melted alloy by cooling the melted alloy at a cooling rate, wherein the cooling rate is at least 1.2° C./sec.