Few days ago, Lego Group presented some new figurines which depicted a father caring for an infant baby, along with the mother in an office outfit. The goal is to “mirror the world we live in today,” Lego says. I came across the news through one of the major Italian newspaper, La Repubblica. The title, translated…
Dr Carolyn Ee is one of my favourite bloggers. She is a pure example of an inspired and dedicated mum who not only does not see any contradictions between her motherhood role and her professional role but also she is brave enough to get online and promote her perspective of balancing family and work. The link below is her recent post on her perspective about combining PhD and motherhood. I feel so related to this post as I had my first baby during my PhD and the second during my postdoc. I wish I had known her earlier in my journey to take advantage of her high energy and motivation to push myself further along the track during those moments of struggling and frustration.
I initailly started blogging with a different set of goals, such as communicating my science, improving my writing skills, raising my concerns and promoting girls into engineering and science, etc., but along the way I realised that this practice of blogging is actually helping me to be a better engineer and scientist and here is how:
- To write a good blog post you need to read a lot, just to keep yourself up to date, refresh your memories, learn new science and broaden your vision.
- The importance of networking to one’s career is very obvious. As you start blogging, you get noticed and that provides a great opportunity to expand your global network and indentify resources that provide you with the perceptiveness required to deliver value to your career as well as your readers.
- Blogging fuel your creativity. Blogging involves a lot of learning that provide you with a continuous brain exercise you need to boost your creativity. In addition, to write a good blog you have to ask yourself a lot of questions. Asking questions map your mind and a lot of mindmapping is another requirement of being creative. Blogging also requires a lot of courage as you have to give up on all the risks involved in going online. Worrying too much about the risk is the number one killer of creativity and by blogging you practice to be a risk taker.
- And at last but not least, blogging for me has been a journey of self-disovery. It has expanded my mind and opened my eyes to my core interests and values. I am more aware of my strength and weakness and I know better what works for me and what doesn’t. Blogging helps me to stay focused and relevant to my goals.
If you are interested in microelectronics industry and are following the latest trends, you have heard the buzz about internet of things (IoT) and wearable technology. Although the concept might not be new, the advances in microelectronics now allow smooth adoption of the technology, revolutionizing human’s life once again after the arrival of internet. In this regard, advanced electronic packaging is a key component to successful integration of IoT and wearable technology. Generally, the essence in advanced electronic packaging is integrating multifunctional modules (logic, memory, RF, sensors, etc.) based on 3D integration. 3D integrated circuits (3D ICs) requires an increased Input/Output densities, stacking chips and interconnecting vertically by through silicon via (TSV) technology providing high performance, low power and reliable electronics. However to the best of my knowledge, moving toward 3D ICs does not exempt from complications and a lot of effort is going on in the industry to overcome those complications. Many of these challenges originates from the fact that despite significant experience of industry in 2D flip chip packaging technology, many of those experiences are not transferable to 3D packaging. This is while as the chip technology is approaching the limits of Moore’s law, moving to 3D ICs seems inevitable as it offers ample room for further growth of electronic industry for at least the next 15-20 years.
Generally, a chip contains different metallic components and understanding metallurgical principles governing the behavior and reliability of these components appears vital to further development of the technology. In metallurgical science, size effect is always a concern as reducing the size (of grains, thickness, precipitates, etc) below a certain amount can potentially alter the governing metallurgical principles. For instance, when the grain size is reduced to the range of nanometers dislocation mediated mechanisms will be largely replaced by grain boundary mediated mechanisms. These change of mechanisms are the origin of complications associated with materials behavior when reducing size is inevitable. As already mentioned, the new trend in electronic packaging is all about miniaturising and reducing sizes. Likewise, the well-known metallurgical principles which formed the foundation of reliability experience of the industry for years , changes with this emerging trend of miniaturizing and a new level of understanding is required to keep up with the demand of the market. As an example of this, moving from 2D ICs to 3D ICs requires a significant reduction in the size of bumps to the range of so-called micro-bumps. This continuous reduction in size of bumps can eventually reach to the point where each micro-bump contains only one single grain leading to a significant inhomogeneous behavior across the entire 3D ICs, as each single grain can respond to its environment differently based on its orientation. In this (and following) post, thus, I aim to provide an overview of the current trend in electronic packaging and relevant reliability issues from metallurgical perspective.
TSV and Micro-bumps the main metallic (and the most mischievous) components of 3D ICs
More dense, high performance 3D ICs requires an increasing number of Inputs/Outputs (IOs) obtained through scaling down to fine pitch and micro-bumps less than 50 micron and stacking multifunctional chips (such as logic, memory, RF and MEMS) using TSV interconnect. TSV interconnect are copper vias in polycrystalline form vertically passing through chips providing connection between chips. TSVs are created through a process of etching and electroplating. In this process through holes are etched which are then filled with electroplated copper. The main advantage of TSV is the reduced travel length of signal from one layer to another, allowing an increased interconnect density, therefore increased functionality and performance. The connection between vertical vias in different layer is made through micro-bump technology. Micro-bumps are created by means of electroplating process. As mentioned earlier, due to the compact nature of 3D IC, micro-bumps used in this technology are much smaller than the conventional bump size used in 2D packaging. The thermal mismatch of the components (e.g. difference in coefficient of thermal expansion of TSV copper and silicon) and the small size of micro-bumps leads to some unique characteristics that can create some critical reliability issues (when I was first learning about micro-bumps and TSVs, the first thing that passed my mind was how mischievous these components are as I never knew how much trouble they can create in advanced electronic packaging). In what follows in this part, I try to cover micro-bumps and their reliability concerns briefly and TSVs will be reviewed in a separate post. Please keep in mind that the universe of metallurgy of microelectronics is vast and what is provided here is just a brief summary of the literature.
Microbumps and Intermetallic Compounds Formation
Soldering is the essential part of interconnection technology and obviously will remain so in the future. By definition, soldering is the chemical reaction between the solder alloy and the two surfaces to be joined together. Since 2006, the consumer electronic market has fully transferred to Pb-free solder alloys due to the health and environmental concerns associated with Pb-Sn solder alloys. Replacement of Pb-Sn was initially appeared possible through a near eutectic Sn-Ag-Cu or SAC alloy. However, development of SAC alloys did not lead to a stop to further alloy developments and there has been continuous effort to develop solder alloys for a wide range of applications. Nevertheless, since near eutectic SAC alloy is the most common solder alloy currently and it is believed that it will remain to be the major candidate for 3D ICs, at least in the near future, the information is provided in this post is mainly based on this alloy.
Generally, an interconnect system consists of solder bumps in contact with under bump metallurgy system or UBM. UBM to interconnection technology is like a foundation to a house providing support to the whole system. UBM contains different metallic layers (or thin films) each playing different role. In case of SAC solder alloy, the UBM is composed of a Cu layer as solder adhesion layer, Ni layer as the diffusion barrier layer and Au layer as the oxidizing barrier layer. Cu, Ni, Ag, and Au are known as fast diffusers meaning that their diffusion rate in body centred tetragonal Sn is orders of magnitude higher than self-diffusion rate of Sn . Therefore, the interfacial chemical reaction between solder and UMB leads to formation of IMCs which is on one hand an indication of a good metallurgical bonding, but on the other hand IMCs are brittle in nature and prone to structural defects which can undermine the reliability of solder joints.
The intermetallic product of reaction between solder alloy and UBM varies depending on the solder alloy composition, UBM layer components, UBM layer thickness and kinetics of reaction. In a binary system possible alternatives of IMCs are as follow: Cu6Sn5, Ni3Sn4 and AuSn4. However, through multiple reflow and subsequent aging treatment Cu3Sn  and other alternatives of ternary and quaternary IMCs appears in the solder [3, 4]. Cu3Sn formation is accompanied by Kirkendall voids formation, a phenomena resulted from the difference between the diffusion rates of Cu and Sn which promotes voids formation and eventually crack formation at the interface of Cu3Sn and Cu .
In the conventional flip chip with bump sizes of larger than 50μm, IMCs can form both as a thin layer at the interface and as particles distributed in the matrix of solder depending on the type of IMCs formed. A thin layer of IMC at the interface provides a good bonding and dispersed IMC particles in the matrix can improve mechanical properties of the joints. However in 3D IC technology with the micro-bump size requirements in the range of a few micrometers, solder alloy can potentially turn fully into IMCs that is not regarded beneficial in terms of mechanical properties of the joints (due to their brittle nature). Another example of complications associated with the small volume of micro-bumps is formation of Ag3Sn (another form of IMCs). While Ag3Sn does not seem to be much of a concern in conventional flip chip bump sizes, it can create serious reliability concerns in micro-bumps [6, 7]. The reason for formation of Ag3Sn in microbumps originates from the low volume of solder alloy in micro-bump. Rapid initial consumption of Sn by IMC formation leads to an increase in concentration of Ag which in turn promotes Ag3Sn formation .
Other than being brittle, different IMCs have different thermal expansion leading to formation and propagation of voids at the interface of IMCs in the subsequent aging treatments [8, 9]. Similar to Kirkendall voids, these voids can lead to crack propagation and early failure of the interconnect thereby, creating a significant reliability issue in micro-bumps where a large fraction of solder alloy transforms to IMCs.
As a large fraction of micro-bumos turn into IMCs during subsequent thermal treatment, It should be ,therefore, noted that the reliability standards developed for conventional flip chip bumps could not be transferred to micro-bumps as the reliability of the former is dominated by properties of βSn while the latter is dominated by IMCs. Surely, developing new reliability standards is an exciting field of work from my perspective as a scientist but a tough challenge for industry in the effort to track I/O roadmap.
Micro-bumps and Electromigrations
Electromigration (EM) refers to the mass transport of atoms under applying current density in metals. The flux of atoms in the direction of electron flow leads to fast dissolution of UBM and voids/hillocks formation at the interface of joint and UBM. The former increase the rate of IMCs formation and the latter leads to an increase in current crowding effect and further elevating EM. Extensive IMC formation and voids at the interface can create localized stress concentration while in service resulting in early failure of electronic components and devices.
The smaller size of micro-bumps results in substantial increase of current density flowing through the joints based on this relation:
Where J is the current density, I is the current and A the area. For example, applying a current of 0.05A on a 20μm-microbump results in a value in order of 104A/cm. Increasing current density increases the transport of Cu, Ni, Au and Ag atoms driven by electron current. Additionally, increasing current density leads to a local increase of joint temperature as Joule heating is proportional to the square of current density. Also, field service temperature can reach as high as 100 °C, approximately 76% of the melting temperatures of SAC alloy. High current density and high temperature in the micro-bumps, therefore, promotes the flux of atoms and increase the diffusion rate in the direction of electron flow. Consequently, EM could be significantly pronounced reliability issue in 3D ICs technology with an enhanced rate of IMCs and Kirkendall voids formation, if compared with conventional flip chip technology under the same applying current. However, it is interesting to note that there are evidence of an improved resistance to EM in 3D ICs and that is attributed to fact that IMCs have an enhanced resistance to EM in comparison with the solder alloy [10, 11]. Once the solder alloy transformed fully or largely to IMCs, therefore, a drop in EM rate is expected.
Micro-bumps and microstructural anisotropy
Micro-bumps can have dimensions one to two orders of magnitude smaller than the conventional bumps in the flip-chip technology. This significant reduction in dimension may result in micro-bump having only one grain. As Sn has a body-centered tetragonal crystal structure, its properties (e.g. conductivity) is anisotropic. Even when micro-bumps are fully consumed by ICs, not much changes are expected in terms of isotropic properties as crystalline structure of ICs are mainly anisotropic, as well [12,13]. This would be a serious reliability concern when there are a large number of micro-bumps.
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Maria Forsyth is a professor of electrochemistry from my home institute (Institute for Frontier Materials, Deakin University). She has been hugely successful in her career, making women really proud. She is a true role model for many young girls who dream to be scientists. She released a letter to women in STEM earlier this year for international women’s day which I believe it is worth to share it here as well. Link is provided below:
“It has been hard for me, therefore it will have to be hard for you and I won’t help you with any of it.” I can still hear my Honours supervisor telling me this, when I asked her about her journey as a scientist. She was one of the very few female group leaders in […]
I ran into this fascinating Post today. It is about how Moore’s law has paved the path for the emergence of IoT (internet of things), a breakthrough technology, which is on its way to revolutionise our lives even further. It also reminded me of another fascinating articles titled: “Multiple lives of Moore’s law” by Chris Mack. If you are fascinated by technology and wonder about its boundaries / limits, I strongly recommend reading these articles.
Balancing family and work, childcare requirement while travelling for work and flexibile family friendly work arrangement are concerns of all working mothers/families. If you are like me and my husband and prefer to travel with kids, this is a nice post listing family friendly conferences for mothers/fathers who need or want to travel with their kids. I am aware that this list is in the process of expanding further. Let’s keep all our fingers and toes crossed for the day when all conferences and events provide kids friendly facilities.
We made a beautiful, heavenly delish soup 👌 last night with wild herbs (or so-called edible weeds). I collected them a few days back while we were walking with my kids. It’s a lot of fun for my kids looking for them and finding them😊. In this era of technology while our kids are locked to screens, I try to use every single opportunity to teach my kids to get connected to the nature. Searching, exploring and collecting edible weeds is a lot of fun for kids and it is also a great way to not only get connected but also respect the mother nature and be grateful of its generousity. That helps them to understand nature better and do a better care of nature when they grow up.
Despite being available pretty much everywhere, They are rarely known as edible. That is why – and to show my appreciation for nature – I decided to write a blog post about edible weeds. With a simple google search you can find a lot of information as well as their gorgeous pictures for your local area. They are not hard to find and there are plenty around. You don’t need to be living in countryside, just do a bit of cross-country walk next time you go for a walk and you will find plenty . You can even find them in your backyard or your children’s playground. Picture below is what we collected the other day, I don’t know exactly what they are called but they taste slightly sour and make beautiful soups. I keep posting more images of wild herbs.
Before doing an actual PhD, I used to believe that intelligence and hardwork are the only requirements of doing a successful PhD and personality traits have little or nothing to do with it. However, I learned the hard way that the journey is far more complicated and there are factors other than intelligence and hard work that can appear crucial to your PhD. Although knowing the real world of PhD would not change my determination to be a scientist, knowing those factors beforehand could have helped save enrgy and make my journey much smoother. Here are a few tips based on my personal experience of doing a PhD for those who haven’t started yet or are early in their journey:
Self awareness is the king. Knowing your strengths and weaknesses is the key to perform a fruitful PhD journey. Many of us might be good at knowing our strengths, but when it comes to our weaknesses, our unconscious reaction is to either ignore them or hide them and If you don’t take control of your weakness early in your journey, that could end up to a truly daunting journey. When I started my PhD, I used to be a perfectionist who was also extremly afraid of being judged. That had led me to have trouble approaching people to ask for help or ideas, therefore ,undergoing a lot of unnecessary pressure and loss of energy. I was also a horrible presenter just because I was extremely nervous in front of public. I am still nervous in front of public, but the difference now is that I am not trying to hide the stress anymore. Instead, I try to acknowledge its presence and control my performance rather than my stress.
It is, however, really important to be able to recognise that there is a difference between self awareness and self criticism, the latter being largely destructive. Developing self awareness is also a precondition to developing a mutual understanding between ourselves and co-workers and mutual understanding is obviously the key to conflict free and happy relationships at work.
Resilience is the art of coping with stress, adversity and change through enhancing your confidence, optimism, problem solving and communication. If you are like me and your PhD journey is coincident with migration and its unique complications and on the top of that having a baby in the meantime, be prepared to be challenged massively. There might be numerous circumstances where you see yourself in a dead end with no way out. You feel trapped and empty. This situation is totally understandable and there is nothing wrong with that. It can happen to everyone. You can’t get the results you want, your experimental design doesn’t work, your equipment stop behaving and you have deadlines to meet. In addition to these, you don’t have much common with people around you. They seem to you like aliens from another world. You have also family commitment and have to juggle family and work. These are all normal feelings and phases that everyone can go through. You can’t and shouldn’t avoid such circumstances but you can manage your attitude toward them. The important thing in such situations is to stay confident and positive, pull together your strengths and communicate, communicate and communicate.
Communication and transparency is an important factor contributing to a successful PhD. It is really important that you communicate your expectations from people you work with clearly. Don’t expect your supervisors to be mindreaders. Don’t be shy and let them know what level of freedom or support you need and ask expectations of your supervisors and other team members as well. Be transparent about your planned milestones and ask them the same. That helps a lot in developing a trustworthy and collaborative environment.
Social skills while doing PhD and investing time in learning technical skills, it is really vital to invest in your social and networking skills as well. Don’t waste even a single chance to network. Get your work out and let others outside of your team know what you do. You never regret hearing other perspectives on your work. Sometimes, that can be the only way to free you from the nightmare of not getting anywhere. In addition, I guess it is not necessary to mention that your network can appear extremely valuable in your job search following your PhD.
However, bear in mind that it is very normal if you don’t feel comfortable with networking. Referring to self awareness here again, just try to find out why. For me, being an introvert had little to do with my uncomfortable feeling at networking but it was mainly due to lack of enough confidence originated from my fear of being judged. Work on the origin of your uncomfortable feelings and develop strategies to obtain control over them.