Method of laser soldering

Claim 1

A method of soldering one or more electrical lead strands to a printed electrical path, comprising the steps of: (a) planting said path on a ceramic substrate and attaching at least one solder pad to a portion of said printed electrical path; (b) forcing a surface portion of said electrical lead strand into full interengagement with said pad with an engaging force sufficient to permit heat conduction therebetween for soldering, said portion of said electrical lead strand and pad constituting a soldering assembly; (c) directing a defocused laser beam onto said soldering assembly, and a beam-on time effective to exert a controlled thermal radius on said soldering assembly to reflow only a preselected portion of said pad and effect a solder joint between said pad and said strand portion, said joint having a strength of at least 400 grams; (d) the parameters of beam power, beam spot diameter, and beam on time-are optimally correlated by the following equation: ##EQU7## the critical thermal radius a is the Gaussian radius at 1/e 2 , ln is logarithm, Tm is the melting temperature of the solder minus the specimen temperature, P is the laser beam power it watts, A is the surface absorptivity of the solder at 10.6 microns, R is the thermal resistance per unit area of the system, tc is the ciritical time to bring the solder to the Tm temperature, c is the heat capacity of the system.

Claim 2

A method of soldering one or more electrical lead strands to a printed electrical path, comprising the steps of: (a) planting said path on a ceramic substrate and attaching at least one solder pad to a portion of said printed electrical path; (b) forcing a surface portion of said electrical lead strand into full interengagement with said pad with an engaging force sufficient to permit heat conduction therebetween for soldering, said portion of said electrical lead strand and pad constituting a soldering assembly; (c) directing a defocused laser beam onto said soldering assembly, and a beam-on time effective to exert a controlled thermal radius on said soldering assembly to reflow only a preselected portion of said pad and effect a solder joint between said pad and said strand portion, said joint having a strength of at least 400 grams; (d) the parameters of beam power, beam spot diameter, and beam on time-are optimally correlated by the following equation: ##EQU7## the critical thermal radius a is the Gaussian radius at 1/e 2 , ln is logarithm, Tm is the melting temperature of the solder minus the specimen temperature, P is the laser beam power it watts, A is the surface absorptivity of the solder at 10.6 microns, R is the thermal resistance per unit area of the system, tc is the ciritical time to bring the solder to the Tm temperature, c is the heat capacity of the system.

Claim 12

A method of soldering one or more electrical lead strands, extending from an annular integral leadframe encapsulated within an annular nonconductive housing, to a printed electrical path carried on a ceramic substrate recessed within said housing, the method comprising: (a) planting at least one solder pad in engagement with a portion of said printed electrical path; (b) bringing a portion of said electrical lead strand into continuous engagement with said pad with a force of 50-150 grams while aligning the mass of said portion of said electrical strand with the center of said pad, said portion of the electrical strand and pad constituting a soldering assembly; (c) wetting said assembly with a flux and, while shrouding said soldering assembly in a shielding flow of nitrogen gas, directing a defocused laser beam, having a beam diameter of 0.03-0.08 inch at the interface of said beam and solder pad, onto said soldering assembly, said beam having a power and beam on-time effective to exert a critical thermal radius on said soldering assembly to reflow a portion of said pad and effect a sound solder joint between said pad and strand portion, said critical thermal radius being computed by the equation: ##EQU8## where: c is the critical thermal radius a is the Gaussian radius at 1/e 2 point ln is logarithm T m is the melting temperature of solder minus the specimen temperature P is the laser beam power in watts A is the surface absorptivity of the solder at 10.6 microns R is thermal resistance per unit area of system t c is critical time to bring solder to T m C is the heat capacity of the system.

Claim 13

A method of soldering one or more electrical lead strands, extending from an annular integral leadframe encapsulated within an annular nonconductive housing, to a printed electrical path carried on a ceramic substrate recessed within said housing, the method comprising: (a) planting at least one solder pad in engagement with a portion of said printed electrical path; (b) bringing a portion of said electrical lead strand into continuous engagement with said pad with a force of 50-150 grams while aligning the mass of said portion of said electrical strand with the center of said pad, said portion of the electrical strand and pad constituting a soldering assembly; (c) wetting said assembly with a flux and, while shrouding said soldering assembly in a shielding flow of nitrogen gas, directing a defocused laser beam, having a beam diameter of 0.03-0.08 inch at the interface of said beam and solder pad, onto said soldering assembly, said beam having a power and beam on-time effective to exert a critical thermal radius on said soldering assembly to reflow a portion of said pad and effect a sound solder joint between said pad and strand portion, said critical thermal radius being computed by the equation: ##EQU8## where: c is the critical thermal radius a is the Gaussian radius at 1/e 2 point ln is logarithm T m is the melting temperature of solder minus the specimen temperature P is the laser beam power in watts A is the surface absorptivity of the solder at 10.6 microns R is thermal resistance per unit area of system t c is critical time to bring solder to T m C is the heat capacity of the system.