Solder Basics

Folks,

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Cheers,

Dr. Ron

• Five Solder Families and How They Work (Eric Bastow)
• Solder Preform Basics (Paul Socha)
• A Quick Guide to Solder Preforms (Jim Slattery and Paul Socha)
• The Basics of Soldering (Chris Nash)

For reasons that I will discuss in a post later this year, a common factor that is emerging in the area of copper-pillar microbump 2.5D and 3D joining, is the adoption of thermocompression (TC) bonding for flip-chip flux/microbump soldering. TC bonding is now being predominantly adopted instead of reflow. Some of you may have the same response as I got at iMAPS 2011 from one well-known expert in packaging technology. He looked askance at me when I mentioned TC bonding for flip-chip and retorted: “That’s for bonding wafers, not soldering flip-chips!”. Even good old Wikipedia (at time of writing) seems to have the same problem – basically that the industry usage of the term has moved into the packaging arena.

I spent a little time talking to people in the industry, and on Google, putting together a buyer’s guide for those of you looking at who-is-doing-what in TC bonding. This is just a prototype guide and necessarily incomplete – if I have missed your company out then I apologize, and will add it in: just give me all the details!

我自己是一个电子焊接材料销售人员，虽然是B2B，但是生活中当我评估B2C的销售人员时，也常常想到自己的客户们……最近我们想把花园里的一棵斜树砍了，朋友介绍了他曾经用过的4家公司。于是我逐一联系比较。

第一家：没人接电话，留言了也一直没有回复。---客户服务不及时，比较差，不考虑了。

第二家： 一个老头接了电话，我们约好了时间来看树。老头准时出现在我家门口，整齐的穿着，礼貌亲善的销售，职业的打招呼和握手。看了树，问了我的需求后，他用专业的纸条写下了情况和报价，并详细解释了我对砍树的一些问题：几个人来砍，时间多长，有多复杂等。 此外，他还适当“销售”了一下自己和自己的公司： 经验丰富，口碑好等。最后，老头留下了自己公司的保险和营业执照等信息，就礼貌职业地握手离开了。--- 这一家总体感觉不错；但是我不知道他的价位是否合理，再比比看。

第三家：没人接电话，于是留言了；第二天一个年轻人回复了并来看树了。礼貌职业的握手，但是全身十分肮脏的穿着（或许他刚给别人砍完树吧）。年轻人看了树后，报价了；价格区间和第二家老头的不相上下。但是当我问到砍树的一些细节时，他的回答和老头的回答有比较大的差别（这些差别让我感到他是故意在显得自己的报价已经是相当的实惠了）。后来我说我会考虑的，因为我还在货比三家，年轻人显得有点点急了，说他可以明天就来完工，还问我找了哪几家…..最后他还是职业礼貌地离开了；没有留下报价单或是其他信息。---总体感觉一般；虽然待客户比较职业，但是少了老头那种特别专业和比较从容、真诚做生意的感觉。价格区间既然和老头那家差不多，就应该是这个范围了。

第四家---电话约好了时间，但是一个中年人提早了1个半小时就来敲门了，说他自己在同一时间被安排2个客户，所以只能提早到我这里（但是为什么没有任何提前通知呢？万一我不在家呢？）。没有因为提前的抱歉，没有职业的招呼。他看完树后，表现出很难办的感觉，并开了一个天价。 我说再想想吧，那个中年人居然什么都没有说，头也不回地甩门就走…… --- 看来是没有最差，只有更差啊！

我录用了第二家老头的砍树公司。联想一下自己平时做电子焊接材料(Electronic Solder)销售，代表的不仅仅是个人，更是整个公司在客户面前的形象，我自己做到以下这些基本的要求了吗？

---能否完成这项工作。我联系的这四家公司是朋友曾经用过并推荐的，起码都完成过砍树工作。 如果换成是Indium的产品，那么就是我们产品的质量好坏(Product Quality)，以及产品质量的稳定性(Product Quality Consistence)。这对客户们的产率(Yield Rate)和不良率(Defect Rate) 都有很大的影响。而我作为销售，对自己公司的产品了解吗？有信心吗？这些，都会在和客户交流的过程中潜移默化地表现出来的。

---基本的销售要求 (Basic Sales Requirement): 认真准备，准时，有礼貌，打招呼，整齐的穿着等。

---销售的技巧(Sales Skills): 是否了解客户的真正需求而有针对性的销售；是否有销售的工具协助；是否会“过犹不及”或是没有给客户全面的有用的信息等。(Goal Oriented---Begin With The End!)

---客户服务和支持(Customer Service & Support): 及时准确地给客户信息回馈和支持。

---其他：价格，客户关系和客户关系的维系（特别是对于B2B的客户来说），公司的形象、口碑和声誉等等……  

时常对照反省，自勉之。

Cheers!

Pic:Google Image

It was a pleasure interviewing Pat Gallagher, who developed the first automated photovoltaic solar cell tabbing and stringing machine back in 1979. (Before I was even born!) Pat has seen the tabbing industry mature, and he was kind enough to help answer some questions about the process that I’ve grown to love. 

Jim: What were the initial design goals? How have they evolved over the years with customer’s needs?

Pat: Our primary goal was to replace variable hand labor in soldering with a machine and a process. That still holds today. Back then, solar cells were very expensive, thick, brittle, and not very efficient. So the biggest issue was to avoid breaking cells. Our first advice to the cell people was to turn the crystal 45 degrees to the bus bars so that the sides of the cell wouldn't break off along the solder joints. That little trick remains in place today.


Jim: Were the first machines designed to tab and string separately, or in a combined process?

Pat: Our first design was to make strings of cells in one shot. The two-step process, fronts then backs, was a holdover from hand soldering and there was no reason to do that anymore. Surprisingly, however, we ended up accidentally inventing the mechanized tabber on the way to creating a fully automated one-step stringer.

Jim: So that’s where the stand-alone tabber came from! I would have guessed it was the other way around. Have there been any changes to the heating method?

Pat: Oddly, the first thing we tried was induction heating. It was wonderful except that it took 5,000 watts to bring a small solar cell to temperature. It seemed rather wasteful, but that was the smallest industrial RF system available. Then we tried IR light, which also worked well. That's what we used in the first automated system.

Jim: Early tabbing ribbon must have been pretty crude. Have you noted anything that has changed with the copper or solder coating used over the years?

Pat: Basically, it's the same flat conductor that we started with in the 70's. The coating chemistry has changed dramatically. Taking cues from the electronics people, we started with lightly tinned copper and that was it. Solder was introduced on the cell so the ribbon did not need a heavy solder coating as is common now.

No-Lead (Pb-free) has been challenging mostly because the process window is smaller and simply hotter. The cells can be hurt if heat exposure is too long or too fast.

If you’d like to meet Pat (the President of Solar Automation) and learn more, you can email him by clicking here or visit the Solar Automation website.

Since I couldn’t find a good beginners guide to c-Si metallization paste, (not even from Wikipedia) I thought I’d provide an explanation of this important module assembly material:

The silicon solar cell has a low-temperature glass-frit paste applied to the active surface. This combination of glass, Ag, and other binder materials is printed onto the solar cell and fired around 850-1000degC to form the solderable metallization on the cell. This glass-silver mixture recombines during the firing process to break through the passivation/antireflective coating layer on the cell and form a strong bond to the cell. During firing the glass and silver are suspended in a mixture with silver forming an electrically conductive path from the top to the bottom of the deposit – and ideally a silver-rich layer is formed on top. This silver is the surface that tabbing ribbon is soldered onto when interconnecting cells.

Because the structure of the glass-silver is formed in the firing process, the firing can impact the solderability of the final metallization. That is the reason it is so important to determine the bond strength and diffusion/intermetallic formation of the interface between the cell metallization and tabbing ribbon solder coating.

Now here’s my challenge to you:

If you know of another good description, post a link to the document in the comments field below!

Thanks,

          ~Jim H.

Folks,

Let’s assume you are trying to improve the productivity of your SMT assembly lines. You work at a facility that manufactures high volume products with a high mix. The first aspect of your process that you attack is stencil printing. You find that your current paste stiffens up when the printing process pauses. So, after a pause, you have to wipe the solder paste off of the board and reprint the board.  

Let’s then assume that you replace this paste with a better one that has good response to pause (i.e. you don’t have to reprint a board after a pause). Let’s also assume that this elimination of reprinting boards results in a modest increase in productivity, say 2%. In other words, you are able to process 2% more boards in the same amount of time. No big deal right? Hardly matters? Wrong! In a typical assembly facility, a 2% increase in productivity results in more than a 6% increase in profits!

You continue working and garner 2% more increase in productivity by balancing your placement machines, improving assist time, and developing a preventive maintenance plan. You have now increased productivity by 8%, but profitability increases by more than 24%!

The calculations that support these conclusions, from ProfitPro,™ are shown as a graph of profitability increase versus productivity increases. All of these calculations support the “Law of Exponential Profits.” This “law” (OK it was developed by me, so maybe it isn’t a law) states that an incremental (e.g. 2%) increase in productivity results in an exponential (e.g. 6%) increase in profits.   The results will vary depending on the assembly facility, but the basic idea is always true.

The bottom line: Investing in productivity always pays.

Folks,

My interest was piqued by a recent article from the pen of good friend, Ray Rasmussen. In this piece, Ray reviews a presentation by Phil Plonski of Prismark Partners on a tear down analysis of an Apple iPad. Ray comments on the profoundly dense interconnection, with package-on-package (PoP), flip-chip, etc. The result is a product with super-dense IC packing with a minimum of printed circuit boards (PCBs) needed. The micrograph, from Prismark, shows this impressive packaging design. 

Ray goes on to lament that these types of designs require fewer and fewer PCBs. He then states: “It won't be long before they learn how to build the iPad and iPhone without a PCB altogether.”

Whoa, slow down! Let’s think this statement through.

The PCB provides at least two fundamental functions:

1.      It provides mechanical support for the electronic components.

2.      It interconnects the components to each other and provides input/output connections so that the electronics can interact with the user.

(PCB experts will point out that there are many other functions such as heat transfer, electrical impedance matching, electromagnetic shielding, and a few other things the PCB provides in addition to those mentioned above. Many product designs require all of these functions of the PCB, however, even the most basic designs require 1 and 2.)

It is an imperative in any electrical design to minimize the number of components, PCBs, connectors, etc. in order to minimize cost and increase performance. However, the minimization of PCBs often results in those used becoming more complex and hence having more “value added.”

In looking at the micrograph cross section, one could strongly argue that the PCB has never been so important or so strongly a “partner” in the design. The multilevel, fine feature, high density interconnection provided by this PCB is truly a miracle of modern PCB manufacturing. Any other “PCB-less” design would require these functions and would essentially, by any other name, be a PCB. As an example, let’s say all of these functions were performed by the case of the electronic device. To manufacture this new PCB-less device, the processes that are used to make a PCB would be needed to form these functions in the product’s case.  In addition, solder paste printing, component placement and reflow soldering of the case would likely be a challenge!

So expect the PCB to be alive and well for some time to come…..and never more needed.

Cheers, 

Dr. Ron

The following is an example of how a simple procedure like cleaning a soldering iron tip can make a world of difference in the quality of a solder joint. Eric Bastow responded to a customer after doing some testing in the lab – and confirming that a clean iron tip contributes to a clean solder joint:

“As I mentioned in our conversation, I did not think that a flux coated preform would fare any worse than a cored wire in a hand soldering application where charring is concerned. Rosin is rosin is rosin, regardless of whether it is within a cored wire or coating a solder preform. I did a quick experiment to see what would happen.

Using a Weller WS80 soldering station, set to an abusively high temperature of 850F/455C, I soldered some .250” square x .005” thick Sn63 preforms (folded-up as small as I could do by hand), flux coated with 1% NC9, to a nickel metallized FR4 test coupon. The contact time of the iron to the solder was ~5 seconds. The results look pretty good. The charred flux that you do see is flux that burnt to the iron and was transferred to the solder from the previous preform. I would anticipate this sort of appearance with a flux cored wire, as well, used under these conditions. I believe that with frequent cleaning of the tip, the amount of unsightly flux residue with be minimal, especially if a more appropriate iron temperature were used.” -Eric

The bottom image is what happens when you don't clean a soldering iron tip.

Soldering Basics

I’m really excited about a new option for those of us who are prototyping solar assemblies or evaluating new tabbing ribbon materials. I’ve been waiting for something like this – everything you need to solder solar cells together in one package. The turn around time is key too – you may recall an older post where I learned how quickly these materials shipped.

On the website where these kits are offered, the description reads:

“Tabbing ribbon kits come with everything you need to evaluate how Indium Corporation materials will work with your solar cells and assembly process. The kits can be used to:
- Evaluate which tabbing ribbon size is best for your design
- Determine which flux is best for your operation
- Experiment with new solder coating alloys
- Assemble a few solar panels”

The tabbing ribbon kits come in 3 flavors:

- Standard Sn/Pb/Ag (62Sn/36Pb/2Ag)

- Pb-Free (96Sn/4Ag)

- Low Temp Pb-Free (58Bi/42Sn)

I have a feeling the Low Temp Pb-Free kits are really going to be the most popular of the 3 that are offered though. Application temperature ranges will determine which kit to use, but all three versions of the kits are said to offer similar base copper sizes and tolerances:
“The ribbon itself is industry standard CDA 110 (99.9% Cu) core flat wire, coated with a precisely controlled layer of solder. Each ribbon is manufactured using our proprietary softening process so you can increase the yield of your stringing process.” Basically, this means that the softer tabbing ribbon will help eliminate the breakage of thinned cells during the heating/cooling cycle.

It also includes some matching bus ribbon to complete your panel build. If you’re trying to find the right flux, this kit serves dually as a flux evaluation kit as well. The kit is loaded with VOC-Free flux, rosin-based flux, and resin-based tabbing fluxes. I prefer GS-5454 as a flux for most tabbing operations, but you can see how the others stack up as well.

Let me know how you like the kit after you try it out!

~Jim

(jhisert@indium.com)

Those of you have been watching this blog for a while will know that I’ve been keeping tabs on the status of the European ELV (End-of-life vehicle) legislation on lead (Pb), mercury (Hg), cadmium (Cd) and hexavalent chromium (Cr^VI). It’s been both galling and heartening at the same time, to find that when I Google “elv legislation”, this (my) blog keeps coming up as one of the top 10 sources on the subject. OK: enough of the bloggy, solipsistic prevarication...

My friend, Geert Willems of IMEC late last week let me know that the EC (European Commission) had given its final decisions on Annex II ("the exceptions"), and pretty much adopted the recommendations of the Öko Institute from their 127 page report of September last year (2009). I have to say my hat is off to Dr. Otmar Deubzer of IZM and Stéphanie Zangl of Öko for the very thorough and logical background to this legislation.

The decisions that affect those of us in the semiconductor (flip-chip) and power semiconductor arena are primarily the ones on lead (Pb) in solders, that were formerly covered by section 8.a/ and 8.b/ of the old, outdated Annex II to Directive 2000/53/EC, and are now covered by this new legislation.

A quick visual summary of the legislation relevant to lead (Pb) in electrical interconnects is given below, and please consult the original document for confirmation, as I may have missed some subtlety of the legalese in my quest for brevity. Also, frankly, subsection 8 (b) led to some Transatlantic confusion over whether finishes on pin connectors and PWB's were covered(?), but I think the below is correct:




Refer to the table below for the timeline for of each subsection/exception:



Note that the last review of exemptions was carried out in 2009, with potential effect by 1/1/2011. This implies that the legislative hammer will potentially fall on each of those usages slated for future review on January 1st two years after the review year. Lead (Pb) for most electronics attach usages of interest to those of us in semiconductor and power semiconductor packaging may therefore be "legislated out" by 1/1/2016.

Basically, the use of Pb-containing solders in solder paste, die-attach paste, die-attach wire, solder preforms, and thermal interface materials (TIMs) in automotive electronics assembly is safe for now, and changes will not be forced on the automotive electronics assembly industry at a time when even current manufacturing practises may be leading to still-unresolved safety incidents.

Cheers!  Andy

At the heart of it, NanoFoil^® is simply the aluminum and nickel chemical reaction just waiting to happen.  A lot of energy and a lot of heat strapped into thousands of alternating layers of atoms.  Each atomic layer of aluminum is waiting for just the right energy to move into the nickel layer and combine - to release up to 1250 Joules of energy per gram of material and as much as 1500ºC (2730ºF).

But, why don’t nickel and aluminum just react in real life? And more importantly, how do we make the NanoFoil react to release heat precisely where we want it? 

The former question is answered by going back to basic chemistry and a concept called activation energy.  Activation energy is defined as that energy that must be overcome in order for a chemical reaction to take place.  In regular use, when aluminum and nickel come into contact with one another they do not react, and this is a good thing. Imagine if your nickel-coated nickel reacted with your aluminum money clip in your pocket…hot!  The activation energy of the reaction is too high to promote this reaction naturally.  ‘

 There are a few ways to reduce this activation energy, the most common being via a catalyst, which is a substance that modifies the transition state which, in turn, lowers the activation energy of the reaction.   In the case of the NanoFoil, instead of a chemical modifier we have taken advantage of a physical modifier, surface area.  By layering the aluminum and nickel atoms very thinly and in a very precise method, we rely on the increase in surface area to decrease the activation energy necessary to start the reaction…in most demos we use only a 9 volt battery!   

 The second question is a frequent one fielded by Indium engineers, and worth a deeper look!

 How is the NanoFoil Activated/Ignitied?

The reason I will use the term "activation" over "ignition" is that ignition implies the beginning of a sustained burn, where the NanoFoil is a reaction that lasts for less than a millisecond, and only requires activation.

The reaction will start with 250ºC of localized heat, or a very localized form of energy.  The trick is getting a very concentrated form of energy to come into contact with the NanoFoil.  Touching the NanoFoil with the point of a resistance soldering iron that is at 250ºC is much more likely to activate the NanoFoil than throwing the NanoFoil on a hot plate that has been heated to 250ºC.  In general, there are three types of energy you can put into foil to activate it.

1. Mechanical Energy
2. Thermal Energy
3. Electrical Energy
   

Mechanical Energy – In the case of mechanical energy, dropping the NanoFoil on a concrete or hard surface could activate it IF it lands on its edge and all of the impact energy is concentrated on the corner.  Generally, the NanoFoil does not go off with contact, but friction between the NanoFoil and itself, in the form of a small shard, has produced enough energy to activate the NanoFoil.

Thermal Energy – In the case of thermal energy, as discussed above, a concentrated amount of 250C heat will activate the NanoFoil.  In the case of ohmic heating, which is what we do in demos, by shorting the leads of a battery, the current must be 100-120Amps for a 15um contact diameter, and 250-300 Amps for a 300µm contact diameter.  A hot filament or flame, such as a lighter, will also activate the NanoFoil.

Electrical Energy – In this case a spark will activate the NanoFoil, but it is about concentration of power, or power density.  With a momentary point contact from an electrical probe, 10 Amps and 5 Volts is sufficient as long as it is POINT contact.  The foil can be activated remotely through the use of a dedicated trace on a board, and this requires testing to determine the amount of energy that will travel the distance of the trace.

In my next blog post I will talk about Laser Ignition, ESD sensitivity, and some of the tools that Indium has developed to control the activation.

Reflow profiling can be broken down into several phases. I generally use the following;

Preheat

Pre-reflow

Reflow

Cooling

Preheat Phase preconditions the PCB assembly prior to actual reflow, removes flux volatiles, and reduces thermal shock to the PCB assembly. Because the preheat phase is often the longest of phases the ramp rate (rate/rise of time vs. temperature) is often established in this phase.

Pre-reflow Phase involves flux activation to remove surface oxides (on mating surfaces as well as the solder paste particles themselves), further pre-conditions the PCB assembly before reflow, and can be utilized for the soak portion of the profile, if needed. A soak profile may be suggested to diminish any delta T between components if there are both very small and very large components or the physical size of the PCB assembly is very large in and of itself. A soak profile is also often suggested to reduce voiding in area array type packages, though with Pb-free chemistries, this is often not as effective as with SnPb.

Reflow Phase is where the mechanical/electrical connection is made through the formation of intermetallics. Peak temperature and TAL (time above liquidus) help define the actual reflow portion of the profile. Peak temperature 20-40°C above liquidus and TAL of 30-90s is common.

Cooling Phase determines the grain structure when solidified and is defined as the solder cools from the peak temperature to solidus. A fast cooling rate is desired to create a fine grain structure (most mechanically sound) but is limited by the differences in CTE (coefficient of thermal expansion) of the joining surfaces. If excessive, stress can be exerted on the solder joint or component, fracturing or tearing can occur. Cooling rate of 4°C/s is commonly suggested.

Ramp to Peak profile depicted

For more please see “Best Practices Reflow Profiling For Pb-free SMT Assembly"

Forming gas is a complicated topic, so I will provide some preliminary background in this section, then get into the soldering part next time.

Don't you mean "formic"?
 

Forming gas is a mixture of hydrogen (H2) and an inert gas (usually nitrogen, N2) that is used to reduce oxides on metal surfaces to water. Please don’t confuse this with formic acid (HCO2H), which I hope to touch on in another posting later this year.

Safety

The reason for the dilution of hydrogen by the inert gas is to keep the hydrogen below 5.7% (by volume), as this is the point above which the hydrogen can spontaneously combust. Gas companies such as Linde and Air Products consider forming gas at less than this level to be an inert mixture, so the fittings used for gas cylinder attach are the standard CGA580 type used for nitrogen, argon, helium and so on. Depending on the gas supplier, they may allow a maximum of either 5.0% or 4.0% hydrogen, to ensure they are within safety margins.

All this notwithstanding, 100% hydrogen furnaces are used around the world in a variety of different processes, and I have also seen soldering processes around the world where 10% and even 20% hydrogen/nitrogen forming gas is in use. I am not saying that >5% H2/N2 can not be safely used, but you have to be careful when using it.

 
Gas Supply

There are three ways of supplying gas for forming gas-based soldering processes:

1/ Mixing hydrogen and nitrogen in a special panel. Sometimes this may also incorporate a catalytic reactor that reacts ppm traces of oxygen, with hydrogen to form water: the water  is then removed by adsorption. This process makes a very "clean" forming gas that will have optimal reducing properties. Usually, the nitrogen source is from vaporised cryogenic N2, and the hydrogen is from a cylinder or "tube"-based sources.

2/ Cylinder supply. A single cylinder, or a manifolded bank of cylinders may be used to provide the gas as a mixture. Usually, this is used as-received without being cleaned up.

3/ Ammonia cracking. Basically, NH3 -> 3N2 + H2. This is feasible, but results in a fixed 3:1 ratio of N2 to H2, and is never used (to my knowledge) in soldering. It is also massively inefficient in terms of costs and power usage to make the ammonia, plus the ammonia usually has a much higher moisture content than a nitrogen plus hydrogen gas mixture.

What does it do in soldering? I’ll get into that next time: I'll be talking thermodynamics and kinetics, and there WILL be a test.

Cheers! Andy

Today I made my rounds in the office, collecting ideas for you from our tech guys  - ideas to help you speed the alloy and flux selection process.  The team gave me ideas from the start of the design process all the way up to speeding the order process, and all the steps in between.  These are solder basics, but they can help you get your process up on its feet quicker - if you put together a little information up-front:

1) Call a tech guy early, but be prepared by knowing the specifics of your material needs, like powder size, flux type, and any design requirements.

2) If you’re an engineer specializing in component attachment, get yourself involved with the component or board design team. It may mean extra meetings, but it will save many headaches in the long run after you help the team remember the meaning of “design for manufacture”.

3) Define the details of your application, equipment, and process before selecting a material. For instance, knowing the needle size that you will be utilizing in a dispense machine will speed the powder size selection for die-attach solder paste.

4) Be aware of cleaning requirements and your current in-house cleaning equipment and chemicals before choosing a flux or flux vehicle.

5) Understand the operational temperature of your assembly and the maximum processing temperatures of the components. This will make alloy selection much faster.

6) Don’t get hung up at the ordering process – know what size packaging you need. Do you have equipment that only fits a certain size syringe or cartridge?  Knowing this ahead of time will save you a second call to verify while talking with an Account Specialist.

7) For alloy compatibility and metallurgical considerations, be prepared to lets us know the composition and thickness of your surface finish. This will also save a second call, because it is required information in order for us to get you the right alloy and the perfect flux for your application.

8) For solder paste printing recommendations, know the specifications of the stencil you will be using. Aperture size, stencil thickness, and any other dimensions you can provide will help guide which flux vehicle and powder size we will recommend to you.

9) For preform selection, try using thinner preforms. For prototype situations you can stack the thinner preforms to build solder volume, and it is much quicker to order preforms in 1 thickness as opposed to many thicknesses.

10) Understand your process bottlenecks. By letting us know your material needs we can usually suggest a few materials, but perhaps one of those materials can help eliminate a problem that is slowing your process down.

11) Consider your company’s roadmap for the next 5 years. It doesn’t make sense to select a material and need to select a new one only a year later. Save yourself the time involved in a second solder evaluation and know what the future holds regarding safety/environmental concerns. Likewise, understand the roadmap of your supplier, their future materials, and how their current materials will fit your company’s future plans.