最近Indium 公司第一次成功參展了SNEC 第四屆(2010)國際太陽能光伏大會（上海）。 同事回來分享他的大概感受：
1． 展會的規模比Nepcon還要大好幾倍！(Nepcon是中國SMT行業的盛會)
2． 整個中國太陽能生産組裝產業鏈各個環節都相對成熟並具有規模。
3． 太陽能產業中的核心技術（光電轉換率什麽的）做得強的，目前還不在中國。
1． 前端的太陽能板子製造：CIGS Target 铜铟硒化镓濺射靶， 和Metallization Paste含銀導電漿料。這兩款產品都主要是針對薄膜太陽能光伏技術的板子的。(Thin-Film Solar Panel)
2． 后端的太陽能板子組裝：Tabbing Ribbon 鍍錫銅帶。這是太陽能板組裝所用到的材料。
Cheers! 
Pic: Indium Corporation
前段時間有一個客戶，詢問了我們如何解決解決錫膏粘刮刀問題。 產品是Indium公司有鉛產品的黃牌Indium 92J.
詳細了解了客戶的使用過程后，我們給出了以下4個建議：
1． 提高印刷速度print speed. 錫膏的flux設計中，有一種叫做
Rheological Additives的材料。正是這種材料，能使錫膏在儲存的時候，金屬粉和flux能夠均勻融合在一起，不會出現分層現象。而在印刷時，印刷的剪切力使viscosity 下降，能很好的下錫。但是，如果印刷速度太慢，Rheological Additives不能使錫膏很好的滾動起來，就會出現錫膏粘刮刀問題。 所以，印刷速度因該在合理的範圍内。
2． 関掉EAU Environmental Air Control. 很多新買的印刷設備，使用時都會忘記關上EAU，就是“空調”。 太冷了容易使錫膏viscosity 增加，造成錫膏粘刮刀。
3． 檢查金屬成分比Metal Load. 每一款產品，都由最適合自己的金屬成分比。金屬成分比稍微偏高了，也會造成錫膏粘刮刀。 隨著無鉛化和微型化，無鉛的4號粉金屬成分會相對低一點。
4． 檢查錫膏在網板上面的放置。 一般來説，在網板上面不能放置太多的錫膏，使錫膏粘到刮刀的holder上面了。這樣當然會粘住刮刀，不利于印刷。
如果有任何技術問題，歡迎隨時訪問Indium的online support網站：http://knowledge.indium.com/ 或者隨時發郵件給我們：askus@indium.com , china@indium.com
Cheers !
Pic : Indium Corporation
I mentioned in a previous a blog posting that the primary driver for halogen-free electronics is ostensibly environmental, but that the confusion about “which halogens and which molecules and what level?” has seemingly decoupled the laudable desire for an improved environment from the reality and made it more of a marketing tool. All this notwithstanding, there remain some instances where the performance of the final product itself can be directly impacted by the presence of halogens, usually as ionic halides. This is the reason why Indium Corporation recently developed what appears, at first glance, to be an odd combination: a high-Pb (high-lead) alloy halogen-free die-attach solder paste, Indium9.72-HF. 
The halogen-related failure mode for die-attach solder pastes is the corrosion of wirebond pads on the topside of Power Semiconductor die which are soldered to the leadframe with halogen-containing solder paste. Many manufacturers producing high volumes of identical power devices may also use die-attach (sometimes called “soft solder die attach”, SSDA) wire to attach the die to the leadframes in a fluxless process, but many manufacturers prefer the inherent flexibility of a solder paste-based process for medium mix / medium volume applications.
Long term blog readers will recall that I did a posting on solderspatter (a.k.a. soldersplatter or soldersplash), and that it can be caused by bubbles of solvent vapor or moisture outgassing from solder paste deposits during reflow. In bursting, the tiny flux droplets or solder particles from the surface of the bubble can be propelled quite a distance (several feet). While solder on wirebond pads is clearly a failure from a reliability viewpoint, certain wirebond pad metallizations may also be subject to corrosion from flux. A poorly maintained reflow oven may also drip flux condensate (usually in the exit – cooling – zone), and this too can be a cause of organic materials on wirebond pads.
As long as the bondwire is gold, and wirebond pads are covered in a uniform layer of gold, there is no problem (as long as the flux residue is washed off) since gold is unreactive, even in corrosive environments. Aluminum (Al) or aluminum/silicon (Al/Si) bondpads, however, are potentially reactive. Halogenated materials, such as fluxes and overmolding compounds may react with them to either reduce the wirebond pull strength and/or increase the wirebond junction resistance, leading to localized heating and subsequent thermal-related joint failure. Even covalently-bonded (C-X, where X is a halogen) materials may dissociate at high temperatures: which is how the banned brominated flame retardants work, of course.
The biggest danger of halogenated flux corroding wirebond pads is when:
1/ Completed assemblies (between the reflow process and the cleaning process) are left for a long time before cleaning; particularly if they are exposed to high humidity (high %RH) before cleaning.
2/ The cleaning process is inadequate: either due to poor selection of the cleaning solution, or poor bath maintenance, or inadequate “scrubbing” energy being imparted to the surface to be cleaned, or simply if inadequate time is allowed for cleaning.
Note that even optimizing 1/ and 2/ may still lead to bondpad corrosion.
The Indium9.72-HF paste is available in both type 3 and 4 powder, in the standard high-Pb alloys, Indalloy 151 (92.5Pb/5Sn/2.5Ag) and Indalloy 163 (95.5/2Sn/2.5Ag), and for larger die that need a higher reliability joint, we also offer the Indalloy 164 (92.5Pb/5In/2.5Ag). A Product Datasheet is available for download, of course.
Cheers! Andy
This blog has been in existence for a little over two years now, and we would like to thank our readers for the feedback and inquiries you have provided. I welcome your comments on what you would like from us. Leave a comment below, or email me at jhisert@indium.com.
And now a look back on past topics of interest:
Grid Ink, Silver Ink, Conductive Ink
Bismuth/Tin Tabbing Ribbon, A Low Temperature Pb-Free Alternative
Plated Metallization for C-Si Solar Cells
Increase Packing Density for Evaporation Crucibles
Photon’s 5th PV Tech Show 2010 USA
Solder Shelf Life as Explained by Eric Bastow
Tips to Speed Your Solder and Flux Selection
What's Happening in the Technical Service Department 
A Day in the Life of a Tech Guy
3rd Renewable Energy Expo 2009 in New Delhi, India
Solar Products and Representatives
Kleenex®, Google™, FedX®, CIGs?
Indium Solar Products Reunited
Trade Show Visitors Love the Ground Floor
Intersolar 2009 – What Barrier to CIGS Technology? 
Concentrator Photovoltaic Systems - Will they reach 50% Efficiency?
Standards for Solar Panel Manufacturing
Solar Panel Certification: “Barrier and Benefit” Reviewed by Eric Bastow
Low Temperature Metallization Paste
What Will Your Interest Be At InterSolar? Meet the Bloggers And Let Us Know.
SAC vs. Sn/Ag for Solar Soldering
Solder Thickness for PV Interconnect
Standard PV Interconnect Ribbon Sizes
Eric Bastow - East Coast Technical Support
Mario Scalzo - West Coast Technical Support
More Information About Metallization Paste
A year in the Life of a Solar Blog
23rd European Photovoltaic Solar Energy 
Conference and Exhibition
TCO choices for CIGS manufacturing
CIGS Absorber Layer Electroplating
CIGS - Can sputtering make a breakthrough?
Fluxes for Soldering Tabbing Ribbon
Computer Brain vs. Solar Photovoltaic 
Selection of the Optimum Lead-Free Solder for Solar Tabbing Ribbon
Record Makes Thin-Film Solar Cell Competitive with Silicon Efficiency
Why Thin-Film Solar Cells are Here to Stay
Hot Rooftops to Flashy Digital Cameras
Graping is a phenomenon which appears as un-reflowed solder particles, typically seen on the surface of the solder joint. 
Cross-section of “graped” solder joint
The graping phenomena has become more common due to some of the following issues:
A combination of any of these factors may exhaust the capability of the solder paste flux to remove surface oxides. This depletes the flux and exposes solder paste particles to oxidation, which means the solder particles do not coalesce into the solder joint.
To avoid the graping phenomenon, use the following tips in setting up your reflow profile. The intent here is to decrease the amount of heat the solder paste experiences during the reflow process.
Whatever you call it, low temperature metallization paste is the silver- (Ag) filled material used to electrically connect thin film solar cells. So why does it go by so many names? Perhaps because it is a relatively new product with no industry standards referencing it. (Feel free to leave a comment if a standard is added…) For a very short period of time, silver ink was planned to be included in the upcoming IPC solar standards: ‘IPC PV Module Technical Standards Committee’. It was recently decided that the standards would initially focus on C-Si module assembly – thin film assembly may be included in later revisions.
So who is right? Honestly, we are quite accustomed to calling it ‘metallization paste’, but we know what you are talking about if you use the other terms or describe what you are looking for. I think that in future blog posts you will notice Indium Corporation using the terms ‘grid ink’, ‘silver ink’, and ‘conductive ink’ much more to describe the material, since all those terms are correct. What term do you think best describes this material?
Shelf life means different things in different situations. Post something on the internet and it will last forever. Leave fresh fruit out on a hot day and it won't make it through the afternoon.
Jim Hisert recently wrote a blog post on Solder Shelf Life where he teams up with Eric Bastow to discuss Solder Shelf Life. As they state, Solder Paste has a very defined shelf life because of the flux component. The shelf life of Solder Preforms, on the other hand, is defined by the solder alloy's propensity to form oxides on the surface of the metal.
How do you minimize Solder Shelf Life issues? There are several ways:
1) Order quantities that are reflective of your usage. It is attractive to get a large-volume price break, but you need to be able to use the product when the time comes.
2) Request that the solder preforms be packaged in quantities that you use them. Getting a year's worth of preforms in one bottle may be cheaper, but the constant opening of the jar will only cause the remaining parts to oxidize and become unusable. If you consume 120,000 preforms per year, consider having your order shipped 10,000 pieces per month to assure a fresh supply of material.
3) Store preforms in their original, unopened containers, in a nitrogen dry box.
4) Once you have opened the jar, keep the lid on while it is at the work station. At the end of the day, return the jar to the nitrogen dry box with the lid off so the nitrogen can purge the oxides that may have begun to form.
5) Consider tape & reel packaging.
After just finishing her department’s monthly activity report, Patty took a break to stare out of her window, admiring the beauty of last night’s fresh snowfall. Her mind quickly went to the events of the past week. Rob had “popped the question” and Patty had quickly said yes. Her and Rob’s mothers were ecstatic. Both Patty and Rob liked and enjoyed each other’s parents. Patty recognized this as a blessed situation, but both mothers were now spending 10 hours each day planning the wedding. A result, Patty and Rob were both fielding 3 or 4 calls a day from each mom. Patty decided to go “with the flow” and count her blessings that both she and Rob had great parents.
She briefly looked down at the ring Rob had given her. It was a striking two carat emerald with 0.4 carat diamonds on either side. Rob was concerned that Patty might not like an emerald, but he explained that the price of diamonds is controlled and that “you could pave your driveway with diamonds for each equally good sapphire, ruby and especially emerald that exists in nature.” He went on to tell her that “all of the emerald mines of Colombia produce only one or two good 2 carat emeralds per year.”
Well one of them was right there on her finger. In addition to the uniqueness of emerald, the setting was in rhodium, the hardest and rarest of the precious metals. “Five hundred times more rare than gold,” Rob told her. She was especially impressed when she looked up rhodium on the internet and found this quote: “Rhodium has been used for honours, or to symbolize wealth, when more commonly used metals such as silver, gold, or platinum are deemed insufficient.” Gold and platinum insufficient!?
Rob was really secretive about how he found such an apparently rare ring. But it was consistent with his many other successes in life. She was thrilled to have him as a future hubby, even if she did beat him at golf.
These happy and a little stressful thoughts were interrupted, by Pete coming to her door.
“Hey, kiddo, get packed, looks like will be going on another trip. Guadalajara, this time. Como es su espanol?” Pete said with gusto.
“Mi espanol es muy bueno. Why do you think we will go to Guadalajara?” Patty asked.
“Well, I just talked to Pedro and he said that they performed our productivity audit. Uptime is 29%, and all lines are time balanced to +/- 2%, about as best as could be hoped.”
Patty and her team developed a “Productivity Audit” from what they learned with The Professor in their recent adventures together.
“So then what is the problem?” Patty inquired.
Pete responded, “Jane, the finance exec we met on our trip to South Carolina, implemented a company-wide profitability software program. It was implemented and Guadalajara is 10% too low. No one can figure out why. I think we’ll want The Professor for this one.”
Patty called and was stunned that The Professor was again available. Apparently this was his off term teaching at Ivy University, as he teaches over the summer.
When our trio arrived at ACME’s Guadalajara facility they all spoke in Spanish. Patty had taken Spanish starting in 4th grade through high school, Spanish was one of the 7 or 8 languages The Professor spoke and Pete was second generation from Puerto Rico. They were surprised that the site GM, Harry Hopkins, asked them to speak in English.
“Give me a break, I grew up in Boston, I can barely speak English,” he joked in his heavy Boston accent. “We want you to help us find that lost 10%, we must be doing something wrong. Help us find it,” Harry implored. “One thing I can tell you is that I am really proud of my team, they are really working hard, you can tell by all of the product that is out there. It makes me proud just to walk out on the shop floor and see all of the product!”, he went on.
Patty was relieved that Harry was so supportive. Apparently Jane had sent the “good word” about how the trio had helped ACME’s South Carolina plant.
As the trio went on a tour, one thing immediately struck Patty, there was hardly room to walk around. There were partly assembled boards all over the place.
At the end of the tour Patty spoke up, “This facility is striking in how much partially completed product is on the shop floor.”
“And there-in lies the problem,” responded The Professor.
How can profits be off when uptime and line balancing are so good? Could it be that Guadalajara uses poor performing solder paste, fluxes, or performs? Will our illustrious trio find the problem? Does Patty really like her emerald engagement ring? Stay tuned for the latest.
Cheers,As a supplier of electronics materials, Indium Corporation is constantly faced with customer requests for “halogen-free” (HF) soldering fluxes and associated materials. This is an interesting trend, but we face several challenges here:
1/ What is “halogen-free”? We have not seen any consistent message from our customers on what they mean by a halogen-free flux. As a materials-supplier this is an absolute show-stopper.
Based on several conversations with interested parties, my understanding of the IPC status is as follows, and apologies for any misunderstandings to Tim Jensen (Indium Corp.) and Tom Newton (IPC). The IPC’s 4-33a Task Group, which was looking at a universal halogen-free material standard (J-STD-709), saw a failure of a second ballot on the standard, even when it got downgraded to a guideline. The 4-33a group faced numerous differences of opinion: on what materials should be included; what halogen levels are allowable; or even whether a single component could be considered a "homogeneous material” to be ground up and analyzed for halogens and so on. The task of defining HF will now reportedly be taken up by two separate groups from IPC and JEDEC.
Meanwhile, in March of this year, the Japanese organization JEITA quietly released their understanding (ET-7304) of what is meant by HF fluxes and solder pastes, using a 1000ppm halogen limit. This definition is clearly at odds with the IEC's definition of HF. That is, 900ppm by weight maximum of chlorine or bromine atoms, or a maximum of 1500ppm of both: the so-called “9-9-15” limit. .
2/ Which halogens? The strict definition of a Group VII element (halogen) is one of Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I) and radioactive Astatine (At). From environmental reasons, chlorine and bromine in halogenated fire-retardant (HFR) materials that emit dioxins and similar compounds when heated should be eliminated. However, some customers are also throwing fluorine and iodine into this definition, too. This may be based on fears of electrical reliability, but from my perspective the customer is becoming defocused from the necessity of meeting environmental concerns.
3/ Does halogen = halogenated fire retardant? Not every halogen found in an electronic material is an indicator of a halogen-containing fire retardant!
Greenpeace is the main driver behind this, and I have, to date, been unable to get a response from them on how they will detect halogens on circuit boards. The fear from our customers, and our customers’ customers, is that an electronic device (iPhone / flat-screen TV or other) will be obtained by an environmental group; pulled apart; and X-ray fluorescence (XRF) used to detect halogens. The minimum sample size that can give a quantifiable result for halogen-free is reportedly 2grams: contrast this with the milligrams of material (residue) present from a no-clean flux in a cellphone, and you can see the issues in quantifying halogen levels based on flux residues. We can’t do it reliably.
4/ How can we confirm “zero” halogens? Contrast the JEITA standard with the requests from many customers for zero halogens / “no intentionally added” halogens / “elemental halogen-free” fluxes or some such. However, since many customers insist on third-party data-reporting, we are reliant on these analytical labs to reliably give us data. One of the challenges we face is when, for example, a lab reports “63ppm of chlorine”, based on a reported limit of detection (LOD) of 50ppm. Our customer is outraged: “You said it was halogen-free!”
Those of you familiar with the statistics of analytical chemistry will immediately see the two fallacies here: the first is that they have reported not the method detection limit (MDL), but the much-lower LOD. The MDL is a function of analytical equipment PLUS the errors in sample preparation and handling. The second fallacy is that you can not report 63ppm as a reliable, reproducible number, since the limit of quantitation (LOQ) – the limit at which you can actually give a figure for the concentration – is more than 3 x the MDL. The limit of analytical capability to reliably quantify the amount of halogen present is therefore around 150ppm or greater.
Instead of reporting “63ppm halogen”, a more accurate statement is: “In our single test, we found a small peak in our spectrum at the same elution time as a halide-ion. It may be a halogen, or it could be one of the millions of anionic organic species that elute at the same retention time. The quantity found is well below the method detection limit, so we have no way of knowing if it is from contamination during the sample preparation, and we certainly can not tell how much is present.”
5/ What is a ‘homogeneous material”? Some customer standards require the level of halogen in a homogeneous material to be reported. We can probably safely say that a flux is a “homogeneous material”, but is a solder paste truly homogeneous? Both JEITA and Indium Corporation can agree that the flux-content needs to be the focus of the analysis, but a solder paste supplier may, for example, take the analysis of a 90%w/w metal solder paste, and report the results as “890ppm chlorine”, knowing that the level in the (10%) flux is 8,900ppm chlorine, essentially diluted by the 90% metal content.
Conclusion:
As a global electronics materials supplier, we at Indium Corporation can see three possible solutions to all these dilemmas:
a/ Adopt the JEITA specification – even though it goes against the 9-9-15 EIC recommendation. This allows us to be on a level footing with our Japanese competitors, but appears to put us at odds with the needs of some of the semiconductor assembly and electronics assembly industries.
b/ Adopt a three-tier specification based on the IPC/IEC recommendation – the Indium Corporation approach is given here (below). 
Why three levels? Because our more discriminating customers are telling us that truly halogen-free fluxes are simply not as effective as those that contain small amounts of halogen. For those who are concerned about end product reliability, a “halogen-compliant” tier allows the best of both worlds.
c/ Report the atomic chlorine and bromine levels present in the flux component, and allow the customer to choose what they want, based on this.
If you are a user of Indium Corporation materials, or even a competitor of ours - what makes most sense here? Or is there a fourth or fifth way?
Cheers! Andy
Testing standardization is important to all products, although it is often overseen when new technologies are first introduced. Low temperature metallization paste is a product that fits this "new technology" catagory. Once the flux has been formulated and scaled up it then must be mixed with solder powder. The mixing procedure and equipment must be capable of providing a homogenous product batch after batch after batch. However, because certain aspects of the solder paste must perform in a certain manner an optimum viscosity must first be determined by finding the proper powdered solder to flux ratio, or the “metal load”. Metal load is expressed in percent by weight.
Particle size impacts viscosity. Therefore the metal loading must be adjusted accordingly. The appropriate particle size for a paste is determined by the aperture sizes for stencil printing and the gauge (inner diameter) of the needle for dispensing applications. (Simply put, smaller particles are needed to fit through smaller holes.)
Each flux vehicle, solder alloy and particle size will have its own unique optimum metal load. The metal load is also application dependant. The optimum metal load for stencil printing will be higher than the optimum metal load for dispensing.
It just so happens that the optimum metal load (for stencil printing), expressed by weight percent, often equates to about 50% metal and 50% flux by volume.
Example Solder Paste Metal Load:
Application | Particle Size | Metal Load |
Stencil Printing | Type III | 90% |
Stencil Printing | Type IV | 89.5% |
Dispensing | Type III | 85% |
Dispensing | Type IV | 84%* |
*The dispensing metal load is often not as sensitive to particle size as stencil printing. It could very well be that a metal load of 85% with type III may also be appropriate with type IV powder as well. The amount of powder has already been reduced enough to offset compaction due to smaller particle size.
All solder pastes have a shelf life. As mentioned earlier the flux can begin to react with the solder powder when both are in direct contact with each other in the paste form. The flux itself, often containing organic ingredients, can degrade over time. At a point, the paste will no longer be usable. The best way to optimize the shelf life of a paste is to keep it refrigerated.
The second main ingredient in solder paste is the flux (vehicle). Flux is a very complex group of chemicals/materials that must be able to do a number of things, some of which must happen simultaneously (a partial list is below). This requires the knowledge of experienced chemists and material scientists.
1) The flux must not react with the powdered solder while in storage (shelf life). This is aggravated by the wide variety of solder alloys available in paste form. Each alloy family typically requires it own unique flux formulation.
2) The flux must effectively remove any surface oxides present on the solder powder itself and mating surfaces prior to and during the melting of the solder.
3) It must effectively prevent re-oxidation of the solder powder and mating surfaces at the elevated temperatures associated with reflow soldering, especially when performed in an air environment (air is ~21% oxygen).
4) The performance of the flux must be unaffected by a wide range of temperature and humidity conditions.
5) If the flux is “no-clean”, the residue must be non-corrosive and non-conductive per J-STD-004.
6) If the flux is RMA, the residue must be easily cleaned with commercially available chemicals.
7) If the flux is water soluble/washable, the residue must clean thoroughly with heated and pressurized deionized water.
8) In a stencil printing application the flux must adequately fill and release from the apertures of a wide variety of sizes, shapes and stencil types, not stick to the squeegee, not dry out too quickly on the stencil, retain a brick like shape both at room temperature and elevated reflow soldering temperatures (minimize slumping), provide sufficient tack to hold components in place, not spatter during soldering (boiling of flux solvents), not outgas excessively (voiding) and provide effective wetting of the solder to a wide variety of board metallizations, component lead platings and package bumps.
9) In dispensing applications, the paste must dispense smoothly and consistently through a variety of needles sizes either through manual application or a variety of dispensing equipment types and technologies without clogging (and do many of the things listed in item 8).
10) Provide a cosmetically appealing solder joint and flux residue (if “no-clean”).
Much to the surprise of the SnPb and SAC alloy consuming world, there are a number of alloys available in solder paste, each with their own unique melting points and soldering and mechanical properties. Below is a table containing some of the alloys possible in solder paste (not exhaustive).
Alloy Composition | Melting Temperature (Solidus/Liquidus) |
46Bi 34Sn 20Pb | 96C (Eutectic) |
52In 48Sn | 118C (Eutectic) |
58Bi 42Sn | 138C (Eutectic) |
57Bi 42Sn 1Ag | 139C/140C |
97In 3 Ag | 143C (Eutectic) |
80In 15Pb 5Ag | 149C/154C |
100In | 157 (Melting Point) |
43Pb 43Sn 14Bi | 144C/163C |
70In 30Pb | 165C/175C |
62Sn 36Pb 2Ag | 179C |
60In 40Pb | 173C/181C |
63Sn 37Pb | 183C (Eutectic) |
50In 50Pb | 184C/210C |
Sn Ag Cu (SAC alloys) | 217C/220C |
96.5Sn 3.5Ag | 221C (Eutectic) |
95Sn 5Sb | 235C/240C |
80Au 20Sn | 280C (Eutectic) |
88Pb 10Sn 2Ag | 267C/299C |
92.5Pb 5Sn 2.5Ag | 287C/296C |
92.5Pb 5In 2.5Ag | 300C/310C |
95Pb 5Sn | 308C/312C |
None of these individual metals are found pure on the planet. They have to be mined, refined and then alloyed to the proper proportion with the other metals to create these solder alloys. Oftentimes a wet chemical technique is required after alloying to validate that the alloy meets the allowable tolerances and purities spelled out in J-STD-006 or a customer specific specification.
For many in the electronics assembly world solder paste is often treated as a near commodity item. However, from within the walls of a solder paste manufacturer it becomes quite obvious that it is a highly engineered and intricate material.
The simplest answer to the question “What is solder paste?” is “It is a mixture of powdered solder alloy suspended in a flux (vehicle).” But the real answer is deeper than that very cursory response.
Later, we will discuss these topics in detail:
That grey “sludge” that you use for soldering did not come out of a tar pit deep in a jungle somewhere. It is a sophisticated material that requires intelligent engineering and chemistry, equipment and proper handling at all levels of manufacture
Ignoring the solder selection as part of your design process is risky business. 
As Terry Costlow, the IPC online editor of EMS Now noted in an article ‘Controlling the Explosion of Lead Free Solders’, the choice of the right solder alloy can affect the manufacturing process, the cost, and the field performance of the product.
Initially it was thought that the move to Pb free solders was just a matter of changing reflow profiles but major issues such as tin whiskers, brittle intermetallic layers and other concerns soon pushed solder selection into the forefront.
With over 200 published alloys and over 300 custom alloys shipped each year, we have seen the need for considering the solder design first. Before you settle on a solder you have to consider:
· Surface metallizations
· Operational temperature of your product or device
· Form of the solder you want to use (solder paste, solder preform, solder wire, etc.)
· Temperature of subsequent soldering steps
· Thermal coefficient of expansion
· Tensile strength
And these are just a few of the considerations. Let us help you make the right selection. Contact us at: askus@indium.com.
Feel free to discuss solder selection with our industry professional, Dr. Lasky on November 11th, IPC is having a materials conference: Engineering for Compliance in
Steve Adamson of Asymtek
I had the chance to discuss fluid dispensing recently with my fellow-Brit, Steve Adamson of Asymtek.
After you've reflowed solder in contact with a flux, you're always left with a certain amount of flux residue. There are no clear industry guidelines on how you refer to the residue, and new terminology is emerging all the time. If you leave it up to me, here is what I recommend : 
1/ "No clean" flux residues:
- Standard Residue: >40%
- Low Residue (LR): Between > 10% to 40%
- Ultralow Residue (ULR): Between >2% to 10%
- Near Zero Residue (NZR): Between 0 to 2%
Each % is given as the weight percent of flux residue after a real reflow process, and refers to the fraction of the raw flux, or flux component of a mixture (such as solder paste or metal-filled epoxy). Note that the exact amount of residue will vary with the reflow profile; the mass of flux or solder paste studied; and the rate of gas flow over the sample material, as well as secondary factors, such as the oxygen level in the reflow atmosphere.
Thermogravimetric analysis (TGA) is a pretty poor method for determining post-reflow residue levels. Results from the use of a platinum TGA sample cup with nitrogen flowing over it have been found in our testing to vary significantly with the mass of sample present, probably because the headspace in the cup acts as a "dead zone" for entrapment of vapor: TGA may therefore give artificially high % residue readings, compared to the results on a flat leadframe or other substrate.
From the viewpoint of a standard semcionductor assembl process, now consider the situation of a low-clearance direct chip attach "flip-chip" or package-on-package application, where the flux is essentially entrapped in a "cage" of I/O's, sandwiched between two flat diffusion barriers. As well as issues of flux residue, this also raises the question of how the electrical properties of the flux will be affected, if more of the solvent and other volatiles from the flux are trapped in the residue.
2/ "Water-soluble" (same principles apply for "Solvent cleanable") flux residues:
- Water-soluble: Residues can be truly dissolved in water to leave a transparent liquid: the color of the this rinse liquid is immaterial,
- Water-dispersible: Non-transparent rinse liquid with any hint of translucency or turbidity
I know that the differences here will be very dependent on rinse-water quality and temperature; chemistry of any cleaning agents; stage of bath-life and so on, but to my mind, if the rinsed liquid is not transparent, then the solids from the flux must be suspended as fine particulates. These particulates usually have refractive indices different from the bulk liquid: the result - turbidity. There may be a means of bath-life end-point determination by turbidity or dynamic light scattering (DLS) or a similar technique; possibly in combination with the standard refractive index measurement that is most commonly used.
In conclusion, note that ULR and NZR fluxes are showing increased usage in flip-chip applications, since these types of material interfere less with the curing of underfill polymers. NZR fluxes are becoming critical for copper-pillar bumping applications.
Just my thoughts - let me know what you think.
Cheers! Andy
A reader of this blog recently mentioned: "I am interested in what products could be sold by manufacturer's representatives." That is a large question, considering the evolution that we have come to expect in the solar industry. To answer that question involves first breaking up the industry into 2 separate sections, front and back end solar assembly. Front end assembly involves the process of making the solar cell. Back end involves connecting cells together and assembling them to create a useable device.
Both front and back end products are going to be geared to the customer's technology. For instance, if I was purchasing materials for a large thin-film manufacturing company and someone boldly offered me glass filled high-temperature metallization paste, I would tell them to come back when they know what they are talking about. (In reality, I'd be nice – even though it's an incredibly ignorant mistake.) With that in mind, let's focus on what back end products a representative might be offering for crystalline and thin film solar customers – assuming that back end begins after metallization:
I probably left out as many possible line items as I included, but I hope you get the idea. Feel free to add the ones I forgot in the comment section below.
~Jim
From an upcoming SMTAI presentation dealing with PoP solder paste: "...Formulation, particle size, and metal loading are all key factors in the design of a PoP-specific solder paste. The time spent evaluating these new products is well spent. Electrical opens on your boards when using standard SMT materials or outdated dipping pastes can result in costly and time-consuming rework down the road. With the proper material and process, insufficient solder transfer and head in pillow defects can be a thing of the past." 
... 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
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