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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.

Background
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, Japan^{9, 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 175^oC to 195^oC, 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.

Phenomena
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)

The following list outlines key findings in relation to a variety of lead-free compositions. Studies on fillet-lifting can be summarized as follows^{9, 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.

References

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, et.al, 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.