Fragility of Pb-free Solder Joints

Recent investigations have revealed that Pb-free solder joints may be fragile, prone to premature interfacial failure particularly under shock loading, as initially formed or tend to become so under moderate thermal aging. Depending on the solder pad surface finish, different mechanisms are clearly involved, but none of the commonly used surface finishes appear to be consistently immune to embrittlement processes. This is of obvious concern for products facing relatively high operating temperatures for protracted times and/or mechanical shock or strong vibrations in service. While fragility problems and the associated embrittlement mechanisms have long been known for both electroless and electrolytically deposited Ni/Au coatings, soldering to Cu has been viewed as 'safer' as far as robustness is concerned. However, recent observations suggest the existence of two or more embrittlement mechanisms in Pb-free solder joints on Cu pad structures, each leading to brittle interfacial fracture at the pad surfaces. With risks of embrittlement associated with all the commonly used solderable surface finishes, the electronics industry is currently confronting very difficult problems. The variability in the manifestations of these embrittlement mechanisms does, however, lend hope that some of these problems may be avoidable or controllable. While it is imperative for each company to pursue its own interests in this arena, it is also clear that there is a common interest within the electronics industry in solving the reliability problems, associated with these solder joint embrittlement problems, particularly considering the short time frames associated with the transition to Pb-free solder technology. The consortium concept may be very usefully applied to this problem, which the industry is now confronting. A consortium effort could provide insurance for the member companies against overlooking critical phenomena or understandings. A consortium could also act as a common forum for advocating the infrastructure changes which will almost certainly accompany the required solutions to these problems. 2 Universal Instruments Introduction The The microelectronics packaging industry relies on solder joints for the robust mechanical attachment and electrical interconnection of a wide variety of components. Thermal excursions and mechanical shock or vibration often lead to substantial loads on these joints. Notwithstanding, we have a detailed technical understanding, based on decades of experience, by which to assess and predict the consequences for Sn-Pb soldering technology. Over the last few years a significant amount of work has also been done in developing Pb-free soldering technology. Although we are still far from the level of experience and understanding reached for the Sn-Pb system, the commonly preferred Pb-free (Sn-Ag-Cu) alloy system is usually claimed to offer superior or comparable thermomechanical fatigue resistance and, at worst, a minimal reduction in mechanical shock resistance. These claims are still the subjects of intensive research, notably in terms of the effects of the evolution of the solder joint microstructure in thermal cycling and with time at elevated temperatures. Recent reports do, however, suggest some unexpected embrittlement problems associated with both Cu and electrolytic Ni/Au-coated solder pad surfaces. In fact, apparently no commonly used solderable surface coating is consistently immune to embrittlement problems. This circumstance may pose a serious reliability concern and infrastructure problem for the microelectronics industry, as it moves towards the implementation of Pb-free soldering technology. However the variability in the manifestation of embrittlement mechanisms, at least in the Cu pad system, lends some insight and hope to the prospect that some of the embrittlement mechanisms can be controlled. In simple terms, the mechanical forces on a solder joint originate from externally imposed forces on its card assembly or from mismatched thermal expansions within the structures to which the solder joint is attached. The plastic deformation properties of the solder serve to limit the stresses in the solder joint at sufficiently high stress values. Even moderate thermal cycling usually requires some joints to survive loads which induce significant plastic deformation in each cycle, making it paramount for the interfacial intermetallic compound structures in the solder pads to survive loads corresponding to the solder plastic flow stress. The solder plastic flow stress is invariably higher under externally imposed mechanical loading, in particular loads associated with system mechanical shock, because of the higher imposed strain rates. Accordingly, even intermetallic compound structures that are strong enough for thermal cycling may still be the ultimate weakest link in a shear or pull test. However, this is not necessarily immediately critical since externally imposed mechanical loading may often be limited, by design, to a level that does not involve much plastic flow of the solder or, at least does not exceed the critical interfacial tensile load required for interfacial fracture. Still, a switch from failure through the solder to failure at a pad surface or within the intermetallics in such a test is invariably an indication of an ongoing weakening. In general, solder joints which demonstrate brittle interfacial fracture without significant plastic deformation of the solder joints, represent an inherent problem in applications, where shock loading of the solder joints can be anticipated. In such instances very little energy is dissipated in the solder joint in the fracture process and the solder joint structures are inherently prone to shock reliability problems. Some of the embrittlement mechanisms may also cause sufficient weakening to allow for premature solder joint failure even under a CTE mismatch stress in some applications. In fact, continued interfacial void growth in the intermetallics may even cause failure at very low load values. While issues with soldering to Ni/Au coated pads have long been recognized, recent observations appear to involve new phenomena, as outlined below. In contrast, until now Cu pads coated with OSP, immersion Ag, immersion Sn, or solder have been viewed as 'safer' in this respect. This does not mean that degradation mechanisms are completely absent, even for Sn-Pb soldering. In fact, rapid diffusion of Cu through both the Cu3Sn and the Cu6Sn5 intermetallic layers commonly leads to the growth of Kirkendall voids at the Cu/Cu3Sn interface [1], [2] and/or the Cu3Sn/Cu6Sn5 interface [3]. However, these voids often remain very low in density and too small to be visible by optical microscopy 1[1], [2]. They are therefore usually not considered to be of practical concern. Recent reports of rapid mechanical weakening of Sn-Ag-Cu solder joints on Cu pads in thermal aging have caused considerable stir in the microelectronics packaging community [4], [5]. The effect appeared to be caused by the growth of Kirkendall voids along the Cu3Sn/Cu interface (Figure 1). Extensive voiding was observed after only moderate aging (20-40 days at 100oC) making it an obvious practical concern, at least for products facing elevated operating temperatures and mechanical shock or vibrations in service. In fact, the apparent temperature dependence might suggest a