Pb-Free Sn-Ag-Cu-Al or Sn-Cu-Al Solder

Abstract

A solder alloy includes Sn, optional Ag, Cu, and Al wherein the alloy composition is controlled to provide a strong, impact-and thermal aging-resistant solder joint that has beneficial microstructural features and is substantially devoid of Ag 3 Sn blades.

1 . An alloy consisting essentially of about 3 to about 4 weight % Ag, about 0.7 to about 1.7 weight % Cu, about 0.01 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         2 . The alloy of  claim 1  having a solidus temperature of about 217° C. 
     
     
         3 . The alloy of  claim 2  having a liquid plus solid temperature range less than about 5 degrees C. 
     
     
         4 . The alloy of  claim 1  having tin dendrites, a ternary eutectic between the dendrites, and pro-eutectic particles adjacent and/or within the tin dendrites in an as-solidified microstructure. 
     
     
         5 . The alloy of  claim 4  wherein the pro-eutectic particles comprise Cu 6 Sn 5 . 
     
     
         6 . The alloy of  claim 4  wherein the alloy is as-solidified at less than 5° C./second. 
     
     
         7 . The alloy of  claim 1  having an Al content selected to form particles in the solder that comprise Cu and Al. 
     
     
         8 . The alloy of  claim 7  wherein the particles comprises substantially Cu 33 Al 17 . 
     
     
         9 . A solder alloy consisting essentially of about 3.4 to about 3.6 weight % Ag, about 0.8 to about 1.1 weight % Cu, about 0.03 to about 0.20 weight % Al, and balance consisting essentially of Sn. 
     
     
         10 . A solder alloy consisting essentially of about 3.45 to about 3.55 weight % Ag, about 0.75 to about 1.0 weight % Cu, about 0.04 to about 0.15 weight % Al, and balance consisting essentially of Sn. 
     
     
         11 . A solder alloy consisting essentially of about 3 to about 4 weight % Ag, 0.95-y weight % Cu, and y weight % Al and balance consisting essentially of Sn wherein y is about 0.01 to about 0.25 weight %. 
     
     
         12 . A solder alloy comprising Sn, Ag, Cu, and Al and having a solidus temperature of about 217° C. and a narrow liquid-solid mushy zone with a liquidus temperature not exceeding about 5° C. above the solidus temperature. 
     
     
         13 . A solder alloy for BGA applications, consisting essentially of about 3.45 to about 3.55 weight % Ag, about 0.80 to about 1.0 weight % Cu, about 0.10 to about 0.20 weight % Al, and balance consisting essentially of Sn. 
     
     
         14 . A solder alloy consisting essentially of about 0.7 to about 3.5 weight % Cu, about 0.01 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         15 . The solder alloy of  claim 11  consisting essentially of about 0.8 to about 3.2 weight % Cu, about 0.03 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         16 . The alloy of  claim 11  consisting essentially of about 0.95 to about 3.0 weight % Cu, about 0.15 to about 0.20 weight % Al, and balance consisting essentially of Sn. 
     
     
         17 . A solder alloy consisting essentially of about 3.20-y weight % Cu, and y weight % Al and balance consisting essentially of Sn wherein y is about 0.15 to about 0.25 weight %. 
     
     
         18 . A solder joint comprisng a Sn—Ag—Cu—Al solder alloy soldified in contact with an electrical conductor wherein the solder joint has an as-solidified microstructure that comprises tin dendrties, ternary eutectic between tin dendrites, and pro-eutectic Cu 6 Sn 5  particles adjacent and/or within the tin dendrites and that is substantially devoid of Ag 3 Sn blades. 
     
     
         19 . The joint of  claim 18  having an interfacial layer comprising Cu 6 Sn 5  and an adjacent metastable intermediate rejected solute region as a zone of intermediate hardness between the interfacial layer and the solder tin matrix. 
     
     
         20 . The joint of  claim 18  wherein the solder alloy consists essentially of about 3 to about 4 weight % Ag, about 0.7 to about 1.7 weight % Cu, about 0.01 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         21 . The joint of  claim 20  wherein the solder alloy consists essentially of about 3.4 to about 3.6 weight % Ag, about 0.8 to about 1.1 weight % Cu, about 0.03 to about 0.20 weight % Al, and balance consisting essentially of Sn. 
     
     
         22 . The joint of  claim 21  wherein the solder alloy consists essentially of about 3.45 to about 3.55 weight % Ag, about 0.75 to about 1.0 weight % Cu, about 0.04 to about 0.15 weight % Al, and balance consisting essentially of Sn. 
     
     
         23 . The joint of  claim 18  formed on an electrical wiring board. 
     
     
         24 . The joint of  claim 18  formed about copper electrical conductors. 
     
     
         25 . The joint of  claim 18  having particles in and/or adjacent a solder joint interface layer wherein the particles comprise Cu and Al. 
     
     
         26 . The joint of  claim 25  wherein the particles comprises substantially Cu 33 Al 17 . 
     
     
         27 . A thermally-aged solder joint comprising a Sn—Ag—Cu—Al solder alloy in contact with an electrical conductor wherein the solder joint has an interfacial layer thickness that is about the same as the thickness of the interfacial layer in the as-solidified solder before thermal aging. 
     
     
         28 . The joint of  claim 27  wherein the interfacial layer thickness is no more than 30% greater than the thickness of the interfacial layer in the as-solidified solder before thermal aging. 
     
     
         29 . The joint of  claim 27  wherein the solder alloy consists essentially of about 3 to about 4 weight % Ag, about 0.7 to about 1.7 weight % Cu, about 0.01 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         30 . A solder joint comprisng a Sn—Ag—Cu—Al solder alloy soldified in contact with an electrical conductor wherein the solder joint has an as-solidified microstructure that comprises an interfacial layer comprising Cu 6 Sn 5  and an adjacent metastable intermediate Al-containing rejected solute region between the interfacial layer and the solder tin matrix, wherein the hardness of the Al-containing rejected solute region is intermediate the hardness of the interfacial layer and the solder tin matrix. 
     
     
         31 . The joint of  claim 30  formed on an electrical wiring board. 
     
     
         32 . The joint of  claim 30  formed about copper electrical conductors. 
     
     
         33 . The joint of  claim 30  wherein the solder alloy consists essentially of about 3 to about 4 weight % Ag, about 0.7 to about 1.7 weight % Cu, about 0.01 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         34 . The joint of  claim 33  wherein the solder alloy consists essentially of about 3.4 to about 3.6 weight % Ag, about 0.8 to about 1.1 weight % Cu, and about 0.03 to about 0.20 weight % Al, and balance consisting essentially of Sn. 
     
     
         35 . The joint of  claim 34  wherein the solder alloy consists essentially of about 3.45 to about 3.55 weight % Ag, about 0.75 to about 1.0 weight % Cu, and about 0.04 to about 0.15 weight % Al, and balance consisting essentially of Sn. 
     
     
         36 . The joint of  claim 30  having particles in and/or adjacent the solder joint interfacial layer wherein the particles comprise Cu and Al. 
     
     
         37 . The joint of  claim 36  wherein the particles comprises substantially Cu 33 Al 17 . 
     
     
         38 . In a soldering process, the step of solidifying a molten Pb-free solder consisting essentially of about 3 to about 4 weight % Ag, about 0.7 to about 1.7 weight % Cu, and about 0.01 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         39 . The process of  claim 38  wherein the solder is solidified on an electrical wiring board. 
     
     
         40 . The process of  claim 38  wherein the solder is solidified about copper electrical conductors. 
     
     
         41 . The process of  claim 38  including forming buoyant particles comprising Cu and Al in the molten solder. 
     
     
         42 . In a solder paste reflow or BGA solder process, the step of solidifying a molten Pb-free solder consisting essentially of about 3 to about 4 weight % Ag, about 0.7 to about 1.7 weight % Cu, and about 0.01 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         43 . The process of  claim 42  wherein the solder is cooled at a rate to form an as-solidified microstructure that comprises tin dendrites, ternary eutectic between the tin dendrites, and pro-eutectic Cu 6 Sn 5  particles adjacent and/or within the tin dendrites and that is substantially devoid of Ag 3 Sn blades. 
     
     
         44 . The process of  claim 42  that forms an interfacial layer comprising Cu 6 Sn 5  and an adjacent metalstable intermediate rejected solute region as a zone of intermediate hardness between the interfacial layer and the solder tin matrix. 
     
     
         45 . The process of  claim 42  wherein the solder is solidified on an electrical wiring board. 
     
     
         46 . The process of  claim 42  wherein the solder is solidified about copper electrical conductors. 
     
     
         47 . The process of  claim 42  wherein the solder alloy consists essentially of about 3.45 to about 3.55 weight % Ag, about 0.80 to about 1.0 weight % Cu, about 0.10 to about 0.20 weight % Al, and balance consisting essentially of Sn. 
     
     
         48 . A solder ball comprising an alloy consisting essentially of about 3 to about 4 weight % Ag, about 0.7 to about 1.7 weight % Cu, about 0.01 to about 0.25 weight % Al, and balance consisting essentially of Sn. 
     
     
         49 . The solder ball of  claim 48  having a liquid plus solid temperature range less than. about 5 degrees C. 
     
     
         50 . The solder ball of  claim 48  wherein the alloy consists essentially of about 3.4 to about 3.6 weight % Ag, about 0.8 to about 1.1 weight % Cu, about 0.03 to about 0.20 weight % Al, and balance consisting essentially of Sn. 
     
     
         51 . The solder ball of  claim 50  wherein the solder alloy consists essentially of about 3.45 to about 3.55 weight % Ag, about 0.75 to about 1.0 weight % Cu, about 0.04 to about 0.15 weight % Al, and balance consisting essentially of Sn. 
     
     
         52 . The solder ball of  claim 50  wherein the solder alloy consists essentially of about 3.45 to about 3.55 weight % Ag, about 0.80 to about 1.0 weight % Cu, about 0.10 to about 0.20 weight % Al, and balance consisting essentially of Sn.