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Waferbumping processes have evolved in the last 10 years. The semiconductor assembly industry has gone from bumping processes using solder paste printing (with all its concerns of voiding, coplanarity, stencil-life, and spatter) to plated solderbumps, and now to plated copper pillars with microbumps (solder endcaps). as shown in figure 1.

Figure 1: Evolution of Flip-Chip Bump Metallization and Structure

The reasons for the move from standard solder bumps to copper pillars are primarily to:

• allow high I/O ultrafine pitches (<80microns) without solder bridging
• maintain high stand-off (chip - substrate clearance) to reduce stress on the chip surface
• eliminate or reduce electromigration issues caused by current crowding in solder near the UBM

The basic process flow for copper pillar/microbump formation is show in Figure 2.

Figure 2: Copper-Pillar Solder: from Plating to finished Microbump

In order to eliminate voids in the final reflowed joint, it is critical to have a reflowed solder microbump surface that is perfectly hemispherical, smooth, organic- and inorganic-residue free, and coated with a thin semi-passivating layer of tin monoxide (photoresist). There are two instances in which the solder bump or microbump must be reflowed:

1. after the photoresist and seed layer are stripped, the rough electroplated bump is contaminated by various oxides and hydroxides of tin (from reaction with the highly corrosive resist strip material), plus organic debris entrapped in the rough plated bump surface.

2. if the wafer is probe-tested in a way that coins (damages) the top of the solderbump, this bump may also need to be re-reflowed to return it from its coined condition to a pristine hemisphere. This eliminates coin-related voids in the subsequent flip-chip joint.

I will be going into more detail on the usage of specialty waferbumping fluxes and critical aspects of process control to generate pristine microbumps, in a subsequent set of blog posts.

Cheers!

Andy

Folks,

We tend to think of mixing as something that can completely even out those things being mixed.  As an example, let’s assume you are making chocolate chip cookies and would like to have 10 chocolate chips in each large cookie.  You make enough batter for 100 cookies and then mix in 1,000 chocolate chips.  After mixing for a long time you put 100 dollops of the batter on the baking pan and bake up the cookies.  Upon inspecting the cookies, to your dismay, you find that you have only 13 cookies with 10 chocolate chips.  More than 40 cookies have 30 percent more or 30 percent less than 10 chips.  Worse yet, 3 cookies have 4 or less chocolate chips and 7 have 16 or more.  See the graph below. You decide that you did not mix them enough, so you make another batch and mix for 4 hours.  The results are the same.

Statistics tells us why the above scenario is so.  In a case like this one, the number of chips in a cookie is described by the Poisson distribution.  The mean will be 10 chips, since we are using the Poisson distribution, the standard deviation will be the square root of the mean or 10^0.5=3.16, or about 3 chips.  One way to assure a more even distribution of chocolate would be to divide each chip into 10, so we would have 10,000 smaller chips in a batch.  On average each cookie would now have 100 chips and the standard deviation would be 10.  Plus and minus one standard deviation is about two thirds of the data, so two thirds of the cookies would have +/- 10% of the desired amount of chocolate, a much better result.  If we divided the chips into even smaller sizes, we would further tighten the distribution.

How does any of this relate to solder preforms or solder paste?  In the new world of lead-free solder pastes, where it is common to have 3 or 4 alloying elements, some in very small concentrations, it can be difficult to control the concentration of the alloying elements throughout a sample of the alloy.  The limits of mixing are just part of several processes that are required to assure that a modern lead-free solder has a consistent formulation.  These are some of the topics you should discuss with your solder supplier to assure that you get consistency in any solder alloy you purchase.  Asking to see assay analysis of a solder alloy is often a good idea, too.

Cheers,

Dr. Ron

The attachment of concentrated photovoltaic (CPV) cells is the perfect application for NanoFoil^®. Due to the isolated heating during bonding, less stresses are imparted due to coefficient of thermal expansion. Unlike conductive adhesives or epoxies, NanoBonds® are full metal interfaces which offer higher conductivity values. These 2 points together reveal how NanoBonding incorporates the main advantages of other bonding technologies. This sounds great, doesn’t it? Well, here’s the catch:

Most people have experience gluing parts together, and many handy engineers have learned how to solder wires, pipes, or other common items in the past. In contrast, very few people have ever dealt with a bonding process like NanoBonding. The principal of NanoBonding is simple, but it does require a small amount of research. Luckily, you’re in the right place. From here you can browse the many posts regarding the NanoFoil^® material and the NanoBond^® process. After learning the basics, simply click on one of the contact buttons on this page or follow this link for tech service. Our technical support team can help you become confident with the technology quickly.

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

Engineered solders are solders that can make a HUGE difference with your thermal management, IGBT, die-attach, medical device, hermetic sealing, or connector assembly application. The possibilities are endless.

One of my personal favorite engineered solders is Solder Fortification® Preforms. Obtaining the correct amount of solder to ensure a strong solder joint is critical in electronics manufacturing. Solder Fortification® Preforms are the solution for many challenging manufacturing issues from miniaturization to tightly fitted components to achieving just the right amount of solder in just the right place.

Solder Fortification® Preforms are generally rectangular pieces of alloyed metal that do not contain any flux. The preform is added to a deposit of solder paste using standard pick and place equipment. Since the alloy for both the preform and the solder paste is the same, the preform will reflow at the same temperature as the solder paste, with the solder paste providing the necessary flux. The preform increases the volume of solder over what could be achieved with solder paste alone, especially for stencils with a pitch of 0.3mm or less.

Tell me where engineered solders, especially Solder Fortification® Preforms, might help you. I'll take it from there.

Seth

As the world began transitioning to Pb-Free solder in the early 2000’s, the electronics industry determined that SAC387 (95.5Sn/3.8Ag/0.7Cu) was the most appropriate alloy to replace eutectic SnPb.  While it did have a higher melting point than eutectic SnPb, SAC387 was seen as the best option relative to solderability and usability.  The industry quickly shifted to SAC305 (96.5Sn/3.0Ag/0.5Cu) because its lower Ag content resulted in a lower price.

At that time, the industry didn’t realize the performance impact of the Ag content.  We now know much more.  Ag has a significant impact on a solder alloy's reliability. When thermal cycling higher Ag content SAC alloys (3-4% Ag), the performance tends to be quite good (Ag adds creep resistance to the alloy).  However, because of the alloy's rigidity (more Ag - more rigid), it is more prone to brittle fractures during mechanical shock.  We achieved significantly improved mechanical shock resistance at the expense of sacrificed thermal cycling performance.  The diagram depicts the balance between mechanical shock resistance and thermal cycling performance.

Over the past several years, the Indium Corporation has developed an alloy that minimizes this SAC solder alloy composition compromise.  SACM™ is a low-Ag alloy that is doped with Mn.  This not only improves the mechanical shock resistance over other low-Ag alloys, it also enhances the thermal cycling performance, making it comparable to SAC305.  For more information about SACM™, check out our website at www.indium.com/SACM.

*This post is part of the Introducing SACM™ series.

Folks,

I am a global warming skeptic.  This term, however, requires some explanation.   I believe in climate change.  The climate is always changing.  However, I don’t think it is clear that the world has been getting warmer in the last decade.  I also am not convinced that humans are the main drivers of whatever  climate change has occurred.  The following explains why.

Point 1: For the Last 12 years the World Not Getting Warmer

The global warming scenario that exists today is that human emissions of carbon dioxide are the main reason that the climate is warming.  So, it is natural then to ask, is the climate really warming?  One look at USA Today’s cover story “Why You Should Sweat Climate Change” on 1 March 2013 would appear to settle the story.  Just look at  Figure 1 below.

Figure 1. Graph from USA Today Cover Story March 1,2013.

A quick scan of the graph shows 55.34°F last year and 50.56°F in 1895.  A 5 degree increase.  Wow!  A little closer analysis reveals that these temperatures are individual data points.  If you look at the years 1900 and 2010, you get 53°F for both years, essentially no change.  The thick red line is the long term trend and admittedly it increases from about 51.3°F to about 53°F in the 117 year period. 

Note the icon in the upper right corner, it shows the globe with a dramatic upward trend line.  However, this causes you to now note that this graph is for US temperatures, not world temperatures.  What was the world temperature like?  As this thought crosses your mind, you remember that 2012 brought Europe its coldest winter in recent memory. So you go on the internet and find out that 2012, for the world,  was the ninth hottest year on record.  Scary.  But then you think, “Wait a minute, that means that eight years were hotter.” So you wonder, what do the last 10 or so years look like for world temperatures.  The graph is below in Figure 2:

Figure 2. World Temperature 2001-2012. Graphed by Dr. Ron

Note that for the last twelve years, the trend is flat (actually a little down).  Where are all of the headlines sharing this important information?  So it is not clear that the world is continuing to get warmer.  

Am I the only one that finds it troubling that the media seem to universally tout the scary stories about global warming, but don’t seem to mention obvious counterpoints such as the graph above?  This information is profoundly important.

Point 2: In the Past, Nature Along has Delivered Stunning Climate Change by Itself

I am writing this post from my home Woodstock, VT. I look out my window and view two beautiful, large rocks, each about the size of a house.  These monoliths were likely left as the glaciers in the last ice age retreated, these rocks probably originated in Quebec.  Woodstock was under thousands of feet of ice during the last ice age, Canada was completely under ice.  New York State's Long Island is a glacial terminal moraine. The extent of the ice coverage is shown in Figure 3 below.  However, the forces of nature alone, raised the temperature of the earth by 12 degrees Celsius (with no help from mankind), melting the glaciers and allowing me to live in the Green Mountain State (Vermont = Ver (green) mont = mountain, in French.)  

Figure 3. The Extent of the Ice Coverage in the Last Ice Age  http://www.iceagenow.com/

The natural processes that caused the warming are many.  They include the precession of the earth on its axis, variation in the output of the sun, changes in the ocean and atmosphere, and others.  These processes  have resulted in the past temperature changes as shown in the Figure 4 below.

Figure 4. Temperature of the Earth in the Past 800,000 Years.

This figure shows as much as a 20°C (36°F!) temperature swing produced by nature alone.  The change in world temperature between 1900 and 2010 would be about as thick as the line in this figure. 

I find the proposition that the main driving force in global warming (if it is occurring)  being human produced CO₂ alone is hard to accept, when we see what mother nature has given us in the past.  It would be similar to someone taking the position that the only thing that affects stencil printing quality is the stencil.  When others point out that it might be the solder paste, or the print head, or separation speed, etc., they are shouted down as being unscientific.

Point 3: It was Warmer in the Middle Ages than Today

The United Nations commissioned a panel to study climate change in 1988.  The Intergovernmental Panel on Climate Change IPCC  was established.  In 1990, the panel came out with an assessment of past world temperatures as shown below in Figure 5.  The estimating of temperatures before the mid 1800s is difficult due to lack of records and thermometers before this time.

Figure 5. The First IPCC Assessment of World Temperatures, 800AD to the Present

There is some argument that the Medieval Warm period and Little Ice Age were local events, however they clearly existed and profoundly affected much of the Northern Hemisphere.  But more recent temperature IPCC plots lose them, as seen in Figure 6. below.   The Medieval Warm Period enable the Vikings to settle in Greenland and red wine to be grown in England. When the Little Ice Age came, the Vikings had to leave and England has not been as warm since. 

Figure 6. Third IPCC Temperature Assessment.  Note the Medieval Warm Period and Little Ice Age Disappear.  Because of the abrupt change in temperature after 1900, this graph has earned the moniker, The Hockey Stick Graph.

The controversy over the Hockey Stick graph  is interesting reading and is the source for Figures 4-6.  

In 2003, MacIntyre and McKitrick presented a detailed criticism of the IPCC 3^rd Assessment's Hockey Stick Graph in Figure 6 .  I find their criticism compelling.

I could go on and on,  but to summarize why I am a global warming skeptic:

• For the  past decade the world has not gotten warmer
• Natural forces overwhelm CO₂ as a driving force for climate change
• Sloppy science is behind the hockey stick graph

Please share your science- and fact-based comments.

Cheers,

Dr. Ron

Folks,

In my last post we saw how you could measure density with only a scale.  In this post, we will expand on that technique and learn how to measure metal content in gold/quartz ore.  In principle, this technique could be used for other ore, but the ores can only be two part (e.g. gold and quartz) systems.  Gold is a “natural” for this analysis as it is typically pure gold with quartz.

Gold is often found “veined” in quartz.  I was certain that this was the origin of the “Golden Fleece.”   The fleece being the white quartz with the gold on top.   However, a little research did not clarify this belief.

Anyway, let’s assume you take a few weeks off from work.  Leaving the world of solder paste, TIMS, ITO, wave solder flux and solder preforms behind, you set out for the west in search of some large gold nuggets.  Fate was with you in that, in a short time, you find a gold/quartz specimen as shown below.   The images, and the new “wet gold” weighing technique I will discuss, are from Bill and Linda Prospecting.

You are so excited you are shaking.  The only tools you brought are a scale, some string and a beaker.  To determine that gold content, you need to measure the weight of the gold in air and under water.  But you only have the scale as shown below.  What can you do?

After measuring the weight of the ore in air, fill the beaker part way with water, place it on the scale and zero the weight.  Then insert the ore on a string as shown below.  The scale will now read the weight of the volume of water that the ore displaces.  Let’s call this weight of the water displaced W_D .  The wet weight of gold (weight of gold under water) will be the weight in air minus W_D.  So we now have the weight in air and the weight in water.

The derivation of the equation that tells us how much gold is in the ore is at the end of this post.  The final equation we need is W_Au = 3.07W_W – 1.91W_Air.  For our ore sample W_Air = 25.1 pennyweight (pw). A pennyweight is 1/20^th of a troy oz.  W_D as shown in the photo above is 8.3 pw.  So W_W = W_Air – W_D = 25.1-8.3 = 16.8 pw.  So W_Au = 3.05*16.8 – 1.91*25.1 = 3.635 pw.  Subsequent analysis showed that the gold content was actually 3.9 pw and error less than 7%.  Not too bad for a simple field measurement.  At $1600/oz our ore sample contained. a little over $300 dollars of gold.

This technique could be used to measure the density of an alloy as in the last post.

Cheers,

Dr.Ron

The Derivation of the Equation

Folks,

In the category of interesting requests, Ron, a gold worker, from Guyana, sent me the following note:

Dr. Ron,

My colleagues use a “wet” gold technique to measure gold alloy density.  Is this valid?  Where does the formula come from?

Sincerely,

Ron

Well, to tell the truth, I had never heard of it and was skeptical.  How can you measure density (mass/volume) by only measuring weight?  So, I investigated. The technique claims that one can measure density with only a scale, by measuring the alloy’s weight in air and in water.

I could find no derivation, so I thought about it and derived it on my own.  As far as measurements go, as stated, you only have to measure the weight in air and water.  If you don’t have a scale that can handle being immersed in water, you can use a hanging scale (think weighing a fish).  So, after weighing the alloy in air, you immerse it in water. It will weigh the amount of water it displaces less.  The derivation is below:

As an example, let’s say you have a gold alloy ingot that weighs 1,000 grams (OK, I know grams is mass, but we are all sloppy and use it as weight, too) in air.  You weigh it in water and it weighs 930 grams. From the formula below, the alloys density is:

r = 1000/(1000-930) = 14.29g/cc

Since the density of gold is 19.3g/cc, the alloy is not pure gold.  If you knew the alloying element, say copper, you could use Indium’s Solder Alloy Density Calculator to determine that the alloy was 69.8% gold, 30.2% copper.  If there are multiple alloying elements, since most of the common elements have a density of about 9 g/cc, you can even estimate the fineness of the gold.

Could this technique be used to measure the alloy density of say a handful of solder preforms. Sure, you could put them in a woven bag of non-hygroscopic material and weigh them in air and water.  Admittedly, measuring the density of solder paste, with this technique, would be a challenge.

Next posting, I will show how this technique is used to measure the quantity of gold in gold/quartz ore.

Cheers,

Dr. Ron

Folks,

I was at APEX 2013 San Diego this past week.  San Diego is a great venue for the show, but I always forget how cold it can be (55-65°F) this time of year.  The folks at iConnect 007 interviewed me at the show; the topic was lead-free reliability and has cost for consumer electronics been demonstrated.  You can see the interview here.

These are topics I think about often, so let’s discuss them a bit. First, let’s consider reliability.  RoHS was enacted on 01 July 2006, more than 6 ½ years ago.  Each year more than $1 trillion-worth of electronics are made, therefore, in this period of time, something over $3 trillion worth of consumer electronics have been manufactured.  There have been no “the sky is falling”-type of reliability issues in this time.  How can I say this?  Well, my office at the Thayer School of Engineering at Dartmouth is across the hall from the IT (information Technology) Dept.  They purchase all of the millions of dollars worth of PCs, printers, displays etc. that Thayer uses.  Several years ago (say early 2011) I stopped by when most of the department was in and cheerfully asked if the reliability of the equipment they purchase has gone down since lead-free assembly was enacted.  They asked me in unison, “What’s lead-free assembly.”  After I explained what lead-free assembly was, they confirmed that they have noticed no changes in reliability.  Since RoHS, my family has purchase about 100+ electronic devices, a few have had reliability problems, about as many as in the past.  Most were attributed to hard drive fails.  Of the scores of friends and colleagues I have, no one has ever commented that they have noticed an increase in electronics fails. So, my conclusion is that consumer product reliability is not "practically" worse if my family and  these many  other folks haven’t noticed it.

I have made an informal study of reliability data of lead-free vis-a-vis tin-lead solders published in papers.  A statement from Rockwell Collin’s JCAA/JGF-PP No Lead solder Project: -55C-125C Thermal Cycle Testing Final Report  sums up my overview conclusion nicely: “Test vehicles assembled with lead-free materials (notably tin-silver-copper) exhibited lower reliability under some test conditions.”  Nay sayers might be quick to suggest that this statement  says that lead-free is no good.  However, the statement could be reworded to say: “In considerably more than half of the test conditions, test vehicles assembled with lead-free materials had higher reliability." Counting the comparisons in the Rockwell Collins paper shows lead-free better in 51 cases, tin-lead better in 31 cases, and one draw.  However, it is disturbing that a small percentage of lead-free assembled test vehicles had much much worse reliability than tin-lead test vehicles.  This later information makes me believe that lead-free is not yet ready for mission-critical, high-reliability, long-life products.  These small numbers of much poorer reliability assemblies must be understood and corrected before lead-free is ready for mission-critical prime time.  The much shorter life cycle of today’s consumer electronics may also mask this concern.

What about cost?  I don’t at all want to minimize the expense that many went through to go lead-free and RoHS compliant.  In about 2007, one of our colleagues estimated that it cost the electronics industry $20 billion to become RoHS compliant.  I think this number is low, but, from a consumer’s perspective, there has been no cost hardship.  The price of a PC continued to go down during and after RoHS implementation, as shown in the figure below.  While performing my non-scientific survey of co-workers, family, and friends on reliability, I also asked about cost.  All agreed, electronics are cheaper than ever.

Challenges still exist, even in consumer electronics with the Head-in-Pillow, Graping, non wet opens, and other defects.  However, we can all purchase lead-free, RoHS compliant products at a reasonable cost and reliability.

Cheers,

Dr. Ron

The source for the image is :http://thomaslah.wordpress.com/2010/02/03/apple-and-intel-defying-gravity/

Folks,

Our discussion of Weibull Analysis continues.....Let’s say you have worked hard and assembled some SMT lead-free PCBs for thermal cycle testing.  You used the best lead-free solder paste, and some lead-free solder preforms as you assembled several through-hole components with the Pin-in Paste process.  You were a little concerned with the assembly process as the board was thermally and physically massive and the reflow process needed to be a bit above the recommended temperature and time.

The results of the thermal cycle testing are shown in Figure 1 below.  You dutifully report the characteristic life (or scale) as 2,387 cycles and the first fail at 300 cycles.  You were quite disappointed, as in the past similar, but slightly smaller boards, had a slightly higher scale, but more importantly, the first fail was about 1,000 cycles.  Anyway, you write you report up and file it away.

Figure 1. A Weibull Plot of the Thermal Cycle Data

Hold on!  The data are screaming at you the something is going on.  Look at the same data in Figure 2.  Note two distinct lines shown in green.  These two separate lines suggest very strongly that there are multiple failure modes.  The line furthest to the right is likely the typical failure mode observed in the past.  The line to the left is a new early failure mode.  It could be due to something like oxidized pads or some other phenomena not seen when testing similar but smaller boards.  Root cause failure analysis should be performed to try and understand to new failure mode.

Figure 2. A Weibull Plot of the Thermal Cycle Data with Multiple Failure Modes Noted

Now for a human interest note:

One of the rewarding aspects of being a professor at Dartmouth is the outstanding nature of many of the students.  They are not just good academically, but often are talented artistically, athletically, etc.  This point was brought home to me recently.  In a class I teach, ENGS 1: The Technology of Everyday Things, we were recently discussing the conservation of angular momentum (CoAM).  One of the most striking ways to demonstrate CoAM is an ice skater’s spin.  I went on the internet and could not find a good video of a spin.  I then remembered that one of my former students, Julia Zaskorski was on Dartmouth’s figure skating team.  I asked her if she had a video she could share.  It appears below.  She is a materials science and physics major.  Who knows, maybe we will see her at APEX or SMTAI in a few years. 

Here is a little bio in her own words:

               My name is Julia Zaskorski, and I’m a junior from Wellesley College taking part in the 12 College Exchange Program at Dartmouth.  At Wellesley I am majoring in physics with the intent to pursue mechanical engineering.  Despite Wellesley’s relationship with nearby MIT, Wellesley does not have its own engineering program, so I sought out the more self-contained curriculum and atmosphere at the Thayer School of Engineering.  In addition to the draw of the Thayer School, the Dartmouth Figure Skating team was also a hugely motivating factor for my exchange, as Wellesley does not have a team, let alone a rink.  I have known the coach of the Dartmouth team for several years now, and to finally see my name on the roster for the team is a dream come true.  The engineers, as well as the winter activities here in Hanover, pulled my heart to Dartmouth long before I’d ever set foot on campus. 

  Cheers,

  Dr .Ron                           

Folks,

In continuing our discussion on Weibull Analysis, let’s assume we assembled some SMT and through-hole PCBs with lead-free solder paste.  On this board are also some bottom-side terminated (BTC) components (often called QFNs), that are also assembled with solder preforms.  A stress test is performed to test the BTCs.  In such a test, the first fail in Weibull analysis is the most important data point.  No matter the results of remainder of the data, these later fails cannot undo the effect of a very early first fail. 

To understand this concept, let’s look at the Weibull chart below.  In many high reliability applications, there may be a requirement that some small percentage of the components under test have at least some minimum reliability.

Figure 1.  Weibull Analysis with an Early Fail.

As an example, let’s say that 1% of the components cannot have less than 500 cycles of life.  By looking at Figure 1, we see that 1% have less than 150 cycles of life (see arrow.)  This one early outlier dramatically affects the Weibull Analysis.

However, if that outlier was removed, as seen in Figure 2, the data suggest that 1% of the components will have a life of 900 cycles.  We can see the dramatic effect the first fail has on this result.  Note that the first fail does not affect the “scale” or characteristic life much (2647 vs 2682).  Hence, the characteristic life, is not a robust metric to use in a high reliability environment.  However, the shape or slope is dramatically affected by the early fail as it changes from 2.22 to 4.23 when the early fail is “censored.” 

Figure 2. Weibull Analysis with the Early Fail Removed (Censored).

Why might an outlier like this exist?  Almost certainly there is something unusual about the early fail.  It might be something like an oxidized pad preventing good wetting of the solder.  Perhaps something like this failure mode might be discovered in root cause failure analysis.  However, I am typically opposed to censoring data, even with supportive failure analysis.  I think the test should be done over.  It is often too easy to talk yourself into accepting inconclusive failure analysis.

What is your opinion?

Cheers,

Dr. Ron

Folks,

Let’s step away from electronics assembly challenges, and deep considerations of solder paste, solder preforms, and wave soldering, to ponder where electronics have gone in the last decade or so. 

The mobile phone of the early 2000s was just that, a phone. Today it is a phone, music player, PalmPilot-type organizer, camera (still and video), video player, gamer, TV remote, GPS system, web portal, etc.  There is almost nothing electronic that it can’t do.  The USB memory stick of 2002 with 0.5GB of memory cost $500, today $5 will get you 4GB one, a cost reduction of 800 to one, the equivalent of halving in price about every year.

There is no reason to expect any less dramatic advancements in the future.  But, predicting the future of electronics is never easy.  In the January 2013 edition of Scientific American  Ed Regis wrote an article titled, The Bold and Foolish Effort to Predict the Future of Computing. In this article, Regis interviewed eight computer luminaries, including Stephen Wolfram and Nathan Myhrvold,  to ascertain their perspectives on where computing will be in 150 years.  The conclusion was that no one can predict the future of computing, as interviewee George Dyson said, “All I can guarantee is that any prediction will be wrong."

One person less humbled by the difficulties of computing predictions is Ray Kurzweil.  His prediction success level of more than 80% would seem to support such confidence.  Kurzweil also just got a new job at Google. I am finishing his new book “How to Create a Mind: The Secret of Human Thought Revealed” and, while I  finding it fascinating, I think he goes too far.  He believes the mind is a sophisticated computer and that, when computers get to a certain point equaling and surpassing the human mind's computational ability, they will be considered human.

Supporting this point, he hopes to, someday, resurrect his father, as Bloomberg states:

“Among the stranger things Ray Kurzweil will say to your face is that he intends to bring his father back to life. The famed inventor has a storage locker full of memorabilia—family photographs, letters, even utility bills—tied to his father, Fredric, who died in 1970. Someday, Kurzweil hopes to feed this data trove into a computer that will reconstruct a virtual rendering of dear old Dad.”

Call me a religious fanatic, but I think there is something more to each of us than our memories and our brains computing ability. 

Kurzweil endorses IBM's computer system, Watson’s victory in Jeopardy  in February of 2011 as a major step in the direction of computers as humans.  IBM provided commercial support for these Jeopardy episodes.  In the commercials they strongly reminded us that Watson was not thinking, but only doing what it (not “he”) was programmed to do.  Someone summed it up nicely, Watson won, but did he know he won?

I think there are a few major things that people like Kurzweil minimize when they propose that computers will be recognized as human.  These points are:

1. Humans are sentient (they would know whether or not they won or lost Jeopardy, we have emotions and feelings).  I know of no progress in sentience development for machines.
2. Humans have a will.  We get up in the morning, we decide what we will do that day and do it.  There is no progress (thankfully?) in giving computers a will.
3. Humans have a biological body.  We smell the newly cut grass, feel a refreshing breeze, get tired, enjoy a meal, enjoy sports etc.   It is easy for some to minimize the importance of the body in being human. Again no progress in this area.

However, I don’t want to minimize much of what Kurzweil predicts.  In her ground breaking book, Alone Together, Sherry Turkle tells us that, in addition to the fact that the average teenager in the US sends 200 text messages a day, electronic companions already exist.  As time goes by they will become more realistic and will be capable of interesting and stimulating speech and interaction.  Having all of the world’s knowledge at their fingertips (pun intended), these companions will likely be more stimulating than people, they will easily pass the Turing Test, and, for good or ill, will make us more “alone together” than ever.  But our companion will not love, fear, hate, or know that it is a companion.

As has been pointed out, this brave new world is coming whether we like it or not.

BTW, on another topic,  the History Channel has produced a terrific video series, Men Who Built America http://www.history.com/shows/men-who-built-america.  It is a the spell-binding story of Vanderbilt, Rockefeller, Carnegie, J. P. Morgan, Edison, and Henry Ford.  If you missed it, it is coming out in DVD in January.

Cheers and Best for the New Year,

Dr. Ron

One question that I often hear from customers is; “Once out of the refrigerator, how long do I have to wait to allow my solder paste and/or flux to reach room temperature in order to use it?”

It is indeed very important for solder pastes and fluxes to be at ambient temperature (approximately 23^oC) in order for them to exhibit optimal performance, as the rheology of these materials will differ in a refrigerated state. Additionally, "cool" materials condense atmospheric moisture onto their surfaces (like a glass of cold water in the humid summer air). This condensed moisture is an unwanted ingredient in high quality soldering.

In order to quantify the amount of time necessary for these materials to reach room temperature, we refrigerated both solder paste and flux in 6oz cartridges.  We then recorded the time it took to reach room temperature by placing thermocouple leads in the center of the materials through small holes that were drilled in the containers.

The following is a graph of the time versus temperature of warming both paste and flux:

For the testing above, the ambient temperature was approximately 22-24^oC.  The flux required approximately 2.5 hours to reach room temperature, whereas the solder paste required approximately 2 hours.  It would be expected that the solder paste would reach room temperature faster than flux alone, as the metal content of the paste increases the thermal conductivity of the material. 

Of course, your particular conditions (refrigerator temperature, container size and shape, ambient temperature, etc.) will make your situation somewhat different.

For questions regarding the proper handling and storage procedures for solder materials, please contact Askus@indium.com.

We’ve heard about the solder paste “graping” defect, but the same oxidation challenge occurs in other solder forms as well, such as solder washer preforms used to attach small connectors. 

Solder graping is a defect that arose as a result of small solder paste deposits and higher (than were used in the past) reflow temperatures.  The peak reflow temperature for SnPb alloys were just over 200°C, but. with SnAgCu alloys, that peak is as high as 260°C.  New solder pastes have been developed that address this defect by utilizing flux chemistries that function as an oxidation barrier.

But what about other solder forms? 

I was recently working with a customer who was evaluating solder preforms for a connector application.  These solder washers (outside diameter of 0.025”), like the small solder paste deposits, had a large surface area with a tendency to oxidize on their heated journey to melting.  These washers were made of SnAgCu, so the peak temperature required of them was no different than it would be for Pb-Free solder paste.

I found that the same 1°C ramp that causes graping of solder paste can do so for preforms as well.  The difference though, is that the non-molten portion of solder takes the shape of its un-reflowed form, that of a small washer in this case, as opposed to the “grapes” of aggregated solder powder.  

Fortunately, the issue can be addressed in 3 ways, and I encouraged my customer in this case to process their assembly using the combination:

1. Adjust the reflow profile. Ed Briggs' recommendations work well here.  Further, use a fast ramp to peak. I tested the reflow on a hot plate set to 250°C; this improved the wetting.  Preforms are unique from solder paste in that they are solid metal, as opposed to a mixture of metal and flux.  They do not benefit from the same heating controls that solder paste requires. 
2. Reflow in nitrogen.  By purging out the environmental oxygen during the reflow process, the solder preform will not oxidize.
3. Apply a tacky RMA flux, such as Tac007.  RMA fluxes provide stronger oxidation barriers than other non-rosin flux types. 

In the second image here, it is evident the improvement these changes made in terms of the spread and coalescence of the solder preforms. Note that the addition of tacky flux left an amber-colored no-clean residue, however, this can easily be washed away using a mild solvent.

Any questions?  AskUs@indium.com!

前段时间我们接触了“Strategic Selling” 和 “Conceptual Selling”的相关内容。 我觉得Conceptual Selling很值得一听，到现在我还经常翻阅那本书。

Conceptual Selling 主要是以客户的角度出发来想问题，说话和办事。作为许多销售和技术销售人员，我们很容易陷入一个固有的销售模式，在和客户接触时我们讲多过于听，很容易抛出一大堆我们这个产品和服务的优点，特点，为什么客户要用我们之类的 “习惯性销售套词”： 因为每天，对每一个客户，基本上每一次见面，这些套词我们都重复过无数遍，在我们心目中滚瓜烂熟，讲起来滔滔不绝。 但是，这真是客户需要的方案吗？我们先清楚了解客户的真正需求了吗？
 

举个简单的例子，比如说卖焊锡膏，我们一见面就会经常搬出这样的套话：这款最新的很好用的呀，润湿好（good wetting)，应刷性能好(good printting)，空洞低(low voiding),没有“枕头效应”（eliminate Head-in-Pillow), 保证你的产率大大提高......客户听着，甚至点头微笑。其实，2个小时前这个客户刚刚接见了另外一个电子焊接材料供应商，他们的销售说了一模一样的关于他们产品的优点。 但是在客户心中，最关心的都不是这些，其实这些问题客户都没有.   最近客户在发愁怎么样改进探针的ICT测试，因为客户发现探针测试的时间最长，是生产的瓶颈(bottle neck)。 如何减少探针测试时间，缩短生产周期cycle time，才是最关键的。客户最想知道的是，究竟是要换一款软一点焊接残留物的锡膏(soft residue solder paste), 让探针好刺穿呢，还是请ICT的设备商来，换探针头和调试一下机器。 如果能大大缩短cycle time, 客户就可以为公司解决一个大问题，就可以得到他老板的赏识，甚至是升职和更多的奖金......

另外有一家供应商来了。这家供应商没有急于向客户“推销产品”，而是先问客户具体的问题，耐心倾听，清清楚楚了解到客户想达到什么效果后，再看看自己有没有适合的产品和解决方案。这家聪明的供应商向客户提供了一款soft residue的锡膏，并且和客户的ICT设备供应商联手，解决了客户ICT测试时间长的问题。结果是，这家供应商拿到了客户所有的焊锡膏订单；而客户因为给自己的公司带来了“巨大的价值”，得到了老板的认可，加薪升职，过年带自己的全家人去旅行了；这家公司也以shorten SMT mfg cycle time而赢得了更过的生意。当然，焊接材料供应商也有更多的来自这家公司的订单，甚至这家公司还向别的公司推荐这个供应商。这是真正双赢的结局。
 

Cheers!
 

Pic: Google Image


 

There are a lot of parameters to consider when choosing the right solder for your application:

1) The operational temperature of the final device

2) The metallizations that you are soldering to

3) Temperature sensitivity of any components that you are soldering

4) Need for Pb-free

5) Drop test requirements

6) Reliability requirements

Each individual application will, no doubt, have additional requirements.  But, generally, one of the first considerations will be the melting temperature of the solder.

Many applications need a low-temperature solder that will reflow below 180C.  For example, LED attach, optics assembly, and MEMS mounting all have low temperature solder requirements.  There are two metals, in particular, that help fulfill this need.  One is indium and the other is bismuth.

There are five common solder alloys that are well suited for your low-temperature, Pb-free requirements.  As a matter of fact, they are so popular we have packaged them together in a kit (in the solder paste version) for you to evaluate.

Our Low Temperature Pb-Free Solder Paste Research Kit allows you to evaluate any two alloys in a side-by-side comparison to determine the optimum paste for your application. The five alloys that you can choose from in our Low Temperature Pb-Free Solder Paste Kit are:

• Indalloy 1E (52In 48Sn) Eutectic at 118C
• Indalloy 281 (58Bi 42Sn) Eutectic at 138C
• Indalloy 282 (57Bi 42Sn 1Ag)  140C/139C
• Indalloy 290 (97In 3Ag) Eutectic at 143C
• Indalloy 4 (99.99In) Melting Point of 157C

In this kit, each alloy is matched with the proper flux vehicle so you can comparatively test multiple alloys to see which is the best option for your application. 

The kit is available on line and our Application Engineers are available to help steer you in the right direction as to which two alloys to choose.

Over the years, solder alloy choices have been pretty stable.  In the last century, SN63 and SN62 could be found at any company making any kind of electronic device, and both alloys were the backbone of every company making solders.

But, when lead was identified as causing health issues, it was legislated out of everything from paint to gasoline to electronics, including solders.  In 2003, RoHS (Restriction of Hazardous Substances) was passed in Europe to restrict the use of lead (as well as mercury, cadmium, hexavalent chromium, and polybrominated diphenyl ethers: PBDE) in electronics and electronic equipment.

The electronics industry is now focused on SAC alloys (so named because they contain Sn, Ag, and Cu).  But, there is also SnAg, which was used in the lead era when a higher melting point was required.  The addition of the copper (in SAC) offers the benefit of improving wetting and potentially reducing the silver content from a non-copper alloy like 96.5Sn 3.5Ag. 

But, there are many applications where SnAg works well. Changing from it would require customer and/or government approval, and that involves a lot of extra money and time. This lead-free alloy works well in the assembly of a variety of medical devices that use non-traditional metallizations and fluxes.  The Cu addition (in a SAC alloy) probably would not improve the results enough to warrant the cost of requalifying an existing medical device through government agencies, so they stay with what works. 

So, if you are using 96.5Sn 3.5Ag (or 96Sn 4Ag), don't be afraid to stick with it.  Indium Corporation offers both of these solder alloys (and over 250 other alloys) in a variety of forms: preforms, wire, paste, and ribbon.   And, if you want to look at the SAC alloy family to see if it works better in your application, we will help you with that, too.  Just contact our Application Engineering Staff for help.

Carol

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^® .

It is hard to believe, but 50 years ago, on October 9th, the first visible light LED was demonstrated by Nick Holonyak, Jr. of GE Advanced Semiconductor Laboratory in Syracuse, NY (just down the road from Indium Corporation's global headquarters).  Fortunately, he did not listen to his critics who said mixing gallium arsenide and gallium phosphide would not work to make a visible light LED, because they were wrong!

According to Roberto Baldwin at www.wired.com, Holonyak predicted that LEDs would one day replace the incandescent light.  Today there are many efforts underway to make that happen.

Today, Indium Corporation is involved in several LED applications:

1) NanoFoil®:   High brightness LEDs use NanoFoil® to bond thermal pads to heat-sinking substrates.  By using the NanoFoil®, the standard reflow process (that can negatively impact brightness, color, and life time) is avoided.

2) Gallium Trichloride: This is used as the starting material for making gallium-based metal organic precursors, such as tri-methyl gallium, which are used in the LED industry.

3) Indium Trichloride Also used as a starting material, but for indium-based metal organic precursors such as tri-methyl indium.  These compounds are also used in the manufacture of LED lighting as well as batteries and other applications.

4) Engineered Solder Materials: Attaching an LED to a substrate can also be done by using a variety of specialty solders, including bismuth-based (low temperature) solder preforms or solder pastes.  We also have a variety of thermal solutions that can help dissipate the heat generated by the LED.  Flux-coated solder preforms provide solder and flux in a consistent volume, for use under larger LEDs to reduce or eliminate voiding. 

It may have taken 50 years to see the widespread adoption of this technology, but there is no doubt that it is here to stay!

Contact us for more information or visit www.indium.com and see our new web site.

cgowans@indium .com

Carol Gowans