The Role of Bismuth (Bi) in Electronics, Part 2

Click here to see Part 1 of this column series.

Part 2 of the series outlines the Bi effects on 63Sn37Pb solder material, which have been substantiated by years of field performance prior to lead-free implementation. This should serve as the sound baseline for further discussion on the subject.

The incorporation of Bi in Sn-containing solders is expected to affect both physical properties and mechanical properties of the resulting solder materials. This includes melting temperature, wetting ability, strength, plastic strain and fatigue behavior. The direct addition of a sufficient amount of Bi to a eutectic alloy (e.g., 63Sn-37Pb) also alters its eutectic behavior or deviates from eutecticity. The DSC thermogram below of 63Sn37Pb plus 1 wt.% Bi indicates that 63Sn37Pb essentially maintains its eutectic property. However, at 2 wt.% Bi, the range of melting starts to appear, departing from the eutectic point.

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Figure 1: DSC Thermogram of 63Sn37Pb added with 1 wt.% Bi.

An extensive study was carried out on the effects of a minor addition of Bi to SnPb eutectic solder. The following table summarizes the effects of addition of Bi to 63Sn37Pb up to 5 wt.% on the basic mechanical properties and the melting temperature. The dosages of 2 wt.% and 5 wt.% Bi were added to 63Sn37Pb, respectively, by separately replacing Sn or Pb. In addition, tests were performed on alloys with Bi replacing an equal amount of both Sn and Pb. This resulted in six solder alloy compositions. The solder alloy compositions along with their melting temperature (Tm), yield strength (sy), tensile strength (sTS), Young’s modulus (E), plastic strain (ep) at fracture, and fatigue life (Nf) at a total strain of 0.2% are summarized in Table 1. All compositions are expressed in weight percent unless otherwise specified. Also included is the reference solder alloy of 63Sn37Pb.

As exhibited in Table 1, the addition of 2wt.% Bi depressed the original melting temperature of 63Sn37Pb by 2–3°C. There was almost no distinction in the melting temperature change when 2 wt.% Bi replaced Sn or Pb or both Sn and Pb in an equal amount. At 5 wt.% Bi, both the alloy liquidus temperature and solidus temperature were lowered. The melting temperature for the solder alloys with 5 wt.% Bi in place of Sn (Alloy 5) was about 2–3°C lower than that with 5 wt.% Bi in place of Pb (Alloy 6). This indicates that Bi at 5 wt.% lowers the melting temperatures of Pb-rich phase more effectively than Sn-rich phase.

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Table 1: Melting temperature range and mechanical properties of 63Sn37Pb containing 2 – 5 wt.% Bi.

Comparing the strength of the solder alloys containing 2 wt.% Bi with that of 63Sn37Pb, the Bi addition largely increased the alloy strength and plasticity. There were no measurable differences in the tensile behavior among the solder compositions containing 2 wt.% Bi in place of Sn (Alloy 2) or Pb (Alloy 3) or equally both Sn and Pb (Alloy 1). When the content of Bi in 63Sn37Pb increased to 5 wt.%, the strength maintained, but the alloy plasticity reduced. The differences in tensile behavior among the solder compositions containing 5 wt.% Bi in place of Sn (Alloy 5) or Pb (Alloy 6) or equally both Sn and Pb (Alloy 4) were within the data-scattering range.

The content of Bi up to 2 wt.% was the most effective amount to increase both the alloy strength and plasticity. Any further increase in the Bi content from 2 wt.% to 5 wt.% exhibited little effect on the alloy strength, however significantly reduced the alloy plasticity.

The fatigue life (Nf) increased with the Bi content up to 5 wt.%, a contrast to the reduction in plasticity at 5 wt.% Bi. This is attributed to the amplitude of fatigue strain range used (De = 0.2%), which is well below the plastic strain at fracture (ep = ~12%). Under the relatively low fatigue strain range, the high strength is a leading factor contributing to the high fatigue resistance.

In a Bi-Pb system, a solid solubility of Bi in Pb is 23.5 wt.%, and a Sn-Bi system indicates a solid solubility of Bi in Sn is 21 wt.%. When Sn-Pb-Bi constructs a ternary system, the underlying basic physical interactions are not expected to be grossly changed. Overall, the Sn-Pb-Bi ternary solder alloys containing 2 wt.% Bi demonstrated a much higher strength, a higher fatigue life, as well as a higher plasticity than 63Sn37Pb eutectic solder. The alloy melting temperatures slightly decreased. When Bi increased to 5 wt.%, the strength and fatigue life of the Sn-Pb-Bi still remained higher than 63Sn37Pb, but their plasticity decreased significantly.

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2017

The Role of Bismuth (Bi) in Electronics, Part 2

12-08-2017

Part 2 of Dr. Jennie Hwang's article series outlines the Bi effects on 63Sn37Pb solder material, which have been substantiated by years of field performance prior to lead-free implementation. This should serve as the sound baseline for further discussion on the subject.

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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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