The Dangers of Using Lead (Pb)
Lead (Pb) and its
compounds are highly toxic and remain stable over time. As such, their
entry into ecological systems, natural water supplies, and plant and
animal food chain has become an environmental issue that's of utmost
importance all over the world. Lead is a heavy metal that accumulates in
the body over time, resulting in various health problems that can
ultimately lead to death under the worst circumstances. Lead has been
confirmed to hamper neurological and physical development, which is why
it is considered most harmful to children under six years of age.
Low level of
exposure to lead can result in physical retardation, low IQ,
hyperactivity, hearing loss, behavioral changes, insomnia, etc. High
level of exposure to lead can cause lead poisoning, which is said to
have occurred if the blood contains more than 25 mg of lead per dL. Lead
poisoning is characterized by symptoms like anemia, hypertension, mental
retardation, convulsions, coma, and as mentioned, even death.
years, much has been done to arrest the contamination of our environment
with lead. These include enactment of the necessary legislation in
various countries to limit the use of lead, launching of programs to
effectively and efficiently recycle lead, and replacement of lead
compounds with lead-free alternatives.
Pb Usage in the Semiconductor Industry
semiconductor industry, lead (Pb) is primarily used in a process known
as lead finish
('lead' here refers to the external pins of the package for connecting
the device to the outside world, and not the element Pb). Lead finish
is the application of a layer of metal over the leads of the device to
improve its solderability, protect it from corrosion and mechanical
damage, and improve its appearance. Tin (Sn)-Lead (Pb) solder is
commonly used as lead finish material, which are deposited either by
solder plating or solder coating.
There are a
number of reasons why the Sn/Pb binary system had become a popular
choice as lead finish material. 85Sn/15Pb solder has a low melting
temperature of around 220 deg C, making it more processable and less
stressful to devices than other alloys that melt at a higher
temperature. Pb as an additive to Sn to form solder also has its own
advantages. Pb lowers the surface tension of pure Sn, which equates to
better wettability/solderability. Pb as an added impurity has also been
shown to prevent tin pest, which is the transformation of gray or alpha
tin to white or beta tin, the reaction of which causes loss of
structural integrity in Sn. Pb also acts as a solvent metal that
facilitates the formation of intermetallic bonds between the joint
constituents, e.g., Sn and Cu (from the leadframe of the device
package). Lastly, Pb is widely available and low-cost.
Alternative Lead Finish Materials
alternative "lead-free" (Pb-free) lead finish materials for use in the solder
plating or coating operations of semiconductor
and electronics industries has not been easy. Stringent visual,
mechanical, electrical, and chemical requirements need to be satisfied
by the alternative solder before it can be released for 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, while remaining to be reliable and cheap. Some
candidates are described below.
Tin has good wettability/solderability over a large range of substrates,
making it an excellent choice for lead 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
Sn-9wt.%Zn has a melting temperature of 198 deg C, making it a close
alternative to eutectic Sn-Pb solder. Once solidified, it exhibits
large grains with a fine and uniform two-phase eutectic colony.
Sn-0.7%Cu is the eutectic composition of the Sn-Cu binary system. This
solder alloy is relatively cheap, has fine grains, and exhibits good
solderability. However, due to the high percentage of Sn, it is also
prone to tin whiskers and tin pest. It melts at 227 deg C.
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 wherein Sn% is high.
If slowly cooled, large grains arise, the boundaries of which may serve
as precipitation points for Sn, resulting in cracks.
Sn-3.5%Ag exhibits good solderability and mechanical properties and has
the longest history of reliable usage as a lead-free solder. However,
it is expensive, prone to tin whiskers due to the high Sn content.
Increasing Ag% to > 5% will result in drastically higher melting
52In-48Sn has likewise been used as "lead-free" solder material in SMT
applications. In-Sn is eutectic at 50.9In49.1Sn. In-Sn solder exhibits
a substantially lower melting temperature.
Sn-AG-Cu solder, which is eutectic at 3.9% Ag and 0.6% Cu, exhibits a
melting temperature of about 217 deg C. Copper may be difficult to
stabilize in this alloy.
Ni-Pd as an alternative lead-free solder was introduced by Texas
Instruments to the semiconductor industry in 1989. Since then, TI has
shipped millions of units that used this "lead-free" lead finish
Manufacturing Impact of Lead-free Alternatives
mounting of units on boards require the exposure of the device package
to high temperature to melt the lead finish for board soldering. A lot
of the alternative "lead-free" solder materials being considered for use in
IC assembly today require a peak soldering temperature of about 250 to
260 deg C, versus the peak temperature of 230 to 235 deg C for Sn-Pb
solder. This means that lead-free IC's will need a higher
temperature for board mounting, and will therefore be subjected to more
severe thermomechanical stresses during the process.
aspect of developing a "lead-free" solder plating process for IC assembly is the
reliability testing of existing package designs to determine if they
will still withstand the board mounting process at the higher
temperature under the same moisture sensitivity classification. If not,
either the package's MSL classification should be downgraded or a
new material set or package design should be implemented to ensure that
the new "lead-free" board mounting process does not introduce any
WEEE; IPC/JEDEC MSL's;
Lead Finish; Tin
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