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paste service-life (while on the stencil), printability and effects on stencil and squeegee life, fatigue characteristics, alloy interactions with board and component surface finishes, corrosion resistance, mechanical shock resistance, and numerous other properties will have to be understood, especially for the dense, high-end electronic assemblies that are expected to last many years in the field. It may be less an issue for consumer electronics that are considered obsolete within a couple of years. However, ruggedness, especially for portable consumer electronics, is a concern. Many of the lower melting point solders have precious elements, whereas some of the inexpensive alternatives wet poorly, are prone to oxidation during soldering, and tend to corrode. A few of the most favored alloy systems will be briefly discussed. 45.5.1.2 Bismuth Alloys. Bismuth (Bi), an elemental by-product of Pb smelting, is generally associated with lower melting point solder alloys. It is in short supply and Bi solders cost about twice that of Sn-Pb alloys. Bi is compliant like Pb and shares several of its properties, such as high specific gravity and ductility.2,3,4 Pure bismuth melts at 271.3 C, about 50 C lower than that of Pb. Very easily oxidized, bismuth alloys are best soldered with the aid of highly activated fluxes or in a nitrogen environment. Bi forms a binary eutectic with Sn in the proportions 58Bi:42Sn (m.p., 138 C). There are numerous bismuth alloys in use, many composed of two or more metals in addition to bismuth. The Sn-Bi eutectic, if contaminated with Pb, can be problematic as it is known to form a ternary alloy with a melting point of 96 C adversely affecting solder-joint fatigue characteristics. In some applications, solder joints will fall apart if the service temperature is high and the low melting point Sn-Bi-Pb alloy is formed. This becomes all the more critical with lowvolume Sn-Bi solder joints. The Pb can come from solder predeposited on component leads, Sn-Pb hot-air solder leveled pads, or both. Bismuth expands upon freezing, whereas tin contracts. A phenomenon called fillet lifting has been reported5, 6 (see Fig. 45.2). It is mostly associated with bismuth ternary alloys such as Sn-Cu-Bi and Sn-Ag-Bi used in plated through-hole (PTH) wave soldering, but has also been observed with Sn-Bi system.
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FIGURE 45.2 The expansion upon solidification in a bismuth-containing solder alloy can cause solder fillets to fracture and lift from the plated through-hole annular pads (right). Compare this to the schematic on the left, which is indicative of a conventional tin-lead solder joint.
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As the board cools, it contracts at a different rate than the solder, and the fillets may lift, especially if cooled slowly or contaminated with Pb. Rapid cooling (~3 C/sec.) has been advised but may not completely eliminate this phenomenon. Care must be taken not to exceed the component manufacturer s specified heating and cooling ramp rate. Too high a cooling rate may result in component cracking or degraded component reliability. Fillet lifting may also result in lifted PTH annular ring (bonding pad) if solder-joint strength is high and bond strength of the ring to the board is low.
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Some Bi alloys have too low a melting point for many applications. Despite its excellent tensile strength and thermal cycling endurance (better than Sn-Pb), Sn-Bi eutectic (m.p., 138 C) would not be practical for use in most automotive applications or even for high-end computer assembly, where the service temperature may approach the melting point of this alloy. Bi also oxidizes rapidly, making Bi alloys difficult to maintain during wave soldering due to copious oxide (dross) production. Bismuth soldering alloys are noted for their brittleness7,8; nonetheless, several bismuth solders are used commercially, the most popular alloys containing Bi as a minor constituent (in the 2 to 14 percent range). Aside from bismuth alloys inappropriateness for high-temperature applications and Pb contamination, safety is an issue. According to C. White and G. Evans,9 safety is particularly a concern when using alloys with high concentrations of bismuth. Cadmium (Cd), a poisonous metal, is a commonly associated bismuth contaminant. Care is recommended in handling of this solder, and special venting requirements may also be mandated for its use. On the other hand, the RoHS legislation has reinforced prior EU restrictions on cadmium, limiting it as a constituent or contaminant to less than 100 ppm by weight. Bismuth and indium alloys are useful for step soldering, which refers to the use of two different solder alloys with two different melting temperatures used on the same circuit board.The higher temperature alloy (e.g., Sn-3%Ag-0.5%Cu, melting onset, 217 C) is used to solder surface-mount components and an alloy of lower melting temperature (e.g., 58%Bi:42%Sn, m.p., 138 C) is used for wave-soldered components.The temperature experienced at wave solder will not cause the Sn-Ag-Cu to reflow, thus preserving the integrity of surface-mount technology (SMT) solder joints. In the case of repair or replacement of components, the use of low melting temperature solder will have no adverse thermal effect on adjacent or reverse-side solder joints. In fact, use of low melting point solder reduces the possibility of pad delamination or through-hole barrel cracking; failure mechanisms associated with localized overheating, especially at repair. 45.5.1.3 Sn-Ag-Cu. As previously stated, the Sn-Ag-Cu (SAC) family is receiving the most attention worldwide. Its tensile strength is superior to that of Sn-Pb, but its shear strength is worse. It is easily fluxed and moderate in melting temperature roughly 35 C higher than eutectic Sn-Pb solder. The elevated temperature regime makes this alloy more difficult to flux since the flux tends to dry up and activate too early in the process.This problem is overcome with higher molecular weight resin fluxes and volatile organic compound (VOC)-free flux formulations since there are no alcohols or other low boiling organics present to aid in evaporation. The SAC family has good compatibility with most Pb-free board surface finishes and Pb-free component platings. Its process window is narrower than that of Sn-Pb. It has been demonstrated that SAC can be used with parts containing Pb, but the reliability of the solder joint is less than if soldering Pb d to Sn-Pb or Pb-free to SAC. SAC solders sell at about two-and a-half to three times the price of Sn-Pb solder alloys. A comparison of Sn-Pb solder to SAC solder is offered in Table 45.4.
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TABLE 45.4 Comparison of Sn-Pb Solder with Pb-Free SAC Solder Tin/Lead Sn63:Pb 183 C ~215 10 C 250 C 260 C Adequate Adequate Tin-Silver-Copper Sn-(3.0 4.0%)Ag-(0.5 0.7%)Cu ~217 C ~221 C ~240 10 C 260 C 270 C (with higher preheats and longer soldering dwells) Collecting data Collecting data
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