Reliability of lead free solders joints and manufacturability during the SMT process

In systematic investigations the failure behaviour of lead free solder joints for different solder alloys as well as substrate metallization was analyzed. The emphasis of the investigations was the optimization of the processing parameters for the printing of different lead-free no-clean solder pastes in the standard pitches from 1.27 mm to 0.4 mm. Furthermore, evidence was provided of the aging-determined changes of the solder joints by means of a sensitive resistance measurement system and in correlation therefore illustrated on the basis of metallographic cross sectioning. Therefore, a demonstrator was developed on FR4 with the electroless tin and electroless nickel/immersion gold (ENIG) surface metallizations and assembled with the leadless chip components R0402, R0603, and R0805 with pure tin metallization. As alternative lead-free solders were examined the eutectic alloy SnAg3.8CuO.7, SnAg3.5 and the SnAg0.2Cu1.0 solder. The aging and/or failure behaviour of the solder joints was detected and evaluated statistically for thermal shock aging from zero to 2000 cycles with -40 C to 125°C by means of a special 4 probe measurement. With the help of these investigations, not only the number of failures as a function of the cycles, but also the influences of the solder alloys and metallization on the failure behaviour could be determined.