Indium Corporation Blogs

Folks,

A reader writes:

Dear Dr. Ron, I need to measure the void content of an alloy.  Is there an easy way to do it?

After a little thought, it occurred to me that the densities of the voided and unvoided material will likely hold the answer.  I derived the result below.  Assuming we know the density of the unvoided material, we can measure the density of the voided material with the Wet Gold Technique, discussed in recent posts, if the voids are not connected (closed cell.)  If the voids are connected (open cell), you could machine the foam to the shape of a rectangular parallelepiped and determine the density of the foam as the mass divided by the volume.

As an example, let’s say you have a closed cell aluminum foam. We use the wet gold technique to measure its density at 1.5g/cc. The density of solid Al is 2.7g/cc.

So the volume fraction of voids is:

Sadly, this technique could not be used to find void content in solder joints, or in BTC (e.g. QFN) thermal pad connections (which are so handily mitigated by using solder preforms.)

:   :   :   :   :   :   :

Global Warming Musings:  My recent post on GW generated many comments.   I will be sharing additional reasons why I am a skeptic at the end of posts like the one above. 

It is important to state the distinction between a GW Skeptic (me) and a GW Denier.  As a Skeptic, I am not convinced that the warming trends are alarming or unusual, especially since the atmosphere has not warmed in more than a decade.  Also, I am not convinced that the main driving force for the warming trend up to the late 1990s can conclusively be attributed to human activities.  Lastly, I’m not convinced that even with Draconian measures, we could affect a change that would matter.

The Carbon Cycle

In this post, I would like to share the data relating to how much carbon dioxide is produced and put into the atmosphere.  More specifically, what percent of carbon dioxide generated each year is from human activities. Would it be 30%, 40%, 50%?  The answer is 3%.  The remaining 97% of carbon dioxide generated on the earth each year is generated by natural processes in the oceans and on the land.  See the image below.  The GW argument is that even though human activities are only 3%, this amount offsets the delicate balance that nature provides.  Working with and modeling data all of the time, I find this argument unsatisfying.  Collecting accurate data and developing an accurate model on data like this is difficult.  Making incontrovertible conclusions (it is certain GW is caused by humans) more so. Freeman Dyson, arguably one of the most accomplished physicists of this era, has a similar view:

The models solve the equations of fluid dynamics, and they do a very good job of describing the fluid motions of the atmosphere and the oceans. They do a very poor job of describing the clouds, the dust, the chemistry and the biology of fields and farms and forests. They do not begin to describe the real world we live in...

It is interesting also to note that throughout history the temperature of the earth determined the carbon dioxide content in the atmosphere, not vice versa.

Cheers,

Dr. Ron

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It is no secret that automotive semiconductor customers are becoming increasingly demanding. The "under the hood / bonnet" electronics environment is arguably one of the most thermally stressful environments on the planet. Electronics close to the engine block can experience extremes ranging from frigid winter cold to tropical heat, with the added heat source of the adjacent internal combustion engine.

The moisture sensitivity level (MSL) standard from JEDEC / IPC was developed to cover the moisture-absorption and "popcorning" effects of polymeric overmolded materials, but has been expanded in usage to cover a variety of different packaging situations and failure modes. The standard does allow for a certain amount of delamination, even under the MSL1 conditions usually required by automotive semiconductor customers. However, now "zero tolerance for delam" is the most common request from automotive design engineers. In order to meet this need, both overmolding materials manufacturers and leadframe suppliers have been working on how to drive to zero delamination. Leadframe manufacturers have developed a variety of approaches to their products that enhance the adhesion between the leadframe metal itself and the overmolding compound. Usually, this takes the form of physical and chemical texturing of the copper, using a process such as brown oxide formation.

It is no surprise that this need for adhesion enhancement (AE) drives leadframe treatments that are antithetical to the need for formation of void-free, high conductivity electrical connections between the die and the leadframe - basically, it messes with the solderability of the preform or solder paste. In order to get around this issue, leadframe manufacturers have increasingly moved to the use of spot-plating of silver onto copper, with thicknesses ranging from 2-9microns. Why is the silver so thick, in comparison to silver sputtering onto the die surface? Simply because copper diffuses very quickly into the silver, so a thicker silver layer leads to a longer shelf-life for the leadframe. Note also that plating does not have as good process control as sputtering, but it is a lot cheaper and faster.

You can see (below) a schematic of solder paste printed onto one of these leadframes.

An emerging failure mode is one of incomplete wetting onto the leadframe, leading to failures at the sites where solder has failed to flow over the silver plated area completely - "delamination sites" - (below). The flat, shiny, silver finish is not a suitable surface for overmolding compounds to bond to.

So why isn't the solder wetting well? The answer becomes clear pretty quickly when you do some back-of-the-envelope calculations of the expected final silver content of the finished joint. Let's assume some bondline thicknesses (BLT) is (25,75microns) of a solder containing 2.5%Ag (such as Indalloy 151 or 163) and the plating thickness is (3-9)microns. Typical plating thicknesses of 2-9microns may be seen, based on a recent customer survey), with a mean around 3microns.

So what is the silver content of the final joint, assuming all the silver is dissolved?

The calculations, therefore, show that it is from 6 to 27% silver. The 27% level is well beyond the solubility limit of silver in these types of solder, and in fact in most solders, at the expected soldering temperatures. The mechanism of non-wetting is clear: solder can no longer wet onto silver, once it has become filled with insoluble intermetallic particles.

The message to power semiconductor component suppliers is:

• Maintain the silver thickness at a consistent, low level: set up tighter specifications on the silver spot-plating from your supplier.
• Update your incoming quality control inspection so you can be sure you are getting what you paid for in terms of thickness of silver and consistency.
• Manage leadframe inventory so you run leaner, so you do not run into leadframe lifetime issues with copper diffusing through the thin silver layer and oxidizing (solderability / voiding problems).

You do have an alternative (moving to an alternate solder type), but then you are into a lengthy requalification procedure.

As always, please contact me if you need assistance.

Cheers!  Andy

Printed circuit boards containing both surface mounted and through-hole components are common, and are often referred to as "mixed technology" boards.  In mixed technology assembly, solder paste is used to attach the components to the surfaces and wave soldering attaches the components that are inserted through holes in the board.  For low volume production, hand soldering is often utilized - for the attachment of through-hole components.  Both of these methods require additional steps after the original reflow of the solder paste.

To increase your profits (saving you time and money while improving your quality and productivity) InTEGRATED PREFORMS^® have found a place in mixed technology assembly.  InTEGRATED PREFORMS^® are interconnected solder washers, designed to fit the pin pattern of a through-hole component.  These arrayed solder washers are sized to deliver the precise solder volume required to fill the holes and to produce excellent solder fillets at each joint. 

In some cases, to add even more solder, solder paste is deposited over the holes and the InTEGRATED PREFORM^® is placed into the paste. The component is then inserted through the solder preform, the solder paste, and the hole. 

In other applications, TacFlux® (that is compatible with the solder paste's flux vehicle) is applied to the preform before it is placed on the component's pins, or is placed directly on the board, and the component is inserted as described above.  Whichever method is used, only one reflow step and only one cleaning step are required.

In traditional wave soldering, components with long pins are a special challenge because they are very difficult to attach without getting alloy on the pins during wave or hand soldering.  InTEGRATED PREFORMS^® can be applied to the top or bottom side of the board and are reflowed along with the components held down with solder paste.

InTEGRATED PREFORMS^®  are designed and built to address the unique characteristics of each specific application.  To build your InTEGRATED PREFORMS® we require the following information, so the solder volume and washer spacing are correct for your specific pin configuration:

• Hole size
• Pin size
• Board thickness
• Center to center spacing of the pins (within the row, and row to row)
• Solder Alloy
• Is the preform going to be used to add to the volume of solder from the paste?

Separate (individual) solder washers can also be used in place of connected InTEGRATED PREFORMS^®.  They can be designed to deliver the same consistent volume of solder required for each joint.   Care must be taken, however, to place only one preform on a pin, and not miss any.  This is what makes InTEGRATED PREFORMS^® desirable.  The solder washer array is designed and manufactured to fit the pin configuration so only one washer goes on a pin.  If extra solder volume is required, InTEGRATED PREFORMS^® can be easily stacked.

With today's drive to optimize profits, InTEGRATED PREFORMS^® present an excellent opportunity.  The biggest advantage of InTEGRATED PREFORMS^® is the fact that quality can be improved while costs are reduced.  If you are looking for any easy way to cut costs, increase production, improve quality, improve customer satisfaction, and, ultimately, increase your profits, talk to me about InTEGRATED PREFORMS^®.

Paul Socha psocha@indium.com

BONUS: Read our white papers regarding InTEGRATED PREFORMS^® .

Folks,

One of the nice aspects of being a Professor at Dartmouth is teaching a course like ENGS 1: The Technology of Everyday Things.  This course is designed for non-engineering students and fills a technology and applied science requirement for them.  In the course, we cover the technology of the automobile, mobile phone, GPS, DVD players, the personal computer, etc.

This year, I purchased an iPad 2 and an Amazon Kindle Fire and added them to the topics.  I had three basic goals in this effort:

1. Discuss how they work
2. Compare them
3. Answer the question, “Will they replace my personal computer?”

So this week, instead of discussing solder paste, cost of ownership, solder preforms, or productivity, let’s see what I learned.

I did this analysis with two of my graduate students and reviewed the basic conclusions with Dartmouth IT staff and students in the class.  Here is a summary:

The iPad is a terrific device for organizing and consuming content such as videos, music, photographs, and, perhaps most importantly, games.  It is extremely intuitive.  My four, five and six year old grandchildren use it for games with no coaching.   With the iCloud, content can be organized and stored very simply.

However, for creating verbal or mathematical content, it is not intuitive or simple.  As an example,  assume you have a letter or paper from a colleague on a USB memory stick that you need to edit.  Apple so controls the “experience” of the iPad that you cannot load anything from a memory stick or any other device, like your PC, onto it.  You must send an email or load the content into iCloud.  The recommended software (or apps) for word processing, spreadsheets, or presentations for the iPad will work with MS Office, but I didn’t find the apps that great.  Saving and filing documents is also not that straightforward.  When I tried to create letters, papers, spreadsheets, or presentations on my iPad 2, I found myself longing for my laptop. I’m not saying it can’t be done, I’m just saying it wasn’t easy for me.  An external keyboard (about $70) makes data entry much easier. 

The Amazon Kindle Fire is only about 25-35% of the cost of an iPad.  In my opinion, the Android software is pretty good.  For consuming content, such as movies, books, photos, etc. the Fire is very good, but not quite as good as the iPad.  I don’t do games enough to make a comment. (Sometimes I feel as though I am the only one in the world who does not play Angry Birds.) For verbal and numeric content creation, the Fire makes working with MS Office documents easier.  It is also easy to store and load documents from a PC with a USB connection.

Both devices are beating the rest of the competition with their seamless connection to their respective stores.  I think this advantage that Apple and Amazon have over other devices has been understated. I believe Amazon Prime is a strong reason to consider the Kindle Fire.  In addition to free two-day shipping for purchases, members get many free videos and have access to a free lending library.

Steve Job’s said it best when he announced the iPad.  Quoted in Walter Isaacson’s outstanding  biography of Jobs, he said something like, “We have the iMac and we have the iPhone, now we have something in between.”  I think that is a fair summary.   The portability of tablets can make them ideal for passing family photos around, or for police to have a larger than mobile phone photo of a suspect to share with colleagues and witnesses.  Tablets can be stored in a suitcase or briefcase when going through airport security, a definite advantage.  But they are not a replacement for the full functionality of a PC.

The higher resolution of the iPad 3 enables viewing x-rays and CT scans and, of course, HD video.  The 10 hour battery life frees one from needing to have a power cord during the day, so it could be beneficial in meetings.  However, I found the Penultimate app, which allows writing on the screen, not that usable.  It was harder to get neat writing than with pen and paper, and the words were too large.  See the image.  The documents formed would (again!) have to be emailed to get them from the iPad to another device.  It would make more sense, to me, to take notes on paper and scan the paper into a PDF. Modern scanners make this act a snap. Apple makes it almost a necessity.

In summary, for a user like me who creates papers, blog posts, Excel® spreadsheets and PowerPoint® presentations, a tablet is a weak substitute for a laptop.  For consuming content like videos, books, music, and photos, or for playing games, they are tough to beat.

Which of the two do I use the most?  The Kindle Fire, mostly due to the connection to the Amazon store for books and free videos from Amazon Prime.  I think this device, at a fraction of the cost of an iPad3, may be more of a threat to the iPad than many people think.

I'd like to hear your thoughts and opinions. Please comment!

Cheers,

Dr. Ron

Folks,

Let's look in on Rob:

Rob looked at the new photo of Patty and their twin sons Peter and Michael. What a handful those two 18 month-olds were. Just like their mother! Rob was still pinching himself that he was lucky enough to have Patty as his wife. Rumors were that she would make VP soon, and a few of his buddies asked him if her success bothered him. 

He would always respond, “Let’s see: beautiful, successful, athletic, fun to be with, great mother, and most of all she loves me. What’s not to like?”

Rob really meant it. He felt Patty deserved her success. One of her great assets was her high energy level. She went to bed at 11:30PM and was up at 5AM to run two miles, lift weights, shower, and then take care of the kids. Rob just couldn’t keep up on less than seven and a half hours of sleep. So he got up at 7AM. Rob had to insist that they have some quiet time each night after the boys were in bed, to talk and maybe even watch some mindless TV. But Patty would often sneak her laptop out to work while NCIS was on. Patty, the workaholic!

Rob and Patty spoke Mandarin at home one day each week and Spanish another night. The boys were picking up all three languages. It was amazing to both Patty and Rob as they watched this miracle.

Well anyway, Rob did have one thing up on Patty: math. Rob was close to a math genius and also good at writing software. He was the “go to” guy for math modeling and writing software for the math models. He even helped the Professor improve ProfitPro. Rob also wrote a program that could be used to design an SMT line for maximum throughput. The software could do what Arena did in hours of simulation, in seconds.

Rob was startled from his daydreaming by the phone ringing. It was Sam the site GM.

“Hey Rob, we need your help in our plant in Guadalajara. Can you come right down to discuss it?” Sam asked.

“I’ll be right there,” Rob replied.

Rob walked to Sam’s office with a feeling of exhilaration. It was always fun and exciting to be sent on a trouble shooting mission.

“Rob, thanks for coming right down. This issue is QFN reliability. About 5% of the Druid mobiles phones in our Guadalajara plant are coming back with some of the QFNs burned out,” Sam began.

“Sounds like a voiding issue under the QFN thermal pad,” Rob interrupted.

“Wow, you seem to know quite a bit about this type of problem,” Sam remarked.

“Remember how I pleaded with you to go to SMTAI,” Rob teased.

“Yep,” Sam replied.

“Seth Homer gave a talk on this issue at the show last week. It was a terrific overview of the problem. From what you described the connection may need more solder. We may have to use solder fortification® preforms to solve this. Optimizing the solder paste printing process may not be enough,” Rob summarized.

“Well, go there and solve the problem. The warranty issues are costing us a fortune,” Sam commanded.

After a moment of contemplative silence, Sam asked, “Do you need anything?”

“It would be nice to have Pete come. He knows the people there and is well connected. His Spanish is also terrific,” Rob said.

“OK, no problem. Since you sleep with Pete’s boss, you can work out the details with her. I need you to go this week,” Sam said.

“No problem,” Rob said.

As Rob left the office, he was elated with his new assignment. He had to admit though, he thought it was unprofessional and a little annoying of Sam to say, “since you sleep with Pete’s boss, you can work out the details with her,” but it wasn’t the first time someone said this. Truth be told, Patty might be a little annoyed. She really depended on Pete.

Will Patty be angry at Rob for taking Pete to Guadalajara? 
Will Rob solve the QFN problem?
How does Patty get by on only 5.5 hrs of sleep each night?
Stay tuned for the exciting conclusion.

Cheers,

Dr. Ron

Power amplifiers and transistors come in many shapes and sizes. The performance requirements vary as well. Attaching them can be a critical aspect of your design.

Both Pb and Pb-free alloys can be manufactured as a solder preform with a flux coating.(Learn more)  Selecting the right alloy and flux coating can be crucial to meeting your void criteria.  

A high-tech SOLDERING solution might include NanoFoil®, which effects a solder joint while minimizing heat exposure to your components.

There are also thermal interface materials such as the HEAT-SPRING® which utilize the unique properties of indium to create a superior thermal connection, similar to a solder joint.

There are many different attachment methods available, contact me with your design parameters and we can find your solution.  

Integrated® Preforms are connected units of solder that can be placed in an application to provide the sole solder for the joint. They can also be used to add to the volume of solder, when used in conjunction with solder paste - to fortify the joint. Integrated Preforms are available in most alloys that are currently incorporated in common solder pastes.

The only difference between connected preforms and separate preforms is that the Integrated (connected) Preforms are attached to one another and can be placed in the application as one multiple unit of solder. Connecting each individual solder preform is a small strand of solder that is designed so narrow that it will separate during reflow (to prevent bridging).

When manually reworking boards to add additional solder to joints becomes too large of a task, Integrated Preforms can be incorporated prior to reflow so any rework is not necessary. In through-hole applications, the connected preform can be placed on the component and then inserted into the holes of the board that have solder paste placed on top of them. There is no need to flux the Integrated Preform because the flux vehicle in the paste will provide the fluxing.

When used as a stand-alone connected preform, or with solder paste to fortify the joint, Integrated Preforms can be reflowed using the same temperatures and profiles as for the paste by itself.

To learn more about Integrated Preforms and solder fortification®, plan to attend the presentation of a paper on this topic at Apex 2011 (Las Vegas, NV, 10-14 April) given by Dr. Ronald C. Lasky (see sidebar →), Indium Corporation Sr. Technologist and Instructional Professor, Thayer School of Engineering, Dartmouth College.

The next post in this series will address the various shapes of Integrated Preforms and how they are packaged.

Spending back-to-back weeks at two electronics assembly trade shows is a unique experience, to me.

First up was SMTAI in Orlando, FL where I helped present a paper on Solder Fortification® Preforms. 

How does one "help" present a paper?  Have you ever listened to a great paper and then, at the end, seen so many people rush around the speaker that no one can get a chance to make a further connection?  My role was to be available as an additional resource for answering questions after the presentation.

The second show was IMAPS 2010 in Raleigh, NC.  This is one of the best showcases for our Engineered Solder Materials.

There are many facets to the trade show experience. 

• There is the booth, which has to have the right message and the right pictures to draw the right people in. Our message is "From One Engineer to Another". Most of our staff IS engineers. Our team is extremely knowledgeable about materials and assembly processes.  We provide the best service when we are talking with Process or Design Engineers that need our soldering or bonding expertise.   
• There are the technical presentations, as mentioned above.
• There is the networking with industry leaders.  Being able to visit a variety of complimentary suppliers and discussing topics we have in common is tremendously valuable.

Even in the face of the myriad technology devices that keep us connected 24 hours a day, there is still nothing like that face-to-face experience.  The IMAPS Global Business Council presented two speakers during the show: one from DuPont and one from Dow.  One of them (I am sorry I can't remember which one) stated the overwhelming importance of getting their Application Engineering staff into the field to work face-to-face with customers.  

So if you want to more about what we know, check out our blogs.  When you see something you like or want to know more about, call us or email us.  We are glad to work with you!

And if you want to hear another viewpoint on trade shows, check out Andy Mackie's blog.

I love watching a good basketball game, and one of my favorite local teams is the Syracuse Orangemen. If you go to a Syracuse home game, notice the shot clock – it was made with Indium Corporation solder. There are a lot of places you can see our products in your everyday life. That smart phone in your pocket, the electrical components in your car, the thermal interface in the computer in front of you. That’s one of the things that makes this job rewarding, being part of so many various applications.

In addition to learning about these different applications, we also get a good reference for what assembly trends are developing, and which material technologies are becoming more popular. 

I’ve watched the halogen-free trend explode and fade, as it was adopted by some large OEMs and their contract manufacturers, but has not spread to most other companies. Another trend that is fading away from the spotlight is Pb-free die-attach solder, since the EU has not found a suitable replacement and has pushed back the exemption deadline. 

A long-existing topic that has had recent mention is solder jetting. The trend towards soldering smaller components is not new or surprising, but for smaller components (01005s and 0201s) we have seen a trend towards dispensing instead of jetting – which seems to suit those applications.

For small component printing, transfer efficiency is critical. Outside of solder paste optimization, “nano-stencil” technology is an upcoming technology that may take-off and improve paste release characteristics. Solder paste is being used in some other creative ways too, like low temperature alloy dipping paste for rework operations. Manycompanies are now using or evaluating specialized solder applications to replace components without fully reflowing the rest of the components on the board.

Integrated preforms are finding their way into more and more applications recently as well. These connected preforms are being used to reduce the need for component pallets and selective soldering operations.

All these applications are great ways that our customers are taking soldering technology to the next level, using materials and assembly methods that were not common before. I look forward to learning how you’d like to use solder in your application!

Integrated Preforms®, like separate solder preforms, are available in all of the common shapes. Since not all applications are alike, these connected preforms will vary, not only in the alloy, but also in the size and shape of the solder preform required to deliver the correct amount of solder to the joint.

Some Integrated Preforms are made into discs, squares and rectangles for surface mount applications. Generally, the most common shape is that of a washer - for attaching through-hole components to a printed circuit board. Regardless of the shape, each is designed with specific dimensions that will fit the application. The spacing between the washers also varies because the pins on through-hole components can have different pitches.

Integrated Preforms are manufactured in 5” x 9” sheets with the designed shapes positioned within the borders. In preparation for shipment, they are stacked with lint-free paper and backing board and placed in a vacuum-sealed bag for delivery to the customer.

The customer can easily cut the shapes from the sheets with scissors or an X-Acto® blade. Care must be taken to handle the solder preforms with gloves so as not to contaminate the sheets with unwanted surface organics which could lead to voiding.

The next blog in this series will address the actual use of the Integrated Preforms and the many ways they can be modified to fit any application.

A reader of this blog recently mentioned: "I am interested in what products could be sold by manufacturer's representatives."  That is a large question, considering the evolution that we have come to expect in the solar industry.  To answer that question involves first breaking up the industry into 2 separate sections, front and back end solar assembly.  Front end assembly involves the process of making the solar cell.  Back end involves connecting cells together and assembling them to create a useable device.

Both front and back end products are going to be geared to the customer's technology.  For instance, if I was purchasing materials for a large thin-film manufacturing company and someone boldly offered me glass filled high-temperature metallization paste, I would tell them to come back when they know what they are talking about.  (In reality, I'd be nice – even though it's an incredibly ignorant mistake.)  With that in mind, let's focus on what back end products a representative might be offering for crystalline and thin film solar customers – assuming that back end begins after metallization:

• Outsourced Solar Cells
• Tabbing Ribbon
• Bus Ribbon
• Tabbing Flux
• Solder Paste
• Preforms
• Solder Wire
• Tacky-type Fluxes
• Tabbing Equipment
• Rework Equipment
• Test Equipment/Services
• Packaging Materials
• Junction Boxes
• Laminate Materials
• Silicone/Sealing Materials
• Passive Components
• Ovens
• Frames
• Gloves / Lab Coats / Safety Equipment

I probably left out as many possible line items as I included, but I hope you get the idea.  Feel free to add the ones I forgot in the comment section below.

~Jim

Figure 1: 15% Voiding with air reflow

Figure 2: ~0% Voiding after vacuum reflow

Figure 3: Multiple voids

While at the Semicon West 2009 show in July, I had a chance to sit down with Herr Klaus Roemer of Pink GmbH. PINK is most famous in the die-attach and power module manufacturing world for their reflow ovens with vacuum, but are also known in the medical and aerospace industries for manufacturing extremely high precision, one-off, vacuum equipment for applications as diverse as particle-accelerators for ion bombardment, and large-volume chambers for helium leak-detection. I asked him some questions about Pink vacuum soldering technology.

Cheers! Andy

I'm excited about the May 2009 issue of SMT magazine, which will be highlighting photovoltaic soldering applications.  Not only does it combine soldering and PV (which is our niche), but it will be including an article written by Karl Pfluke of the Indium Corporation.  (Abstract shown below)  Karl's one of my personal heroes, he's a veteran technical engineer and leader in solar applications.

Photovoltaic Stringing in Solar Cell Module Assembly

By Karl Pfluke

Indium Corporation

"As more and more contract manufacturers (CMs) look for opportunities to diversify and fill factory space, photovoltaic (PV) solar cell module assembly seems to be the popular choice.  Commonly referred to in the solar panel industry as the "back end process", the assembly is done in lines and has some similarities to SMT. 

Solders, solder pastes, solder wire, solder preforms, and fluxes are used for interconnects in PV assembly.  Since RoHS and WEEE initiative does not apply to this industry, many manufacturers use SnPb solder for interconnects, Sn60 and Sn62 being the most popular. However, SnAg is occasionally used, and some manufacturers are exploring the use of SAC alloys (SnAgCu), specifically SAC305.

One process, "cell stringing" or "tabbing", uses products quite familiar to the SMT industry.  Tabbing ribbon, a precisely straight and flat piece of copper that has been coated with a tightly controlled thickness of solder, is used to connect multiple PV cells. 

This article discusses the common solders, reflow technologies, and materials used in PV cell stringing for solar module assembly."

Nice work Karl!

~Jim

A set of wires prepared for connection

Properly joined thermocouple wires

Thermocouples do a lot for us.  We use them for profiling reflow ovens, checking material temperatures, and a host of other temperature related measurements.  They see a lot of abuse and they are bound to break with enough rough use.  So how do we fix broken thermocouple wires?  I have a method that works very well.  It may not be the cheapest way to fix the wires but it has the following advantages:

• Pb-free joint
• Highest thermal conductivity of any current method
• Good for use up to 280°C
• Strongest connection (40,000psi tensile strength)
• Requires no specialized equipment

This method can also be used to convert leaded thermocouple wires to pb-free.

Here’s how:

1) Clip the ends of the thermocouple wires so they are even

2) Strip the sheathing back as shown in the picture (1/4inch)

3) Use a razor blade or emery paper to scrape the oxide layer off the wires, then twist the ends together

4) Put a very thin (~.001”) layer of NC 506 flux on the surface of a ceramic coupon and the exposed thermocouple wires

5) Place an 80Au/20Sn preform or a sphere(s) of the correct volume on the flux layer

6) Place the coupon onto a hotplate set to 400°C

7) Bring the wires over to the Au/Sn (which should now be molten)

8) Dip the wires into the solder

9) The solder should wick onto the wires, when it does – remove the wires.

You can leave the no-clean flux residue on the wires, or wipe it off using a solvent and rag.  You now have a high-temp pb-free thermocouple.

If you'd like to discuss this with me, click here or just give me a call @ (315) 853-4900 x-7592. 

Have you ever had to hand-place solder washer preforms on the pins of a connector and found it to be very labor intensive? Indium Corporation has a product called InTEGRATED Preforms which enable the user to place multiple preforms at one time in less than 15 seconds.

InTEGRATED Preforms are joined in a matrix by fine, precise strands of solder which, during the soldering process, melt and flow to adjacent pads to give complete preform separation.

In addition to time saved, the quality of the joint is assured because only one preform is delivered to each joint eliminating the chance that a pin will be missed or two washers placed on the same pin. Each scenario will create a quality problem that reduces the yield and increases the time spent on rework.

Unique preform designs and complicated configurations can simplify difficult soldering jobs. An example is a thru-hole connector with multiple rows of long pins that are difficult to reach in the center with hand soldering. InTEGRATED Preforms eliminate the need to reach these remote areas. The connected washers can be placed on top of the board, under the connector. After reflow, equal volumes of solder are delivered to each of the pins in the connector.

In order to get the full benefit of InTEGRATED Preforms they must be uniformly fluxed on both sides including the connecting strands. Also, they need to be flat in the application and uniform heat must be used to reflow them.

Many printed circuit boards have mixed technology. The surface mount components can be placed in the paste first followed by the thru-hole components using InTEGRATED Preforms. The board can then be reflowed and cleaned once, eliminating time consuming steps.

To learn more about this time saving product patented by Indium Corporation, please contact the Applications Engineering Staff at Indium Corporation.and we would be happy to design an InTEGRATED Preform to fit your solder requirements.

For additional information regarding reflow methods using InTEGRATED Preforms, check out the June 2007 issue of SMT Magazine.

When considering the choice of flux for preforms, in addition to factoring in the amount of flux required on a preform, the type of flux is also important. Fluxes are categorized using the nomenclature given by the IPC, the Association Connecting Electronics Industries.

In the Joint Industry Standard 004A, the IPC specifies the testing methods used to categorize a flux. The end classification includes a designation for the type of ingredients used, the flux residue activity level, and the amount of halides included in the flux. A sample designation would be ROL1, indicating that the flux is a rosin-based flux with low residue activity, and does contain halides.

Fluxes are specified for their use based on this nomenclature. The correct flux is chosen based on the method of cleaning to be used post-soldering, the method used for reflowing the solder, the method in which the flux is to be applied, the reflow temperature it will be exposed to, and the substrate it is intended to be used on.

The ideal flux for an application will activate during reflow and any residue left at the end of solder processing (including any required cleaning) will be inert. The flux will be deposited in a repeatable way and the flux chosen will be the least active flux possible which can remove oxides from the substrates soldered.

For use with preforms, flux options include liquid fluxes which are water-soluble, rosin-based, or no-clean, Tacfluxes® with similar options, or pre-fluxed preform coatings including no-clean and rosin options.