Low temperature-wetting tin-base solder paste

Claim 2

A low temperature-wetting tin-base solder paste comprising a mixture of a first and a second solder powder, said first powder being formed of a predominantly tin metal having a first melting temperature, said second powder being formed of a tin alloy containing a metal selected from the group consisting of indium and bismuth and having a melting temperature less than the first melting temperature.

Claim 3

A paste in accordance with claim 1 wherein the first powder is substantially lead-free and is composed of at least about 90 weight percent tin.

Claim 4

A paste in accordance with claim 1 wherein the second powder is formed of a lead-free alloy comprising between about 40 and 60 weight percent indium and the balance tin.

Claim 5

A paste in accordance with claim 1 wherein the mixture comprises between about 10 and 20 weight percent of the second powder.

Claim 6

A low temperature-wetting tin-base solder paste comprising a liquid vehicle and a mixture of solder powders suspended in the vehicle, said mixture comprising a major portion of a first lead-free tin-rich solder powder and a minor portion of a second tin alloy solder powder, said first powder being formed of a metal containing at least about 90 weight percent tin and having a solidus temperature, said second metal powder being formed of a lead-free tin alloy containing a metal selected from the group consisting of indium and bismuth and having a liquidus temperature less than the first powder solidus temperature.

Claim 7

A low temperature-wetting tin-base solder paste comprising a liquid vehicle and a mixture of solder powders suspended in the vehicle, said mixture comprising a major portion of a first lead-free tin-rich solder powder and a minor portion of a second tin alloy solder powder, said first powder being formed of a metal containing at least about 90 weight percent tin and having a solidus temperature, said second metal powder being formed of a lead-free tin alloy containing a metal selected from the group consisting of indium and bismuth and having a liquidus temperature less than the first powder solidus temperature.

Claim 8

A paste in accordance with claim 5 wherein first tin alloy consist of up to 5 weight percent silver and the balance tin.

Claim 9

A paste in accordance with claim 5 wherein the second alloy consists of about 40 to 60 weight percent indium and the balance tin.

Claim 10

A paste in accordance with claim 5 wherein vehicle includes a rosin flux compound.

Claim 11

A paste in accordance with claim 5 wherein mixture comprises at least 80 weight percent of the first powder.

Claim 12

A paste in accordance with claim 5 wherein mixture contains not more than 8 weight percent indium and bismuth.

Claim 13

A soldering process comprising applying a tin-base solder paste to a faying surface, said paste comprising a mixture of a first and a second solder powder, said first powder being formed of a predominantly tin metal and having a first melting temperature, said second powder being formed of a tin alloy containing a metal selected from the group consisting of indium and bismuth and having a second melting temperature less than said first melting temperature, heating the paste to a temperature above the second melting temperature to melt the second powder to form a liquid phase that initiates wetting of the faying surface, and further heating the paste to a relatively higher temperature sufficient to melt the mixture, whereupon the first powder dissolves into the liquid phase, and cooling the liquid phase in contact with the faying surface to form a solid solder body bonded to the faying surface.

Claim 14

A soldering process comprising applying a tin-base solder paste to a faying surface, said paste comprising a mixture of a first and a second solder powder, said first powder being formed of a predominantly tin metal and having a first melting temperature, said second powder being formed of a tin alloy containing a metal selected from the group consisting of indium and bismuth and having a second melting temperature less than said first melting temperature, heating the paste to a temperature above the second melting temperature to melt the second powder to form a liquid phase that initiates wetting of the faying surface, and further heating the paste to a relatively higher temperature sufficient to melt the mixture, whereupon the first powder dissolves into the liquid phase, and cooling the liquid phase in contact with the faying surface to form a solid solder body bonded to the faying surface.

Claim 15

A process in accordance with claim 11 wherein the first powder is formed of a lead-free alloy composed of at least about 90 weight percent tin.

Claim 16

A process in accordance with claim 11 wherein the second powder is formed of a lead-free alloy composed of about 40 to 60 weight percent indium and the balance tin.

Claim 17

A process in accordance with claim 11 wherein the powders are dispersed in a liquid vehicle comprising a rosin flux compound.

Claim 18

A process in accordance with claim 11 wherein the mixture comprises between about 10 and 20 weight percent of the second powder.

Claim 19

A lead-free soldering process comprising applying a lead-free tin-base solder paste to a first copper faying surface, said paste comprising a liquid vehicle containing a rosin flux and a mixture of a high melting tin-base powder and a low melting solder powder dispersed in the vehicle, said high melting powder being composed of at least about 90 weight percent tin and having a first melting temperature, said second metal powder being formed of a tin alloy consisting essentially of between about 40 and 60 weight percent indium and the balance tin and having a second melting temperature less than said first melting temperature, arranging a second copper faying surface in contact with the paste applied to the first surface to form an assembly, heating the assembly to a temperature above the second melting temperature to activate the flux and to melt the low melting powder to form a liquid phase that wets the faying surfaces, further heating the assembly to a relatively higher temperature and for a time effective to complete melting of the first powder and to form a liquid phase having a uniform composition, and cooling the assembly to solidify the liquid phase to form a solid solder connection that bonds the faying surfaces into an integral structure.

Claim 20

A lead-free soldering process comprising applying a lead-free tin-base solder paste to a first copper faying surface, said paste comprising a liquid vehicle containing a rosin flux and a mixture of a high melting tin-base powder and a low melting solder powder dispersed in the vehicle, said high melting powder being composed of at least about 90 weight percent tin and having a first melting temperature, said second metal powder being formed of a tin alloy consisting essentially of between about 40 and 60 weight percent indium and the balance tin and having a second melting temperature less than said first melting temperature, arranging a second copper faying surface in contact with the paste applied to the first surface to form an assembly, heating the assembly to a temperature above the second melting temperature to activate the flux and to melt the low melting powder to form a liquid phase that wets the faying surfaces, further heating the assembly to a relatively higher temperature and for a time effective to complete melting of the first powder and to form a liquid phase having a uniform composition, and cooling the assembly to solidify the liquid phase to form a solid solder connection that bonds the faying surfaces into an integral structure.

Claim 21

A process in accordance with claim 16 wherein the mixture comprises between about 10 and 20 weight percent of the second powder and wherein the solder connection comprises not greater than 8 weight percent indium.