A "Lead-Free" Semiconductor Industry

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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.

<Proceed to Page 2 - Alternative Pb-Free Lead Finishes>

LINKS: ROHS; WEEE; IPC/JEDEC MSL's; Lead Finish; Tin Whiskers

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