STR/04/033/PM 203 Investigation of the Effects of Surface Finish and Simulated Service Aging on the Characteristics of the Intermetallic Layer Formed in Various Lead-Free Solder Joints A. C. Spowage, L. M. Sim, C. M. Thong, P. A. Collier and G. Y. Li 1 1 School of Material Engineering, Nanyang Technological University, Singapore. Abstract – One of the most pressing challenges facing today’s electronics packaging industry is to identify reliable and cost-effective solder al- loys to replace toxic lead containing solders. Besides evaluating new alloy compositions and improving the soldering process, it is also impor- tant to understand how surface finishes applied to the copper metallization affect joint character- istics. This study investigates the impact of printed circuit boards (PCB) finish and solder type on the interfacial intermetallics. Five types of solder and four types of finishes were used. Sn37Pb (SP), Sn3.5Ag (SA), Sn3.5Ag0.7C (SAC), Sn2Ag0.5Cu4Bi (SACB) and Sn3Bi8Zn (SBZ) solders, combined with immersion Silver (I-Ag), electroless Nickel-immersion Gold (ENIG), Organic Solderability Preservative (OSP) and immersion Tin (I-Sn). It was shown that both the SP and SAC solders follow a para- bolic growth model during ageing and that the surface finish has a significant effect on the in- termetallic morphology and growth kinetics. A combined SEM and XRD investigation were shown to be suitable methods for characterizing nanoscale intermetallics. Keywords: Lead-free solder, Finishes, Inter- metallics, SEM, Layer thickness 1 BACKGROUND International regulations regarding the presence of toxic materials, such as lead, within electronic devices are having a major impact on the elec- tronic manufacturing sector. Although the impli- cations of the legislation have been known for over a decade, a recent study has shown that manufacturers in many countries are still con- solidating materials data, developing reflow processes, selecting components, testing board finishes and investigating reliability [1-3]. One of the major activities has been to find replacement alloys that can operate in the various application domains currently dominated by SP solders. The metallization materials used are considered equally important and have a number of roles to play; they may act as a diffusion barrier, electri- cal connectors and they form a mechanical bond to the solder. Copper is currently the most widely used metallization layer. However, copper met- allization on PCBs and other devices oxidizes readily which results in a number of problems. Surface finishes are commonly used to protect the copper metallization. Some surface finishes also function as diffusion barriers preventing the metallization layer form being consumed. A ro- bust stable bond between copper and the solder is essential for functionality and reliability. The bond develops via the formation of intermetallic compounds between the wettable metallization and solder. The selection of an appropriate fin- ish and solder alloy combination therefore influ- ence the properties and reliability of the joints [1- 7]. The extent and nature of the interfacial reac- tions may be strongly influenced by the process- ing parameters during reflow and the associated liquid phase reaction. Additionally, during service the joints may be subjected to elevated tempera- tures (a significant fraction of the solders melting point), resulting in nano and micro-structural changes. Characterisation of these changes is difficult and time consuming. The complex mul- timaterial structure makes TEM foil preparation difficult, the difficulty in attaining smooth sur- faces generally rules out AFM, and the small sampling volumes makes conventional X-ray diffraction (in cross section) difficult. SEM has the advantage of relatively simple preparation, examination in cross section (or plan view), ele- mental sensitive imaging, in addition, the tech- nique is widely used within the electronics sec- tor. 2 OBJECTIVE 1. Investigate the effects of surface finish on the nature of the intermetallic phase formed between copper metallization and tin based lead free solders. 2. Investigate the effects of solder type on the nature of the intermetallic phase formed be- tween copper metallization and tin based lead free solders. 3. Investigate the effects of simulated service