Some years ago, a leading IC manufacturer decided that their R&D effort could benefit if analyzed for possible improvement - by hiring professionals from the arts (e.g. singers, actors, artists etc.) In this spirit, I am going not quite so far from electronics assembly, to ask one of Dartmouth’s premiere engineering students to take a look at some of the activities currently going on in electronics assembly. The student is Alison Stace-Naughton (ASN). I will chat with Alison for a while and then let Indium Corporation's Brook Sandy take over to answer some questions Alison has about SACm™.
Dr. Ron (DR): Alison, tell us a little about yourself.
Dartmouth's Alison Stace Naughton
ASN: I just graduated cum laude in June 2013 with High Honors in Engineering Sciences, receiving a lifetime membership to Tau Beta Pi, a Bachelor of Arts degree and a Bachelor of Engineering degree from Dartmouth College.
DR: You just recently founded a start-up, tell us about it.
ASN: During my time at Dartmouth College, I cofounded a medical device startup, Spiral-E Solutions LLC. Since then, I have always been interested in the start-up world and different products that solve real-world needs.
DR: You recently won two national prizes right?
ASN: Yes, I won First Prize in the 2013 National Biomedical Engineering Society’s BMEStart Innovation Competition as my senior project developed a novel filtration and isolation system for fecal transplantation, making the process more efficient while maintaining efficacy. I also won First Prize in the 2012 National Institutes of Health (NIH) Innovation Competition for the same fecal transplantation filtration device. Dartmouth published an article on this as well.
DR: Well if people are interested in learning more about you, they can go to your Linkedin account. Tell us about your interest in Indium Corporation.
ASN: This summer I was working with Dr. Ron and I became interested in Indium and SACm™. But, after reading the paper titled “Achieving High Reliability Low-Cost Lead-Free SAC Solder Joints via Mn Doping, ” by Dr. Ning-Cheng Lee et al. I did have a few questions.
DR: OK for questions, I am going to turn you over to Indium Corporation’s Brook Sandy-Smith (BSS).
ASN: OK. Brook, from a materials perspective, what are a few critical materials science concerns that an engineer may have regarding solder?
BSS: The role of solder in printed circuit board assemblies is to make a connection that is conductive, both electrically and thermally, as well as mechanically robust. Reliability is also a major requirement because devices are expected to have a long lifetime. All of these factors will influence the choice of alloy and the form of solder used. Process requirements may also be a key factor in choosing the correct materials.
ASN:Can you share a little bit about the history of SAC alloys?
BSS: The electronics industry used primarily eutectic 63Sn37Pb solder alloy until the Pb-free conversion almost a decade ago. In finding a replacement, there were a few key factors: similar melting characteristics within the temperature ranges acceptable for assemblies, resistance to drop shock, resistance to thermal cycling, and mechanical properties similar to SnPb solder. Alloys consisting primarily of tin (Sn) with small amounts of silver (Ag) and copper (Cu) (SAC alloys) performed most similarly to SnPb. Because 63Sn37Pb is a eutectic alloy (meaning that when it melts, all phases of the alloy melt at one temperature), a near eutectic composition was the first to be adopted, SAC387, containing 3.8% Ag and 0.7% Cu.
Shortly after the adoption of SAC387, the industry encountered the defect that we call “tombstoning”. This defect occurs during reflow and results in only one end of a component being soldered. The solder surface tension at this end forces the component to stand on its end resulting a in tombstone-like appearance. One solution to this problem was to use a different composition of SAC alloy, SAC305, containing 3% Ag and 0.5% Cu. This alloy change helped because the diminished difference in wetting forces during reflow allows the components to stay placed while the solder deposits melt and wet. This alloy is not eutectic, so there is a range of temperatures during which the alloy melts.
SAC305 was widely adopted because of its lower incidence of defects (having 0.8% less Ag also helped from a cost perspective). It seemed that this alloy might become standard, but, with the additional drop shock requirements for smaller mobile devices, and a sharp rise in Ag prices during 2010-2012, alloys with less Ag have also been adopted, such as SAC105. SAC105's drop shock performance is superior to SAC305, but its thermal cycle performance is poorer.
ASN: I read about drop shock testing and thermal cycle testing in the paper. Can you clarify what they are and why they are important?
BSS: Our electronic devices have additional requirements, other than just conducting electrical signals and displaying information. They also must be robust to have a long working life. Robustness is characterized in many ways, but, for solder alloy fundamentals, we look at drop shock resistance and thermal cycling reliability.
Drop shock resistance testing simulates dropping your device many times under controlled conditions to assess when there is potential for components on a circuit board to fail. For SAC alloys, drop shock is better with smaller amounts of silver, so SAC105 is better than SAC305.
Thermal cycling reliability was originally developed to simulate the effects of heating caused by turning large devices on and off. Large devices and older electronics had more challenges with thermal management and would endure larger thermal cycles than the smartphones or modern devices we use today. This aspect of performance is still important because devices must still be robust at many temperature conditions, and thermal cycling gets additional attention in some applications, such as automotive electronics. For SAC alloys the trend is that Ag and Cu add reliability and extend the number of thermal cycles to failure. Therefore, for thermal cycle performance SAC305 is better.
The crucial balancing act for SAC alloys, then, is dependent on application and how resistant to drop and thermal conditions the assemblies must be. For personal electronics, drop shock resistance is crucial, whereas devices that are more stationary or protected will have a different balance of requirements.
ASN:So, SAC105 replaced SAC305 in mobile devices and saves money, as less silver is used. Does SAC105 have any disadvantages?
BSS: When choosing materials, there are always trade-offs. There is rarely a magic material that has all of the properties you want in all of the categories you want. So, with SAC105, there is the benefit of better drop shock resistance, but the trade-off is earlier failures in thermal cycling, as mentioned earlier. Another difference is a slightly higher and wider melting range, making reflow temperatures even closer to the temperature limits for assemblies.
ASN: Now I see the need for SACm™, to achieve better thermal cycle performance for a SAC105 type alloy.
BSS: Correct. In order to have a material with additional drop shock resistance without sacrificing any of the properties enjoyed with SAC305, changing the silver and copper compositions alone is not enough. This is why we evaluated many different dopants, or elemental additives. SACm™ has a very small amount of manganese (Mn) which improves the performance of the alloy and achieves even better performance than SAC305, with less silver.
ASN: That’s quite amazing; it isn’t often that a material can make two competing metrics (i.e. drop shock and thermal cycle performance) better. How does SACm™ achieve this?
BSS: In order to assess how SACm™ is different than other SAC alloys, we look at the microstructure of a solder joint in a scanning electron microscope. The microstructure has finer intermetallic structures. As you know from materials science class, cracks in solder joints tend to propagate along this intermetallic plane close to the copper pads. Because Mn is present, the intermetallic area is more robust, and able to resist drop shock.
ASN: Is SACm™ a drop-in solution?
BSS: I assume by drop-in solution you mean that it can be swapped into an existing Pb-free process without changing printing and reflow parameters. SACm™ is a drop-in solution for SAC305 and SAC105. Also, Indium Corporation offers SACm™ in some of our most popular Indium8.9 Series paste formulations, so paste performance lives up to expectations.
ASN: Are there any manufacturing challenges in producing SACm™?
BSS: Indium is expert at making many different alloys and SACm™ does not present alloying challenges. However, powder production is a challenging process, and some special manufacturing techniques were developed to make high quality SACm™ powder. Because this alloy contains some elements in small quantities, it is important that all of the elements are represented in the powder. Powder development has been very successful and SACm™ is now offered in paste as well as spheres.
DR: Alison, Brook, thanks for your time and this interesting discussion.