The Role of Bismuth (Bi) in Electronics, Part 4

To link science and technology with commercial applications, Part 4 of this series continues to address two pivotal questions: Why SAC is not able to be a universal interconnecting material for electronic circuits, and why a quaternary alloy system offers a more wholesome approach (note: a quaternary system referred herein does not include SAC incorporated with one or more doping elements).

The baseline for designing a viable solder joint material for applications in advanced electronic circuit boards that require increasingly higher functionalities and higher power in a small form factor is to deliver reliable physical properties and mechanical performance that are not lower than 63Sn37Pb alloy.

Another baseline characteristic is to provide an alloy that can adapt to the established electronics manufacturing infrastructure including production flow, process parameters and compatibility with the thermal stability of a PCB and a variety of components. One of the critical process parameters is the required minimum peak temperature in the reflow process that enables mass production capable of delivering high throughput without introducing production defects and/or insidious thermal damage to the internal structure of a PCB and components. To this end, the alloy shall possess "agile" wetting ability and a compatible melting (liquidus) temperature. In this regard, wetting ability controls not only interfacial metallurgical interaction but also the rate of interaction that needs to be in sync with the inherent characteristics of the reflow process.

With these baselines in mind, a viable alloy should possess an adequate thermal fatigue resistance to withstand the increasingly adverse and harsh conditions in microelectronics and electronic applications while providing a moderate melting temperature (170°C–215°C, more desirably 175°C–213°C) suitable for sound manufacturability without causing undue thermal damages.

This was the genesis of designing quaternary alloys, as stated in many of my professional development courses, workshops, webinars and publications since the late 1980s, including the U.S. Patent 6,176,947 (1999).

The ability to deliver a higher performance level in thermal fatigue resistance is particularly critical to connecting those high-power, large-size IC components onto the circuit board, as these components impose a larger amount of thermal stress on solder joints during power-on/power-off and/or elevated temperature excursion. In mobile electronics, these thermal excursion-related stresses may be compounded with machinal-shock related stresses that could be occurring during the product’s useful life.

SnAgCuBi is one of the quaternary systems studied. Again, it is important to accentuate that the scientific base to design the SnAg-CuBi system was not to add an element (in this case, Bi) to an SAC alloy. Rather, it was a material innovation holistically using the underlying science and engineering of metallurgical principles by taking the commonly-occurring solder joint failure mechanisms into consideration. In other words, the objective was to mitigate those likely failure mechanisms so that solder joints can reliably connect the ever-powerful semiconductor chips to the outside world by serving as electrical, thermal and physical conduits at chip level, package level and on circuit boards.

How do the proper compositions of the SnAgCuBi alloy system (containing 2.5–3.5% Ag, 0.2–2.5% Cu, 0.5–4.0% Bi, balance Sn) perform in comparison with the standard alloys? (Note: All compositions expressed herein are in weight percent.)

Comparison with SnPb Eutectic—63Sn37Pb

As an example, take Sn3.0Ag0.5Cu2.0Bi as the composition. It offers higher strength as well as more than 200% higher fatigue life than 63Sn37Pb in accordance to ASTM Standard E606-92 (Standard Practice for Strain-Controlled Fatigue Testing).

Comparison with SnAg Eutectic—96.5Sn3.5Ag

The composition of Sn3.0Ag0.5Cu3.0Bi has a melting temperature 209-212°C that is 9°C lower than the eutectic 96.5Sn3.5Ag (221°C). When comparing the basic mechanical properties with 96.5Sn3.5Ag, Sn3.0Ag0.5Cu3.0Bi composition performs better in strength and fatigue life—more than 150% higher in fatigue life.

Comparison with SnCu Eutectic—99.3Sn0.7Cu

Sn3.0Ag0.5Cu3.0Bi demonstrates significantly better performance in strength and fatigue, but lower plasticity than 99.3Sn0.7Cu. Its melting temperature is 15°C lower than 99.3Sn0.7Cu.

Comparison with SnAgCu Near-eutectic—Sn3.0Ag0.5Cu (SAC305)

Sn3.0Ag0.5Cu2.0Bi exhibits high strength (both yield and tensile strengths and higher thermal fatigue life). Another important advantage of SnAgCuBi over SnAgCu is the ability to offer lower liquidus temperature. The composition of Sn3.0Ag0.5Cu2.0Bi offers 7°C lower than SAC305. Further, the intrinsic wetting ability of SAC system does not measure up to that of SnPb or SnCu. With SAC305’s high liqiudus temperature, the tendency to use a process peak temperature below the optimal temperature often leads to a marginal process, which further aggravates the SAC305’s lower wetting ability, thus increasing potential production defects.

Focusing on the integrity of a printed circuit board assembly, the liquidus temperature of the interconnecting solder alloy plays an important role in alleviating any potential defects or thermal damages to components or PCB, which can be detectable or undetectable on the production floor or during quality control verification. Concentrating on solder joint reliability, the thermal fatigue resistance sits front and center to the performance and reliability of a circuit board.

Overall, the SnAgCuBi system offers more robust performance than any of practical binary alloys, such as 63Sn37Pb, 96.5Sn3.5Ag, or 99.3Sn0.7Cu, and ternary alloys, such as SnAgBi and SnAgCu. In comparison with SAC305, Sn3.0Ag0.5Cu2.0Bi exhibits higher strength (both yield and tensile strengths). More importantly, its thermal fatigue life is higher under harsh conditions (e.g., a large temperature swing, high-temperature excursion). Further important advantages of SnAg-CuBi over SnAgCu are its lower melting temperature and superior intrinsic wetting ability. The practical compositions can offer as much as 9°C lower in melting temperature than SAC305.

Melting at a few degrees lower, SnAgCuBi compositions are advantageously positioned for circuit board manufacturing. Embracing diverse PCB assemblies and process window requirements to achieve high yield, low-production defect rates, an alloy having a melting temperature below 215°C is considered necessary to deliver robust manufacturability.

Part 5 in this series will outline the underlying operating mechanism among the four elements (Sn, Ag, Cu, Bi) of SnAgCuBi system and elemental dosages in relation to desirable performance properties.

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2018

The Role of Bismuth (Bi) in Electronics, Part 4

06-04-2018

Dr. Jennie Hwang's column series continues in Part 4, which addresses two pivotal questions: Why SAC is not able to be a universal interconnecting material for electronic circuits, and why a quaternary alloy system offers a more wholesome approach.

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New Year Resolutions and Best Wishes

03-12-2018

The New Year stands before us, like a chapter in a book, waiting to be written. We can help write that story by setting goals. But the true challenge is to keep these goals from falling into a wish list and to know how to stick to those goals and when. I hope that in this year to come, goals give us direction in whatever we do, be it on AI, 5G, mixed reality and quantum computing or the next chip design.

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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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