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Solder Materials
December 31, 1969 |Estimated reading time: 8 minutes
By Jennie S. Hwang
Around the globe, a main action item for 2003 is implementation of lead-free processing. This third step focuses on its technology and manufacturing.
Any viable lead-free solders used in lieu of Sn/Pb eutectic or near-eutectic compositions could not escape from the Sn-based system (i.e., a minimum of 60 wt percent of tin). This was concluded based on both fundamental materials science and practical applications. Fundamentals include metallurgical bonding capability on commonly used substrates, dynamic wetting ability under the practical reflow conditions and metallurgical "interactions" or alloying phenomena between elements. Practical factors cover the availability of natural resources, manufacturability, toxicity and cost. In selecting elements and their respective dosages, their alloying abilities with Sn and properties in melting point depression while alloying with Sn are the two crucial material characteristics in the design of lead-free solders.
Relative wetting ability of lead-free alloys measured by the Wetting Balance test. Three specific compositions can deliver significantly improved wetting performance over those of the near-eutectic alloys.
For an Sn-matrix, candidates that can serve as viable alloying elements are in quite a small number — practically being limited to Ag, Bi, Cu, In, Sb, Ga. Metallurgical interactions (reactions) and microstructure evolution in relation to rising temperatures and temperature fluctuations comprise the critical scientific basis in developing new lead-free solders. Binary phase diagrams provide the general information about the conditions and extent of metallurgical interactions, albeit the complete phase diagrams beyond the binary system are scarce. Nonetheless, binary phase diagrams offer a useful starting point.
After a 12-year sustained research period, it was found that the actual test results of the designed multiple-element alloy compositions came very close to the anticipated features in properties and performance between a candidate element and Sn-matrix.1,2,3To illustrate, examples include:
- Se and Te were found to readily embrittle the Sn-based alloys
- Sb, in an improper amount, quickly jeopardized the alloy's wetting ability
- The distribution of In atoms in the Sn host lattice is sensitively reflected in the fatigue performance
- The level of Bi second-phase precipitation is closely associated with the mechanical properties of the Bi-bearing alloys
- The formation of intermediate phases and intermetallic compounds between Sn and Cu, Ag, or Sb, remarkably affects the strength and fatigue life of the alloy, which in turn depends on the concentration of each element as well as on the relative concentration among the elements.
Since the general performance is as predictable as stated, a high-performance alloy composition demands a stunningly intricate balance of the elemental constituents. In each compositional system, the useful products often are a specific composition or a narrow range of compositions at best.1,4,5,6 New solder alloys must possess the characteristics that are compatible with practical manufacturing techniques and end-use environments. The basic material properties such as liquidus/solidus temperature, electrical/thermal conductivity, intrinsic wetting ability on surfaces commonly used, mechanical properties and environmental shelf stability must be gauged. Under the current framework, however, conductivity and shelf stability are not as sensitive to the makeup of a specific system as are intrinsic wetting ability, mechanical performance and phase-transition temperatures.
Alloy Choices for SMT Solder Connections
From the outcome of research, there are several viable lead-free solder alloys that offer a set of balanced performance characteristics including the superior mechanical properties.
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The top three criteria for a viable composition are its mechanical properties to be equal to or better than the established reference (63Sn/37Pb), its physical properties comparable with the reference and its application characteristics that are compatible with the practical SMT manufacturing infrastructure. Based on these criteria there are several selection options, depending on the production requirements; Table 1 lists the alloys as ranked by melting temperature.
Surface Mount Manufacturing
With reference to manufacturing, the primary differentiation among these selected alloys goes to two physical characteristics: 1. Relative melting temperature. As listed in Table 1, it should be noted that within an alloy system, only the specified compositions deliver the desired performance. This is not different from Sn/Pb systems — the specific composition (63Sn/37Pb) is the optimum. 2. Relative performance in intrinsic wetting ability. The latter feature of a solder alloy is defined as the affinity and effectiveness of a molten metal to wet a substrate surface (such as Cu or Ni) under a given set of conditions. This is an important performance parameter because it directly affects the integrity of solder interconnections as well as controls production yield and throughput under a dynamic soldering process — wave soldering or SMT reflow. Additionally, it is well recognized that the solderability of substrates of both printed circuit boards (PCB) and components by a given solder alloy, together with the effectiveness of flux chemistry, are the factors contributing to the quality of solder joints.
For the past two decades, the intrinsic wetting ability of solder alloys has not been a subject of study and discussion because the Sn/Pb eutectic (63Sn/37Pb) has not been subject to "making choices" and its wetting ability has been taken as a given. By design or accident, 63Sn/37Pb has an excellent wetting ability and a "just right" melting temperature for most electronic end-use applications and for wave and reflow assembly processes.
When a selection is in order, the understanding of the wetting kinetics of various lead-free compositions becomes crucial in choosing a suitable lead-free material, thus ensuring the overall quality of the solder joints as well as the PCB yield. Based on a systematic study as exhibited in the figure, at the temperature range of 235° to 245°C, the three systems — Sn/Ag/Cu/In, Sn/Ag/Bi/In and Sn/Ag/Cu/Bi — having well-designed compositions, can deliver a significantly improved wetting performance over that of Sn/Ag/Cu near-eutectic.7 Specific compositions include 91.4Sn/4.1Ag/0.5Cu/4.0In, 91.5Sn/3.5Ag/ 1.0Bi/4.0In and 93.0Sn/3.1Ag/3.1Bi/0.5Cu. The observed superior performance is particularly evident in solder fillet formation and appearance, which determines the manufacturing yield, particularly in through-hole joints. The true successes demonstrated in real-world manufacturing are very much congruent with the wetting ability measured.
Soldering Temperature. Closely related to wetting performance is the operating (soldering) temperature, which in turn is pegged with the solder alloy's melting temperature. Generally, the wetting time of solders decreases with increasing temperature, gradually reaching a relatively steady state.
How is the melting temperature related to the soldering temperature? The higher the melting temperature, the higher the soldering temperature required. An adequate soldering temperature is mandatory to carry out an assembly process that is capable of delivering quality performance in all aspects ranging from production yield to long-term solder joint integrity. It should be emphasized that the misfit due to a "marginal choice" in each part of the manufacturing setup can be augmented monstrously in actual day-to-day production. Simply put, the materials and process parameters must have adequate performance latitude.
Under the existing establishment, it is a prudent practice to work under the following temperature framework: The wavesoldering temperature should be kept within 245°C and reflow peak temperature within 240°C (preferably 235°C). This translates to the fact that the melting temperature of the alloy selected for the process must be below 215°C. Any stretch beyond those parameters would make the manufacturing vulnerable to process defects.
Fatigue-resistant Lead-free Materials
It is reasonably well substantiated that the common thermal fatigue failure for solder interconnections is linked with the Pb-rich phase, which cannot be strengthened effectively by Sn-solute atoms due to limited solubility and Sn precipitation. At room temperature, the limited solubility of Pb in an Sn matrix renders limited improvement in the plastic deformation to slip. Under temperature cycling (thermomechanical fatigue) conditions, this Pb-rich phase tends to coarsen and eventually leads to solder joint cracking. Therefore, it is expected that the absence of a Pb-phase of a properly designed lead-free tin-based solder may impart improved mechanical behavior, resulting in strengthened solders.
Strengthening Approaches
Under high-temperature conditions (above room temperature) that solder joints typically are exposed to, the mobility and dislocations of atoms increase. Other crystallinic defects such as vacancies also increase. Additional slip systems are introduced and metallurgical stability is unfavorably affected. Also, environmental effects (oxidation, corrosion) become more pronounced.
Approaches that potentially can hinder the above material phenomena are expected to enhance the performance of solders, which in turn will meet the level of performance required for new and future applications. This includes:
- Microscopic incorporation of non-alloying dopant
- Microstructural strengthening
- Alloy strengthening
- Macroscopic blend of selected fillers.
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These approaches involve both process and material factors. For example, solid solutioning, where solute atoms typically reduce the stacking-fault energy and favorably control the diffusion behavior, is a well-adopted strengthening mechanism. A "stronger" solder material to serve as interconnections between the chip and the substrate can better accommodate applications possibly destined to harsh external conditions or subjected to intrinsic high-heat dissipation. Among the viable alloy compositions and within the upper limit of 215°C melting temperature, 88.5Sn/3.0Ag/0.5Cu/8.0In, 91.5Sn/3.5Ag/ 1.0Bi/4.0In and 92.8Sn/ 0.7Cu/0.5Ga/6.0In are the top three high-fatigue-resistant materials as listed in descending order in Table 2.
Recommendations and Conclusions
Although lead-free is a new material process system, all fundamental principles and production-floor practices that have been developed for lead solders during the last 20 years remain equally valid and applicable when implementing. Much information related to lead-free technology and implementation is available in the form of textbooks1, prints and lecture series. An optimal composition should be determined by the need of the performance level and the required process conditions for a specific application. For surface mount PCB assembly, a solder alloy having a melting temperature below 215°C along with a superior intrinsic wetting ability provides the necessary process window. Overall, technological advancement has been made in enhancing creep and fatigue resistance in some lead-free alloys, which are identified for high fatigue-resistant applications.
REFERENCES
1. J.S. Hwang, Environment-friendly Electronics — Lead-free Technology, 2001, Electrochemical Publications, Great Britain, ISBN 0-90-115040-1.
2. J.S. Hwang, Modern Solder Technology for Competitive Electronics Manufacturing, Chapter 15, 1996, McGraw Hill, New York, ISBN 0-07-031749-6.
3. Ibid (list of 54 references).
4. G.K. Lucey, Composite Solders, U.S. Patent No. 5,520,752.
5. J.S. Hwang, High Strength Lead-free Solder Materials, U.S. Patent No. 5,985,211.
6. J.S. Hwang, Lead-free Solders, U.S. Patent No. 6,176,947.
7. J.S. Hwang, "Lead-free Solder Interconnections for SMT Manufacturing," Proceedings, NEPCON West, San Jose, Calif., December 2002.
Dr. Jennie S. Hwang, an SMT Editorial Advisory Board member, a member of the National Academy of Engineering and an inductee of the WIT International Hall of Fame, has been a highly recommended advisor to the industry and the U.S. Government. She is the author of more than 200 articles and textbooks and is president of H-Technologies Group Inc. Contact her at (216) 839-1000 or e-mail JSLHWANG@aol.com.