Life-time test of laser soldered joints on flexible boards

Laser soldering experiments were carried out in respect of lead-free solder materials and flexible board combinations. Polyimide (PI), polyethylene-naphthalate (PEN) and Polyester (PET) flexible copper clad foils with temperature limits of 270degC (PI); 180degC (PEN); 150degC (PET) were tested using the popular SAC (SnAg3.8Cu0.7) lead-free solder with melting point of 217degC, or the low temperature 58Bi42Sn lead-free solder with melting point of 138degC. Substrates with different adhesives and cover layers were used. Application oriented test boards were designed to investigate the laser soldering process and the quality of the resulting solder joints, regarding such field applications as soldering surface mount components onto flexible boards and attaching flexible board/cable to rigid board, in particular if the temperature limit of the flexible foil is lower than the melting point of the solder. The paper discusses the performance of laser soldered joints on flexible boards regarding optical inspection, shear strength, joint resistance and life time tests.