SMT Perspectives and Prospects: The Role of Bismuth (Bi) in Electronics, Part 7: A Case Study in Fillet-Lifting

In my column series on “The Role of Bismuth (Bi) in Electronics,” I have addressed the properties, safety, resources of elemental bismuth (Bi), the effects of Bi in SnPb, and the effects of Bi on the properties and performance of solder interconnections when Bi is not a constituent element in lead-free solder alloys.

The intricate interactions of the four elements—Sn, Ag, Cu, Bi—in a Sn-based lead-free SnAgCuBi or SnAgCuBi+ systems (“+” denotes other doping elements incorporated in the system) were also previously highlighted. Additionally, the plausible underlying operating mechanisms and resulting mechanical behavior of a lead-free alloy containing these commonly and likely-used elements were outlined. This installation focuses on a case study illustrating a problematic phenomenon called fillet-lifting, what lessons have been learned, and how these findings have helped and will continue to help develop new lead-free alloys.

The fillet-lifting phenomenon has “inspired” fruitful thoughts and deeper examinations in the role of Bi in a Sn-based alloy system. This understanding has substantially helped shorten the development time to reach viable alloy formulae that can deliver the desired performance under thermal fatigue environments that microelectronic/electronic products inevitably encounter.

A decade before the Restriction of Hazardous Substances Directive (RoHS) was proposed in December 2000 and adopted in February 2003 by the European Union (and then implemented by the U.S., Japan and other countries), much research and development efforts have been conducted by individual laboratories in the U.S.1–8, Japan9, 10, and consortia. One of the primary performance targets was to have a “drop-in” or “nearly drop-in” replacement for SnPb eutectic alloy so that the SMT manufacturing infrastructure, including PCB materials, reflow, and wave soldering processes could remain intact without being subject to disruptions.

With comprehensive studies and thorough examinations of the potential of “logical” alloys, including perusing the entire periodic table, it was found that the most challenging property to be delivered was (and still is) to keep the melting temperature (liquidus temperature) of the lead-free alloy in the range of 175oC to 195oC, while meeting all other necessary properties and manufacturing requirements.

Without delving into historical details and granularity, in order to keep the melting temperature low enough, one approach adopted was to add the element Bi to the Sn-based system.


A phenomenon termed as fillet-lifting was reported in the late 1990s and early 2000s, which refers to the partial separation (crack) of the solder fillet between the solder and the through-hole land on the PCB after the completion of wave soldering (Figure 1). It is worthwhile noting that this phenomenon has hardly been observed in surface mount solder joints; however, it repeatedly occurred with through-hole joints. It should also be noted that such fillet-lifting phenomena were evident immediately after processing (before being subjected to any accelerated reliability testing).

Overall, key observations were:

  • Fillet-lifting were associated with through-hole solder joints after wave soldering
  • There was no detectible solder joint separation (crack) associated with SMT components
  • The solder joint crack often started from the far end of the through-hole joint (Figure 2)

Jennie_Apr23_Fig2_cap.jpgThe following list outlines key findings in relation to a variety of lead-free compositions. Studies on fillet-lifting can be summarized as follows9, 10, where fillet-lifting occurred in:

  • More than 90% of through-hole joints with Sn3.4Ag>4.8Bi
  • More than 90% of through-hole joints with SnAg>7.5Bi
  • Severe occurrence in through-hole joints with various lead-free alloys (in the absence of Bi) when SnPb-coated components were used
  • Some through-hole joints with Sn3.5Ag0.5Cu1Zn
  • Some through-hole joints with Sn2.6Ag0.8Cu0.5Sb
  • 30% of through-hole joints with 96.5Sn3.5Ag
  • 0% of through-hole joints with 58Bi42Sn
  • 0% of through-hole joints with 63Sn37Pb
  • 0% of surface mount joint cracks with any alloys tested

Factors and Causes
Based on the observation that the fillet crack appears to initiate from the far end of the land from the barrel in conjunction with the finding that the crack occurs only with through-hole joints, fillet-lifting was attributed to the excessive stress generated during the cooling and solder solidification of an assembly. The contributors to this excessive stress may emanate from several sources:

  • Cooling rate
  • Maximum temperature gradient (solder pot temperature)
  • PCB construction (land design, board thickness)
  • Solder alloy composition (melting temperature, metallurgical pasty range, metallurgical phases)
  • Solder alloy (stress and strain behavior)
  • Wetting ability (intrinsic alloy wetting ability)
  • Wettability of PCB land

In most practical cases, fillet-lifting was highly likely contributed to from more than one of the above factors.

Metallurgically, the phenomenon was also considered a result of segregation during solidification and/or the formation of low melting Sn-Bi eutectic phase, when applicable. If or when low-melting SnBi eutectic or other low-melting phases are formed, low-melting phases may be a culprit. Nonetheless, low temperature Sn-Bi phase does not always form when Bi is just present.

Fillet-lifting may or may not cause a circuit board failure or a product failure. However, even when there is no mechanical or electrical failure, the fillet-lifting phenomenon should be examined and remediated.

Remediation and Prevention
To alleviate the problem from the assembly operation (when PCB design is given and not subject to change), the following areas are to be considered:

In process:

  • To lower the cooling rate
  • To avoid using high soldering temperature, if feasible

In material:

  • To choose alloy composition properly (e.g., possessing adequate plastic strain)
  • To choose alloy composition with narrow pasty range (less than 10 degrees)
  • To choose alloy composition having good intrinsic metallurgical wetting
  • To assure adequate wetting condition on PCB land
  • To assure the compatible soldering flux

What Was Learned
Achieving one performance property that could jeopardize another performance parameter should be avoided at the outset (i.e., design stage). This may sound as though it’s stating the obvious, but the mishap resulting from the lack of holistic understanding of process, materials, and the compatibility between the process and materials has happened, and happened too frequently.

Bi is a unique element which can deliver significant utilities that are beneficial to electronic products, but it has to be used properly and scientifically.

The interplay of the four elements—Sn, Ag, Cu, Bi—in Sn-based lead-free systems is intricate in the underlying metallurgical interactions and reactions. For Sn-based lead-free alloy system, adding or removing an element, solely reducing or increasing the dosage of an element to target one property or performance parameter is not a robust approach from the reliability perspective.

Manufacturing process, materials, and compatibility are closely intertwined in relation to reliability; setting, selecting, and designing an electronic system must consider process, material, and compatibility as an inter-dependent triad.

Caution (Awareness)
The discussion in this writing is on a Sn-based system that contains Bi element—in practice, the “base” is practically defined as the element that constitutes 50 vol% or greater of an alloy (i.e., serving as the metallurgical matrix). Distinctions should be drawn between Bi-containing Sn-based alloys and Bi-based alloys. This distinction is profoundly important when we design, select, and use lead-free solder alloys for microelectronic/electronic packaging and assemblies.

  • Bi-containing Sn-based alloys

e.g., 93Sn3.0Ag1.0Cu3.0Bi

e.g., 93Sn3.0Ag1.0Cu1.0Bi2.0XYZ

  • Bi-based alloys

e.g., 57Bi41Sn2Ag

e.g., 57Bi41Sn1.5Ag0.5XYZ (dopants)

The distinction in properties, performance, and most importantly to the reliability of electronic products, will be discussed in future columns.


  1. U.S. Patent 5,520,752: “Composite Solders,” May 28, 1996.
  2. U.S. Patent 5,985,212: “High Strength Lead-Free Solder Materials,” Nov. 16, 1999.
  3. U.S. Patent 6,176,947 B1: “Lead-Free Solders,” Jan. 23, 2001.
  4. Internal Research Reports, Z. Guo and J. S. Hwang, H-Technologies Group, 1991–2002.
  5. Internal Research Reports, J. S. Hwang and H. Koenigsmann, H-Technologies Group, 2000–2007.
  6. “High Strength and High-Fatigue-Resistant Lead-free Solder,” by J. S. Hwang and H. Koenigsmann, SMT007 Magazine, March 2000, Page 55.
  7. “Effect of Bi Contamination on SnPb Eutectic Solder,” by J. S. Hwang and Z. Guo, SMT007 Magazine, September 2000, Page 91.
  8. “Effect of Pb Contamination on lead Free Solder—Part 1 to Part 8,” J. S. Hwang and Z. Guo, Chip Scale Review, December 2000–December 2001.
  9. “Research and Development for Lead-free Soldering in Japan,” by K. Suganuma, Proceedings, IPC Works, 1999.
  10.  “Challenges and Solutions for Lead-free Soldering of Large PCB Assembly,” by T. Baggio,, Proceedings, IPC APEX EXPO 2000.

Appearances: As a part of IPC Engineering Education Webinar Series, Dr. Hwang will present lectures on “PoP and BTC Package and Assembly: Materials, Process and Reliability” April 4, 6, 11, 13, 18, and 20; and on “Lead-free Reliability for Harsh Environments Electronics,” May 16–18 and Nov. 7–9; and “Top Lead-free Production Issues – Causes, Prevention & Solutions” July 11, 13, 18, and 20.

This column originally appeared in the April 2023 issue of SMT007 Magazine.



SMT Perspectives and Prospects: The Role of Bismuth (Bi) in Electronics, Part 7: A Case Study in Fillet-Lifting


In my column series on “The Role of Bismuth (Bi) in Electronics,” I have addressed the properties, safety, resources of elemental Bismuth (Bi), the effects of Bi in SnPb, and the effects of Bi on the properties and performance of solder interconnections when Bi is not a constituent element in lead-free solder alloys.

View Story

SMT Perspectives & Prospects: Creating a Better World Through Engineering


What is the role of an engineer? In my definition, an engineer plays a crucial role in bridging science and society. In this spirit, The National Academies of Engineering (one of the triad Academies of The National Academy of Science, Engineering, and Medicine) recently initiated a video interview series with the theme, “Today’s Engineers—Creating a Better World.” I was invited to be one of several interviewees for the series. What follows are excerpts from that conversation.

View Story


SMT Perspectives and Prospects: Cybersecurity Requires an Active Approach


I last wrote about cybersecurity nearly 10 years ago in my column, “Cybersecurity—from Boardroom to Factory Floor.” So, where do we stand on cybersecurity? As the digital world continues pushing ahead, it comes with new challenges in the cyberspace. Individual systems and/or infrastructure systems are subject to attacks by increasingly savvy adversaries who can leverage new and emerging technologies. A cyberattack can be surreptitiously detrimental, crippling business operation, the national economy and security, or just jeopardizing an individual laptop. This pervasive and persistent security threat is one of the most formidable challenges of our times.

View Story

SMT Perspectives and Prospects: In Search of Wisdom


After two-year COVID-19 hiatus, I made a point to attend the Berkshire Hathaway (BH) shareholders annual meeting in person, so I could listen to legend Warren Buffett and observe the dynamic of the aspiring people from all walks of life. The event was full of high-spirited, eager attendees; it was quite festive as well. Buffet, now age 91, and his long-time business partner Charlie Munger, age 98, are as sharp, quick-witted, and as humorous as ever. Their deep and profound thought process and broad-based knowledge are inspiring. It was a completely worthy trip; I personally felt intellectually nourished. Their investment wisdom is vividly manifested by the indisputably phenomenal results.

View Story

SMT Perspectives and Prospects: Critical Materials—A Compelling Case, Part 2


When I wrote Part 1 on this topic in January, the global geopolitical landscape could be characterized as “status quo”—testy, challenging, yet absent of “war” in any region of the world. Now with Russia’s invasion of Ukraine, which elevates the peril and uncertainty of metals, minerals and materials into overdrive, the title of the article may warrant: “Critical Materials – A Precariously Escalated Compelling Case.”

View Story

SMT Perspectives and Prospects: Critical Materials, A Compelling Case, Part 1


It has come the time for a national strategy, in a deliberative and comprehensive manner, to address the critical materials/minerals. Doing so is increasingly critical to the long-term economy, national security, and the nation’s global competitiveness. With the handling of conflict minerals as an exemplar, there is perhaps an even more urgent need to rally another concerted effort to tackle the critical materials/minerals. Overall, critical materials/minerals will have an overarching impact on the entire supply chain to all industries, and once again, electronics/microelectronics is on the front line.

View Story


SMT Perspectives and Prospects: Reliability Primer—A Pragmatic SMT Perspective


When we look at the reliability of a product, be it associated with a physical product or virtual service, there is a set of performance expected from the users or the customers.

View Story

SMT Perspectives and Prospects: Digital Manufacturing—Just-in-Case or Just-in-Time


Under the dynamic global-macro factors and the burgeoning digital manufacturing platforms, the construct that is solely based on just-in-time inventory management as a stand-alone practice could be proven inadequate. Considering both just-in-time and just-in-case appear to be a pragmatic model to operate in the digitized enterprise; perhaps a “comforting” approach as well.

View Story

SMT Perspectives and Prospects: Ebullient Trade Events Going Virtual


Jennie Hwang reflects on past and current versions of CES and IPC APEX EXPO, and outlines her two presentations at APEX.

View Story


SMT Perspectives and Prospects: Joint Industry Standard IPC J-STD-00-Electronic Solder Alloys, Part 2


In Part 2 of her column series on requirements for electronic-grade solder alloys and fluxed and non-fluxed solid solders for electronic soldering applications, Jennie Hwang addresses questions raised regarding the subject industry standard IPC J-STD-006. She also summarizes relevant background information, the options for plausible naming systems, and the logic behind the decision to adopt the current naming system.

View Story

SMT Perspectives and Prospects: Rethinking Manufacturing–Bracing for and Embracing a Post-Pandemic Decade


Against the potent backdrop of current events, how should our industry respond? How should we manage and rethink manufacturing? And what are the main issues at hand in near-term and long-term horizons? Dr. Jennie Hwang explores these questions, as well as three tangible areas of business and manufacturing.

View Story

SMT Perspectives and Prospects: Smart Factory Implementation—How Smart Is Smart Enough?


As we are moving further into the Industry 4.0 era, rigidity is out, and flexibility is in; stiffness is out, and agility is in; and sluggishness is out, and swiftness is in. Dr. Jennie Hwang explains how manufacturing companies need to develop a thorough understanding of the available technologies that can be utilized to translate business objectives into business roadmaps targeting operational excellence to produce competitive, reliable, and economic products that perform in a timely fashion in the marketplace.

View Story

SMT Perspectives and Prospects: Joint Industry Standard IPC J-STD-006—Electronic Solder Alloys


It has been nearly two decades since the global electronics industry adopted lead-free conversion from leaded electronics. Readers who have been in the industry during this period will recognize the changes and challenges the industry has faced and appreciate the fact that taking the element lead (Pb) out of electronics has not been a straightforward path.

View Story


SMT Perspectives and Prospects—Revisiting Globalization: Technology, Jobs, Trade


In 2004, Dr. Jennie Hwang wrote a column titled “Globalization: Technology, Jobs, Trade,” which was published in the July issue of SMT007 Magazine. Amid the protracted and roller-coaster trade uncertainty between the U.S. and China, and the renewed debate on globalization, she revisits the topic. What has changed over the last 15 years? Where do we stand today? Is globalization undergoing a retreat or reverse course?

View Story

Learn From the Wise


How can we get ahead in this digital world inundated with a gargantuan amount of information available to all? More sustainably, how can we stay ahead of the curve? Knowledge and wisdom are the fuel to propel us ahead; learning from the wise is the speedier path to acquire the fuel.

View Story

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


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).

View Story

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


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.

View Story
Copyright © 2023 I-Connect007 | IPC Publishing Group Inc. All rights reserved.