Sputtering Target

I have been asked to explain why someone would want to use rotary sputtering targets instead of planar sputtering targets.

Certainly there is some expense involved with larger targets and new equipment (assuming you are currently using a planar system), but in a high volume process (such as roll-to-roll thin film deposition) the advantages lead to a lower cost of ownership:

• Compared to planar targets, rotary targets generally have more surface area per given length.

• Rotary targets have much more surface area, so the magnetron power can be spread out over a larger area in a given amount of time. This helps keep the target running cooler, decreases nodule formation, and reduces the occurrence of arcing.

1. Since rotary sputtering decreases nodule formation, targets can have longer continuous runtimes.
2. There is generally more material available to sputter on a rotary target, which increases runtimes.
3. Rotary target utilization is usually ~80%, as opposed to ~30% for planar targets – which decreases scrap and increases runtimes.

• Rotary targets are well suited for continuous sputtering processes. Continuous processing increases throughput since there is less time wasted preparing the sputtering chamber.

• Rotary targets are more cost effective for high volume processes. They provide a good platform for long runtime processes, with less chance of defects and downtime.

• Planar targets are still best suited for prototype work or elemental experimentation, especially when large amounts of material are not needed at once.

If you are interested in discussing sputtering targets, contact our team at: Solar@Indium.com

~Jim

It is common to hear people that are skeptical about CIGS technology ask questions like:

• "Aren't indium and indium tin oxide (ITO) thin film deposition processes wasteful and inefficient?"
• “Aren’t we going to run out of indium soon? Doesn't the world use more than we produce!”

What are the truths?

Here they are:

WASTEFUL: A well-run process is NOT wasteful. Why? Recycling!

At first glance, a process like indium planar target sputtering seems ridiculous – generally only 30% of the indium actually makes it onto the substrate it is destined for (and that’s in a well-tuned process). As it turns out, the material that doesn’t land on the substrate is too valuable to just scrap. This translates into recycling, a lot of recycling…

According to presentations given at Minor Metals 2012: “indium production will total 1,500-1,700 tonnes in 2012, with virgin supply accounting for around a third of total output”.  It’s incredible that recycling accounts for such a large percentage of the indium used in the world today.

INDIUM AVAILABILITY AND SUPPLY: Another important conclusion made at the conference was (as reported in Metal Bulletin):

“proven indium reserves from existing mines at 50,000 tonnes, a volume that will be sufficient to satisfy demand for the next 75 years”.

While it’s not news at Indium Corporation, it is definitely assuring news for those looking to get involved with CIGS technology.

~Jim

I recently had a discussion with Frank Zimone (VP of Business Development at Angstrom Sciences) about sputtering target utilization. He stressed the point: although "material by weight sputtered off the target" is how most folks define the ‘target utilization’, when judging the efficiency of a process, it is only the material that makes it to the product that counts. Frank said:

“What is happening now, is that we are seeing that many companies, after the rush to set up a process to create a good product and “get to market”, are now working on dialing in the process to save money by lowering production costs. This can be achieved by putting more of the target material onto thin-film cells, and wasting less by depositing less of the material elsewhere in the production tool.

“We have recently completed a study with a major photovoltaics company which evaluated enhanced magnetics from multiple competitors.  In a back-to-back comparison with identical process conditions (power density, line speed, etc.) both targets were utilized ~85%  as measured by weight loss.” (Remember folks – this is application specific.) “The main difference between the seemingly similar depositions was that one set of magnetics yielded 20% more material on the substrate.”

I asked the obvious question at this point: “How was that possible?”

Frank replied:

“One set on magnetics was able to get the 2 erosion racetracks more closely aligned, and more perpendicular to the target surface. This translated into higher dynamic deposition rate and less wasted material on the chamber shields.”

It’s a simple concept if you understand the physics of sputtering, and Frank agreed that most customers know this from an academic point but do not have the time/resources to properly test. He said, “More established customers are now looking into these particular issues.”

To learn more about moving magnetics, contact Frank Zimone at FZimone@angstromsciences.com or stop by and see him at InterSolar this week!

I'll be there, as well. Look for me in the Indium Corporation exhibit #5325.

~Jim

Q1) What the heck are we looking at in this picture?

A1) It’s a CuGa (copper gallium) target being sputtered at Angstrom Sciences, Inc. test lab. Since CuGa rotary sputtering targets are becoming more popular in the CIGS deposition industry, we wanted to see how they work with AS cathodes. The result: a winning combination!

Angstrom Sciences Lab








Q2) Why haven’t Cu-Ga rotary targets been more popular for production of CIGS solar cells (a thin film technology)?

A2) The big problem has historically been segregation of the copper and gallium in traditionally cast targets. This was a hot topic for those who stopped by the booth at the Society of Vacuum Coaters TechCon and checked out our full size CuGa display target. It is only natural to question if a display piece actually works well in a production sputtering process. In order to make this product work, we had to manufacture it using our proprietary hybrid consolidation technique.

Without giving away all the juicy details, I can tell you that it was a learning experience and that there were some setup issues that led to improved applied power settings. Our customers have been pleased with the results of our CuGa targets, although the fine tuning is proprietary to them and we cannot share their learnings. Now we have a much better understanding of the maximum power we can use for this type of target. That's why it was so important to work with an equipment supplier.

One thing that is obvious from looking at the spent target is the lack of an erosion groove from magnet dwell - a nice feature of the magnetron that was used. The spent target is on display in my boss’ office. It serves as a reminder of the time we spent with the Angstrom Science guys sputtering the target, gathering data, and learning from the team.

This year’s Society of Vacuum Coaters Technical Conference was certainly as focused on vacuum deposition as it has been over the years, but we were delighted by the new emphasis placed on solar cell fabrication. As I mentioned before , one of our key topics was thermal evaporation material - although we also presented on the topics of nano-bonding sputtering targets and the availability of indium and gallium. Along with these topics, the audience was treated to themes of cell fabrication, increasing solar cell efficiency, and roll-to-roll processing.

At the show, we had a chance to discuss new ideas with many of our existing and potential customers. Improved throughput and eliminating alloy segregation were hot topics at the Indium booth. Many customers wanted to learn more about CIGS materials like indium forms for evaporation, or full-size Cu-Ga, CIG, & In rotary targets for magnetron sputtering.

2011 raised the bar for the Society of Vacuum Coaters Tech Convention, I can’t wait for the 2012 event to top it!

这两年美国的西北部有越来越多的光伏太阳能公司在这里开设工厂，有些公司是前些年在加州硅谷地区把技术研发成功了，然后再把工厂开设在美国西北部（利用这里相对廉价的总成本，和联邦政府和州政府的补贴或投资的政策和条款）

在太阳能板子的组装方面，平时客户们问得最多的就是互联条和汇流带了(tabbing ribbon; bus ribbon)。这两种产品和普通的焊接带有一点区别，它们两一般是镀锡铜带。一般的客户都会有自己对互联条和汇流带的详尽规格说明(specifications)，比如说要求铜的规格是什么，镀锡的合金、厚度、误差范围(tolerance)，成品的宽度、厚度等。其中，与普通焊接带(solder ribbons)特别不同的是，互联条和汇流带一般有以下四点规格要求：

---Camber 曲弧度:简单来说，就是一条线拉直了，曲翘的程度不能超过多少。

---Elongation 延伸率: 一般有最小的百分比要求。

---Yield Strength 屈服强度: 材料开始产生宏观塑性变形时的应力。一般互联条要求的范围值比汇流带要求的范围值会低，毕竟每一段互联条要链接相邻太阳能板子的正反两面，要比较相对容易形变一点。

---Tensile Strength 拉伸强度: 是指材料产生最大均匀塑性变形的应力. 

Indium公司还提供各种太阳能溅射靶材(Sputtering Target)，太阳能低温焊锡膏(metallization paste)。 www.indium.com/solar

Pic：Indium Corporation

PS: 卖各种焊接产品给太阳能公司的生意不容易做啊。但是有机会，有潜在客户，总比根本没客户没机会好：-）最近有一个潜在大客户的进展很不错，让我顶着大肚子都往那里跑，常常为它忙乎着：-）

ITO is one of the materials that makes the magic of flat panel displays (monitors, TVs, etc.) possible. When sputtered on in a thin layer, it acts as a transparent conductive film. However, the process of sputtering is very inefficient in its use of ITO. And, with the explosion of flat panel display sales, large amounts of ITO end up as "waste".

It is important to understand the economics of ITO. The main precursor of ITO is indium (metal). Indium is a semiprecious metal and trades on the open market like gold and silver. It is subject to price fluctuations just like the other precious metals. So, there is an economic impetus to reclaim as much of the unused ITO as possible.


As mentioned earlier, flat panel displays employ a sputtered ITO coating. The indium metal must first be converted to indium oxide and then blended with the appropriate amount of tin oxide. The result is a pale green powder.

In order for the ITO to be usable, it must first be compressed in to a sputtering target. Planar sputtering targets are the dominant form of sputtering target used for sputtering of ITO. The geometry of the ITO sputtering target is often a rectangle or disc. Compressing the powder causes it to take on a darker color.

Inherent to the sputtering process is the uneven erosion of the sputtering target. The target material erodes in a "race track" pattern. These images of a spent nickel-vanadium sputtering target show the classic "race track" erosion pattern (valley).

“Hi, this is Jim Hisert. I just read an interesting article and I wanted to share it with you. In the 2010, October edition of PV magazine, the cover story on page 70 was pretty interesting, so here it is…

It’s called Rotatable Rotatables, and it discusses rotatable targets. The really nice thing about the article is: it goes through the physics of sputtering and gives you the background of what magnetron sputtering is all about. Then it talks about different kinds of targets. It talks about the difference between rotatable targets and planar targets, the different compositions of targets. And then it also goes into talking about the supply of indium and gallium, different target materials, as well as the difference between thermal vapor deposition and sputtering – the advantages and the disadvantages. It even wraps up with refining and recycling of the material and talks about the process overall.

So I advise that you check this out. It’s in the latest (October) edition of PV magazine. Thank you.”  ~Jim

Keywords: Solar, sputtering targets, copper indium gallium, CIG, indium, thin film technology

Are you scaling up to a rotary sputtering system from your current planar target line?

That’s a big jump! New equipment, new materials, new process challenges. I hope we can help you make this an easier transition. One of the issues you may be concerned with is the segregation of gallium in the new targets. Fortunately we produce our rotary targets using a hybrid consolidation process. Although I cannot share the processing details of this method, I can say that it sets Indium Corporation’s targets apart from what few competitors are out there right now. We offer Copper Indium Gallium sputtering targets as well as Copper Gallium sputtering targets. If you’re interested and would like to learn more just follow this email link: solar@indium.com

Wandering through the references to indium metal on the internet, I sometimes see it referred to as, "that 'rare' metal."  But is it really so rare?  I recently talked to my colleague, Claire Miko, Director, Metals and Chemicals for Indium Corporation and asked if the reports of the rarity of the metal (like the death of Mark Twain) were greatly exaggerated.

Question:  The element indium is widely used today in many electronic (glass coating, low temperature solder, hermetic sealing and thermal interface material) and solar applications (CIG solar panels), but very little is known about it.  Can you tell us where indium metal comes from?

Claire:  Indium is a by-product of several base metals such as zinc, lead, copper, tin and other poly metallic ores. It is very abundant on the crust of the earth (much more than silver for example and the annual silver production is at least 40 times bigger than the annual indium production). Geographically indium is abundant in South America, Canada, Australia, China and the CIS, i.e. the reserves are widely spread.

 
Question:     Does indium have to be refined after it is mined?

Claire:    Indium is present in the base metal ores at ppm levels. It first needs to be separated from the base ore and concentrated. This is done at the base metal smelter (for example during the refining of zinc, lead, copper, tin etc). It is then further refined and purified at indium refineries.

Question:  Indium Tin Oxide (ITO) is the one of largest indium-containing products today.  How much of the indium mined goes to making ITO?

Claire:   About 50% of the indium refined is used for making ITO. A larger percentage is needed to start the ITO target productions but the sputtering process used (when putting the ITO layer onto the glass) is inefficient and generates a large quantity of indium which is reclaimed and is then recycled and put back into circulation.

Question:     Is there enough indium available to meet the current and future needs of the marketplace?

Claire:   The indium production has always expanded to meet growing demand. Indium production grew from 70MT (metric tonnes/year to over 500MT/year over the last 20 years. At the moment only one-third of the indium mined yearly is being refined in indium metal, another third accumulates in residues that are more expensive to treat but they remain available for future processing, and the last third is currently lost because it does not reach a base metal smelter which has the equipment to separate it from the base metal ore. Investments at these smelters would enable the extraction and refining of these quantities if the need arose.

Question:    Are there recycling programs in place to recover unused ITO from the targets used to deposit it onto the glass surfaces where it is used?  What is the rate of recovery?

Claire:   There is ample capacity to treat spent ITO targets (as per point 3) and the recovery process is now mature and very efficient. The cycle time of this process has also now become very short enabling a very quick return of the refined indium for new consumption.

Question:    Are there any viable alternatives to ITO?

Claire:   A far as we know ITO remains the best material for LCD and other flat panel displays applications. It offers the best performances in terms of optical transparency, electrical resistivity, uniformity of both transparency and resistivity, chemical and mechanical stability, resistance to corrosion, and, finally, uniformity of etching.

The cost of the ITO on 42” TV represents less than $2 and less than 1% of the display cost. It is a small cost to pay to ensure that the quality of the display is maintained. Alternative materials have shown significant process problems with resistivity, uniformity and chemical and mechanical stability.

The SNEC 4th International Photovoltaic Power Generation Conference & Exhibit in Shanghai is known as one of the BIG solar shows of the year. Indium Corporation’s Bill Jackson (Director of Solar Products) commented that the 2010 SNEC was: "A busy, well attended show exuding with confidence about the beginning of a worldwide economic recovery and good solar-related growth for the foreseeable future".  That’s good to hear! Luckily, we had a strong team there to handle technical inquiries. Attendees from Indium Corporation included:

• Bill Jackson
• Thomas Tong
• William Aw
• Tommy Fan
• Michael Qiu
• David Hu

Even Indium Corporation President Greg Evans stopped by to visit the booth, to network, and to take the pulse of the industry.

This year, the SNEC was especially important for us. We had a chance to show off some of our new technology and sputtering target capability. The visitor interest seemed to mirror this thought with “…high interest in rotary CIG (Copper Indium Gallium) and Cu-Ga targets, also high interest in target bonding with NanoFoil®".

I also wanted to take a second to thank the people behind the scenes that help to make shows like this possible. Special thanks to Bill Wilson for helping to make sure our display targets looked their best, Gene Loparco and his team for dealing with the logistics of transporting our materials to/from the show, and Anita Brown for helping out with the details of coordinating the show.  These Indium Corporation employees help us all shine at solar trade shows!

~Jim

Okay, I admit it, I was sneaky and grabbed a photo of Tommy Acchione during his presentation of “Measuring the Performance of Low Melting Point Alloy Sputtering Targets Bonded At Room Temperature” while at the SVC show in Orlando a few weeks back. Congrats to Tommy for presenting our paper after doing so much background testing and preparation work for it!

~Jim

The Society of Vacuum Coaters 2010 technical conference took place last week, so this week I’ve been rounding up the display sputtering targets and evaporation sources to display at our next event in China. If you’re not familiar with it, the SVC (Society of Vacuum Coaters) conference focuses on deposition materials, equipment, and processes. The Indium Corporation has a specific interest in both sputtering and thermal evaporation since we provide materials for these processes. There are too many applications to list, but some specific sputtering/evaporation apps that are close to my heart are:

• Absorber layer (ex. Copper Indium Gallium) for thin film solar cells
• TCO layer (ex. Indium Tin Oxide) deposition – also for the solar industry
• Sputtering target bonding with NanoFoil®

Last week we set up a very impressive array of sputtering targets (shown in the picture), so I hope they all make it safe and sound to SNEC next week!

~Jim

This blog has been in existence for a little over two years now, and we would like to thank our readers for the feedback and inquiries you have provided. I welcome your comments on what you would like from us. Leave a comment below, or email me at jhisert@indium.com.

And now a look back on past topics of interest:
 

Grid Ink, Silver Ink, Conductive Ink

Bismuth/Tin Tabbing Ribbon, A Low Temperature Pb-Free Alternative

Plated Metallization for C-Si Solar Cells

Increase Packing Density for Evaporation Crucibles

Photon’s 5th PV Tech Show 2010 USA

IPC Solar Standards Update

Solder Shelf Life as Explained by Eric Bastow

Tips to Speed Your Solder and Flux Selection

What's Happening in the Technical Service Department 

A Day in the Life of a Tech Guy

A Clean Laboratory

CIGS for Beginners

3rd Renewable Energy Expo 2009 in New Delhi, India

Solar Products and Representatives

Kleenex®, Google™, FedX®, CIGs?

Indium Solar Products Reunited

Trade Show Visitors Love the Ground Floor

Solar Product Data Sheets

Intersolar 2009 – What Barrier to CIGS Technology?

Concentrator Photovoltaic Systems - Will they reach 50% Efficiency?

Standards for Solar Panel Manufacturing

Solar Panel Certification: “Barrier and Benefit” Reviewed by Eric Bastow

Low Temperature Metallization Paste

What Will Your Interest Be At InterSolar? Meet the Bloggers And Let Us Know.

Share Your Solar Images

SAC vs. Sn/Ag for Solar Soldering

Solder Thickness for PV Interconnect

What is Bus Ribbon?

Standard PV Interconnect Ribbon Sizes

No-Clean Flux

Photovoltaics in EMS Sector

PV Interconnect Products

Eric Bastow - East Coast Technical Support

Mario Scalzo - West Coast Technical Support

Au/Sn Sputtering Targets

SMT Goes Solar

A Trip Down Memory Lane 

More Information About Metallization Paste

A year in the Life of a Solar Blog

CIG Target

23rd European Photovoltaic Solar Energy Conference and Exhibition

TCO choices for CIGS manufacturing 

CIGS Absorber Layer Electroplating

No Slump Metallization Paste

Meet the Bloggers

CIGS - Can sputtering make a breakthrough?

Fluxes for Soldering Tabbing Ribbon

Computer Brain vs. Solar Photovoltaic

Beam it down from space

Selection of the Optimum Lead-Free Solder for Solar Tabbing Ribbon

Record Makes Thin-Film Solar Cell Competitive with Silicon Efficiency

Why Thin-Film Solar Cells are Here to Stay

Hot Rooftops to Flashy Digital Cameras

Synchronize Your Solar Cell

Solar Conversion Efficiencies  

Government Support is the Key

It's Just a Beginning ...

After returning from San Francisco, I’ve had a chance to think about the Photon PV Technology show. This show was hosted at the Moscone Center, which also hosts the rapidly expanding Intersolar show each summer. Other than location and industry, these two shows have little in common.

With only 6 active aisles, the Photon show was easy for most visitors to cover in only half of a day. Since many exhibitors chose not to bring equipment, there wasn’t really a lot to see either. The silver lining to all of this to a visitor – you could spend a good amount of time at almost any booth you wanted without feeling rushed or distracted by a crowd of people. It was a good atmosphere to discuss technology.

I like to see other versions of the products I work with, so it was a let down to only see one booth with tabbing ribbon at the bottom of a display, and no solar sputtering targets (there was 1 target there, but it was aluminum – used to show equipment utilization.) 

The price to just attend the show was almost nothing ($29) although that doesn’t cover the exciting part the technical conference. It’s common to charge extra for admission to the tech sessions at shows, but over $600 per day for 3 days (if my memory serves me correctly) is a huge burden on an engineer that needs to justify that cost to his boss. In my opinion the conference attendance was crippled by this cost.

Will this show survive? After discussing this question with others at the show, the idea of combining the Photon USA and APEX shows began to sound very logical. The point was made during discussion, that APEX is starting to have a small solar focus and the Photon attendance seems to fill that need. Another good argument for the combination is that some of the OEM engineers and material suppliers go to APEX anyway, this is a good way to eliminate one costly trip.

Tommy: NanoFoil...it's kind of like aluminum foil in thickness and look, but a little stiffer and when you put energy into it, like a spark, it heats to 1500 degrees Celsius (which is hotter than lava in a volcano) for less than a millisecond!

My engineering and non-engineering friends display puzzled looks, you know the ones you get when you start explaining that your favorite sport is water polo and it has nothing to do with horses...ok the look you have right now

Engineering Friend: Umm so what can I blow up with it?

Non-Engineering Friend: So wait, if it's that fast and that hot, I bet you could make really fast grilled cheese. Have you called Healthy Choice yet?

Good Grief!

Al + Ni -> AlNi  (You didn't know there'd be chemistry involved did you?)

I just sat down to talk to Tommy Acchione (pronounced “akki-OWN”) Applications Engineer with Indium Corporation’s  new product line, Reactive NanoTechnologies’ (RNT) NanoFoil^®, about the technology, and its offerings into the semiconductor, power semiconductor assembly, LED and display assembly industries.

[ACM] First of all: welcome to Indium Corporation! Can you tell us, in just a few words, what the basis of the RNT Technology is?

[Tommy Acchione] NanoFoil^® technology is a thin metal sheet (“foil”) made up from alternating ultrathin layers of aluminum and nickel (Al and Ni). The reaction between these two metals is stoichiometrically very simple:

            Al+2Ni -> AlNi₂

And extremely exothermic (heat-generating). This reaction (see picture) is started by a very localized heat or other high-energy source, such as a 9V battery or even a laser beam. For a fraction of a second, the alternating thousands of sandwiched layers reach temperatures as high as 2000degC, and this isotropic heatwave radiates away from the initial hot-spot through the foil at speeds of about 5-8meters/second.

Just banging two lumps of Ni and Al together will never initiate a reaction this intense, as the two large pieces of metal act as very effective heat sinks, but by layering the metals together, the heat-generating reaction propagates by allowing the adjacent layers of Ni and Al to rapidly interdiffuse, so giving out more heat, causing the nearby layers of Ni and Al to interdiffuse and so on.

[ACM] How are these materials manufactured?

[Tommy Acchione] First, we pull a high vacuum, equivalent to those vacuums found in outer space, then we sequentially deposit the alternating layers by a sputtering process onto a specially-made metal block.

For a bonding material, a layer of a specialized brazing material is initially deposited onto the metal block, then the Al and Ni are put down, then a final capping layer of braze is deposited. The initial brazing layer both enhances subsequent bonding and also helps with easy removal from the surface of the metal block.

[ACM] I understand that the uses of these materials are expanding all the time. Can you give some examples that you can talk about?

Well, as you know we have about 30 patents on this technology and 35 outstanding patent applications, but I still have to be careful talking about newer applications, which are emerging all the time.

The biggest uses are in sputtering target manufacture (which is a little ironic, since that is how they are made!); Component mounting; and what we can call “reaction initiation”, or “energetics” - things requiring an instantaneous heat-source.

Sputtering Targets: For sputtering targets of non-refractory metals, standard indium or diffusion may be the preferred method. For most refractory metals and ceramics, solder wetting and CTE mismatches make bonding with standard processes difficult. NanoFoil^® allows for these materials to be bonded at room temperature, thus removing any CTE mismatches during bonding or subsequent cooling processes.

However, as targets get larger for flat panel displays (and we are seeing needs for up to 3m x .4m targets with higher generation depositing), indium starts to become too weak to take the weight of the indium-tin oxide (ITO or InTO) target itself, and only the strength of a NanoBond^® is sufficient to hold the target in place. Another key factor is that a manual bond of a large target to its backing plate starts to become simply physically unwieldy for an operator, as its size and weight increase. NanoFoil^® becomes the elegant and simple solution here.

Component Bonding: One major market that we are seeing is in component bonding. I can’t talk too much about this, but for high-brightness LED’s (HB-LED’s) and photovoltaic concentrators (CPVs) there is a growing demand for a high-temperature stable, thermally-conductive flux-less bonding material able to provide low junction temperatures over the lifetime of the device.

Energetics: Here we are talking about fuses and timed devices, with specially-shaped initiators that take advantage of the ignition properties and the reaction rate and energy produced by the NanoFoil^®.

[ACM] Tommy: very interesting! Many thanks for your time.

If you've checked out our data sheets lately, you may notice that they look a bit different.  We've updated and provided more data and images for sputtering targets, chemicals, PV interconnect ribbon, fluxes, metallization pastes, and the rest of our offerings.  What prompted us?  We wanted to provide more information.  Honestly, we are continuously looking for which bits of data we should include - and the answer to that question is in your hands.  Feel free to send me any suggestions of data that you would like included in our solar data sheets so we can put out the best literature for you. 

In addition to Au/Sn sputtering pucks, Indium Corporation can also manufacture full size Au/Sn sputtering targets.  80Au/20Sn is a versatile solder that is used in a wide variety of specialty applications within (and outside of) the photovoltaic world.

Au/Sn is commonly used as a Pb-free solder to attach Au surfaces.  The 80/20 alloy melts at 280°C, and can be used with a mild flux or a reducing atmosphere.  Another key point of 80Au/20Sn solder is that it has a tensile strength of 40,000psi.  This material's high melting point, combined with the non-hazardous nature of Au and Sn have made it very popular for Pb-free assemblies that must endure subsequent Pb-free reflow temperatures (~245°C).