RoHS Compliant Finishes

Of the many RoHS-compliant leg finishes vying for industry acceptance as a replacement for Tin-Lead (Sn-Pb), THAT Corporation currently offers three different types: 100% Tin (Sn) (on 2181Sx products), Tin-Bismuth (Sn-Bi) (on 4301S and 4311S products), and Nickel-Palladium-Gold (Ni-Pd-Au) (on 1200, 1240, 218xLx, so far). Below is a brief description of each.

100% Tin (Sn)
Tin has good wettability/solderability over a large range of substrates, making it an excellent choice for lead (leg or pin)) finish through tin plating. However, it has some distinct disadvantages in its pure form. Pure tin has a tendency to exhibit 'tin pest' at temperatures below 13 deg C, making it structurally inadequate in low-temperature applications. Pure tin also has a tendency to form tin whiskers which can cause lead-to-lead shorting. Modern IC packaging companies have found methods to mitigate tin whiskers through special processing, however, some amount of "whiskering" with pure Sn is inevitable.

Tin Bismuth (Sn-Bi)
Sn and Bi form a eutectic alloy at 42%Sn and 58% Bi, which melts at an excessively low temperature of 138 deg C. However, at 3% Bi the melting temperature is about 215-220 deg C. Sn-Bi solder tends to be brittle and can also exhibit tin whiskers at compositions when the Sn content is high. If slowly cooled, large grains arise, the boundaries of which may serve as precipitation points for Sn, resulting in cracks.

Nickel-Palladium-Gold (Ni-Pd-Au)
This is a family of pin finishes accomplished by successive applications of Nickel, Palladium, and Gold via a plating process. Nickel-Palladium (Ni-Pd) plating was first introduced by Texas Instruments to the semiconductor industry in 1989. Since then, TI has shipped millions of units that used this "lead-free" pin finish material. Ni-Pd-finished components are compatible with both Sn-Pb and Pb-free solder pastes. Nickel-Palladium-Gold (Ni-Pd-Au) is a refinement of Ni-Pd, which offers somewhat enhanced performance due to a quicker wetting time when compared to Ni-Pd and Sn-Pb finishes. TI and other semiconductor companies (including THAT) have shipped many more millions of units using this plating combination. We are particularly enthusiastic about Ni-Pd-Au plating because it appears to be compatible with lead-free solders as well as more conventional tin-lead solders.

There is a manufacturing impact to all of these RoHS compliant alternatives. In soldering the pins to printed-circuit boards, the device pins must be exposed to temperatures high enough to melt the lead finish for board soldering. Many of the alternative RoHS compliant solder materials being used in IC assembly today require peak soldering temperatures of 250 to 260°C, versus peak temperatures of 230 to 235°C for Sn-Pb solder. This means that RoHS compliant ICs must survive higher temperatures in the soldering process.

These requirements are more significant for surface-mount packages than for through-hole ones, since surface-mount soldering is performed by (reflow) heating the entire component to soldering temperatures. By comparison, through-hole components are somewhat shielded from the solder temperature by the circuit board and the component legs. Nonetheless, both component types will be subjected to more severe thermo- mechanical stresses during RoHS compliant soldering than with conventional processes. This means that the plastic encapsulation material must be designed to withstand higher temperatures during RoHS compliant soldering than with conventional soldering. All THAT's surface-mount RoHS compliant parts are compatible with peak reflow soldering temperatures of 260°C. For more details, refer to IPC / JEDEC standard number J-STD-020C entitled "Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices" This document and others related to the handling and soldering of surface mount parts are available at the JEDEC web site