Solder Sphere

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

Folks,

Back in October, I posted comments on lead-free reliability.   In this post, I mentioned that I chaired a session at SMTAI on “Alternate Alloys”. At this session, Greg Henshall presented a paper on the  Low Silver BGA Sphere Metallurgy Project. This paper was a collaborative effort of six companies.  In addition, Richard Coyle presented an overview of the work of three companies titled “The Effect of Silver Content on the Solder Joint Reliability of a Pb-free PBGA Package.” Both projects evaluated lead-free thermal cycle reliability as a function of silver content and compared the results to SnPb reliability.

Both papers concluded that, as far as 0^oC to 100 ^oC thermal cycle reliability is concerned in their experiments, SnPb < SAC105 < SAC305 < SAC405

Coyle’s presentation summed it up best:

“Each of the SAC alloys outperformed the SnPb eutectic alloy in every test, including the long, 60 min. dwell time test. This tends to diminish the argument that SAC is less reliable than SnPb.”

To be clear, it was two papers by two different groups coming to the same conclusion. It would probably be a stretch to say that the conclusions of either group were “almost unique".

Denny Fritz responded to this blog post with this point:

“No one I know will dispute your ranking of SAC better than SnPb solder using the commercial temperature cycle Henshall uses – 0C to 100C. But, harsh environment electronics have to perform to either -40C or -55C, and most use a top end cycling temperature of 125C. IT IS IN THAT WIDE THERMAL CYCLE TESTING THAT SnPb outperforms SAC solders.”

Denny’s point is well taken. I believe it can be said that SAC alloys have demonstrated acceptable reliability in commercial, non harsh environments (i.e. mobile phones, PCs, consumer electronics, etc.) However, it cannot be said that acceptable reliability for SAC has been established for military (RoHS exempt) and harsh (i.e. automobile engine compartment) environments.

A short time ago, Werner Engelmaier wrote an article on this topic (Global SMT Vol 11, No. 1, Jan 2011, pp 38-40.), which among other things he said:

“Of course, ‘Dr. Ron’ selectively picks data agreeing with the point of view he held from the inception of the Pb-ban under RoHS on a plot with an expanded x-axis overemphasizing the differences and supporting a solder joint reliability ranking of SnPb < SAC105 < SAC305 < SAC405.”

Ouch! My motives were not quite so nefarious, I chaired a session and wanted to share the conclusions.

However, Werner makes good points in his article, data exist disagreeing with this reliability ranking and he suggests some good points on how to conduct reliability tests so that comparisons can be made between data sets.

In reading some of his other articles, I was delighted to find that we actually agree on the state of lead-free reliability in thermal cycle testing. Here is a statement of his circa 2008 (Global SMT, Vol 8., No. 8, Aug 2008 pp 46-48.):fting, and n

“It has been 2 years since the infamous ban of Pb-solders under RoHS. What have we learned? For solder joints, no dramatic differences in reliability are apparent. The data bases for LF-solders have grown, the favored LF-solders might be shifting, and no reliability model exists as of yet. Nevertheless, progress has been made.”

Cheers,

Dr. Ron

A quick trip to discuss roadmapping with one of the world’s top processor manufacturers, and a visit to discuss Pb-free power die-attach materials, left me with a few hours to spare at LAX.

This time around I was trying to work out how much package-on-package (PoP) solder paste we would expect to see for a waferlevel CSP (WL-CSP) or a BGA dipped to half height. The need for some deep thought was driven by a customer who asked at what point a PoP dipping paste needs to go from a type 4 to type 5, 6, 7 and so on (however you define them), based on the PoP/CSP pitch or ball diameter. Good question.

To start with, in order to get consistent quantities of paste on each sphere, the PoP paste metal loading needs to be well below the point at which rheopectic behavior can expect to be seen (that is, much less than 50% by volume of solder powder metal). By doing this, you pretty much guarantee a “monolayer” of solder paste powder particles (radius r) coating the CSP or BGA sphere (radius R). Figure 1 shows the kind of result that is typical for a good paste: in this instance our halogen-free PoP paste Indium 9.88-HF.



Figure 1: 0.4mm pitch CSP with PoP paste

If the metal loading is too high, even at time zero, you will start seeing large variations in the amount of PoP solder paste adhering to the surface of each sphere (bump), even on adjacent spheres: the small amount of paste that is picked up during the dipping process adheres to the main solder sphere in uneven clumps. This is why standard type 4 printing solder pastes just don’t work in PoP applications: not only is the particle size too big – the rheology is all wrong.

If R>>r, then a reasonable first order approximation is that you can treat the sphere surface as planar and so model the number of solder particles based on a series of hexagonally close-packed particles (Figure 2 gives the definitions).
 

Figure 2: Definitions for the PoP paste dipping process

Using the same model of solder powder particle size as in the discussion on waferbumping paste, you can calculate a couple of potentially useful things:

i/ The maximum number of solder powder particles on each solder sphere (bump)

ii/ The mass of solder paste adhering to each soldersphere

The first (i/) is useful for establishing the inherent variability due to the finite size of the solder powder, and I’m going to suggest another Mackie rule of thumb of a minimum 150 solder powder particles per solder bump, based on the maximum allowed particle size (diameter). The table below gives  the result of this rather simplistic analysis:



Table: Effect of Package Bump Diameter on Solder Paste Type Needed

A 400micron bump should therefore be fine even with a type 3 dipping paste, whereas a 200micron bump will need a type 5 paste.

I look forward to someone proving this wrong. The second (ii/) is helpful, because we can easily use it to test the theoretical mass of PoP dipping paste against what we actually find. Note that this is just simple geometry: it doesn't tell us how much paste is really needed to resolve issues such as the 60 - 90micron bowing we are hearing about from our customers, even with the more rigid PoP packages currently available.

Cheers!  Andy

This week a customer in Asia asked why one of our new epoxy fluxes was not allowing the package-on-package (PoP) device to be picked up from the dipping tray. Obviously, the vacuum nozzle must have sufficient force to extract the PoP package from the PoP flux reservoir (yellow, below).



Those of you who know me also know that I am always trying to reduce things to numbers so, naturally I got thinking about how I would model this from a physical viewpoint and came up with the following:

If the downward force (weight of component plus tack force of epoxy flux) is greater than the upward force (air pressure on the bottom of the component), then the component could not be extracted from the epoxy flux. The figure shows the different variables. Expressing this mathematically, this comes out, in SI units, as:

Downward force = m.g + n.Ft.pi.(d/2)^2

where Ft is the tack force in units of mass per unit area, taken from the maximum tack force determined by the Solder Paste Tack Test from J-STD-005, ANSI/IPC TM 650:2.4.44

Upward force = 101000.A.pi.(D/2)^2

where A is the measure (fraction) of atmospheric pressure and denotes how good the vacuum is (zero vacuum is 0.0atm : hard vacuum is 1.0atm).

There are some uncertainties with this approach: How does the vacuum vary across the nozzle diameter? Does the 5mm diameter probe used in the IPC test equate to a complex CSP (chip-scale package) bottom surface, with many rounded solder bumps or solderspheres? And so on. But, at least the model puts us in the right ballpark. Just to give you a feel for how this works, the second figure shows some results. Note that scenario (iv) is the only one showing problems (negative force balance).

The data implies that you are only likely to see an issue with inability to pick up components from a dipping flux tray if either:

• Components: Heavy (thick / large)
• Vacuum Nozzle: Too small a diameter and/or the vacuum is weak/poor
• Flux: Very tacky (high tack force)

For many of the newer applications, component sphere/bump immersion to just deeper than the bump height (say 100-110%) is desirable. If the customer dips the whole bottom of the component into a standard (non-epoxy) flux, this potentially opens up a lot of issues including reliability (SIR; electrochemical migration); component displacement (skewing) during reflow; as well as difficulty in picking up the component from the tray. The solution to this series of issues, is to choose either a standard flux with a high pre-reflow SIR, such as our PoPflux 30B, or a low-volatile content epoxy flux.

I'll have more to say on epoxy fluxes in a couple of months, as we are currently nearing the end of extensive testing at several customers in Europe and Asia.

Folks,

I was at SMTAI (Surface Mount Technology Association International) from September 24 and 27, 2010.   As I mentioned, I chaired a session on Alternative Alloys from 2:00-3:30PM on Tuesday 26^th.

At this session, Greg Henshall presented a paper on the Low Silver BGA Sphere Metallurgy Project. This paper was a collaborative effort of six companies. In addition, Richard Coyle presented an overview of the work of three companies entitled The Effect of Silver Content on the Solder Joint Reliability of a Pb-free PBGA Package. Both of these projects evaluated lead-free thermal cycle reliability as a function of silver content and compared the results to tin-lead reliability.

Both papers concluded that as far as thermal cycle reliability is concerned SnPb<SAC105<SAC305< SAC405. Coyle’s paper summed it up the best:

Each of the SAC alloys outperformed the SnPb eutectic alloy in every test including the long, 60 minute dwell time test. This tends to diminish the argument that SAC is less reliable than SnPb. (See Coyle’s figure. Data curves to the right are more reliable.)

Henshall’s paper also showed that the addition of dopants, to improve shock resistance, in SAC105 does not reduce thermal cycle life.

So, it appears, at this time, that, from a thermal cycle and drop shock perspective, it is looking more and more like SAC based solders out perform tin-lead solders in these two reliability arenas.

At the end of the session a noted lead-free curmudgeon came over to introduce himself.  We have had a jovial disagreement on several blogs etc. in the past re: lead-free status and issues, but had not met in person.   I should mention that this person is a college graduate, a former technical leader at several influential technological companies, and he owns a PE license. I asked him what he now thought about lead-free reliability after hearing the talks. He claimed that he is a little less likely to think that lead-free reliability is a disaster. He still refuses to purchase any lead-free products. He buys old units (pre-2006) on eBay.

I mentioned that over $2 trillion of electronics has been placed in the field since 2006 with no unusual reliability issues.   I then went on to say that a RoHS-compliant product is much more likely to fail due to a non-RoHS related issue. He did not disagree. So then I asked him why he won’t use RoHS compliant electronics. His answer: “I just don’t trust them.”

Cheers,

最近小忙，少讀書了，也少和大家分享了；不過工作之餘，翻看了一下《A Seat at The Table》一書，覺得裏面有些道理也蠻有啓發的。比如説此書中一直圍繞這個主題來展開了論述“Today, the only thing your customer cares about is value.”

就這個觀點，再對照一下Indium公司的兩個主要系列產品：

²       電路板組裝焊接材料(Solder Materials):  這裡也要分產品而論。對於技術含量較高，工藝使用要求較多的焊錫膏(Solder Paste)材料，重視成品可靠性的客戶們會更多的關注產品帶來的“價值”。 如果只圖便宜的材料，但是用起來“錯漏百出”的，最後還是事倍功半：返工，復修，廢棄率高（特別是浪費貴的不能翻修的板子），產出率低，總體成本也自然高了。 對技術含量較低，工藝已經“模式化”的產品，像錫棒(Solder Bar)，錫綫(Solder Wire),  性价比會更關鍵……在目前日益高漲的金屬原材料市場中，Indium公司考慮到客戶們的成本壓力，也推出了性能可以和SAC305錫棒媲美的有成本優勢的Sn995錫棒。

²       半導體封裝材料(Semiconductor Materials):  整個半導體行業應該算是一個高成本，高投資，高回報（運營得好的話）的三高行業。半導體封裝材料也像是其中的經絡血脈吧，連接各個部分，讓整體最後順暢無阻的工作。半導體各個部分的材料都不便宜，設備更是不菲；對材料性能的表現要求和驗證都很嚴格，畢竟都投資那麽多，不能“功虧一簣”嘛。所以客戶們一般會十分重視產品的價值。 Indium 公司目前提供的半導體材料有：Wafer Flux, Wafer Paste, Micro Spheres, Flip-Chip Flux, Substrate Paste, Ball Attach Flux, Die-Attach Paste/Wire, PoP Fluxes, etc. 

Indium公司還為大家提供散熱界面材料(Thermal Interface Materials)，工程焊料(Engineering Solders)，薄膜光付太陽能板製造材料&太陽能板組裝焊接材料(PV Solar Materials)，和銦金屬及其化合物等。 這些材料使用在比較領先的應用中，新興行業，或是細分市場中，客戶們都十分重視產品和服務能給自己帶來的價值。

Cheers!  

Pic: Indium Corporation

PS: 前些日子看了中央4台的《第三屆漢語橋在華留學生漢語比賽》，感慨不已！除了感嘆這些留學生們對“那麽難”的漢語的精湛掌握，對中國文化和歷史的了解，甚至對中國的熱愛；更感慨的是，這些活動也説明了祖國的強大！現在越來越多的留學生們來中國學習，想進一步了解中國，和中國人民交流；中國話也在慢慢傳播到全世界！以前中國學子們苦讀英語，考TOFEL, 雅思，GRE什麽的；現在金髮碧眼的學生們也在場上比拼誰更了解我們的“四書五經”了，哈哈！

Patty, Rob, and The Professor finished their tasks in Shenzen and were flying to Shanghai for their last set of challenges in electronics assembly.  Then they would head back to the US, Rob and Patty being only a week away from their wedding day.

As usual Rob, conked out as soon as the plane lifted off. Surprisingly, The Professor also drifted off to sleep. Patty was too excited to sleep. Rob’s mother had given her and Rob their wedding presents early … an iPad  for each. They decided to bring only one laptop and one iPad. Patty was a little nervous about using the iPad for presentations but it worked quite well. She was still surprised that the iPad did not have a USB port. The Professor also gave each of them an early wedding present, a Pickett slide rule for Rob and a K&E slide rule for her. She must be the only person in the world right now that was watching a movie on an iPad and solving a math problem with a slide rule!

True to form, The Professor was passionate about how learning to use a slide rule helped improve a person's innate math ability. He showed Patty and Rob how to use them and gave them several assignments. Rob was better with his slide rule than Patty due to the amount of “one on one” time he had with The Professor. She had to admit that using the “slip stick” gave one more of a feel for calculations and it was consistent with one of The Professor’s adages: “Always know approximately what the answer to a calculation should be…..it will help you to avoid errors."

In addition to the iPad and slide rule, Patty was excited to be going to Shanghai at the time of the World Expo 2010. Our trio had scheduled some time at the expo into their busy schedule.

Their plan was for Rob and The Professor to work on some productivity issues and for Patty to take on some of the process materials related problems. The three of them again met with the site GM for ACME’s newly acquired plant in Shanghai, a Mr. Wong. Wong was relieved to find that they all spoke Mandarin, as his English was a little rough. When The Professor addressed him in excellent Shanghainese, everyone was speechless. Patty was determined to ask him about this later. No American spoke Mandarin, Cantonese, and Shanghainese!

They again agreed to stick to Mandarin. Patty headed out to the line, accompanied by a young Chinese engineer, Zhou Chang, who seemed to be taking more interest in her than expected. She tried to make her engagement ring visible, but she wasn’t sure the he knew of the significance of it. When she got to the line that was experiencing yield problems, the Engineering Manager, Fei Ding, met her. He showed her some of the fails and she quickly identified the head-in-pillow (HIP) defect as the likely culprit. After investigating some more fails, looking at stencil printing, some of the BGA components, and component placement, she asked Zhou Chang what spec was used to thermal profile the line.

“I don’t understand what you mean,” Zhou said in Mandarin.

“How do you determine what the reflow profile should be?”  Patty responded.

With more discussion, Patty determined that they had one profile for all products! Fortunately most of the products were of similar, small thermal mass.

“What solder paste do you use for this line?", Patty asked.

The embarrassed silence suggested that Zhou did not know! They grabbed a tube and Patty was relieved to see that it was one of her favor solder pastes. Since profiling was so rarely performed, Patty and Zhou had to go to another part of the complex almost a mile away to find a reflow profiling unit. After taking the profile, the likely solution appeared. The 11 zone oven was very long and the reflow profile had a long thermal  “soak” before the temperature went above liquidus. This long soak probably exhausted the flux, so that when the PWB went above liquidus, there was little flux left, resulting in oxidation and poor reflow.

All during their time together she had mentioned that her fiancé Rob was here, with her on the trip. This information seemed to do the trick.

“Zhou, why don’t you look up the solder paste spec on the web and then set up the right type reflow profile,” Patty suggested.

It was clear that Zhou was troubled. It became obvious to Patty that Zhou did not know how to profile a reflow oven. Patty set about working with Zhou to accomplish this mission. Within an hour they had re-profiled the oven and, over the next two hours, 300 PCBs were manufactured with the yield improved to 95%.

Patty asked Fei if she could give a brief presentation on the head-in-pillow defect to his team and he cheerfully agreed. Fortunately for Patty, her friend Mario Scalzo had given her his presentation that he gave at APEX 2010 on HIP (head-in-pillow). Patty always enjoyed visiting Mario in Utica, NY, as he always knew the best restaurants in town.

Her major points were:

HIP is caused by the failure of the BGA sphere to reflow with the solder paste. There are 3 major reasons for HIP:

1.       Supplier Issues

a.       Solder BGA sphere oxidation

b.      Silver segregation to the BGA sphere surface

2.       Process Issues

a.       Stencil Printing

                                                               i.      Registration accuracy

                                                             ii.      Insufficient solder paste

b.      Component Placement

                                                               i.      Off pad

                                                             ii.      Out of plane

                                                            iii.      Non optimum pressure

c.       Reflow

                                                               i.      Inappropriate reflow profile

                                                             ii.      Flux exhaustion

                                                            iii.      PWB warpage

3.       Material Issues

a.       Poor solder paste transfer efficiency

b.      Insufficient solder flux oxidation barrier

c.      Solder paste slump

d.      PWB or BGA warpage

Patty went on to say that she had investigated all of these issues with Zhou, and that the reflow profile was not optimum as the very long soak time had exhausted the flux. The other possible issues in the list did not seem to be a concern.

At the end of the day Patty, Rob, and The Professor met at the GM’s office to leave together for dinner and the Expo. Patty had to ask, “Professor, how can you possible know Mandarin, Cantonese, and Shanghainese?”

“Actually I speak Min reasonably well too,” he replied.

“How can this be?", Rob inquired.

“Mother and father were missionaries with Wycliffe Bible Translators,” The Professor answered.

“I grew about around many languages during my youth. Mother and father speak more than I do,” he finished.

Patty went on to tell about the interest that Zhou Chang seemed to have in her, and how she had to discourage him.

“The burdens of being a beautiful young woman,” Rob teased.

Patty elbowed him, but they all left the taxi laughing as they headed for a restaurant near the Expo.

Best Wishes,

Dr. Ron 

The Shanghai, slide rule, and HIP images are from: 

http://pool14.files.wordpress.com/2008/12/shanghai_skyline_g.jpg

http://www.hpmuseum.org/powerlog.jpg

http://ppsimanufacturing.files.wordpress.com/2010/03/bga100.gif

Solderspheres or solder spheres, or even solder balls: whatever you call them, Indium Corporation has been making them for years and has rightly acquired the reputation for doing whatever it takes to meet our customers' unique needs.

Unique Alloys:

Hard to find alloys (like multipart alloys; low-melting alloys and even gold/tin (80Au/20Sn)) are our bread and butter. As "Indium Corporation" it should be no surprise that we lead the world in our ability to supply low-melting indium-alloy solder spheres, as well as other forms of these alloys, such as engineered solders or solder pastes.

Unique Quantities:

We don't want you to buy more than you absolutely need. If you just want 100 spheres, we can easily do that: if you want more - we can do that, too. But remember that, because each customer's need is unique, our prices may be higher than our competitors, especially for more standard alloys. Some customers also have unique inventory-control needs, so we work with many customers to ship on-demand by retaining a buffer stock of spheres here at Indium.

Unique Sizes:

Our current dimensional capabilities as of this writing are from 80microns to 0.062inches, or even bigger. Generally, the bigger the sphere - the less spherical it is (within the limits of surface tension and viscosity), and we can't control the laws of physics, so instances where a very large amount of solder is needed, a preform may have better dimensional control. Also, notice that we won't ask that you order in a specific unit of diameter measurement, like the mil or the micron or the millimeter: we're a global company - just tell us what you need.

Unique Packaging:

Often needed for more delicate alloy spheres, we can offer specialty overpacking that eliminates oxides from the atmosphere around the solder spheres, essentially stopping oxidation in its tracks. It's the same technique we use to package our soft solder die-attach (SSDA) wire: a technique that showed that the very reactive wire was still "as new" 3 years later. We also offer spheres in tape & reel packaging (see image) for 24mil, 35mil and 62mil diameter spheres.

Unique Tolerances:

Just as a case in point, a MEMS customer of ours had a need for a low-melting indium-alloy solder sphere with a tolerance of +/-5microns (+/-0.005mm) for a sphere with a 350micron diameter. That demands a tolerance of just over 1% - pretty demanding, but we did it.

Our standard tolerance is +/-1mil (1 thousandth of an inch, or 25.4microns), but as you can see, we have the capability to go to much tighter tolerances using three proprietary manufacturing techniques.

Other Needs:

We are also seeing people asking for doped-alloy spheres; low-alpha emission solder spheres and other things that we could never have dreamed of...

So please just let us know what you need. We'd be happy to help out, and if we can not do what you ask - we'll let you know why.

Cheers!  Andy

In an evaporative deposition process, source material is evaporated and then condensed onto a substrate which is being coated. One of the common uses that our solar team encounters is the deposition of indium (provided as shot) for CIG thin film technology.

It’s an easy concept: if you can fit more indium shot in a given crucible, the evaporation process can run for a longer period of time before material needs to be added. The traditional form of solder shot is a teardrop shape, which is easy to produce as a bulk form of solder. 

A newer version of shot is now available without the tail, we call this ‘round’ or ‘tailless’ shot. This material is similar to solder spheres, but not as precisely spherical. Compared to traditional shot, round shot offers a packing density increase of 15% - 20%. This means you can fit more source material in a given crucible, which can keep your evaporation process running longer, more efficiently, and more profitably.

Shelf life of solder is a concern that is raised by customers on a somewhat regular basis.  Solder pastes typically have a well defined shelf life, however the useable life of a solid form of solder may be much longer and harder to define.  This was the topic of one of my first blog entries, although I think Eric Bastow has a better explaination than I did. Here is Eric's explanation:

“One of the issues surrounding solid forms of solder (no incorporated flux) is determining the usable (or “shelf”) life of the solder. Solder manufacturers have to draw a “line in the sand” somewhere to establish a time limit on the duration of their “warrantee” period. For better of for worse, in our document driven world, many electronics manufacturers live and die by the solder manufacturers stated shelf life, and will even petition the solder manufacturer to recertify the solder after the shelf life has expired so that it can continue to be used. Otherwise, it is excluded from further use and discarded.

There are many solder alloys in existence and each alloy “ages” in a unique way. However, the most common issue with aged solder is oxidation. Is there a level of oxidation at which a solder is no longer usable? With normal storage and handling and use of a flux, the author believes that the answer may be “no” based upon the results of an experiment.

60In/40Pb spheres, 300 microns in diameter, were reflowed in air or nitrogen onto ENIG coupons with two different activity level no-clean fluxes; reduced activity ROL0 (passes J-STD-004A SIR un-activated) and ROL1. The spheres were oxidized to four different levels; fresh, 4 days at room conditions, 5 hours at 85C & 85%RH and 3 minutes of violent shaking. The appearance of the “fresh” was shiny; the “4 days at room conditions” and “5 hours at 85C & 85%RH” were very similar in their slightly dull appearance, and the “3 minutes of violent shaking” were noticeably darkened.

The experiment was performed with a reflow profile that had a peak temperature of 231C. After reflow, the diameter of the wetted spot was measured.

Interestingly, the determining factors were the flux type and reflow environment (air or nitrogen). For a given flux and reflow environment, there was no statistical difference in the size of the wetted spot among the different levels of oxidation.“

-Eric

7:40am

Just got in, fired up the laptop, and made some hot chocolate. This is the best time to get a jump on the day. I clear out my spam that rolled in overnight and prioritize the emails in my inbox. The first tasks that I cleared were:

-         Connected a new potential solar materials rep with the right people at Indium Corporation

-         Recommended the optimal reflow profile for Indium9.88HF PoP solder paste

-         Activated an online Vapor Deposition course

-         Helped specify tabbing ribbon and solder wire for a college student working on a lunar rover project

-         Planned underfill testing for today.

10:00am

After rounding up materials, components, and equipment, Brandon Judd and I assembled some BGAs on a customer's test board. Later today we’ll underfill, and rework some of the components to demonstrate the yield of a reworkable underfill. Each board had 18 components of 2 types. One of them is a very large, coarse pitch BGA. The other one (you guessed it) is exactly the opposite, a small, fine pitch BGA.

Noon

Took a drive and ate lunch.

1:00pm

When I returned to my desk, I noticed a few emails that needed attention. One was regarding the PoP solder paste reflow profile I mentioned earlier. It looks like that will work for the particular application. Another email regarded a barcode design that I am working on for a customer.

1:30pm

Took a call regarding solder sphere attachment. WS3622 was recommended to ease flux cleaning in place of an older tacky flux.

2:00pm

Answered an interesting call regarding thermal management for a cavity CPV assembly. The coolest part – he found my contact information on this blog.   

2:45pm

Worked on editing an interview for Global Solar Technology magazine. I had a chance to discuss many of our solar products in detail, while explaining the advantages of each.

3:00pm

The schedule for the day shifted, so we will reconvene the underfill testing early next week.  This gives me some time to begin the Interfacial Engineering course mentioned earlier. Looks pretty interesting so far.  Spent some time going through the course material and learning some new things.

4:30pm

Posted this blog entry. After looking at the things I’ve mentioned here, I noticed I could tweet all the little parts of my day. If you’re interested, check: http://twitter.com/SolderNinja

... and we're back now with the real situation of solder powder size and its effect on solder paste rheology.

Size distribution - Real solder powder is not monodisperse (single diameter), but has a spread of sizes (typically approximating to a log-normal distribution). Generally, the wider the distribution, the higher the maximum packing fraction. This can be easily understood by looking at the picture from the last post (below) and imagining tiny spheres fitting into the little interstices between the particles: they would have to be around 1/10 of the diameter of the larger spheres. Theoretically, you can get 0.99 or even higher packing fractions with a very specific multimodal distribution, but you never see this in real life.

Boundary layer - Every time a fluid flows over a surface, the part of that fluid closest to the solid surface does not move, relative to the surface. On a molecular level, individual molecules are diffusing in and out of this so-called "static boundary layer", but essentially, the fluid right next to the surface is completely immobile. The fluid just above this static layer is moving slowly, and the next layer out moves faster still, until the velocity is the same as that in the bulk fluid. "Ok" you say, "so what?" Well the fluid around the solder particle therefore forms a kind of shell that is almost like an extension of the particle into the fluid, and the thickness of the shell is not dependent on the particle size. Complicating things further is the fact that solder paste fluxes are plastic, not Newtonian, so the boundary layer goes out even further. To sum up: smaller solder particles have a "virtual shell" around them that means you need a lower metal weight percent to get the same rheology.

Non-sphericality - The sphere is an ideal solid, and any deviation from perfect roundness causes an increase in the "k" factor, which is 2.5 for spheres, and increases as the particle become sincreasingly deformed, more and more fluid being trapped either within or around the particle (see picture). Most powder these days is spherical, so it's not a big deal

Chemical reactions - Activators (see previous posts) are very good at removing oxides from metal surfaces. There is a myth that there is a magical temperature at which the activation (metal oxide plus activator) reaction occurs, but that's exactly what it is: the myth of the "activation temperature". How can I demosnstrate it's a myth? Simply because solder paste has to be stored in a refrigerator, or else it increases in viscosity with time through the so-called "concretion" reaction which is just the slow reaction of activators with metal oxides to form solid reaction products, just in the same way that water hydrates cement to swell the crystals and cause them to change shape and grow, interlocking together into a solid mass.

Air - No matter how hard you try, you will always have a little air mixed in with your solder paste.

A complicated answer to a simple question!

Oh, and by the way, once you've embarked on a study of solder paste rheology, there is is the little matter of "artefacts". A subject for another time....

Cheers!  Andy

A well-respected colleague asked this week if the metal loading of type '4' solder powder in a paste was lower than that for type '3' because the particles were smaller. The implication being that smaller and smaller particles will fill up more and more volume. He is correct in that typically, in solder paste made from type 4 solder powder, the metal loading is usually slightly lower than that for the same alloy in type '3', but not for the reasons he is thinking of. Bear in mind that (per the ANSI/IPC powder J-STD-006A, the type '4' is typically between 25 and 38 microns, while the type '3' is from 25 to 45microns in diameter, so the type '4' is just a smaller range of particle diameters.

There are two aspects of this: the geometry of solder particle packing... and some things we can lump together under the heading of "harsh realities".

Particle-Packing:

Just from a theoretical point of view (monodisperse powder diameter / perfectly spherical particles), the maximum volume that can be occupied by these particles is 0.740. This is the so-called "maximum packing fraction". You can prove this yourself by taking a triangular prism as a unit repeating cell and calculating the volume of fractional-spheres inside it - see the picture for the two ways that hexagonally-close-packed spheres can be arranged - the math is easier for the A-B-A packing. Note that this is [I}independent of the diameter of the particles[/I], which is counter-intuitive, but nonetheless correct. In other words, no matter how small the particles are, you should always be able to get them to achieve a maximum volume fraction of 0.74.

For randomly-arranged spheres, the maximum packing fraction is usually agreed to be somewhere between 0.67-0.69. Of course, at the maximum fraction, any solder paste would just be a solid mass, and the typical volume fraction of a solder paste is from 0.4 to 0.54 by volume of solder (from 75-93%w/w powder, depending on the alloy and the application).

So to achieve the same viscosity needed for the application, you need less volume of solder powder, and hence less % by weight, if the powder is more monodisperse.

Theory is nice, but we'll talk about the "harsh realities" of solder paste in the next post.

Cheers!  Andy

Dr. Andy Mackie recently put together a model to determine the probability that a component can be successfully dipped in solder paste or flux.  Here is a little more from him on this subject:

"A customer in Asia was asking why one of our no-clean package-on-package fluxes, the ultralow residue NC510, was not allowing the PoP device to be picked up from the dipping tray. It turned out that the customer was allowing the flux to coat the whole of the bottom of the component, not just the solder bumps, so the vacuum nozzle had insufficient force to extract the PoP package from the flux . I got thinking about how I would model this from a physical viewpoint.

If the downward force (weight of component plus tack of flux) is greater than the upward force (air pressure on the bottom of the component), then the component could not be extracted from the flux. The figure shows the different variables. Expressing this mathematically, this comes out, in SI units, as:

Downward force = m.g + n.Ft.pi.(d/2)^2

where Ft is the tack force in units of mass per unit area, taken from the maximum tack force determined by the Tack Test Method from J-STD-005, ANSI/IPC TM 650:2.4.44

Upward force = 101000.A.pi.(D/2)^2

where A is the measure (fraction) of atmospheric pressure and denotes how good the vacuum is (zero vacuum is 0.0 : hard vacuum is 1.0).

There are some uncertainties with this approach: How does the vacuum vary across the nozzle diameter? Does the 5mm diameter flat IPC probe equate to a much smaller sphere? and so on, but it at least puts us in the right ballpark.
Just to give you a feel for how this works, the second figure shows some data. Note that scenario iv is the only one showing problems (negative force balance).  The data implies that you are only likely to see an issue with inability to pick up PoP components from a dipping PoP flux tray if either:

- Components: Heavy and have many large PoP solder bumps
- Vacuum Nozzle: Too small and the vacuum is weak/poor
- Flux: Very tacky (high tack force)

and certainly, if the customer dips the whole bottom of the component into the flux, this opens up a lot of issues, including reliability (SIR); component displacement during reflow; as well as inability to pick up the component from the tray. This is why we always recommend a flux dipping height of 40-50% of the PoP bump height, to eliminate these issues."

I have found this model not only interesting, but useful for technicians to use when asked why components are 'only dipped 50%'.  As a technician, it is good to have a scientific reason to refer to - even though experience may have already proven the theory to us personally. 

Something that seems counter-intuitive to many customers, yet seems obvious to a rheologist is why the percent metal loading of solder paste varies so much. The most crucial control variables are:

- Alloy type

- Powder size

- Application / usage

I'll tackle the two last points in a later blog entry, but the first point is fairly simple to explain. Assuming that solder powder is formed of identically-sized, unreactive, perfect spheres, the viscosity of the solder paste will depend only on the VOLUME of solder present, and is therefore related to the alloy density. This finding goes straight back to one of Einstein's first papers* where he showed that, for dilute dispersions of identical spheres, the relative viscosity (the measured viscosity of the suspension divided by the viscosity of the carrier fluid), is 2.5 (the so-called 'k' factor or Einstein constant) times the volume fraction. The picture (right) shows the definition of the volume fraction.

As can be readily seen: the metal density of solders can vary from 6.5 to 14.5g/cm3, and changing the metal density therefore necessitates adjusting the metal weight percent accordingly. This also varies with the usage:

 1/ For printable paste: a 6%w/w spread is possible

 2/ For package-on-package paste: over 10%w/w spread in metal loading may be needed

For both 1/ and 2/, of course, the goal is to maintain the SAME volume fraction, so the rheology of the solder paste remains the same.

Simple algebra will allow you to derive an equation so you can plug in any density of alloy for a solder paste and calculate the required weight percent of that metal. I can email you the solution if you're stuck, just click on the "Contact Me" button (left).

Also note that changing the flux will not only change the density (flux densities can range from 0.85 - 1.05g/cm3), but will also change the rheological properties of the paste significantly. Quick plug: Ron Lasky was good enough to give me a chance to discuss solder paste rheology a few weeks back, and there will be more about this topic in the coming months.

Cheers!  Andy

* A. Einstein, "Concerning the motion of particles in quiescent liquids as required by the molecular- kinetic theory of heat," Ann. Phys., i_7549-560 (1905)

(Follows this post)

Microsphere quality is especially important for bumping wafers.  The dimensional tolerance of microspheres impacts the bump co-planarity across a wafer surface.  In short, more precise spheres directly influence the quality of bumps on the die.  This will of course increase process yield since the spheres will all be closer to the pads they are being soldered to.

Sick of trying to find “off the shelf” solder spheres to fit your application?  We make microspheres on a per-order basis to your specifications.  If you have had trouble getting exactly the size, tolerance, or alloy for your MEMS application, please talk to one of our tech guys (www.askus@indium.com) so we can get you the perfect spheres for your application.

Common features/criteria for MEMS microspheres are:

•          Diameter of 300micron and below

•          Consistently shiny spheres

•          Wide range of alloys

•          High purity alloy (>99.95%)

•          Precise size (+/-25, 10 or 5microns)

•          Non-clumping

•          Non-darkening

•          Argon-purged packaging

A side view of PoP paste Indium9.88HF after .4mm component dipping.

By propping your component up sideways under a microscope (if you don’t have one with a side-view function), you can get a pretty good view of the paste deposit.  There are different theories surrounding the deposit profile and its relationship to ‘performance’, although they are all just theories at the moment.  We have worked with pastes that have uniform and non-uniform profiles - pastes from each category have worked well.  There are so many other characteristics that impact warpage compensation, it’s probably not fair to judge a paste by its profile.  Viewing dipped components from the side does allow you to see roughly how much solder paste was applied, and if the deposit is consistent from sphere to sphere.

Figure 1: Solderspatter (soldersplash) on Gold/Nickel

Figure 2: Phase Inversion during Reflow

Most people are aware that solder paste is a mixture of solder powder and a flux vehicle. The flux has several functions, but  the most important ones are:

1/ Controlling the rheological properties of the paste (dispensing / tack / printing / stability)
2/ Removing oxides from metal surfaces during reflow.

Solder paste is supposed to stay where it is deposited, but occasionally it can turn up after reflow in unexpected places; sometimes a long way (maybe even inches) from where it is supposed to be. This is called "solderspatter" or "soldersplash" (Figure 1).  The first step in eliminating solderspatter is to investigate contamination prior to reflow. For a dispensing solder paste, contamination may occur due to "tailing" of the solder paste between deposits. For a printing process, it may be due to bottom-side contamination of the stencil. If this comes up blank, it is time to investigate the reflow process.

Contact contamination within the oven is unlikely, but once all the obvious sources of contamination, such as indexers or edge conveyors, have been checked and cleaned, it is often found that the problem has not been eliminated. The causes of this remaining solderspatter are not clear, but it has been found that two of the important control variables are lot number of the substrate and oxygen level during reflow. Low oxygen levels can actually 'turn on' the defect.

In theory, solderspatter may result from flux decomposition, or rapid volatilisation, of flux or other materials on the surface of the substrate. However, it does not appear to be related to any of the following:

i./ solder paste type or lot number
ii./ relative humidity or
iii./ time paste is left exposed to air

The usual explanation of solderspatter resulting from solvent evaporation, or absorbed moisture in the solder paste itself does not therefore seem to fit here. What may be happening is that during the reflow process, the solder paste goes through "phase inversion" (Figure 2). That is, the flux goes from being the "continuous phase" to being the "discrete phase", while the solder goes the other way - from a high surface area powder to a minimal surface area reflowed joint. The driving force for reflow is the standard potential-energy minimization, from liquid-solid surface-wetting, and liquid-only surface tension.

A simple equation has been derived, to calculate just how high in the air a reflowed solder paste deposit will project a single unreflowed sphere. The answer will surprise you: send me a note and I'll send you the paper.

Cheers!  Andy