If you remember the basics for a second, NanoFoil, whether it is standard or plated with tin solder on both sides, needs to be in intimate contact with the surfaces to be soldered. Once the NanoFoil is activated (at a thickness of 40 microns) it only stays at 1,500 degrees Celsius for less than a millisecond. So, if the NanoFoil is not in "intimate contact" with the interfaces that need to be soldered (or "wet"), it will not create a great bond. By applying pressure, you, the engineer, can maximize contact with the foil. The best way to do that is to use constant pressure and some foam or compliant material.
Constant Pressure: If you were to personally witness the NanoBond process (imagine you are shrunk down to nano-size and can actually see the NanoFoil reaction begin), you would see a wave of molten solder propagating across the bond area as the reaction occurs. Now, if you were using two static plates to press the assembly together, there would be minimal constant downward pressure while the solder is molten. However, if you were using a spring-loaded, air-driven, or piston-driven pressing device, you would ensure that downward pressure was pressing the assembly together, enabling the molten solder to produce a high quality, low void bond.
Foam (Compliant Material): If you remember nothing else about this flight-induced blog post remember this:
A COMPLIANT LAYER SPREADS THE LOAD EVENLY AND
HELPS TO MAKE THE MOST SUCCESSFUL NANOBOND.
It shouldn't be too much of a surprise to learn that, if you use some foam above your component as you are applying pressure, the load will be spread much more evenly.
Well that is all for now. Preparing for a landing. Not me, the pilot. All I have is this wi fi compatible laptop!
Don't Play Laser Tag with NanoFoil
The last way to activate the NanoFoil is through the use of a laser. Now this makes sense right? A form of high energy, very localized can ignite the NanoFoil. Where it gets tricky is that a laser can also be used to cut NanoFoil as well! I won't go into the gory details here, but with tweaking of the pulse width and power, a laser can either cut through the NanoFoil to make intricate shapes OR it can ignite the NanoFoil. There are great processing implications for being able to ignite foil with a laser. In automation, for example, a laser could be built into a head fixture that simultaneously deposits the NanoFoil, appiles pressure and ignites. Additionally, having a through hole on the backside of a board where the component sits can give a sightline for activation with a laser.
The easiest and cheapest way to ignite foil is with a 9 volt battery. By hooking up wires to the leads and touching both leads to the foil (essentially shorting the battery and generating heat) you can activate the NanoFoil. In demonstrations we have also forgone the leads and simply touched the leads of the 9 Volt to the foil.
American Beauty - The most common small production level tool used by customers and here at Indium is the American Beauty resistance soldering tool. With this tool one lead can be grounded to a press or the part, and a secondary probe can be used to complete the circuit by touching the NanoFoil. Simple, easy, reliable and fits onto a table top, perfect for small scale manufacturing needs.
MPIS (Multi Point Ignition System) - Primarily used for sputtering target bonding with NanoFoil. For sputtering targets larger than 6 inches, the NanoFoil needs to be activated in multiple locations to reduce voiding. This needs a full blog post to explain the in-depth details, but the basics are as follows: when the NanoFoil is activated under pressure between two layers of solder, at the wave front of the NanoFoil is molten solder. If the NanoFoil is activated at one location for a large part, the wavefront of molten solder will spray out the opposite side of the activation causing voiding. If instead the NanoFoil is activated at opposite points around the part, the wavefront of molten solder meets in the middle and causes minimal to no voiding.
A lot of times we get asked: is the NanoFoil ESD sensitive? It is a logical question, with a reactive material that last thing you want is an operator shocking the NanoFoil and activating it. We have had the NanoFoil sent through standard ESD question, and the company responded in true engineering fashion: "The operator would have to have enough electrostatic energy running through him to kill him 10 times over before the NanoFoil would go off" And so the short answer is no, the NanoFoil is not susceptible to ESD!
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 (CrVI). 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.
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. ‘
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.
- Mechanical Energy
- Thermal Energy
- 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 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.
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.
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.
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.
焊片的外面，还可以含一层薄薄的助焊剂，flux-coated preform. 助焊剂的含量通常在3%或以下，能够有效帮助清除焊接表面的氧化物。
预成型焊片，在北美很多的军事/医疗/航空航天等精密元器件的焊接，都有广泛应用。 Indium公司能够提供各种尺寸，形状，和金，或是包装的solder preforms, 我们的技术团队，更是一直致力于为大家解决焊接/工艺问题，提供最佳的方案。
Pic: Indium Corporation
If you’re a reader of this blog, you most likely deal with solder paste, flux, or some other messy material. How can you keep these materials off of you working surfaces?
Step 1) Clean before you make a mess.
Most of us share labs with other people. I hope you trust your fellow co-workers as much as I trust mine - but DO NOT trust that they have sufficiently cleaned your equipment and working surfaces. To keep a clean lab, you need to do at least a quick wipe down of your area before bringing out the chemicals.
Step 2) Keep containers closed and open containers covered.
This step is easy. Yeah, you might think that nothing in your lab is going to fly into the open jar of solder paste on the workbench, but why risk it - and why have it exposed to air?
Step 3) Get movin'.
Most people clean around equipment, but rarely under it or behind it. In the event that you do drop something like a screw or spring around your equipment, let's hope you don't also find 5 years of dust bunnies!
Step 4) Make it easy for others to clean up too.
Getting people to keep a workplace clean is much easier if they don't need to waste their time looking for cleaning items. Keep the supplies in a central area and make sure they are stocked. IPA and some lint-free rags are great for cleaning paste and flux off surfaces, and should not be harmful to most surfaces in the lab.
Do you have some ideas to share?
I recently had the opportunity to discuss several issues in Pb-free die-attach and other solder applications with Jennie Hwang PhD, DSc, and world-renowned consultant in solder and electronics assembly processes.
Most of you have probably seen videos of countless Mentos/Diet Coke experiments all over the internet. If you are one of the few who have not, this is a good site to teach you the basics: Steve Spangler Science. Essentially what you get is an explosion of foam due to the rapid outgassing of the soda. I am still not sure why diet soda works better than regular soda. If you know why, please comment or email me at firstname.lastname@example.org.
What do Mentos and Diet Coke have to do with voiding you ask? Truthfully, not much but it is just one example of things that shouldn't be mixed (unless you are intentionally looking for a mess). Mixed alloys is another example. This is particularly true when you mix Sn/Pb and Pb-free for BGA assemblies. Today, the most common mixing is using Sn/Pb solder paste but using a Pb-Free bumped BGA. There are concerns about overall reliability of these mixed alloys, but the most common problem people have encountered is high amounts of voiding. This comes as a direct result of people trying to improve the reliability by raising the Sn/Pb reflow profile to around 225-230 C and allowing complete mixing of the two alloys. This definitely gives a more homogeneous microstructure, but most Sn/Pb pastes weren't designed for that high of a reflow temperature. High peak temperatures result in more flux outgassing and, therefore, more voiding.
To avoid the excessive outgassing, you could eliminate mixed alloys. Everyone would like to do this, but it is often not possible. Therefore, you best option is to select a solder paste that is more thermally stable and has a high oxidation barrier. This will reduce the outgassing at the elevated peak temperatures and allow you to focus on diet coke outgassing rather than that of the solder paste.
Indium Corporation is a big proponant of using the EN14582 oxygen bomb test method for halogen determination. However, we have seen misleading reports from major test labs when using this test method for solder pastes. Since the industry is concerned about what remains on the final electronics device when consumers receive it, testing of solder pastes should look at the material that remains on the board after assembly. Here are some basic facts about solder paste to make my point clearer:
- Halogens would only be present in the flux portion of solder pastes
- Pb-Free solder paste is approximately 89% metal and 11% flux (by weight)
- During the reflow process, approximately 50-60% of the flux volatilizes
Why are these points important? What I am seeing more and more often is the oxygen bomb report on the solder paste when the flux residue is clearly what is important. If the report shows that a solder paste has no halogens detected (N/D), it still could be in violation of what the electronics companies want. Let's assume that in solder paste form, the actual halogen content was 50 ppm of Br (which would often be N/D due to equipment capabilities). Since the halogen is coming from the flux, that means we have 455 ppm of Br in the flux (50/11%) . During reflow, let's assume that 60% of the flux volatilizes. Very little, if any, of the halogen is part of that volatile constituant. Therefore, if there was 455 ppm of Br before reflow, there will be about 1137 ppm of Br in the flux residue (455/40%). This is higher than the 1000 ppm allowable maximum recommended by the J-STD-709.
Therefore, it is unacceptable to test solder paste for halogen content. Even if there is no halogen detected in the solder paste, the flux residue still could be above the acceptable limit. Be sure the solder paste vendor is at least testing the flux (if not the flux residue).
Soldering to copper has been around since the beginning of soldering itself. Copper wiring and copper pipes are where soldering began, and copper is still the most common wire conductor, and soldering to it is still easy.
Copper for printed wiring boards (PWB's) have been around just as long, dating back to the first "computers", basically room sized calculators. But, since copper oxidizes, one way of protecting it is coating it. The coating is called Organic Solderability Preservative, or OSP. OSP copper is different from the other surface finishes because it is the only surface finish that covers the solderable surface and is eliminated during soldering, rather than consumed. And since copper is usually the base metal that we are soldering to anyway, why pay for the extra metal, such as tin, silver or nickel/gold, if just a "plastic" coating will work. This is especially true since OSP copper does not require any special reflow profiling or needs, such as a high peak temperature, or long time above liquids (TAL).
Like all surface finishes, OSP copper has some issues that we must look out for. First is the fact that since it is a non-metallic coating, any in-circuit testing must be done on a solder joint, as the test probes cannot pierce the coating to get to the copper underneath. One way around this is to apply solder paste to the test probe pads, and allow the solder to wet through the OSP.
Another potential issue is that since OSP is eliminated during the soldering process, multiple reflows, such as for 2nd side soldering or a final step of selective wave soldering, tend to break down the OSP surface and allowing the copper oxidize. Typically, the copper is pretty well oxidized once the board has been sent through the reflow oven twice, and then sent to the wave machine for selective soldering, requiring the use of a strong flux to remove the copper's oxidation.
A great Indium Knowledge Base question came from a customer last week for "recommendations on solder paste height versus component pitch..." Well, the first thing I did was go to our solder paste transfer efficiency and process guru Chris Anglin, and this is what we've come up with. Typically, it is the variation in solder paste deposits during ultra fine pitch component assembly that are measured, and the amount of variation can be observed as an indicator of defect level. The consequence of excessive variation is that ultra fine pitch defect levels tend to be most susceptible. Because there is no single answer for all applications, we must look at the customers' individual processes. Only then the upper and lower specification limits can be defined. So, our initial recommendation, therefore, institute a 5S Methodology practice at the paste print workstation to minimize the root causes for the variation in the paste deposition process. This is to recognize the amount of variation in, for example, solder paste height, eliminating the variation due to all other causes outside the actual printing process. This, by the way would make a great DOE and 6σ Green Belt project, for only through the paste measurement and number crunching in your favorite statistics program, can the variation of the paste heights be put into a number. And, we are not talking about ten boards, here... The minimum amount of boards that I usually start with is about 100 boards. Now, I know that not everyone can get 100 clean, fresh boards to test with, so I have about 20 boards that are reused, cleaning them in a commercially available stencil and board washer. It is often easy to identify root causes for variation after 5S is instituted. Some of the more common causes for variation are related to board support and squeegees. As control of these basic features of the solder paste deposition process is improved, the variation in solder paste height will be minimized, and defect levels will decrease for the ultra fine pitch components. Indium Corporation's Technical Service has some detailed reports from our Process Simulation Lab that we have done to show the typical variations in solder paste deposition across different apertures and stencil types. More information may be found at our Online Help: Indium Knowledge Base or by contacting our Chris Anglin.
When a flux is required to facilitate a bond in an engineered solder application, the flux type depends on the alloys involved, the temperature range and the surface you're dealing with. Herbert Ludowieg is one of our manufacturing engineers involved with engineered solders and flux coated preforms, he has this advice;
"Since most surfaces involved are in good shape, starting with a low percentage of flux is best. Larger quantities can result in excessive cleanup after reflow and can change the dimensions on through holes by reducing the size of an opening. Ultimately, more is not always better. We have several customers who have reduced their flux percentage from more than 2% to 0.5% with excellent results, the parts are easier to handle and have a better overall experience."
These issues along with possible voiding can really make your flux coated preform experience a bad one. For further support and information, please use the following links:(Flux Coatings for Preforms) and (World Class Tech Support)
Soldering to non-metallics is of interest in numerous applications. Bonding to crystal, ceramic or glass is requested commonly. One of the best materials for doing this is indium and only one basic tool is needed in addition to a heat source, indium, and the bonding surface. This is a nickel felt applicator.
Jim Hisert has recently posted a video illustrating just how to do this.
Check it out on Jim's Semiconductor Blog.
I am well aware that many of you are now working in stressed out, short-handed workplaces and your workloads have increased significantly. With hundreds of solder alloys, forms, and flux chemistries to choose from, it is a difficult (and potentially time-consuming) decision to determine which will best suit a product design. I understand why product engineers miscalculate the effects of chip component choices, or let solder materials be assumed based on historical convention, but hasty decisions open the door to product failures in the field. My expert solder team and I are here to help you make the best solder design decision as efficiently as possible so that this doesn't happen to you.
When stretched thin, design mistakes happen to the best of us. For example, according to Andy Patrizio, Apple has encountered the misfortune of field failures with one of their most reputable products – the MacBook Pro. Solder serves a number of purposes (in this case as an electrical and mechanical connection) and on the most basic level, the solder must serve these as well as maintain joint integrity for a given amount of time. This can almost always be achieved with a little thought and engineering. According to Patrizio however, the 9600M discrete NVidia graphics chip in the MacBook Pro was designed with chip-attach materials that are failing. The operational temperature of the chip is melting the solder completely.
In the least arrogant way possible, I'd like to make it clear that this chip attach issue would have been simple to prevent had the expertise of my engineering team been enlisted. Let us be your "call-a-friend" lifeline.
To ensure high quality, reliable solder connections, for most devices it is a requirement to have very clean substrates. Without a clean substrate, the solder metal will melt and may appear to have soldered, but upon further inspection of the solder joint, it will be evident that the solder did not chemically bond and the attachment will be very weak. This phenomenon will be most obvious by taking a cross-section, but in many cases, the bond is so weak that the solder can just be scratched off the substrate surface with a fingernail or spatula.
For basic applications, fluxes take care of the substrate cleaning by removing oxides from the base metals. The oxide is dissolved into the flux during reflow and deposits within the flux residue, which may be later cleaned off.
In some devices, cleanliness is not needed just to allow the soldering process to begin, but to ensure no particulates become trapped in the solder joint which may stress the interface. For these circumstances, a common cleaning method used is plasma. Argon plasma etching is a residue-free method for cleaning surfaces prior to and after soldering. An explanation I have read on plasma cleaning compared it with sandblasting at the molecular level except that instead of the scrubbing material being sand, it is tiny argon atoms.
This is also a good cleaning technique for devices which require cleaning methods other than flux alone because they cannot handle the residues left behind by fluxes. Plasma cleaning has been adopted by many semiconductor manufacturers as well as those manufacturers of precise components who are soldering to gold substrates.
A couple of my colleagues, Jim Hisert and Andy Mackie recently published an article about the use of solders in chip packaging. The article begins with the historical notion that the practice of soldering dates back around 5000 years where it was used to make metal jewelry. It is interesting to take notice that although soldering has been done for so many years, it is not a science which is completely understood. On top of that, many people are still lost in the soldering basics.
It seems that the best place to begin addressing the common misunderstandings or lack of knowledge regarding soldering is to begin with the soldering basics. In Subsequent postings we can build information from this framework.
• Soldering is different than brazing or welding. Soldering is a technique to attach surfaces together using metal fillers (solders) which melt below 350ºC.
• A solder joint is the result of an intermetallic formation between the elements of the low temperature solder (typically some mixture of tin, lead, silver, bismuth, gold, indium or copper) and the substrate metallization being soldered. A comprehensive list of popular solder alloys can be found on Indium's Website.
• The presence of oxides over the metallic surfaces may prevent the ability for solder to co-mingle and form an intermetallic with the substrate metals. For this reason, fluxes are utilized to remove the oxides off the metal surfaces. Fluxes are a chemical mixture which may contain rosin, acids, or halides.
More information on soldering basics is available in the Indium Corporation Technical Library under Soldering 101.
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