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|Novel Solder Alloy with Wide Service Temperature Capability for Automotive Applications|
|Keywords: solder, automotive, reliability|
|While SnAgCu has been the prevailing choice for SMT assembly and some packaging solder alloy at electronic industry, the adaptability toward next generation automotive applications, including automotive LED application, is challenged due to questionable service temperature range capability. For automotive applications, high reliability is a MUST not only under moderate temperature, but also under high service temperature condition. A number of new solder alloys have been attempted by the industry, with major emphasis on enhancing the high temperature capability. However, only limited success has been achieved up to this point, except for one novel solder alloy, with high reliability demonstrated on both moderate and high service temperature conditions. A novel lead-free solder alloy 90.6Sn3.2Ag0.7Cu5.5Sb, designated as Indalloy276, was developed targeting for high reliability with a wide service temperature capability. The alloy exhibited a melting temperature range of 223 to 232C, reflowable at profile with peak temperature 245C and 255C, with ambient temperature Yield Stress 60MPa, UTS 77 MPa, and ductility 28%, and a higher stress than both SAC305 and SACBSbN, the latter two alloys were used as controls. When tested at 140C and 165C, the die shear stress of 276 was comparable with SACBSbN but higher than SAC305, and the ductility was higher than both SACBSbN and SAC305, with SACBSbN exhibited distinct brittle behavior. When aged at 125C and 175C, the die shear strength of 276 was comparable or higher than both controls. When pretreated with a harsh condition, TST (-55C/155C) for 3000 cycles, the die shear strength of 276 was 8 times of that of SACBSbN and SAC305. When pre-conditioned at TCT (-40C/175C) for 3000 cycles, the die shear strength of 276 was 11 to 20 times of SACBSbN and SAC305, depending on the flux type used. Both 276 and SACBSbN are alloys based on SnAgCu, but reinforced with precipitate hardening and solution hardening, with the use of additives including Sb, Ni, and Bi. 276 exhibited a finer microstructure with less particles dispersed, while SACBSbN exhibited more particles with some blocky Ag3Sn plates or rods. 276 is rigid and ductile, while SACBSbN is rigid but brittle. Under the harsh test condition where T was high, the dimension mismatch between parts and substrate became very significant due to CTE mismatch. This significant dimension mismatch would cause a brittle joint to crack quickly, as seen on SACBSbN. The challenge was more tolerable for a ductile joint, as shown by 276. Accordingly 276 showed a much better reliability than SACBSbN under harsh condition, including high testing temperature and large T. Overall, to achieve high reliability under a wide service temperature environment, a balanced ductility and rigidity for solder alloy is critical for success.|
|Ning-Cheng Lee, VP Technology