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

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

NanoFoil^® is a great localized heat source, but it can cause some ugly looking aesthetic defects if the process is not set up correctly. Let’s take a look at how you can use high temperature tape to mask off parts during NanoBonding.

With certain NanoFoil^® thicknesses and assembly pressures, solder may be ejected from the bond area. To protect these parts from “solder spitting”, simply mask them off with high temperature tape. In the picture shown here (right), excess solder and pressure were used with a higher energy version of NanoFoil^®.

Similar parts were again prepared for bonding (left), this time with high temperature tape covering the area around the solder joint.

After a similar reaction, we see the telltale signs of excessive pressure and solder. This time, however, we can simply peel away the excess solder (right).

After bonding, the tape can be peeled off the surfaces, revealing the fresh, clean surface underneath.

This is a very flexible solution - to mask parts during reflow. In high volume, metal shims and shutters can be used to keep solder where it belongs. The best solution is to optimize the solder thickness, assembly pressure, and NanoFoil^® thickness. We can help you do that.

* This post is part of the NanoFoil^® Do-It-Yourself Tips and Tricks series

Folks,

A while ago I discussed the Weibull Distribution and its importance in electronics reliability analysis.  This distribution has been used to evaluate the life of solder joints whether formed in SMT, wave, or even using solder preforms. In the next few posts, I would like to discuss how to interpret Weibull plots.

Let’s consider two Weibull plots from thermal cycle testing of lead-free solder joints as seen below in Figure 1.

Figure 1.  A Weibull Plot of Thermal Cycle Data for Alloy 2 and Alloy 4.

Both alloys have almost exactly the same scale, or characteristic life. You will remember that characteristic life is the number of cycles at which 63% of the test subjects fail.  For Alloy 2 it is 2,593 cycles and for Alloy 4 it is slightly better at 2,629 cycles.  However, these two alloys performed dramatically differently.  The most striking difference is in their “spread.”  We see this much greater spread for Alloy 4, when we plot a fit to the data as a normal distribution, as in Figure 2 below.

Figure 2. The Best Fit Normal Distribution Plot for Alloy 2 and Alloy 4.

In the Weibull plot, the data for Alloy 2 has a very steep slope or shape factor, this indicates a tight distribution.  A tight distribution is desirable as it facilitates more accurate prediction of thermal cycle life.  Alloy 2 is clearly superior.  So, in a Weibull distribution, not only is a large scale factor or characteristic life desired, but so is a steep slope or larger shape factor.

Next time we will talk about outliers.

Cheers,

Dr. Ron

It seems like a fairly simple thing to do.  What could be difficult about soldering a wire to a pad? 

Well, I hear three common complaints and expressions of frustration:

POSITIONING: Typically, in this process, a soldering iron is used. The first problem arises from trying to hold onto the soldering iron AND the wire to be joined to the prefluxed pad AND the solid-core solder wire you are using.  An extra hand would be nice! Some people use a system of fixtures or clips to hold the wire and the pad in the appropriate position. (see image and link, below)*.

COLD SOLDER JOINT: Another common complaint is that, after soldering, the wire easily pulls out of the solder joint.  This is due to the poor wetting of the solder to the wire and the pad - it never really "soldered".  A solution that I share is to pretin both the pad and the wire with the solder, using a flux.  To pretin the wire, I suggest melting some of the solder in a crucible or solder pot.  Dip the wire in the flux and then into the molten solder.  A teardrop should form on the end of the wire.  It can also be pretinned using the soldering iron. Next, pretin the pad. Both pretinned surfaces will have a coating of post-reflow flux residue.  If required, this residue can easily be removed using a suitable solvent.  Now that you have pretinned both surfaces, the pad should be heated with the soldering iron and, when the proper temperature is reached, the pretinned wire should be pressed to the pretinned pad.  The solder on both the pad and the wire will melt together and, when the heat is removed, the joint will be formed.  Usually this can be accomplished without adding additional flux.

INCONSISTENT VOLUME: A third issue is that the volume of solder in the joint is not uniform from piece to piece. If this is your concern, consider using a flux-coated solder preform. They can be produced with the exact solder volume, and the precise dimensions to fit onto the wire you are joining to the pad.  Similar to the process described above, when the pad and the wire are heated, the flux will be activated (removing the oxides) and the solder preform will melt, forming a consistent and perfect solder joint.

Please contact our technical support group with any questions you may have.  We are always ready to help you solve your soldering problem, whether it is large or small.

For more background, read these blog posts on hand soldering:

• soldering iron tip temperature
• hand soldering flux selection
• hand soldering tech support
• the importance of a clean soldering iron tip

Paul Socha

*Image: Harbor Freight sells a product called "Helping Hands" for (US) $6.99, as of this writing. Other companies offer similar products. Consider buying more clamps to hold the wire in place, freeing you to hold only the solder wire and the soldering iron.

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,

In the last posting, we saw how Weibull analysis helped us to determine that SACM lead-free solder (SAC105 with about 0.1% manganese) has comparable (actually better) thermal cycle performance versus SAC305 solder.  Software like Minitab will give us even more detailed information about the performance of the solder joints in stress testing as we see in Figure 1, above.

In addition to the Weibull plot, we also have the Probability Density Function (PDF), the Survival Function and the Hazard Function.  The PDF tells us when it is most likely that a test board will fail in a test population, as shown by the inserted red line.  We see that it is a little less than 2,000 cycles.  The Survival Function shows the percent of surviving test boards.  We observe that the expected life (the 50% point) is quite close to the maximum of the PDF.  The Hazard Function tells us the rate at which the test boards are dropping out.  It increases with time, but there are few boars left so the PDF drops down at the end of the test, even though the fall out rate is the highest.

It is interesting (and perhaps appropriate as Halloween approaches) to consider if human mortality follows a Weibull distribution.  I used some data for the Centers for Disease Control  that are a little over ten years old, for males in the US.  So, the mean life expectancy is a little low at 72 years.  (I was a little lazy, the old data were a little easier to work with than new data, some conversions are needed to make it work.) The data appear above in Figure 2. 

As you can see, just like a solder joint, your life expectancy can be modeled quite well by the Weibull distribution.

Cheers,

Dr. Ron

Derivation of the Weibull Graph

Folks,

Last time we introduced Weibull analysis. Let's now derive the relationships needed to calculate the slope, beta, and characteristic life, eta.

F(t) is the cumulative fraction of fails, from 0 to 1. By choosing Ln(t) as x and LnLn 1/(1-F(t) as y, we would expect a straight line.  See the derivation above.  It can be shown graphically that this fact is so.  So if we plot F(t) versus t on logarithmic graph paper, the slope of the line will be beta. To determine eta, let t=eta, in the first equation below.  The result is F(t) = 1-e^-1 = 0.632.  So the time at which 63.2% of the parts have failed, is eta, the characteristic life.

Let’s consider some data comparing SAC305 and SACM (SAC105 with about 0.1% manganese) BGA solder balls in thermal cycle testing.   The primary test vehicle employed was a TFBGA with NiAu finish mounted on PCB with OSP finish.  SACM is a new breakthrough soldering alloy that has better drop shock resistance than SAC105 and comparable thermal cycle performance to SAC305.  The data follow.  The first column is the sample number, the third and fifth columns are the number to thermal cycles to fail for SAC305 and SACM.  The second and forth columns are rank of the sample number.  One would think that the first number in the second  column would be 100*(1/15) =6.67%, as it represents the cumulative percent of samples failed, but a slight correct factor is needed.   By plotting the log log of rank as shown above (LnLn1/(1-F(t)) vs log of cycles at failure, we get the Weibull plot.  The slopes of the best fit line is equal to beta and the number of cycles at rank = 63.2% is eta.

Fortunately software like Minitab 16 does the plotting and calculating of beta and eta automatically.  The results are below:

We see that the shape (beta) for SAC305 is 1.76 and that of SACM is 6.09, the scale or characteristic life (eta) is 1736.8 and 2016.8 respectively.  These results are a strong vote of confidence for SACM.  Its steep slope (high beta) suggests a tighter distribution, with more consistent solder joints and its characteristic life (eta) is also slightly greater.

I plan on teaching detailed workshops on this topic.  I will keep you posted.

Cheers,

Dr. Ron

Attaining consistent and accurate solder (and flux) volume uniformity in hand soldering has long been a critical quality and performance issue.  Traditional hand soldering creates consistency and quality issues from operator to operator, from shift to shift, from day to day, and even within the same operator within the same day!

Hand soldering always delivers inconsistent solder volume.The human factor is a major contributor to this non-uniformity.

Solder preforms:

• offer a solution to the need for consistent solder volume.
• are the preferred solution in tens of thousands of applications where the uniformity of solder volume is critical.
• provide the correct alloy, the right size, and the exact volume of solder that you require.
• are available in custom sizes, shapes, volumes, and packaging, to suit your production needs.
• can be flux coated with a consistent and precise volume, and type, of flux. Flux-coated solder preforms reduce process time by eliminating a separate flux application step. They can also reduce total flux usage.
• can be ganged together (InTEGRATED^® Solder Preforms) for mass placement. I blog about that here and here.
• can be manufactured using a fugitive dye (in the flux coating) imparting a visible color for easy identification. This helps distinguish visibly-similar but different (alloy, dimensions, etc.) solder preforms.
• can be clad to a dissimilar metal, providing extra strength and/or the ability to bridge gaps.

Using,  applying, positioning, and reflowing solder preforms (and flux) is simple. Simply place the solder preform at the joint site (by hand or robotically), and apply heat.  It’s really that simple.  Each joint will have precisely the same solder volume regardless of who is doing the soldering.

Manage your 1st-pass yields, your quality, your field failure rates, your customer satisfaction, and your profitability by putting solder preforms to work for you.

Paul Socha

23 October 2012

During a NanoBond^® reaction, assembly pressure may determine if you create a quality solder joint. There are many details that can influence how much pressure is actually needed:

• NanoFoil^® thickness
• Solder type/oxidation/thickness
• Surface roughness
• Part flatness

   

In practice, we use anywhere from a few psi (like the clips shown above) ...

... up to 400-500psi (with a press, as shown below) – depending on the criteria listed above.

The main thing to keep in mind is that you want uniform pressure across your interface.

As illustrated in the pressure paper images below, uniform pressure across a NanoBond^® interface is critical for maximum bond strength.

We will discuss this further in "Aligning the Assembly".

*This post is part of the NanoBond^® Process series

Folks,

The Weibull distribution is arguably the most important distribution in failure analysis of leaded and lead-free solder joints.  It is the first thought of someone trying to model thermal cycle, drop shock, or other failure modes associated with through-hole and SMT assembly.

The Likelihood of Getting Heads in 60 Coin Tosses is Described by The Binomial Distribution

The Weibull distribution was invented by Waloddi Weibull in 1931.  This invention fact was recounted by Dr. Robert Abernethy in his famous textbook on Weibull analysis, The New Weibull Handbook. This statement may not seem unusual, until we ponder that all common distributions in statistics were discovered, not invented.  The three most common statistical distributions are the Normal, Poisson, and Binomial distributions. As an example of a discovered statistical distribution, let’s consider the Binomial distribution.  This distribution describes, among other things, the odds in flipping a coin.  If you flip a fair coin 60 times, you are most likely to obtain 30 heads (H) and 30 tails (T), but getting 29 H and 31 T or 32 H and 28 T would not be all that uncommon.  Mathematical analysis shows that the curve below results.  If a coin flipping experiment is performed many times, this curve will faithfully predict the results.  The curve is not invented it is discovered from the deep theoretical underpinnings of the Binomial Distribution.

Waloddi Weibull 1887-1979

The fact that the Weibull distribution was invented suggests that Weibull selected it because it fit many types of failure data.  He defined cumulative Weibull distribution is defined as:

Where eta is the characteristic life or the scale function and beta is the slope, were as F(t) is the cumulative fraction of failures.  Weibull proposed this function because for beta less than 1, F(t) describes “infant” mortality fails.  In this situation the failure rate is decreasing with time. For beta greater than 1, it describes “wear out” failures, where the failure rate is increasing with time.  In electronics, we typically try to weed out infant mortality by using “burn in.” For beta equal to 1, the failure rate is constant.  These three scenarios are shown in the figure below.

So typically, in electronics failure analysis, we are plotting failure data versus time to determine beta and eta, typically with software like Minitab®.

In the next posting we will analyze some failure data to determine eta and beta and discuss their significance.

Weibull himself was a curious character and much of the available information on him is chronicled by Abernethy. 

For sure Weibull was a vigorous man.  His second wife was almost 50 years his junior and he fathered a daughter at about 80 years of age!

Cheers,

Dr. Ron

The first time I was taught how to solder (as a child), I was told: “All the surfaces need to be mechanically cleaned and chemically cleaned.” The person who told me this was referring to pipes, I was learning about plumbing. (I would have never thought we'd be using nanotechnology to create solder joints!) Although your application is probably far from a plumbing job, the basics of soldering remain the same. The best solder bonds are formed when oxides and contaminants are not present.

These two points are taken care of in traditional electronics soldering by using a flux. Flux can flush away light contaminants like dust, and reduce oxides on certain metal surfaces. But, in the NanoBond^® process, we aren’t using flux.

Luckily, NanoFoil^® can power through the soldering process as long as the proper surface finishes are used, and they are ready to be soldered to. Different surfaces are prepared in different ways. Here is a list of some common surface finishes and what preparation they require:

• Gold – Wipe with isopropyl alcohol if aged
• Silver – Wipe with isopropyl alcohol if aged
• Tin – Remove oxides with 10% HCl if aged
• Solder coating - Remove oxides with 10% HCl if aged
• Aluminum – Add solder coating
• Molybdenum – Add solder coating
• Titanium – Add solder coating
• Naval Brass – Add solder coating

And for any surface you don’t see, feel free to contact askus@indium.com so we can find a solution for you.

*This post is part of the NanoBond^® Process series

While on a recent trip to Malaysia, I interviewed two colleagues regarding trends in semiconductor assembly. My previously-published interview with Sze-Pei Lim appears here.

This time, while on a visit to a logic device manufacturer in the North West, I [ACM] talked briefly to Sehar Samiappan [SS], Indium Corporation's Area Technical Manager, about our recently-developed water-soluble pin-transfer Ball-Attach Flux, WS446-NRD, which is designed for BGA applications of 0.5mm pitch, and greater than 1500  I/Os.

[ACM] What is the origin of WS446-NRD?

[SS] The development was driven by a customer need for a guaranteed good quality BGA (ball-grid array) solder joint, but with reduced environmental impact. Our very quality-focused customer uses several different suppliers of organic FC-BGA substrates with copper OSP pads. The customer had serious concerns about occasional poor solderability of SAC305 solder spheres onto substrates. The key defect seen was poor wetting onto the OSP-coated copper pad, which would give rise to variability in both joint strength and bump coplanarity,  and even (in worse cases) missing-ball / “big ball” effects. Some of the pad finishes were seen to be highly oxidized, severely restricting solder wetting during reflow. Variability in the surface finish was found to be not just from supplier to supplier, but also showed up as lot-to-lot variability from lower cost suppliers.

Some of the differences seen could be attributed to the method of mask desmear from the C4 “cage” of the flip-chip (top side) area, which was either a plasma-based desmear or an oxidizing inorganic acid dip, that was clearly having effects on solderability of the opposite (bottom) BGA side of the substrate.

[ACM] What steps have customers previously taken to get around this issue?

[SS] This is a serious issue for many ball-attach flux users, and some customers have gone to the lengths of using a special fluxing step to remove contaminants such as oxide and OSP (organic solderability protectant) coatings. These liquid fluxes are very reactive, but require  separate spraying, reflow, and cleaning stages that add cost and time. The halogenated ball-attach fluxes of the WS446 series have an established good chemistry that allows wetting of SAC105, 305, and 405 onto a variety of metallizations. In the semiconductor assembly industry, the WS446 fluxes are well-known in Taiwan, and throughout South East Asia, for their good solderability and long pot-life in a variety of FC-BGA applications.

[ACM] What was different about WS446-NRD, and why was it developed?

[SS] WS446 fluxes are all colored, using a bright red dye, so the flux can be seen by eye and automatically detected by vision systems. Red coloration also allows automated ball-attach flux dipping replenishment systems to detect flux levels. Normally, colored fluxes are not a problem, but the customer had some environmental concerns with the red color contaminating the water-wash equipment, and building up in their water-recycling system. WS446-NRD was developed from the basic WS446 flux series chemistry, but  without the red dye. The solderability performance of WS446-NRD was excellent, eliminating the variations in OSP solderability without requiring any additional processing steps. WS446-NRD also passed internal process and product requirements, such as cleanability, and the customer was very pleased with Indium’s ability to rapidly tailor a chemistry to their specific requirements.

[ACM] Sehar: thank you. I look forward to sharing a durian with you again when they are back in season.

Folks,

Everyday, we are exposed to the results of surveys and polls.  A typical example might be that President Obama is leading Mitt Romney in a poll by 48% to 45%, but the results are not statistically significant.  A reasonable question might be, “What does it mean to be statistically significant ?”  

To determine statistical significance, typically, the statistician will use the criteria that if there is only a 5 percent or less chance that the conclusion would be wrong, it is considered statistically significant.  So, when another poll would state that President Obama leads by 49% to 44% and it is statistically significant, there is, statistically, less than a 5 % chance that the conclusion is wrong.  The 5 % criteria is not cast in concrete. Sometimes 10%, 1%, or even 0.1% might be used.  However, tradition has given us 5% as the default value for “statistically significance.”  It is also helpful to understand that, the more data points in the sample, the more likely the results will be statistically significant.

But if some data are statistically significant, is it always "practically" significant?  As an example, let’s say that you really like chocolate.  Your favorite brand is in a taste test and it scores 9.6 out of 10, whereas a new chocolate scores 9.7/10 and the results are statistically significant.  On the downside, the new chocolate costs 5 times as much.  Is it worth the extra money to convert to the new chocolate? In this case, we have to ask, is the difference practically significant.  The answer is, in all likelihood, no.  Such a difference as 0.1 point out of 10 is very small, and taste is also subjective.  Here, the result might not be practically significant.  The subjectiveness of a taste test may mean that you either can’t tell the difference or that you still like your favorite chocolate the best.

Let’s consider another less subjective example.  Suppose that, in a certain application, solder voiding  is a critical concern.  So, you measure the voiding of two solder pastes.  After collecting hundreds of data points, you find that the average voiding of one solder paste is 8% and that of the other is 7%.  Analysis with Mintab® software tells you that the difference is statistically significant.  But is the difference practically significant?  Probably not. 

How do you determine practical significance? Typically it would be by experimentation or in some cases by experience.  In our example of solder voiding, suppose experiments showed that, as long as the voiding average is below 30%, there will be no concerns.  In light of this, engineering may have set a specification that voiding must not be greater than 25% on average.  (All of this discussion assumes that the spread or standard deviation of the data is not large, but this subject is the topic of another discussion.)  So, in this case, the difference between 7 and 8 percent voiding may be statistically significant, but not practically significant.  So, a prudent engineer may select the 8% paste if it had other desirable features, such as better response to pause, or resistance to graping, or improved head-in-pillow defect.

So always ask yourself, is the difference both statistical and practical.

The image shows solder joint graping, which is often more of a concern than voiding.

Cheers,

Dr. Ron

Folks,

There is a lot of interest in cleaning PCBs that have been assembled with no-clean solder pastes. 

Recently I discussed the topic with my good friend Mike Bixenman of Kyzen.

Dr. Ron (DR)

Mike, many of the best performing lead-free and lead containing solder pastes today are no-cleans.  They have been designed to solve assembly problems like graping and the head-in-pillow defect.  For the vast majority of applications, the small amount of residue left by a no-clean is not a problem.  However, some assemblers want the performance of no-cleans, but need to clean the no-clean residue as they have extreme reliability or cosmetic requirements.  Are there cleaning solutions for these situations?

Mike Bixenman (MB)

Absolutely!

DR

Can you tell use a little bit about these cleaning solutions?

MB

Several factors come into consideration when engineering electronics assembly cleaning agents. Design factors include the soil make-up, heat exposure, Z-axis clearance under bottom termination components, material compatibility, and cleaning equipment. Typical process goals require that all flux be removed in one cleaning cycle, shiny solder joints (no chemical attack to the alloy), fast production speed, no material effect to labels and other materials of construction, long chemistry bath life, and low operating concentrations.  

Cleaning solutions vary depending on the cleaning equipment. For solvent systems, a solvent cleaning agent is needed - with properties that allow for non-flammability, constant boiling mixture, and being environmentally-friendly to workers and the environment. For solvent cleaning agents that are rinsed with water, the cleaning agent requires a solvent mixture that can be rinsed with water while matching up to the soil and cleaning equipment. For aqueous cleaning agents, the cleaning agent is engineered with properties that provide solvency for the soil, polarity for inducing a dipole and/ or to oxidize and reduce the soil, low surface tension to reduce the wetting angle, buffers to stabilize pH, defoaming to reduce the tendency to foam at high pressures, and inhibitors to widen the passivation range on metallic alloys.

The property most critical is the nature of the soil. As soldering temperatures rise and the time exposed to higher temperatures increase, solder paste material supplies must improve the oxygen barrier and prevent flux burn out. This requires higher molecular weight compositions that may change the nature of the soil and the cleaning solution needed to remove the soil. Other factors such as processing conditions and how these conditions can change the soil’s cleaning properties must be considered. For example, excessive exposure to heat may polymerize the flux residue rending the soil uncleanable. To better understand and plan for these factors, solubility testing and matching the cleaning agent to the soil assist formulators in designing cleaning agents that are effective on a wide range of soldering material residues.

DR

What type of equipment is typically needed?

MB

Two key factors must be matched to clean:

1: Potential energy of the cleaning agent for the soil and

2: Kinetic energy of cleaning machine for delivering the cleaning agent to the soil necessary to create a flow channel needed to rapidly displace the soil.  

The cleaning machine requires energy to deliver the cleaning fluid across a distance and create enough force to deflect fluids under the Z-Axis. The capillary attraction for moving the cleaning fluid into an out of tight gaps is created by fluid flow, spray impingement pressure and surface tension effects. When cleaning under tight standoffs, cleaning agents that wet (form small droplets) improves capillary action, penetration and wetting of the residue. The solubility rate is dependent on the soil, temperature effects and concentration of the cleaning agent needed to dissolve the soil. Hard soils clean at a slower rate and remove the soil in a concentric (tunneling effect) manner. Soft soils clean at a fast rate and remove the soil in a channeling (multiple tunnels) effect.

The Z-Axis gap height has a direct correlation to the energy required to penetrate and remove the soil under components, time required to clean the soil and wash temperature. The irony is that lower Z-axis gaps increase capillary action of the flux for underfilling the bottom side of the component. When this occurs, flux residue dams up and closes any flow channels under the component. Research findings indicate that high pressure coherent spray jets are needed since energy drop is less and defective energy is higher. The wash time needed to clean under a 1-2 mil gap as compared to a 4-6 mil gap can range from 4-8 times longer. Higher wash temperatures increase the softening effect and aid in penetrating and removing the soil. The net effect is that, as components decrease in size, the Z-Axis gap height reduces and the cleaning factors needed to clean the soil increase. These effects favor spray-in-air cleaning equipment over immersion cleaning equipment.

DR

How are the results of cleaning assessed, so that we know that the boards are truly clean?

MB

The first level that we judge cleaning performance by is the visual presence of the residue post cleaning. Most cleaning processes have no problem with removing surface residue from the assembly. The issue is the residue under the bottom side of the component. This complicates the issue since the residue under a specific component is where most failures occur. These site-specific failures may reduce the confidence in existing IPC standards that correlate anion and cation ionic residues over the entire board surface area. So, when designing the cleaning process, we use test cards with bottom termination components and judge cleaning performance by the level of flux residue remaining under those components. To achieve this value, all components are removed and the surface area of the residue under components is graded and statistically analyzed.

Let me finish by adding that highly dense interconnects assembled onto circuit boards is advancing at a rapid pace. Traditional SMT component spacing between conductors was larger. No-clean post soldering residues posed minimal risks to reliability. The information age has spoiled us in expecting higher functionality in smaller spaces. As assembles reduce in size and increase the levels of functionality, cleaning becomes more important.  I hope that the cleaning factors discussed in this interview provide insight into cleaning process design considerations that may be of help.

DR

Mike, thanks.  Who should folks contact if they would like more information on cleaning boards assembled with no-clean solder pastes.

MB

Thanks for letting me share with your readers.   I would be glad to help anyone with the cleaning challenges they face.  Contact me at mikeb@kyzen.com.

Cheers,

Dr. Ron 

While it is important to have at least 10µm of solder on each side of a tabbing ribbon to form a proper solder joint during cell interconnection, more is not always better. What we have found is that thicker solder coatings may provide adequate and consistent solder joints, but at a reduced bond strength.

The test was performed on c-Si cells, with an industry leading flux, and 3 sets of tabbing ribbon with different solder coating thicknesses. The tabbing ribbon was made from the same ribbon stock to minimize any variation between test subjects. The samples were prepared on a Komax X-series tabber/stringer, and the tabbed cells were allowed to rest at ambient conditions for >48 hours after soldering to relieve stresses. Next, the tabbing bonds on each cell were peel tested at 90°F using a XYZTEC Condor 150-3. Average (not peak) bond force values across the cell were recorded. 

We are happy to apply custom solder coating thickness to tabbing ribbon for you.

I hope this helps you make a good decision when you are specifying material.

Shoot me an email!

~Jim

Do you ever have a need for a "low temperature" solder (meaning an alloy that melts at less than 175C)?

You may have delicate components that cannot withstand standard reflow temperatures, or maybe you are looking to reduce costs by lowering the reflow temperature, or you may be step soldering.  Whatever your reason, there are two unique metals that are used extensively in low temperature solder alloys.

The first one I am sure you can guess: Indium.  The other one is Bismuth. While these two elements are used extensively in the over 100 alloys available in the 50C to 175C range, they couldn't be more different from each other.

Indium is a very soft, malleable metal and remains so even at cryogenic temperatures. It melts at 156C.  Bismuth, on the other hand, is very brittle, even at room temperature, and melts at 271C.  But both lend themselves very nicely to solder alloys that melt below 175C.

Let's look at the two most common alloys in these families.

The two alloys:

• 52In 48Sn (Indalloy #1E) Melts at 118C
• 58Bi 42Sn (Indalloy #281) Melts at 138C

What they have in common are:

• Both are lead-free
• Both are tin-based
• Both are eutectic (liquidus and solidus temperatures are the same, with no plastic range)
• Both can be made into a wide variety of solder forms and can be used in low temperature applications

But the indium-based alloy will give you better compensation of coefficient of thermal expansion (CTE) mismatch than the bismuth alloy.  The bismuth alloy has greater tensile strength but has a lower shear strength than the indium alloy and is generally not recommended in applications where the product has potential to be dropped (like cell phones).  The indium alloy will give you greater thermal conductivity than the bismuth, as well.  The bismuth will give you a cost advantage.

So, which alloy do you use?  Well, that depends on the metallizations you are working with and the environment in which your final product will be operating. For example, if you are soldering to two different surfaces that expand at different rates, then you will want to go with the indium alloy - to keep your solder joints from cracking.  But, there are a lot more considerations when choosing a low temperature solder, and we can help you sort through them.  Check out our Low Temperature Solder page on the web or contact us at AskUs@indium.com or contact me directly at cgowans@indium.com and we can answer your questions or put you in touch with one of our local experts to review your entire process for the best solution.

Let us help!

Carol Gowans

Maria Durham, Indium’s new Technical Specialist in Semiconductor and Advanced Assembly Materials, has been doing some research on indium lead (In/Pb) solder alloys. We chatted about her findings this week. 

 [Andy C. Mackie: ACM] Which indium/lead solder alloys are most common, and what are their properties?

[Maria Durham: MD] Firstly, the use of lead-(Pb-)containing solders in some soldering applications is restricted due to local environmental and RoHS compliance, but there are still many applications where they are  allowed. Many military, aerospace, and industrial equipment uses, as well as many applications related to vehicles, are exempt. The table below shows the most common indium/lead (In/Pb) alloys (pink) and their properties, sorted by liquidus temperature; the higher of the two melting points (solidus and liquidus) seen for non-eutectic alloys. In blue are three comparison materials.

Indalloy 205 is the most commonly used, probably because it has the closest liquidus temperature to the tin/lead eutectic (183°C), 63Sn/37Pb (Indalloy 106). This means it can be reflowed using a standard Sn/Pb eutectic profile. The next most common alloys that are used are Indalloy7, 204, and 206.  Besides the melting range, indium has comparable thermal and electrical conductivity to standard materials.

[ACM] What makes indium-lead (In/Pb) solders so attractive, and why have we seen a recent resurgence in their usage?

 [MD] One main attraction to using indium/lead (In/Pb) solder alloys in soldering to precious metal surfaces is that, unlike tin-containing solders, they do not leach gold. That is, gold does not dissolve in them to any appreciable extent. During discussions at Semicon West in 2011, one of our California customers reported going through 8 simulated reflows with Indalloy 205 in contact with a gold surface with no loss of joint strength and no joint embrittlement. That is pretty impressive. Note that embrittlement is often caused by gold-intermetallic formation. It has been noted that even at 250°C, 50In/50Pb dissolves Au at a rate 13 times slower than it does into 63Sn/37Pb, although this, of course, is a kinetic, not a solubility limit, study.

The higher melting Indalloy 164 (92.5Pb/5In/2.5Ag) has the lowest coefficient of thermal expansion (CTE) of all of the In/Pb solders and is able to withstand the higher temperature excursions that can be seen in step-soldering type applications (where a very high melting solder is used to form the first joint, followed by a next lowest melting alloy, and so on). This is seen in applications such as power electronics assembly, where the first step solder is often used for die-attach either as a solder paste, wire, or preform. The high melting point helps the solder withstand the operational temperatures associated with under-the-hood electronics, in applications such as engine control modules, where Indalloy 151 (92.5Pb/5Sn/2.5Ag) or Indalloy 163 (95.5Pb/2Sn/2.5Ag) are most commonly used. In/Pb solder is excellent on very rigid structures such as ceramic-to-metal or ceramic-to-ceramic. The desired solidus / liquidus temperature range can be adjusted by changing the indium:lead ratio, making it very easy to “dial in” the alloy to a specific reflow process.

Another attraction to using In/Pb solders is that they exhibit good fatigue resistance in thermal cycling from -55°C to 125°C.  In testing, the 50In50Pb solder joint fatigue life is about 100 times greater than that for 63Sn/37Pb.

 [ACM] What fluxes are used in these applications, and how are they formulated differently?

 [MD] The fluxes most compatible with the lower melting point (<200°C) indium-containing solders are NC-SMQ-80 (solder paste) or the lower-tack TacFlux® 012 (suitable for use with wire, preforms, and spheres). These are no-clean fluxes, specifically formulated for lower temperature reflow.  Under appropriate low temperature reflow these fluxes leave behind benign residues that do not need to be cleaned off (“no-clean” flux), although they are often cleaned off in most practical applications, usually to ensure reliable wirebonds absent of flux spatter.

 To learn more, please contact us.

 Cheers!  Andy

Reducing the surface oxides of Nitinol is just the first step in getting a good solder joint with this versatile medical assembly material.

Next you have to choose the right solder alloy.  You will probably want to stay away from anything containing lead, cadmium, or antimony, particularly in medical applications.  And you will want something with a high tensile strength.

The best choice is Indalloy #121 (96.5Sn 3.5Ag).  It has a tensile strength of 5,620 PSI and a melting temperature of 221C and is obviously lead-free.  It wets well to the cleaned Nitinol.

If you need a higher melting temperature solder (one that can withstand autoclave temperatures for example) you should consider Indalloy #182 (80Au 20Sn) which melts at 280C, has a tensile strength of 40,000 PSI, and has long been considered a highly reliable solder.  Additionally, this alloy is available in very fine diameter solder wires to minimize waste.

Soldering temperatures should be 25C to 50C above the liquidus temperature of whichever solder you use and proper cleaning should be always be performed afterwards.

Contact us at medical@indium.com for more information about soldering for medical devices or visit our web site at www.indium.com/medical

Carol