The Role of Bismuth (Bi) in Electronics, Part 6

In this installment of my column series on the role of bismuth (Bi) in electronic products, I’ll look at the effects of Bi on the properties and performance of solder interconnections in electronic products when Bi is not contained in the solder alloy for the SMT assembly process (Bi-absent solder alloy composition of solder paste). The effects of Bi in solder joints are created by an unintentional path through the supply chain, which introduces Bi into the resulting solder alloy of solder joints. The performance and reliability of the resulting solder joint impacted by the presence of Bi can vary; it can be beneficial or detrimental or no-detectable-effect.

From the supply chain in electronics assembly, Bi can come from component lead coating, passive component termination coating, and PCB surface finishing that are Bi-containing material (albeit, this has not been a common PCB surface finish in recent years). Accordingly, even in a Bi-absent assembly process (e.g., using SAC solder paste or SnPb solder paste), the changes in the properties and performance of solder joints due to the introduction of Bi into the solder joint could occur. Similarly, for BGA components, the Bi-containing solder ball will make Bi-containing solder joint.

In SMT assembly after reflow, the composition of solder joints is expected to deviate from the composition of the solder alloy used in solder paste. The compositional change in solder joints as a result of Bi contributions from one or both of component and PCB surface finish should not be dismissed. Bi contribution from component leads (component surface coating) to the composition of the solder joint, while using Bi-absent solder paste for SMT assembly, depends on the:

• Type of component

• Configuration and dimensions of the component lead

• Surface area of component leads embedded in the solder joint

• Thickness of the coating

• Resulting solder joint volume (including the solder paste volume)

• Substrate surface metal (e.g., Cu vs. Ni)

As an illustration, one study focused on 20-mil (0.5-mm) pitch QFP208 with Cu-lead or Alloy 42 lead-coated with SnPb or SnBi, using SAC305 or SnPb solder pastes reflowed at 245°C (for SAC305) or 220°C (for SnPb). Test results indicated that SnBi coating/SAC solder paste performed better than SnBi coating/SnPb solder paste, which was better than SnPb coating/SnPb solder paste under accelerated temperature cycling (-40–125°C, 10 minutes dwell). However, with the same system except for the component lead material (replacing Cu leads by Alloy 42 leads), both SnBi coating/SAC solder paste and SnPb coating/SnPb solder paste performed better than SnBi coating/ SnPb solder paste[1–3].

For BGA components, a Bi-containing solder ball is expected to contribute to the resulting solder composition in a much significant proportion compared to leaded-components. Nonetheless, the resulting solder joint composition will contain less Bi than in a BGA solder ball composition when a Bi-absent solder paste is used during SMT assembly. Bi contribution from BGA component to the composition of the solder joint depends on the:

• Diameter of BGA solder ball

• Resulting solder joint volume (including the solder paste volume)

• Substrate surface metal (e.g., Cu vs. Ni)

Regarding surface finish, one study examined the effect of Bi-coated PCB pads on the solder joint integrity using SnPb eutectic solder paste [4]. The PCBs were deposited with 4–6 microinches of pure Bi and assembled using LCC and QFP components under surface mount processes. In comparing the Bi finish with the SnPb HASL finish, the fatigue data exhibited that two surface finishes essentially imparted similar thermal fatigue results in terms of the failure percentage at given temperature cycles. Visual inspection also revealed that the solder joints have the same general appearance after temperature cycling.

Thus, Bi contribution from the PCB surface finish to the composition of the solder joint depends on the:

• Thickness of the surface coating

• Dimensions of pad

• Resulting solder joint volume

Bi is a unique metal that can offer multiple positive effects on solder joint performance (outlined in Part 2, Part 3, and Part 4 of this column series). In Sn-based binary solder alloy, its two-phase phase diagram possesses multiple strengthening mechanisms (Figure 1). There are opportunities to maneuver the microstructure through compositional tailoring and process condition variations.

Hwang-Fig1-May2019.jpgFigure 1: Schematic of Sn-Cu phase diagram.

However, Bi is a brittle metal and has a finite solid solubility in an Sn matrix. The Bi precipitation process is expected to be additive to other strengthening phenomena. There is a natural breakdown in the relationship between yield strength and Bi volume fraction as a result of the transition of the strengthening mechanism. Bi must be used properly to eschew any likely adverse effects in the reliability of solder joint, which may lead to likely product failure.

To utilize the benefits that Bi can offer in forming electronic solder interconnections, a specified dosage in a specific alloy composition system is required; the knowledge of its intricate interplay with other constituents in an alloy composition is indispensable. An understanding of the design-for-performance demands as well as application constraints is also a prerequisite.

Overall, the concentration of Bi in solder joint has to be carefully designed. Unfortunately, many studies and testing programs with an intent to compare Bi-containing alloys with a Bi-absent solder alloy have often selected a Bi dosage apart from what is required or desired. This lack of proper composition (a proper Bi dosage in a specific system) has contributed to highly publicized negative test results. Thus, this has impeded the application of Bi in Sn-based solder alloy systems during the first decade of deployment of lead-free solder interconnecting materials for producing electronic products.

References

1. H-Technologies Group Inc. “Internal Reports,” 1990–1999.

2. Hwang, J. “Lead-free Implementation Series, Part 6—The Role of Bismuth,” IPC Professional Development Courses, 1999–2010.

3. Hwang, J. Environment-friendly Electronics—Lead-free Technology, “Chapter 28: The Role of Bismuth,” Electrochemical Publications Ltd., Great Britain, 2001.

4. Hwang, J. Environment-friendly Electronics—Lead-free Technology, Electrochemical Publications Ltd., Great Britain, 2001, p. 758.

Dr. Jennie S. Hwang is currently CEO of H-Technologies Group providing business, technology and manufacturing solutions.

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2019

The Role of Bismuth (Bi) in Electronics, Part 6

05-10-2019

In this installment of this column series on the role of bismuth (Bi) in electronic products, Dr. Jennie Hwang looks at the effects of Bi on the properties and performance of solder interconnections in electronic products when Bi is not contained in the solder alloy for the SMT assembly process (Bi-absent solder alloy composition of solder paste).

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The Role of Bismuth (Bi) in Electronics, Part 5

03-07-2019

The fifth part of this column series addresses the most interesting, yet intricate, aspect of the subject—plausible underlying operating mechanisms among the four elements (Sn, Ag, Cu, Bi) in a SnAgCuBi system. This article features illustrations on relative elemental dosages in relation to relevant properties and performance.

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2018

SMT Manufacturing: Why Soldering?

11-15-2018

Upcoming AI hardware requires advanced semiconductors, packaging approaches, new architectures, increased speeds and capabilities of inference processing, and system design and manufacturing prowess continually developed to reach the interconnect density. Against this backdrop, packaging and assembly levels will continue to be critical technology and serve as the backbone of manufacturing electronic hardware to deliver desired products with enhanced miniaturization, functionality, and augmented intelligence promptly.

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Artificial Intelligence: Super-Exciting, Ultra-Competitive

09-18-2018

Artificial intelligence (AI) and machine learning (ML) have become common everyday words, however, the present reality and future potential are yet to evolve. This article looks into the key considerations and strategies to better leverage these trends that are expected to transform the manufacturing world.

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The Role of Bismuth (Bi) in Electronics, Part 3

08-08-2018

The third part of this column series aims to answer why SAC isn't able to become a universal interconnecting material for electronic circuits, and why a quaternary alloy system offer a more wholesome approach.

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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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2002

Lead-free Symposium at APEX

04-15-2002

Amid a variety of programs, lead-free technology was "a conference in a conference" at APEX 2002, in San Diego. The program was designed with six focal themes covering components, printed wiring board (PWB) surface finishes, materials, assembly processes, test and rework, and reliability. Additionally, SMT Magazine developed an illuminative panel forum and NEMI discussed a summary report by NEMI

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