The state of Pb-free solder — A joint reliability overview

Over the past decade the electronics components industry successfully transitioned from the use of leaded solder to lead-free (Pb-free) solders in response to growing environmental health concerns related to heavy metals and other substances. Pbfree components in general are in compliance to meet the European restriction of hazardous substances (RoHS) directives. During the transition period to Pb-free surface mount, numerous issues were raised about the selected alloys, the board assembly process and reliability. Early Pb-free reliability concerns were due to incomplete analytical understanding of the Tin-Silver-Copper solder creep-fatigue behavior, difficulty in computing the magnitude of ball grid array (BGA) relative displacements or strains and lack of product field history. Since then the failure mechanisms were characterized and many models are in common use for reliability estimation and design. This manuscript revisits the initial concerns, reliability model use evolution and summarizes the current understanding that has resulted in a decade of reliable field operation for the Pb-free SAC solders selected.