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 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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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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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 “Jurassic Park” was preserved, flux activators are also encapsulated within no-clean flux as it cures during a typical heating cycle.  Unless you plan on fully removing the no-clean flux residue with a commercially available cleaner specified for that flux, it is better to leave the residue alone.  Partial cleaning of no-clean residues can uncover small amounts of flux activators – like uncovering the mosquito in the tree rosin.  The flux activators have the potential to cause corrosion or electro-migration when exposed – so make sure you get the right solvent if you do plan on cleaning these materials! 

 

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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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], as well as tabbing flux.  It is important that both of these products are used during tabbing.  The flux is used to remove oxides on the surface of the tabbing ribbon (the solder coating) and promote wetting to the metallization paste.  Liquid flux is generally used for this application, and it is applied by dipping the ribbon into it.  Feel free to email solar@indium.com to learn exactly how these products should be used in your application.

 

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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 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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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”, Florida. Anybody that reads solar industry related news will know full well that there is no lack of solar activity in Florida. It should also be noted that Florida residents have captured the suns energy for years through roof mounted panels. Swimming pool water is pumped from the pool up into the blackened panels on the roof, capturing the sun’s warming energy, before sending it back to the pool. There are also certain East Coast universities such as UNC at Charlotte that offer programs specific to solar energy.

 

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 British Empire…..”

 

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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 EMS help with solar assembly.  Likewise, I feel there is an advantage working with equipment and material suppliers like DEK and Indium, suppliers who also serve the semiconductor packaging market.  Much like semiconductor materials and equipment, solar products are designed from a base knowledge borrowed from SMT – with very specific application criteria in mind.  As you can imagine, working with this type of vendor gives you the assurance of a proven track record, even in a relatively new market like solar assembly.

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I’d like to introduce you to a member of the Indium Solar Team that is here to support the western half of the United States.  You may have seen Mario if you have been looking at the Indium blogs, he hosts the Tech Support Blog.  Mario has a busy region to take care of, and one of the most influential.  When I asked him today what is most important about his region, he commented “The west coast is leading North America in manufacturing and R&D of solar modules and assemblies”.  That’s where Indium Corporation seems to fit in the best as a company – at the front of soldering technology.

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