The Role of Bismuth (Bi) in Electronics, Part 1

Following the prelude for this series (SMT Magazine, August 2017), we will now sequentially address the topics as outlined therein.

Elemental Properties

Bismuth (atomic number 83 and atomic weight 208) is classified as a metal. It normally appears as grayish white with reddish tinge and can be grown into colorful iridescent crystals. It is soft, but brittle. It has rhombohedral crystal structure in contrast to a cubic structure of lead and tetrahedral of tin. It melts at 271°C (520°F) and boils at 1,560°C (2,840°F). In comparison with tin and lead, its density (9.80g/cm3) is higher than Sn (7.31g/cm3) and lower than Pb (11.34g/cm3).

With respect to conductivity, the electrical conductivity of bismuth is measured at 0.8 (104 Ohm-1 cm-1) versus tin at 9.1 and lead at 4.8, and the thermal conductivity is around 8 (w/m-k, 300°K) compared to tin’s 66 and lead’s 35. Among metals, bismuth’s electrical and thermal conductivity are the lowest. The coefficient of thermal expansion (CTE) is also lower than that of tin or lead, at 13.4 x 10-6/C (Sn=22.0, Pb=28.9). The lower CTE can be leveraged as a useful property to design a proper CTE of solder materials.

Its lower surface tension (378mN/m, 270°C) than tin (574mN/m, 232°C) and lead (465mN/m, 327°C) is also a useful property, contributing to the improvement in wetting ability. This improvement is expected to be observed in bismuth-containing solders with other conditions being equal. This is considered a significant advantage over other elements when the specific performance requirement is needed.

Two other unique properties of Bi are that it has the greatest Hall effect of any metals (i.e., its resistance increases in a magnetic field), and that Bi expands upon solidification by 3.32%.

Overall, bismuth’s versatile properties make it an intriguing element to design alloys.

Table1-DrJennie.jpgTable 1 summarizes the properties of bismuth.

Natural Resources

The level of natural resources of bismuth is about the same as Ag. Elemental bismuth may occur naturally, although its sulfide and oxide form important commercial ores.

It is found most in Australia, Bolivia, Canada, China, Japan, Korea, Peru, Mexico, and USA. However, Bolivia, Australia and China are the only lands where native Bi is available. Due to its lack of wide presence in the native state, bismuth is usually associated with copper, lead, tin, tungsten, silver and gold ores. Yet in China, a major proportion is associated with tungsten. Bismuth has always been produced mainly as a by-product, and its price historically reflected the cost of recovery and the balance between production and demand.

The world mine production in 2016 stands at approximately 10,000–11,000 metric tons, with major contributions from China, Vietnam and Mexico, and the world reserves at 370,000 metric tons (Source: USGS). The United States ceased production of primary refined bismuth in 1997 and is highly import-dependent for its supply.

Bi Safety Data

Bismuth is widely recognized by the scientific community as one of the safest elements available. In practical terms, it has been regarded as non-toxic. Its only regulated uses are in pharmaceutical and cosmetic applications. It was approved by US EPA/NSF for substitution of Pb in free-cutting brasses for drinking water fittings in the U.S. A number of literatures cover the studies of Bi and Bi-compound safety.

Table2-DrJennie.jpgTable 2 lists the relative rank of toxicity per U.S. OSHA – PEL.

Application Areas

Bi-compounds have been used for burn bandage dressings, antiseptic powders, and the treatment of venereal diseases. As reported, other pharmaceutical areas engaging Bi-compounds include its use as a pre-treatment to reduce the lethal toxicity of several forms of cancer therapy and the use as an additive to special polymers for bone implants. In addition, Bi-Ge oxide crystals have their place in diagnostic devices by its virtue of neutralizing lethal gamma rays and improving overall imaging quality.

In the chemical world, Bi catalysts are widely used in industrial organic chemistry, and Bi-compounds are popular pigments for health and beauty care. Toys and industrial applications that require non-toxic yellow to red or green pigments also rely on Bi compounds. For example, bismuth oxychloride pigment’s brilliance and luster is an effective ingredient to generate the pearlescent effect in lipsticks, nail polishes and make-up powders. Another compound, Bi citrate, is found to contribute to improved hair dyes. Additionally, bismuth is used in metallic paints and glass coloration products, as well as in heat/energy absorption coatings, such as solar panels.

For the metal industry, Bi has been successfully used as a lubricant to steel and aluminum, improving the machinability of free-cutting steels and aluminum. In galvanizing, Bi in place of Pb has been used to increase fluidity of the bath and wettability of steel. This Bi function makes the lead-free galvanizing possible. Then there are “green bullets.” U.S. Army engineers have been trying to develop lead-free, combat-ready cartridges since the early 2000s. Reportedly, more than 1,000 indoor military shooting ranges have closed because of the Pb-contamination problem and the resulting high airborne Pb-level. For this purpose, Bi is found to be a successful replacement for Pb.

For electronic applications, Bi-compounds are found to effectively improve and alter the properties of ceramic materials, such as lowering the processing temperature and improving the properties of varistors (zinc oxide). Bismuth telluride, a semiconductor, is found to be an excellent thermoelectric material. Additionally, bismuth has been used in electronic solders, which will be the subsequent topics of this series.

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2017

The Role of Bismuth (Bi) in Electronics, Part 1

10-17-2017

In this column series about bismuth, Dr. Jennie Hwang starts with its elemental properties: where it is usually mined, its safety data, and application areas—in the chemical world, the metals industry, and electronics. She also writes about how bismuth compounds improve the performance some electronics devices, such as varistors.

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The Role of Bismuth (Bi) in Electronics: A Prelude

08-24-2017

When it comes to considering applications in electronics and microelectronics industry, over last three decades, the industry has shied away from using bismuth (Bi), at least not in standard practices in mass production. However, an interest has surfaced recently. This article series is tailored to electronics and microelectronics industry, to provide an overview in its entirety in the areas of importance to industry applications going forward.

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Do Acquisitions Bear Fruit? A Pragmatic Perspective

05-02-2017

Acquisition is an effective tool for a company’s growth as a part of corporate growth strategy; and it is one of the top fiduciary duties of a company board’s governance oversight. However, statistically, the acquisition failure rate is quite high. In her column this month, Dr. Jennie Hwang reflects on her hands-on experience as well as observations on mergers and acquisitions in the corporate world.

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2016

The Theory Behind Tin Whisker Phenomena, Part 5

11-23-2016

In this installment of the series on the theory behind tin whisker phenomena, Dr. Jennie Hwang completes the discussion of key processes likely engaged in tin whisker growth—crystal structure and defects.

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New Year Outlook: China’s Five Year Plan

01-25-2016

In this article, Dr. Jennie Hwang writes about the latest developments in the current global economic landscape, as well as mega-technological trends, which include: the highlights of macro-economy outlook, China factor, oil dynamics, cyber security, and grand challenges in technology and the path forward.

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2015

A Look at the Theory Behind Tin Whisker Phenomena, Part 3

11-05-2015

The third installation in Jennie Hwang's five-part series on tin whisker phenomena continues the discussion on key processes engaged in tin whisker growth. She discusses the energy of free surface, recrystallization, and the impact of solubility and external temperature on grain growth.

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The Theory Behind Tin Whisker Phenomena, Part 2

08-06-2015

In the second part of this article series, Dr. Jennie Hwang writes that a plausible theory of tin whisker growth can be postulated through deliberating the combination and confluence of several key metallurgical processes.

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The Theory Behind Tin Whisker Phenomena, Part 1

05-27-2015

In this first article of a five-part series, Dr. Jennie Hwang goes back to basics as she discusses the theory behind the tin whisker phenomena--the reasons and mechanisms behind its occurrence--as well as how tin whiskers can be mitigated in the plating process.

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New Year Outlook: What Can We Expect in 2015?

03-04-2015

Dr. Jennie Hwang takes a long view on market thrusts in the anticipated global economic landscape, as well as mega-technological trends in selected areas deemed timely and relevant to the industry: macro-economy, oil dynamics, China factor, cybersecurity, and grand challenges in technology and the path forward.

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2014

2014: Year-end Review

12-31-2014

In her latest column, Dr. Jennie S. Hwang reviews how predictions in her January 2014 column actually panned out. She goes through the key sub-topics that directly or indirectly impact the industry in terms of macroeconomics, business environment, technology, and the global marketplace. By and large her 2014 outlook was on or close to target.

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Tin Whiskers, Part 6, Preventive and Mitigating Measures: Strategy and Tactics

09-24-2014

In this installment of the tin whisker series, Dr. Jennie S. Hwang takes a look at the preventive and mitigating measures--the strategy and tactics. She says an effective strategy for prevention and mitigation starts with a good understanding of the causations of tin whiskers. A smorgasbord of material and technique options are offered as a guide to prevent or retard tin whiskers.

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Capsulization

08-06-2014

Since lead-free implementation, concerns about tin whiskers have intensified. For the past 12 years, studies and research by various laboratories and organizations have delivered burgeoning reports and papers, and Dr. Hwang has devoted an entire series to this subject. This article aims to capsulize the important areas of the subject.

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Tin Whiskers, Part 5: Impact of Testing Conditions

05-21-2014

Dr. Jennie S. Hwang says, "Real-life stresses may lead a different tin whisker behavior as in accelerated tests (temperature cycling, elevated temperature storage). The alloy-making process to achieve homogeneity needs to be taken into consideration. For an 'impurity' system, how the process that adds elements into tin could also affect the whisker propensity."

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Tin Whiskers, Part 4: Causes and Contributing Factors

03-26-2014

According to Columnist Dr. Jennie S. Hwang, nucleation and growth can be encouraged by stresses introduced during and after the plating process. The sources of these stresses includes residual stresses caused by electroplatin, additional stresses imposed after plating, the induced stresses by foreign elements, and thermally-induced stresses.

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New Year Outlook: What Can We Expect in 2014?

01-29-2014

In her latest column, Dr. Jennie Hwang takes a long view on market thrusts in the anticipated 2014 global economic landscape, as well as technological trends in selected areas important to the SMT industry. Readers, pay attention--her predictions for 2013 were extremely accurate.

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2013, A Year-End Review

01-09-2014

For this year-in-review column, Dr. Jennie S. Hwang checks on whether her January 2013 column, "Outlook for the New Year," is on or off target. She addresses the key sub-topics that directly or indirectly impact the industry in terms of business environment, technology, and global marketplace to see how her predictions actually panned out.

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2013

Tin Whiskers: Concerns & Potential Impact

11-26-2013

What is the biggest concern about the growth of tin whiskers? A simple answer is "uncertainty." If or when tin whiskering occurs, what are likely sources of uncertainty or potential adverse impact? Dr. Jennie Hwang explains that concerns and impact concerning tin whiskers primarily fall into one of four categories.

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Tin Whiskers: Phenomena and Observations

10-09-2013

Tin whisker reflects its coined name. It has long been recognized to be associated with electroplated tin coating and most likely occurs with pure tin. Its appearance resembles whiskers. However, whiskers can also form in a wide range of shapes and sizes, such as fibrous filament-like spiral, nodule, column, and mound.

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Cyber Security: From Boardroom to Factory Floor

08-21-2013

Cyber attacks are and will continue to be a huge concern to U.S. corporations in the foreseeable future. It's a matter of when, not if. It is not industry-specific and every company will have to deal with this challenge. The earlier preparation is made, the better a company is positioned to fend off the attack.

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SMT Perspectives and Prospects: Cyber Security - From Boardroom to Factory Floor

08-21-2013

Cyber attacks are and will continue to be a huge concern to U.S. corporations in the foreseeable future. It's a matter of when, not if. It is not industry-specific and every company will have to deal with this challenge. The earlier preparation is made, the better a company is positioned to fend off the attack.

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Tin Whiskers: Clarity First

06-11-2013

Lead-free solder comprises a wide array of alloy systems and each system can be modified in numerous ways. A test scheme to represent lead-free is a daunting task with an astounding price tag. Dr. Jennie Hwang advises that any tin whisker propensity study be conducted with a specific alloy composition, as clarity is the name of the game.

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SMT Perspectives and Prospects: Conflict Minerals: A Snapshot

04-03-2013

As the supply chain becomes increasingly complex and global, with an ever-increasing number of suppliers, full traceability of conflict minerals throughout the global supply chain is a daunting task. To comply with the SEC’s reporting and disclosure requirement, a company must formulate a comprehensive program to achieving traceability and transparency.

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SMT Perspectives and Prospects: SAC System, A Revisit

03-13-2013

In compliance with the RoHS Directive initiated by the EU and later deployed globally, SAC305 of SnAgCu (SAC) system has been used as a lead-free solder interconnection alloy for both second- and third-level interconnection since the implementation of lead-free electronics. After a 10-year run, Dr. Jennie Hwang takes a look at SAC305 for IC packages and PCB assembly.

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SMT Perspectives and Prospects: Outlook for the New Year

02-06-2013

After protracted high unemployment and lack of a speedy recovery in the U.S., and in the absence of clear solutions to the Eurozone's financial crisis and China's lower manufacturing activities in 2012, will the grim global economic outlook extend to 2013?

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SMT Perspectives and Prospects: 2012 Year-End Review

01-16-2013

Dr. Jennie S. Hwang compares the past year to predictions made in her January 2012 column, "What Can We Expect in 2012?" including business, technology, and global marketplace issues. She feels that, overall, 2012 was another intriguing year filled with both wanted and unwanted events.

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2012

SMT Perspectives and Prospects: Can Microstructure Indicate a Good Solder Joint? Part IV

11-27-2012

How does one examine solder joint microstructure? Is the microstructure important? This month, Dr. Jennie S. Hwang continues a series that addresses the practical aspects of solder joint microstructure and what it can tell us about solder joint reliability. The focus of this offering is the role of the phase diagram in microstructure.

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SMT Perspectives and Prospects: Can Microstructure Indicate a Good Solder Joint? Part III

11-06-2012

How does one examine solder joint microstructure? Is the microstructure important? This month, Dr. Jennie S. Hwang continues a series that addresses the practical aspects of solder joint microstructure and what it can tell us about solder joint reliability.

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SMT Perspectives and Prospects: Can Microstructure Indicate a Good Solder Joint? Part I

09-11-2012

How does one examine solder joint microstructure? Is the microstructure important? This month, Dr. Jennie S. Hwang begins a series that addresses the practical aspects of solder joint microstructure and what it can tell us about solder joint reliability.

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SMT Perspectives and Prospects: 100 Points on Lead-Free Performance and Reliability, Part 2

08-21-2012

In the final of a two-part series, Dr. Jennie S. Hwang takes a wide, sweeping look at the history, timeline, highlights, and future projections for lead-free manufacturing.

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2011

Reliability of Lead-Free System: Part II, The Role of Creep

10-26-2011

The degradation of a solder joint is inevitable. The solder joint intrinsic degradation process engages two scientific phenomena--fatigue and creep. In this article, industry expert Dr. Jennie S. Hwang continues her look at the reliability of the lead-free system with a closer examination of the latter.

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Reliability of Lead-Free System: Part I, Solder Joint Fatigue

09-14-2011

Industry expert Dr. Jennie S. Hwang continues her look at the reliability of the lead-free system this month with a closer examination of solder joint fatigue. Fatigue is one of the most likely culprits for material failure--regardless of metals, polymers or ceramics.

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