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Identifying alternative RoHS-compliant finish materials for use in the solder plating or
coating operations in the electronics industry has not been easy.
Stringent visual, mechanical, electrical, and chemical requirements
must be satisfied by the alternative solder before it can be used in
production. These requirements include acceptable melting point, good
wettability/solderability, high adhesion/mechanical strength, and
excellent conductivity. The solder must also allow high volume
production, inspection, and if necessary, rework, and, it must be
reliable and inexpensive.
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.
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