Eutectic solders

Roughness has important implications in wetting applications. While the eutectic solder, SnPb, normally forms a contact angle of 15-20° with copper, it completely wets the surface of rough electroplated copper and forms a fractal spreading front [69]. 

The lead-free solders are mostly based on Sn-containing binary and ternary alloys. Among them, the Sn-Ag system is one of the earliest commercially available lead-free solders and has been recommended for general-purpose use as a substitute for Sn-Pb eutectic solder. Addition of nano-particles of second phase helps in improving thermo-mechanical properties such as melting temperature, mechanical strength, mechanical fatigue resistance, creep resistance and solder-joint reliability. 

CO-DEPOSITION OF AU-SN EUTECTIC SOLDER USING PULSED CURRENT ELECTROPLATING... 

Figure 11-15 shows eutectic solder bumps placed onto a 18 x 18 test substrate with 100-pm diameter pads on 250 pm centers. The solder volume deposited per pad is equivalent to a drop diameter of 100 pm. The solder bumps acquire the shape shown in Figure 11-15 is a result of rapid solidification [18]. The bumps were placed at the rate of 400 s by raster-ing the substrate in the horizontal direction of the rows of solder bumps. 

With improved formulations, organic-based conductive adhesives became feasible replacements for eutectic solders. Table 4 shows typical one- and two-part conductive adhesives from the 1970s. The one-part adhesive system employed a latent catalyst for long pot life but required a high-temperature cure cycle. The two-part system, formulated... 

UF-3400/ 3MCo. No-flow, unfilled epoxy resin - FUp-chip/BGA or CSP assembly and CSP bonding to PWBs (tin/lead eutectic solder bumped devices) Automated dispensing (22- gauge needle) CSP assembly and FCOB and CSP bonding to laminates. 

The CSP was mounted on the PWB by using eutectic solder balls (37Pb/63Sn) and paste. The height of solder joints was approximately 0.45 mm. 

Lowest melting (Eutectic) solder fa- electronic applications... 

Figure 630 Evolution of the eutectic solder concentration profile at thermomigration.

Lead-free joints, which have no appreciable lead content (for what appreciable means in this context, refer to the appropriate directive see also Chapter 10) reflow at temperatures 30 to 40°C higher than the melt point of eutectic solder. These temperature increases have necessitated the development of newer resins or blends, different curing agents, and fillers to provide a laminate system that can survive and remain reliable after these elevated heat excursions. The changes to the laminates themselves are covered elsewhere in more detail (see Chapters 6 and 7), but those properties directly applicable to the performance of the PCB are discussed in this chapter in Sec. 27.2.2. 

A minimum temperature of 230°C is required to ensure adequate wetting and spreading by the Pb-free solder on circuit board pads as well as on component leads and terminations. Temperatures exceeding 245°C increase the likelihood of thermal damage to larger, plastic-molded packages (e.g., BGA and QFP devices). When temperatures exceed 260°C, there is the potential for thermal degradation of passive chip components (chip capacitors, indnctors, or filters) as well as to circuit board structures (e.g., vias) and laminate materials. In the case of Sn-Pb eutectic solder that melts at 183°C, the minimum process temperature of 215°C ensures adequate solderability, which is 15°C lower than that of a nominal Pb-free process. The available Sn-Pb process window is 215°C to 260°C, or a AT equal to 45°C, rather than the AT of 30°C for the Sn-Ag-Cu Pb-free solders (T di = 217°C). 

Patanaik, S., and Raman, V., Deformation and Fracture of Bismuth-Tin Eutectic Solder, Proceedings of ASM International-Materials Development in Microelectronics Packaging Conference August 1991, pp. 251-256. 

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