The relationship of components, alloys and fluxes, part 1: call it a love triangle--or a Bermuda Triangle. Either way, the best alloy may be determined by the end-application.

Ed.: For the full article, please see circuitsassembly.com/cms/content/view/2081/

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Lead has been an indispensable element in solders due to its high ductility, reasonable mechanical strength, low eutectic melting temperature with tin, low surface tension and low cost. Lead-free soldering requirements are an earthquake to the lead infrastructure that has been evolving for several decades. Within the past few years, the relationship among components, alloys and fluxes has already reshuffled. Although the dust has not settled, a picture of this new relationship is emerging. Changes in alloys not only directly impact components and fluxes, but also the relationship between components and fluxes, which in turn impacts the alloys.

Alloys vs. Components

High temperature stability. Perhaps the biggest impact on components is the requirement of higher thermal stability. With mainstream solder for SMT changing from eutectic SnPb to higher melting temperature systems such as eutectic SnAgCu (217[degrees]C), eutectic SnAg (221[degrees]C) or eutectic SnCu (227[degrees]C), the soldering temperature inevitably has to be raised, with a consequent increase for component thermal stability. As reflected in IPC/JEDEC J-STD-020C, with an increase in melting temperature to 217[degrees]C from 183[degrees]C, the thermal stability requirement rises about 20[degrees]C. On the other hand, moisture sensitivity would drop one to three levels with 260[degrees]C reflow peak temperature, and new molding compounds would be desired to improve the performance--at additional packaging cost. (1)

The interaction between solder alloy and components is two-way. The impact of a high solder melting temperature is very harsh, particularly for large boards in applications such as servers or telecommunication systems in which the temperature gradient across the board can be greater than 40[degrees]C and the maximum temperature experienced by some components can be over 270[degrees]C. In other words, the thermal stability of components has to be upgraded to that temperature, unless the melting temperature of solder alloys is reduced. The latter appears to be an easier solution, and has been adopted by industry. Table 1 shows melting temperatures of some viable Pb-free solders, including some low melting alloys. A 10[degrees] to 20[degrees]C decrease in melting temperature is sufficient to permit components to survive in large-board situations. Alloys such as Sn86.9Ag3.1In10, Sn88Ag3Cu0.5In8 or Sn89Zn8Bi3 are considered viable for this purpose. Bi57Sn42Ag1 and Bi58Sn are also possible options. The latter has been used by IBM for wave soldering for three decades. However, the melting temperature of eutectic BiSn or near-eutectic BiSnAg1 may be so low that their service temperature range is compromised.

1. Internal Solder Alloys

High melting solders. Due to the process hierarchy consideration, the internal solder joints of a package often need to be high in melting temperature so that the subsequent board-level assembly will not cause remelt of those internal joints. Remelt of internal solder joints may cause die drifting, solder extrusion and damage on wire bonding. With SAC alloys having a melting temperature around 217[degrees]C being adopted as a mainstream solder for SMT assembly, and with components possibly reaching 260[degrees]C upon reflow, internal solder joints should have a solidus temperature above 260[degrees]C, preferably above 280[degrees]C. Table 2 shows some Pb-containing solders with solidus no less than 260[degrees]C. (2) By considering alloys with melting temperatures as high as 500[degrees]C, still very few Pb-free alloys meet this relaxed requirement, as shown in Table 3, and none are acceptable as a drop-in Pb-free replacement for high-temperature, Pb-containing solders. (2) Gaps in performance include melting temperature and range, solder wetting, solder ductility, mechanical strength, thermal fatigue, electromigration and cost. Due to these unresolved gaps, lead is exempted in certain RoHS applications: 1) solder joints with lead content greater than 85%, 2) pin-to-package connection with 80 to 85% lead and 3) lead used in flip-chip solder joints within packages.

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Medium melting solders. For internal solder joint applications where remelting is not a concern, solder mechanical or thermal fatigue behavior becomes the focus. Frear et al. reported in 2001 that eutectic SnCu performed best in flip-chip applications. (3) Clech in 2004 analyzed some published data (see Figure 1 online) expressing the correlations of characteristic life to cyclic shear strain range for bare chip assemblies for SnCu, SnPb and SAC assemblies by trendlines. (4) The analysis indicates that, under high cyclic shear strain range, eutectic SnCu with a higher ductility exhibits a longer characteristic life. The results suggest that, in low-strain applications, a more ductile solder experiences an earlier loss of structural integrity and therefore earlier failure. In high-strain applications, this faster creep deformation results in a shorter life than in low-strain applications. However, this deformation also avoids early cracking caused by excessively large strains, and therefore outlives solders with low ductility. On the other hand, more brittle solders such as SnAgCu or SnAg could not relieve the stress caused by large strains, and consequently suffer early cracking at weak spots of the system. In other words, the best alloy choice is determined by the end-application.

Alpha particle emission. An alpha particle is a nuclear particle that contains two protons and two neutrons. The alpha particle strips electrons from atoms such as [.sup.28]Si as it passes through the electron cloud of an adjacent atom, thus producing charge along their path, leaving a trail of electrons and holes. When enough electrons were knocked out of an IC and accumulated in a capacitor, it switched the capacitor from 0 to 1 or from 1 to 0 and resulted in a so-called "soft error." The distance an alpha particle travels through the semiconductor package ranges from 7 [micro]m for Au to 24 [micro]m for Si and 28 [micro]m for polyimide. For high I/O devices, solder bumps have to be placed everywhere on the IC surface, hence a low alpha emission solder will be required. (5)

Within common solder constituents, lead has been identified as the primary source of alpha particles. Both [.sup.214]Pb and [.sup.210]Pb contribute to alpha emission, with [.sup.210]Pb being the primary source due to its considerably longer half life (see Figure 2 online). Most of the elements involved in viable Pb-free alternatives, either as major constituents or as possible minor additives such as tin, indium, silver, copper, antimony, zinc, germanium, nickel and gold, have no isotopes which will release alpha particles; all considered safe. Bismuth may have an issue due to the existence of radioactive [.sup.214]Bi, which will convert into stable [.sup.206]Pb in about 24 years. [.sup.212]Bi would also undergo alpha decay and cause concerns for soft error. (5)

Other than Bi-containing alloys, low alpha emission can be achieved by controlling lead impurity levels. The alpha emission rate of typical pure lead is about 10-1[0.sup.2] count/c[m.sup.2]/hr. Solders currently in use have alpha emission levels from 0.05 to 0.01 count/c[m.sup.2]/hr. (LC2 level). To reduce alpha emission rate from 1[0.sup.2] to 1[0.sup.-2] count/c[m.sup.2]/hr., the lead content in Pb-free solders will have to be lowered by four orders of magnitude, or to 100 ppm or below. This is easily achievable, since tin is the primary source of lead contamination, and lead impurity in tin metal often ranges from 50 to 200 ppm. In other words, the requirement on maximum lead impurity permitted can be met by screening tin lots for lead content with a reasonably high pass rate. However, the same cannot be said for future requirements in which the alpha emission rate may need to be as low as 1[0.sup.-3] count/c[m.sup.2]/hr. (LC3 level). (5)

Ed.: Part 2, a look at the relationship between alloys and fluxes and components and fluxes, will be published in December.

Table 1. Pb-Free Alloys and Solidus and Liquidus Temperatures

Solder Alloy        Solidus       Liquidus    E=Eutectic
                  ([degrees]C)  ([degrees]C)

SnCu0.7               227           227           E

SnAg3.5               221           221           E

Sn95.5Ag3.0Cu0.5      217           221

Sn95.5Ag4Cu0.5        217           220

Sn95.5Ag3.9Cu0.6      217           220

Sn95.5Ag3.8Cu0.7      217           219

Sn95.5Ag3.5Cu0.9      217           217           E

Sn91.8Ag3.4Bi4.8      211           213

Sn86.9Ag3.1In10       204           205

Sn88Ag3Cu0.5In8       195           201

SnZn9                 199           199           E

Sn89Zn8Bi3            187           197

Bi5742Sn42Ag1         139           140

BiSn42                138           138           E

Table 2. Pb-Containing High Melting Temperature Solders (2)

Solder Alloy           Solidus       Liquidus    Remarks
                     ([degrees]C)  ([degrees]C)

PbIn19                   260           275

Pb88Sn10Ag2              267           290

PbSn10.5                 275           302

PbSn10                   275           302

Pb92.5Sn5Ag2.5           287           296

Pb90In5Ag5               290           310

Pb90Ag5Sn5               292           292         MP

Pb95.5Ag2.5Sn2           299           304

Pb92.86In4.76Ag2.38      300           300         MP

Pb92.5In5Ag2.5           300           310

PbIn5                    300           313

PbSb2                    300           320

PbAg2.5                  303           303          E

Pb93Sn3In2Ag2            304           304         MP

PbSn5                    308           312

Pb97.5Ag1.5Sn1           309           309          E

PbSb1.5                  310           322

Pb91Sn4Ag4In1            313           313          E

Pb98Sb1.2Ga0.8           315           315         MP

100Pb                    327           327         MP

E = eutectic, MP = melting point

Table 3. Tentative High-Temp. Pb-Free Alloys, Solidus Temperature
260[degrees] to 500[degrees]C (2)

Solder Alloy        Solidus       Liquidus    E=Eutectic
                  ([degrees]C)  ([degrees]C)

89Bi-11Ag-0.05Ge      260           360

80Au-20Sn             280           280           E

88Au-12Ge             356           356           E

96.76Au-3.24Si        363           363           E

95Zn-5Al              382           382           E

55Ge-45Al             424           424           E

75Au-25In             451           465

82Au-18In             451           485

Dr. Ning-Cheng Lee is vice president of technology at Indium Corp. of America (indium.com); [e-mail].