I recently had the opportunity to discuss several issues in Pb-free die-attach and other solder applications with Jennie Hwang PhD, DSc, and world-renowned consultant in solder and electronics assembly processes.
ACM: What are the emerging performance needs for Pb-free solders in die-attach and similar applications?
Dr Jennie Hwang: Firstly, high-temperature solder alloys that are equivalent to the SnPb system containing more than 89% Pb with balance Sn are needed. This is a technically challenging task: the alloy must possess a combination of performance criteria, such as:
- High melting temperature (specifically, liquidus above 290oC and solidus not below 260oC ),
- High strength
- High ductility
- Absence of multiple phase transitions
Modifications to the SnAgCu system and similar approaches being currently undertaken by some suppliers will not accomplish the goal.
Secondly, a lower reflow process temperature than that used with SnAgCu alloys is required in some applications. In this case, the melting temperature should be lower than 210oC, making the process temperatures closer to that for the SnPb eutectic process at the high end of the peak reflow temperature range of 215 oC ~ 230oC.
Thirdly, a quaternary or higher alloy system is necessary, although intuitively binary or ternary is preferred. Twenty years ago (in late 1980s), our team was commissioned to embark upon a Pb-free solder study under no legislation mandate, nor industry pressure. Our primary purpose was to enhance solder joint reliability for harsh environment applications, meanwhile reducing or eliminating the use of Pb. Selected findings of the study were included in the textbook:"Environment-Friendly Electronics—Lead Free Technology".
About ten years ago (in late 1990s), I advised the industry that a Pb-free alloy in place of SnPb eutectic solder takes four-element alloy compositions because a ternary alloy lacks the capability of providing the right metallurgy. The statement then (in various publications and lectures) was not welcome to a few, but intrigued others. After ten-years or so of Pb-free manufacturing, several companies are now moving in the direction of four-element solder alloys to solve occurring production and performance problems. Some technical approaches are more effective, and some are less. Overall, it is comforting to see my teachings materialize after ten years.
ACM: Why are the choices for Pb-free solder alloys so limited?
Dr Jennie Hwang: This should not have happened, but it has. Basically five factors influence this:
A. U.S. industry has been in denial of the Pb-free reality in the 1990s, thinking that Pb-free would not come. Pb-free was not part of key R&D strategy and business tactics at most companies, especially materials suppliers.
B. Our industry was eager to have an alloy standard. The sentiment and action are humanly understandable, yet have driven the concerted industry effort toward a different route.
C. Along the route of the single alloy standard, some better alloys as the result of early R&D were not seriously considered, thus without broad-based evaluation.
D. For last two-three decades, U.S. universities have essentially stopped producing graduates specializing in Metallurgical Engineering. Instead, Materials Science and Polymer Engineering have been more prevalent. Meanwhile, many experienced and knowledgeable metallurgists have retired. Consequently, true expertise in metallurgy is lacking. I cannot say enough about the contributions of the Metallurgy discipline to Pb-free development. In fact, all test results and field performance, good performance or defects/failure, are expected and anticipated from the basis of metallurgical fundamentals, which is the foundation of solder joint reliability.
E. Aside from solder alloys, other know-how in the paste formulation and manufacturing process are equally important for practical application of the Pb-free alloys. An integrated knowledge in both solder alloy metallurgy and formulation chemistry is scarce.
The demonstrated performance of SnAgCu system is anticipated (discussed in my previous publications and lectures) because its inherent properties in metallurgical phases and microstructure. Although of higher strength than SnPb eutectic, the SnAgCu surface may crack during solder joint formation and subsequent temperature cycling. Failures under mechanical shock, such as drop conditions, are also not a surprise.
ACM: There are other technologies that may allow Pb-free electrical and thermal joining: what are their advantages and disadvantages?
Dr Jennie Hwang: Polymer-based or other materials that are not classified as soft solders have been actively pursued in the electronic packaging and assembly industries for more than three decades. For instance, conductive adhesives are under development by incorporating conductive particles or developing intrinsic material conductivity into a polymer matrix. This effort is continuing, and significant technical advancements have been made in conductive adhesives.
Top critical performance deficiencies fall in the areas of high-volume automation-adaptability, environmental stability (particularly to moisture) and the level of conductivity, thermally or electrically, depending on the application.
Until the fulfillment of the required performance for this specific industry sector, advantages are not relevant at this point.
ACM: Why are some customers also eliminating antimony (Sb) as well as Pb?
Dr Jennie Hwang: There is discussion and debate about the benign nature of Sb or lack of. In any event, Sb does not impart the same level of health or environmental hazards as Pb in soldering. One reason for such discussions is that some Sb compounds, namely, antimony trioxide are considered highly toxic, but standard reflow soldering of Sb-containing solders does not produce such compounds.
In measures by authorized organizations, such as the Agency for Toxic Substances and Disease Registry (ATSDR), EPA, Toxicity Characteristic Leaching Procedure (TCLP), Occupational Safety & Health Administration (OSHA) air-borne limits, Sb is ranked less toxic than Ag and Cu, and is not classified as a carcinogen.
Scientifically and technically, Sb can be beneficial to the mechanical properties of SnPb and Pb-free solders, particularly creep resistance, as detailed in my book "Solder Paste: Technology and Applications for Surface Mount, Hybrid Circuits, and IC Component Manufacturing", p. 120. However, Sb can also retard spreading at the dosage range of 2wt% to 6wt% or even lower, depending on the alloy composition.
Jennie: Many thanks for your time and excellent technical insights.