SOLAR Materials Science Blog
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Share Your Solar Images
Do you have interesting pictures of a solar installation, solar cells, or solar module assembly that you’d like to share? Do you have a really specialized application that you want the world to learn about? Click on these words to send me anything you’d like to see here and we’ll discuss it on the blog.
I’m always personally interested in new solar applications – even though I realize they are often closely guarded. Nonetheless, if you have pictures, images, even drawings scribbled on the back of a napkin, submit it as a topic for the Solar Materials Science blog. I’m not picky at all, but I do love to see new tabbing, metallization paste printing, or sputtering applications.
Oh, another thing that would be great is sending in pictures of any defects you are seeing. If it is solder related we can help you solve the problem and help other readers at the same time. You can send them in anonymously too, that’s fine.
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SAC vs. Sn/Ag for Solar Soldering
Half of the world is still "in the dark" about which alloy to use.
As a discussion point (read “please feel free to disagree – that’s what the comments section is for”), 96.5%Sn/3.5%Ag is NOT the most well suited pb-free alloy for tabbing. I know not everyone agrees, as evidenced by the amount of Sn/Ag that is requested, compared to Sn/Ag/Cu. Our industry uses Sn/Ag because it is programmed to, but let’s examine why you should consider 96.5%Sn/3%Ag/0.5%Cu (SAC305) instead.
1) Melting Point
The melting point of SAC 305 is 217ºC, compared to Sn/Ag at 221ºC. Why subject your solar assembly to more heat than necessary? This could be worth it for mechanical advantages, but are there any?
2) Reliability Data
Since before the Pb-Free craze started, information was collected on certain Pb-free alternatives, using Sn/Pb as a reference. There is simply more data available for Sn/Ag/Cu. You can find this information @ www.NIST.gov. Click Here
3) Strength and Raw Materials Cost
In addition to slightly higher tensile and yield strength, using an alloy with a small amount of Cu improves wetting – which in turn improves solder joint strength. The electronics industry has learned the benefits of Sn/Ag/Cu, it is time for the solar industry to catch up.
~Jim
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Solder Thickness for PV Interconnect
X-section of tabbing ribbon showing solder thickness
Solder thickness is important whether you are interested in tabbing ribbon, bus ribbon, or (most likely) both types of PV interconnect materials. In almost all tabbing/stringing applications, the solder coating on the interconnect ribbon provides 100% of the solder used to form a metallurgical bond on top of solar cells. With this in mind, the solder coating should be more than just a ‘shiny finish’ on the tabbing ribbon – but what is the proper thickness for soldering?
Indium Corp. has been making precision solder coated ribbon for quite a long time (and not just for tabbing/stringing). This experience has taught us how to control solder thickness, and also what thicknesses work in various applications. If you would like to learn how solder coating thickness affects the reliability of your solar cell, email us at: solar@indium.com.
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What is Bus Ribbon?
Bus Ribbon vs. Tabbing Ribbon (left to right)
Bus ribbon is a very specialized interconnect for photovoltaic modules. Much like tabbing ribbon, bus ribbon is made of a copper ribbon or flat wire which is coated in solder. The solder protects the surface of the copper from oxidation and provides a layer of solder to form a solder joint. These solder-coated copper strips are used to channel current from the top and bottom of solar cells (tabbing/stringing) or to collect the electricity from these strings (bussing).
The main difference: bus ribbon is a bit wider, and sometimes thicker than tabbing ribbon. You can think of tabbing ribbon as roads which travel across a solar cell, and bus ribbon as the highways that connect and tie them together. Bus ribbon is larger in cross-section because it has more electrical current to carry. To give you an idea of the size, bus ribbon is generally anywhere from 5mm-6mm wide, although some applications require bus ribbon more than twice as wide.
As bus ribbon becomes larger and larger, it is only natural to ask: “When will things need to change?” When efficiencies and currents outgrow the physical constraints of bus ribbon, will module design change to meet the requirements? Perhaps there will be a shift in the materials which are used instead – which would likely be even more expensive than a design change. Either way, solder-coated copper bus ribbon has a long life ahead of it as the solar panel interconnection material of choice.
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No-Clean Flux
You may notice that many of the solar grade fluxes and solder pastes are designated ‘no-clean’. This classification is used to describe a flux or flux vehicle that does not need to be removed (cleaned) after soldering. In much the same way as the mosquito in “
It might sound scary, but no-clean fluxes are very common in today’s electronics assembly – including military, medical, and high-reliability applications. There is no question that electronics assemblers and material suppliers have done their due diligence in qualifying no-clean materials. In the end you can put your mind at ease, modern no-clean materials are classified as such after standardized testing for electro-migration and surface insulation resistance. This testing allows flux suppliers to safely provide no-clean materials that can function well in various soldering situations and eliminate the need for a post-assembly cleaning process.
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Tabbing Ribbon Flux for Solar Assembly
If you’re new to PV module assembly (tabbing in particular), you’re probably trying to get a feel for all the needed materials. This part is a little like getting all the materials you need to cook dinner. You need to get EVERYTHING that is required – so you don’t have to go back to the store for ¼ cup of milk (or in this case, a pint of flux). In previous posts we’ve talked about tabbing ribbon [1, 2, 3],
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Standard PV Interconnect Ribbon Sizes
I probably should have mentioned in my last post, you don’t need to start from scratch. Here are a few of the standard sizes that we have just waiting to string your cells:
2 mm wide x 0.10 mm thick
2 mm wide x 0.15 mm thick
1.5 mm wide x 0.10 mm thick
1.5 mm wide x 0.15 mm thick
5.0 mm wide x 0.40 mm thick
Standard alloy coatings include:
Sn96, 96.5% tin, 3.5% silver
SN60, 60% tin, 40% lead
SN62, 60% tin, 38% lead, 2% silver
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PV Interconnect Products
PV interconnect products have a lot of nicknames: tabbing ribbon, interconnect wire, bus bar, stringing ribbon, etc. I wish I could say the dimensions are only as various as the names we use to describe them. We look forward to working with you on custom PV interconnect projects though. To speed the process, just email us at solar@indium.com with your specifications. Along with copper thickness, width, and solder alloy and thickness, make sure to include all the criteria that are important to you. That’s one of the best reasons to work with us on your project – you can get exactly what you think will work best in your application.
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Eric Bastow - East Coast Technical Support
I’d like to introduce another member of the Indium Solar Team. Although the west coast enjoys most of the solar activity, Eric Bastow (East Coast Solar support) values the importance of the east:
[Eric B.] Living in upstate New York, just a few hours south of the Canadian border, with the Great Lakes not far to the west and the Atlantic Ocean not far to the east, with something like 200 cloudy days a year and “10 months of winter and 2 months of bad skiing”, it is almost hard to imagine that capturing the sun’s energy would ever be a worth while approach to supplying our energy needs, at least in this part of the world.
The western United States seem to have the dominant position in the domestic solar industry but we should not forget that the East Coast is a “power house” as well (no pun intended). We should be mindful of the fact that the East Coast is home to “The Sunshine State”,
I may live on the East Coast in a somewhat cold dreary part of it, but there are other places in the East that are forging ahead into this industry.
The West may be strong but there are no British Empires in the solar industry. “The Sun Never Set on the
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Photovoltaics in EMS Sector
http://circuitsassembly.com/cms/component/content/article/199/7190-photovoltaics-in-ems-sector
This article isn’t ‘hot off the press’, but it is worthy of review. In the October 2008 edition of Circuits Assembly Magazine, Darren Brown (DEK) discusses “Photovoltaics in EMS Sector”. I agree that there is a parallel between SMT and metallization processing, and that there are advantages to having an
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