Liquidus temperatures solders

Solders. In spite of the wide use and development of solders for millennia, as of the mid-1990s most principal solders are lead- or tin-based alloys to which a small amount of silver, zinc, antimony, bismuth, and indium or a combination thereof are added. The principal criterion for choosing a certain solder is its melting characteristics, ie, solidus and liquidus temperatures and the temperature spread or pasty range between them. Other criteria are mechanical properties such as strength and creep resistance, physical properties such as electrical and thermal conductivity, and corrosion resistance. 

When adjusted properly, the board meets the crest of the wave and disrupts the oxide skin. In doing so, the fluxed components and board are immersed in the flowing, oxide-free molten solder. If all steps are carried out properly, the solder alloys with the fluxed, oxide-free component leads, component pads, and PTH barrels. Upon exiting the wave solder machine, the assembly cools below the solder liquidus temperature and solder joints are formed. The more the system is used to solder boards, the faster the contamination and dross build-up. 

Service life The number of years that equip-ment/component serves in a specified environment Soldering A fusion process that uses a filler metal with a liquidus temperature less than 538°C. 

Table 9.1 Compositions, Solidus Temperatures, and Liquidus Temperatures for Two Lead-Containing Solders and Five Lead-Free Solders...

TABLE 14 Some Lead-Free Solder Alloys with a Melting Point or Liquidus Temperature Below 150°C... 

Electrolytic capacitors are very susceptible to high-temperature damage, as are wound components, such as relays [2]. Surface mount packages are not rated for temperature exposures beyond 230°C, but some survive up to about 250°C, and a few beyond that temperature. Pin-in-hole components are typically rated to survive up to 260°C. In standard reflow assembly practice, the solder liquidus temperature is less than 200°C (e.g., eutectic Sn-Pb, mp = 183°C) and the reflow temperature is t5q)icaUy below 230 °C. 

Sn-5Sb solder has a near-peritectic composition and a relatively high-liquidus temperature, around 238°C, which is significantly higher than the melting point of eutectic Sn-Pb solder, 183°C. Unlike other alloying elements, the addition of Sb to Sn raises the melting point of Sn gradually as the solid solubility of Sb in Sn increases [49]. The peritectic reaction, which occurs near 246 ° C, produces Sn- Sb microstructures which are complex. A typical cast microstructure of Sn-5Sb alloy solidified rapidly is shown in Fig. 2. This microstructure exhibits coarse dendrites of Sn-rich solid solution with Sn Sb intermetallic compounds dispersed between the dendrites. 

Among seven down-selected alloys for the NCMS manufacturability and reliability assessment [4], only one Zn-containing alloy, Sn-3.5Ag-0.5Cu-lZn (liquidus temperature of 221 °C), was included. The maximum concentration of Zn in the selected alloy was chosen to limit the pasty range, the difference between the solidus and liquidus temperature of a solder, to be less than 30 °C. 

FIG. 53 Wetting equilibrium contact angles of various Pb-free solder alloys, (a) Cu substrate with RMA (GF-1235) flux. (From Drewin et al. Ref. 169.) (b) Cu substrate with RMA (Alpha 611) flux 40"C superheat over the liquidus temperature. (From Vianco et al. Ref. 170.) (c) Cu substrate with RMA flux at 30 °C superheat over the liquidus temperature where the liquid alloy is dispensed onto a heated substrate. (From Pan et al. Ref 171.)... 

In one study [29] the formation of intermediate alloys in Sn-3.4Ag-0.7Cu/metal systems was investigated for Ni and Cu metallization (with the same die and Ni-V die metallization). In producing these samples, a Cu-Ag-Sn solder paste was printed onto Ni-V die terminal pads and reflowed. The resulting solder bumps were fluxed and reflowed a second time in order to attach them to an OSP-coated Cu metallization or Ni/Au/Cu metallization on a chip carrier. The chips were reflowed under standard industrial conditions. A second reflow was conducted to attach the solder to either a Ni or a Cu metallization. This reflow consisted of peak temperatures between 234° and 250°C, and times above the liquidus temperature ranging between 50 and 80 sec. 

FIG. 31 Scanning electron microscope image of Sn-Ag-Cu solder reflowed for 54 sec on a Ni/Au substrate. The Sn-Ag-Cu solder was reflowed on a die with a NiV under bump metallization. The bumped die was then reflow attached to a Ni/Au substrate pad with a time of 54 sec above the liquidus temperature and a peak temperature of 238 "C. 

Liquidus temperature Temperature at which solder alloy is completely molten. <225 C... 

TABLE 3 Limits of Eq. (1) for Calculation of Liquidus Temperature for Sn-Based Solder Compositions where Compositions are Given in Mass Fraction (xlOO)... 

Code Solder compositions Liquidus temperature (°Q Reason for selection... 

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