Eutectic tin-lead

Although commonly used solder paste for both THT and SMT production contains 63-37 eutectic tin-lead solder, other metal formulations are available, including 96-4 tin-silver (silver solder). The fluxes available are also similar, with typical choices being made between RMA, water-soluble, and no-clean fluxes. The correct decision rests as much on the choice of flux as it does on the proper metal mixture. A solder paste supplier can best advise on solder pastes for specific needs. Many studies are in process to determine a no-lead replacement for lead-based solder in commercial electronic assemblies. The design should investigate the current status of these studies as well as the status of no-lead legislation as part of the decision making process. 

As pointed out in Table 10.2, the vapor pressure of water at lead-free assembly temperatures—260°C, for example—is much higher than at eutectic tin-lead assembly temperatures such as 230°C. Figure 10.10 plots the vapor pressure of water, in both mm Hg and psi, versus temperature. At 230°C the vapor pressure of water is near 400 psi. At 260°C, it is close to 700 psi. Therefore, any absorbed moisture within a PCB during assembly can have a much greater impact in lead-free assembly, as the greater pressure stresses the adhesion between the base material components and can also create small voids within the resin system. 

As many PCB fabricators and assemblers transition to lead-free processing, they are faced with solder alloys that have an operating temperature up to 30°C (54°F) hotter than the traditional eutectic tin-lead solders. These increased soldering temperatures raise concerns about the ability of the solder mask (as well as all other materials) to resist embrittlement, discoloration, loss of adhesion, and cracking with repeated exposures to the higher temperatures. Some existing products may not be acceptable for lead-free processing. 

From the very beginning of the electronics industry (not to mention ancient solder appUca-tions dating hack 5,000 years), solder joints have been made primarily of alloys of tin (Sn) and lead (Pb). In particular, the eutectic tin-lead alloy (63%Sn and 37%Pb by weight, eutectic temperature 183 °C, or 361 °F) has been used almost exclusively in electronics, due to its unique characteristics (cost, availability, ease of use, and... 

Second reflow. Component fall-off during the second reflow process has also been studied as a function of component weight and solder surface tension and contact area (Ref 69). The component weight to contact/pad area ratio has been found to be slightly higher for eutectic tin-lead than SAC however, the difference has been found to be statistically insignificant. Therefore, the same rule of thumb (g/in ) can be used. 

Fig. 14 Mechanical properties as a function of gold content in eutectic tin-lead solder joints, (a) tensile strength, (b) shear strength, (c) elongation, as function of Au content in eutectic Sn-Pb alloy. Source Wild (Ref 106), Bester (Ref 107), and Foster (Ref 108), and (d) fatigue lives at room temperature of eutectic Sn-Pb solder joints as function of their Au contents. Source Ref 109...

S. Knecht and L.R. Fox, Constitutive Relation and Creep-Fatigue Life Model for Eutectic Tin-Lead Solder, IEEE Comp. Hybrid Man. Tech., Vol 13 (No. 2), 1990, p 424- 33... 

One of the most uncertain elements of the lead-free transition has been the unannounced arrival of EGA components with lead-free balls lead-free EGA components provided for placement on boards originally fabricated and assembled for, and with, tin/lead solder. Often, these components arrive without any indication thatthe balls are lead-free, andit s discovered that they are not SnPb when they don t flow at eutectic tin/lead reflow temperatures. 

The use of lead (Pb) has been widely accepted in the electronics industry (utilized in a variety of applications for more than five decades), but most notably the use of eutectic tin-lead (Sn-Pb) solders to attach discrete components to printed circuit boards (PCBs). These attachments typically also serve as the electrical interconnection between the attached components and the PCB. Lead-based solders are also used as a coating or finish on metal terminations on PCBs, and on the peripheral leads of components. 

Mixing of Cu and Ni/Au attachment pad finishes, however, can result in the formation of intermetallic structures that are extremely brittle, and have been shown to reduce the mechanical robustness of the solder to attachment pad interfaces. It has been shown that a ternary Cu-Ni-Sn intermetallic region can form at the interface between the eutectic tin/lead solder and a nickel pad finish. The formation of the ternary intermetallic can cause the solder joint to become weak, at the ball to pad, resulting in a non-fatigue failure at the intermetallic layer [15-19] as illustrated in Figure 12. 

During machine startup (warm-up time) more energy is consumed per hour than during wave operation. Typical heat-up time for eutectic tin-lead solder (250°C) is 3 consuming approximately 34 kWh for a standard commercial unit. In contrast, the maximum wave temperature required for Sn-Cu solder is much higher, approximately 280 °C. The heat-up time for Sn-Cu solder is about 5 hours with an energy consumption of 36 kWh. The power consumption required to maintain the solderpot at temperature was determined to be similar for both alloys despite the temperature dilference. 

Surface Mount Technology. Better fatigue life than eutectic tin-lead for both thermal cycle test ranges less fatigue damage than eutectic tin-lead observed in surface mount cross sections. 

Through-Hole Technology. Mixed results with fatigue life, for CPGA-84 better than eutectic tin-lead for CDIP-20 worse than eutectic tin-lead. 

Surface Mount Technology. Fatigue life equivalent to eutectic tin-lead at 0 to 100°C worse than eutectic tin-lead at —55 to + 125°C. 

In the case of 48 I/O TSOP components with Sn Pb-plated Alloy 42 lead, interconnections with eutectic tin-lead solder seem to outperform those with Sn-Ag 2u joints in all the temperature ranges investigated, namely, —55°C to 125°C, —40°C to 125°C, and 0 to 100°C, respectively, as shown in Fig. 35 [94]. 

The discussion in the previous section makes it clear that there are significant differences between lead-free, tin-based solder systems and other solders successfully used in the past. The anomalous crystal structure and allotropic transformations in tin-based alloys can affect the material properties that control the ultimate performance and reliability of these solders in microelectronic assemblies. Table 5 provides a comparison of several solder systems including high-lead and eutectic tin-lead alloys, white and gray tin, a lead-indium solder, gold-tin eutectic, and a fictitious ideal alloy. 

The elastic-plastic-creep behaviors of the two lead-free solders and the eutectic tin-lead solder bumped WLCSP on PCB assembfies subjected to the novel temperature cycle test was discussed in this study. It can be seen that in the case of the same scale to the WLCSP and the WLCSP with imderfill, the difference of the equivalent total strain range exceed an order of magnitude for these three solder joints. 

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