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A "Lead-Free" Semiconductor Industry
As early as the 1980's, the semiconductor and electronics industries have recognized the inherent health dangers and environmental concerns that arise from the use of lead (Pb) and its compounds in some of their manufacturing operations, such as solder plating. This article aims to provide the reader some basic background information on the semiconductor industry's "lead-free" (Pb-free) initiatives.
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.
Over the 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
In the 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
Identifying 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.
Pure Tin. 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 shorting.
Tin-Zinc. 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.
Tin-Copper. 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.
Tin-Bismuth. 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.
Tin-Silver. 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 temperatures.
Tin-Indium. 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.
Tin-silver-copper. 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.
Nickel-Palladium. 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 material.
Manufacturing Impact of Lead-free Alternatives
Surface 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.
An essential 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 reliability risks.
See also: ROHS
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