Mario Scalzo's Tech Support Blog

PART 4- Cool Down

The final element of maximizing tensile strength through a proper reflow is the cool down.  Cool down is last line of defense against a poor solder joint.  This is because the cool down ramp, and it alone, controls the formation of the crystalline structure of the metal lattice.  The smaller, tighter and denser we can make the crystal lattice is, the higher the joint strength.  Because, it is along these facets of the crystal that the joint breaks, and the longer, larger and sparser the crystal facets are, the easier they are to cleave.

One way of visually investigating whether the solder joint is tight is to look at the post-reflow surface finish of the solder joint.  A joint that seems to have good wetting and good flow yet is grainy and gray may have been exposed to a slow cool down.  One way to test this is to heat it up with a soldering iron.  After it goes molten, remove the heat.  If it becomes brighter and shinier, it probably needs a faster cool down.  This may also happen if the joints around the perimeter of the board, or where the components are lightly populated, are bright and shiny and the densely populated areas have solders joints that are dull and grainy.  This is because the more densely populated areas take longer to cool off, and affect the cool down rate of the board.  I would reposition the thermocouples used in profiling to the denser area, and re-map the profile to meet their cooling needs.

More information may be found at Online Help: Indium Knowledge Base (IKB).

PART 3-TAL & Peak Temperature

For our purpose here, Time Above Liquidus (TAL) and Peak Temperatures both have the same affect on the solder joint.  Look at it as "total heat input", as you can have a longer TAL and lower peak, or a higher peak, and shorter TAL.  As it is, together they play arguably, the most vital role of the reflow process.  The name of the game is heat.  Heat is responsible for solid intermetallic formation and a homogeneous solder joint, as well as proper flux deactivation.

A short TAL or low peak may result in insufficient intermetallic formation, which results in low tensile strength.  It is the intermetallic that gives the joint its strength, as you always want the joint to fail during testing at either the board-side of the pad, or in the middle of the solder joint, not along the intermetallic.  This is the same for the homogeneity of the joint, which is a metal solution.  If the joint is not thoroughly mixed, then it is where the edges of the metal layer is where it fails, which is poor intermetallic formation.  Another issue with a short TAL or low peak is not deactivating the flux.  Improper flux deactivation causes a multitude of sins, including poor Surface Insulation Resistance (SIR) and continued etching of the metals.

On the flip side, a long TAL or high peak temperature may increase the dissolution of the base metallizations, and possibly increase the MP of the final joint.  Too much dissolution of the base metals also forms a higher number and larger of intermetallics.  Eventually, this may lead to the complete dissolving of the pad or component lead.  Any time you increase the size of the intermetallic crystals, it is easier for them to fracture along said layer.  A long TAL or high peak also increases joint stress, again giving another avenue for fracturing.

Ramp Rate

Ramp rate is literally the first step in the four-part reflow process and plays an important role in the formation of the intermetallics.  Ramp rate, from room temperature to peak, needs to be watched for a few reasons.  The ramp rate determines both the spread and volatization of the flux, and has a hand in voiding and oxidation build up.

A slow ramp tends to allow more solvent volatization, or "out gassing".  Slow ramps for solder pastes are usually 0.75-1°C per second.  (For reference, a "typical" reflow profile has a ramp rate of 1-2°C per second, which generally poses a balance between spread and out gassing.)  This slow ramp keeps the flux close to where it's been applied, reducing spread and slump.  This also gives enough time for the full volatization of the solvents in the flux, usually reducing voiding, as well as keeping the ΔT of the board well under 10°C.  All this extra time may have a detrimental effect on some other points of interest, though, especially oxide build up of both the component and substrate metallizations, as well as the solder alloy itself. 

On the flip side, a faster ramp reaches the softening temperature of the flux quicker, and therefore the flux (and paste) spread to cover a greater area, which increases the area of the joint.  It may also allow for some of the activators to be saved for the actual liquidus of the alloy.  Of course, there are downsides to this approach, which are the possibility of voiding (sometimes severe) and a high ΔT across the board.

At a recent customer visit, I had the opportunity to discuss "the process".  What we typically call "the process", is that magic that happens from when the separate parts go in at the start of the line, and the finished product comes out of the reflow oven.  This discussion was focused on reflow, and why it is important.  Reflow is the high-wire balancing act of the SMT circus.  Reflow is a balancing act because a good profile is a split between too little and too much.

Typically, we configure the reflow profile to work with the available solder and components, to give the highest tensile strength possible.  So, we know what the end goal is, and we adjust what we have to achieve that goal.  Besides tensile strength, some secondary goals are good wetting, solid intermetallic formation, homogeneous solder joint and a small, tight crystal structure.  All of these are achieved through process management of the reflow process.

There are four parts of the reflow process that are adjusted to achieve the goals we have in mind, namely highest possible tensile strength.  They are ramp rate, time above liquidus (TAL) peak temperature and cool down rate.  Each one of these has its own effect on the final solder joint, and each one is important.

At what point in new product development does the solder get updated with the rest of the product or process?  "We" as an industry have just finished a "mandatory" update of soldering products, because of the European Union's "...restriction of the use of certain hazardous substances in electrical and electronic equipment".  Commonly called RoHS.  Where several components of the electronics we use in daily life have gone through a redesign, mostly to remove Lead.

Now that the rush is over, many of the exempt applications have been updating their designs, without trying new solders.  One example is company that I have been working with that updated their entire process with new equipment.  Because of the great rush for new equipment, this technology company has also bought new printers, placement machines, reflow ovens and x-ray machines.  They updated everything across the "board" (pun intended).  Except the solder paste that they have been using since the dawn of man.  Well, this is an exaggeration, as this particular application has always been a niche application, using a specialty solder.  But, since the inception of the product, they have been using the same material; an older formulation for which we developed a replacement flux vehicle specifically designed for the alloy that they were using.

Imagine, you purchasing one of the new "retro" muscle cars (insert your favorite…), yet still having the bias-ply tires you remember on the original.  At what point does the performance of the total package suffer from the flaws in the original design?  Something that is not directly related to the output of the car yet can have a measurable impact on the total package.

The same scenario occurred with this customer.  After they went into production with the updated process, the solder quickly became the weak link in the chain, and they called us for help.  It seems that under the old process, the board-by-board processing technique covered the flaws in the older formulation paste that was handed down from project to project, and once everything else was under control, it stood out.  This is where the "new" formulation, specifically designed for the alloy that they were using, was introduced.  And it worked.  Perfect.

So, the moral of my story is where does the solder fit in?  Is it a modern component in your modern process? Or is it bias-ply in a radial world?

More information may be found at Online Help: Indium Knowledge Base (IKB).

Ok, so it happened again. Another urgent request was brought to me for action. This time it was a customer who had performed a Red Dye Penetration test on a Ball-Grid Array (BGA) that was attached to a board using our paste. A single BGA on a single board was the evaluation (Sample size is a WHOLE other topic of conversation!) The picture is one that I took of the pad that showed no wetting. There are a number of causes of why the paste did not wet only these two pads. Including poor paste transfer, BGA contamination and board contamination. Paste transfer is certainly an issue that is at the forefront of every BGA issue. The first issue that I think of is a contaminated stencil. If their cleaning process were incomplete or sloppy, then this would definitely cause some issues. Especially if the stencil apertures were not completely cleaned out or if the solvent used was not completely removed. Solvent left in the apertures when paste is introduced would wreak havoc on reflow. But, in this case there was sufficient paste printed, because the analysis of the solder joints confirmed that the spheres of the good joints were the same volume as the suspect BGA solder joints. Had the paste had issues with transfer efficiency, those spheres would be comparably undersized. As with Head-in-pillow, the solder spheres on the component may also play a role in the situation, where again, the increased silver content or higher than normal oxide contamination would hinder good wetting. But, would this have shown itself as non-wetting to the pad? Probably not, but rather as head-in-pillow, or a "cold" solder joint. So, where does that lead us? Directly to board manufacturing and storage, or direct pad contamination. Board manufacturing issues would show itself, I believe, more widespread than just two localized pads. Especially since these are regular production boards. This is the same for poor board storage, where an oxide layer had may build up on the pads. But, again, it would probably be more widespread. Therefore, my money is on contamination. We have all been to production facilities, and very rarely do I see any of the operators using gloves. In fact, at one facility, where they were placing solder preforms by hand, we brought up the idea of using gloves. Their defect rate dropped to <1%. As a matter of fact, we do not permit the wearing of silicone bracelets, as the silicone has shown to rub off and contaminate some testing. The silicone actually prevents the wetting of the flux and solder! Even our body's natural skin oil is an inhibitor to wetting. Which is exactly what I believe happened here. More information may be found at Online Help: Indium Knowledge Base.

Package-on-Package (PoP) use in the SMT process is slowly creeping from a niche application into mainstream use. From experience, I know that some customers are using off-the-shelf Wireless Local Area Network (WLAN) and Bluetooth Personal Area Network (PAN) integrated into their current PCB designs. Systems that already exist suddenly become user friendly with the addition of easy Bluetooth, such as portable navigation systems or even my new motorcycle helmet. So, with that, I'd like to try something different, and post an interview with our Semiconductor Application Engineer, Jim Hisert, and resident Package-on-Package expert. Jim, what is the biggest issue facing the widespread use of PoP's in the world today? Well, Mario, it is a different process. I would say component dipping is "easier" than standard SMT into solder paste, because customers are just so accustomed to paste printing. There is also a limited material set and knowledge base for this process, so I find many people feel they are pioneering and become uncomfortable. We try to provide as much knowledge as we can up-front to shorten the learning curve. Lets talk about the component dipping process. Can you diagram the process of PoP use in the SMT world? There are actually two different process sequences that are mainstream. Process "A" -Stencil print solder paste on the entire board (this will be used for all standard SMT components and the bottom component in the PoP stack). -Place standard SMT and bottom PoP components. -Dip second level PoP component in PoP solder paste or flux. -Place second PoP component on top of bottom PoP component. -Repeat if necessary for +2 level component stacks. -Reflow. Process "B" -Stencil print solder paste on the entire board. -Place a pre-assembled PoP stack along with standard SMT components (They can be purchased pre-assembled by the manufacturer in some cases, although this limits the modular application flexibility of separate PoP components). -Reflow. Based on your experience, what can we do to help existing and theoretical processes? There are so many things to do! We need to evaluate existing materials, new materials, and competitor materials. We should keep track of new process developments and possible issues to share with our customers to make their life easier. It is important to help customers select materials, and then be there on the line to help set up the process and get the equipment settings dialed in. I feel that many customers take too much on by themselves; why "re-create the wheel"? Thank you to Jim Hisert, Semiconductor Application Engineer for Indium Corporation. More information may be found at Jim's Semiconductor Blog or Online Help: Indium Knowledge Base.

Head-in-pillow defects related to solder paste or flux performance are classified as Materials Issues. These include poor transfer efficiency on standard apertures, insufficient wetting (fluxing) capacity, low oxidation barrier and low activity. The key to over coming head-in-pillow defects is to get each component sphere to contact, and stay in contact, with the soldering material, mainly the solder paste. If the solder paste itself has poor or inconsistent transfer efficiency, then how do we know that there is even going to be contact between the sphere and the paste? Low area ratios can account for lot of the transfer issues, especially if the stencils are not electro-polished or Electro-formed (e-fab), but with that said, you must match the material set to the process and stencil design. The solder paste can cover a lot of overlap. The second half of the solder paste equation is the fluxing action. There are three parts to this; activation, oxidation barrier and stencil / tack life. High activation is an obvious choice because this is the working part of the flux, which removes the oxides from the solder and the spheres. Oxidation barriers, such as a higher rosin content of the paste's flux, are useful because it will protect the alloy from forming new oxide, which means there's more activation for the component's oxide. Also, it usually adds tack, which is a huge benefit. Because if the paste stays tacky, and the package does warp, the paste will stretch to provide a continuum, so the solder and component will be a single alloy mass upon reflow. There are artificial ways to add an oxidation barrier and additional activation, such as nitrogen reflow or a flux / paste dipping process. Nitrogen reflow PREVENTS the formation of additional oxides during the reflow process, but does not REMOVE oxides and hydroxides already formed on the components. Flux or paste dipping are viable options because this adds activation directly on the component, rather than leaving it to chance on the board. Plus, this flux or paste can be used for rework on the back-end as well. Of course, Material solutions can overcome both the Supply and Process Issues. More information may be found at Online Help: Indium Knowledge Base.

Head-in-pillow defects caused by on-line processing issues are categorized under Process Issues. These include printing, placement and reflow. Printing issues not directly related to the properties of the solder paste are poor registration, imperfect or improper printer setup and poor stencil design. Poor registration leads to printing off-pad or pump-out, and is only part of the printer setup. Printing too fast or too slow will alter the amount of solder that is printed, as well as pulling paste out of the apertures (scooping). Stencil design is probably the most important of the process issues, as a bad stencil design can lead to insufficient solder deposits, which can cause the component to not even touch the paste as well as not having enough flux to overcome the oxide on the sphere or in the paste. Area ratio plays huge role, as well as transfer efficiency. Placement is another danger zone, as offset or off-plane placement can affect head-in-pillow. As well as the placement pressure and down stop, which if the component doesn't sit far enough into the paste, and just floats, then not all the spheres may be touching the paste. The majority of the head-in-pillow comes from the reflow process. This is where the warping of the component actually lifts one edge, opposite edges ("Pringle effect" or "potato chip") or even the corners or center spheres. This is why it is important to read the component manufacturer's recommendations, so the reflow temperature doesn't exceed the maximum processing limitations. This was brought to light to us as a OEM called us and said that they had seen a reduction in head-in-pillow because when they switched pastes, we recommended a lower peak because they were running really hot, and we reviewed the components' ratings, and they had been running too hot for many years! Another issue in reflow is flux exhaustion, where the flux loses its activation because the reflow profile is too long. And I've seen them as long as 15 minutes! Seriously, process issues are where the majority of the head-in-pillow defects are caused, but can be minimized through careful process setup. More information may be found at Online Help: Indium Knowledge Base.

Head-in-pillow defects caused by everything before the components go on-line can be grouped into Supply Issues. Some specific issues within this group are oxidation, hydroxidation and silver segregation. Oxidation is the most known. This is where during the manufacture, packaging, shipment or storage of the bumped components, these bumps have formed a hard-to-solder oxide coating on their surface. This is wrongly called "Black Ball" because the surface oxidation coating is not always dark, and on the flip side, dark sphere are almost always not hard-to-solder. Poor air introduced into the manufacturing, un-sealed packaging or in the storage of the components contributes to this surface oxidation. Hydroxidation, a little less well known, is when a surface hydroxide is formed, but is usually limited to the manufacturing process as it is commonly caused by the molten spheres exposed to higher humidity. Hydroxidation is extremely hard-to-solder. Another issue that we have seen at customers is called "silver segregation". At this particular customer, the head-in-pillow was such a problem, that the components were sent for analysis, and the soldering surface of the sphere was found to be 36% silver. The actual causes of the silver segregation have still a mystery to me, but my thoughts are leaning towards the manufacturing of the sphere, particularly the cooling rate. The attached picture is actually of that issue, where a "silver tail" has formed on the sphere, much like a tin whisker. Supply type head-in-pillow defects are something that we look for when addressing customers, but are usually outside Indium's sphere of influence. (Pun intended, ha!) More information may be found at Online Help: Indium Knowledge Base.

I know it's been a while since my last post, but we've been so busy. In fact, one such incident was yesterday. One of our VP's came to me and asked me to do a 10-15 slide presentation on head-in-pillow defects (Also called "Hidden Pillow" or Ball-in-Cup (BIC) defects). Sure, no problem. So, last afternoon, we're on a conference call with a MAJOR OEM, and he announces "Here to speak about Head-in-pillow, is our Application Engineer, Mario..." Let's talk about SURPRISE. Well, my torture is your benefit... Head-in-pillow is the incomplete wetting of the entire solder joint or a Ball-Grid Array (BGA) or Chip-Scale Package (CSP), or even a Package-On-Package (PoP). From cross-sections, it actually looks like a head has pressed into a soft pillow. Two issues, poor wetting and component warpage cause head-in-pillow defects. They both look the same, but you can identify them because random head-in-pillow defects are from poor wetting and edge or center defects are from warpage. I have separated all the head-in-pillow defect types into three areas. These are Supply Issues, Process Issues and Material Issues. They can be defined like this: Supply issues are everything before you put them on the line. This includes oxidation, or any other oxidation/hydroxide effects. Process Issues are everything that is on-line. Including printing, placement and reflow. Material Issues is everything that has to do with the soldering itself, such as wetting capacity or flux exhaustion. I will go in depth with each one of these issues in further posts. More information may be found at Online Help: Indium Knowledge Base.

It seems that the topic of Halogen-Free Solder Paste has grown by another leap and bound. The big confusion is what are the definitions of Halogen-Free Solder Paste. I mean, who decides what techniques to use for the determination of Halogen-Free Solder Paste. The different testing and procedures come from many sources, including the IPC and individual Original Equipment Manufacturers (OEM's). So, lets take a few minutes and discuss not the techniques used for testing Halogen-Free, but the procedures and reporting specifications. For procedure references, please check original post, Halogen-Free Solder Paste. The first specification that comes my mind, is the IPC's "J-Standards" or J-STD. The section of the J-STD that covers the halogen and halide specifications is J-STD-004. The current revision "a" states that the halogen and halide content is tested on the reflowed solder paste, by Ion Chromatography. Here is the IPC's test method IPC-TM-650 2.3.28.1. As we have discussed in the past, Ion Chromatography is good for testing ionic halides, but may miss the covalent halogens, and of course any volatiles that contain halogens and halides. It is by this method, that some solder paste that are indeed halogen containing, can still advertise themselves as halide-free per J-STD-004. I'm not going into details about the procedure, but it involves a molten solder pot, a Petri dish, and a plastic bag. Sound very scientific to you? Me neither; more like a 9th grade science fair project. By the way, the IPC's definition of Halide-Free is <0.05% (500ppm) total halides. No mention of halogen-free. Another specification that is preferred by some OEM's states that a test board be printed with solder paste, then reflowed and tested by Ion Chromatography. At least this is board-level testing, but the definition of halogen-free versus halide-free remains, as well as their definition of halide-free, <0.09% (900ppm). I have personally tested pastes to this specification, and on one test, knowing that there is >0.5% (5000ppm) total halogens in the sample of the raw flux, have it come back on this test as <0.001% (10ppm). Knowing this is not bad, if only based on the fact that the halide-containing activators may have burned away. Lets get into the big-ticket topic; "no intentionally added halogens". One of the only test methods of determining total halogen content is Parr Oxygen Bomb, then ion chromatography of the ash, on the raw solder paste or flux. This will give you both the halogen and halide content of the flux vehicle. It is no secret that some OEM's looking into truly halogen-free solder pastes, after testing that has showed that their "halide-free" per J-STD-004) pastes that they are using now, is failing due to corrosion or dendritic growth. The statement "no intentionally added halogens" sums it all up, where in reality, only the naturally halogen-containing compounds would be detected, such as the rosin. Even on the post-reflow flux residue, ion chromatography may still miss the covalently bonded halogens, but not oxygen bomb testing. More information may be found at our Online Help: Indium Knowledge Base.

For the second half of Solder Paste Transfer Efficiency, I would like to talk about tolerances and the Rule of 10% Standard Deviation (σ). From part one, we have a target height of 0.005" (5 mils), which is the stencil thickness. Therefore we assume for this application 5 mils is 100%. We can now calculate the Capability Indices of the for the various ultra-fine pitch components using the Standard Deviation of the ratios. In the picture to the right, we have reasonable print tolerances, which are plus or minus one-half of the 5 mil thickness; or 2.5-7.5 mils. Using the Rule of 10%, the tolerance for 10% standard deviation could be used to characterize the solder paste prints (until further process measurement shows otherwise). So lets calculate the Upper and Lower Spec Limits (USL & LSL): LSL = (Ratio) x (Nominal Height) = (50%) x (0.005") = 0.0025" USL = (Ratio) x (Nominal Height) = (150%) x (0.005") = 0.0075" If the LSL is 0.0025" and USL is .0075", then we can calculate the upper and lower Capability Indices, for 6 Standard Deviations (6 sigma): Cpu = (Height % - 50%) / (3 x Std. Dev.) = (100-50) / (3 x 10) = 50 / 30 = 1.7 Cpl = (150% - Height %) / (3 x Std. Dev.) = (150-100) / (3 x 10) = 50 / 30 = 1.7 Cpk = (Cpu + Cpl) / 2 = 1.7 As far as I'm concerned a Cpk of 1.7 is acceptable, but the measured average height will indicate whether we need to make the tolerances higher or lower. A wider tolerance will give you a higher Cpk, but again, a Cpk just below 2.0 will be easier to monitor. It is common knowledge that no matter what you use for your Cpk (as long as its tracked), it will go down when your process is losing control and up when it gets better (or your measurement system has been compromised). To maximize the Cpk; a good stencil printer cleaning, new squeegee blades and better board support will make marked improvements. Starting with these features not only takes a lot of the variation out of the process, but will give you a better baseline. Again, thanks to Chris Anglin for his help on this posting. More information may be found at our Online Help: Indium Knowledge Base or Chris Anglin.

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.

I just returned from a business trip and encountered a common problem, solder paste squeegee hang up. But like nearly all of the cases I've seen, it was not a paste issue, but an operator training issue. It was on the second and third shifts, which are usually short staffed. It was the operators' believed it was a time saving save time idea to add the entire tube of solder paste to the stencil, and they wouldn't have to return to the printer for several hours. All 600g of paste were added, and as soon as the solder paste touched to the bottom of the squeegee blade holder, it was stuck. Instant insufficients!

Lets look at it from another point of view. Solder typically has 30-40g of "tack" per 5.5mm². Since the underside of a typical 250mm long blade holder is 3125mm². You can do the math!

Of course there are other factors to cause solder paste squeegee hang-up, such as too little paste and exceptionally dry paste, but those occurrences take a far backseat to too much paste.

Lets look at some ways to prevent this. First, training is the key! Since the vast majority of soldering issues can be traced back to printing, this is where the operators should be the most diligent. Every engineer has their own set of guidelines that they use for setting up the "perfect" production line, and mine for adding solder paste to the printer is shooting for a single bead of solder paste, ½" to ¾" wide spanning from one inch off the left side of the stencil design, to one inch off the right, added every 45 minutes to one hour. The easiest way to remind the operators is use a timer.

Another way to prevent solder paste hang-up is to increase the print speed. During the printing process, solder paste is shear thinned, effectively lowering the viscosity of the solder paste to liquid. It is this drop in viscosity that alloys the solder paste to "fall" into the stencil aperture opening just as much as the squeegee blade pushes it in. As a result, the solder paste has a "recovery time", where it will return to a thick consistency. As the viscosity falls, so does its tackiness. If you can print quickly, then lift the squeegee blades before the solidification of the solder paste, it will flow off easily. Usually, 50mm per second (2"/s) is fast enough to drop the viscosity of the solder paste enough to eliminate squeegee hang-up. As you increase the print speed, you may need to lower the pressure, as increasing the print speed and sheer thinning the paste more, more paste will flow in to the apertures.

More information may be found at our Online Help: Indium Knowledge Base or Eliminating Solder Paste Squeegee Hang-Up Application Note.

Soldering to aluminum is the opposite of soldering to gold; both difficult and simple to solder to do. Whereas soldering to gold can use almost any flux and almost any solder (depending on the thickness of the gold), soldering to aluminum has a much narrower selection for both the flux and the solder types. I don't know if this makes it harder, but having a limited selection sure sounds easier in my book.

There is a very specific type of flux that is needed when soldering to aluminum. Said flux must contain Zinc Chloride, which is the specific activator that effectively etches the aluminum and allows the formation of the intermetallics needed for a strong solder joint.

Also the opposite to gold soldering, aluminum soldering works best with a high Tin content alloy, such as Indalloy 121 (Ind121; 96.5Sn 3.5Ag, 221°C eutectic) or any of the "SAC alloys" (Tin-Silver-Copper or SnAgCu) like Ind256 (96.5Sn 3.0Ag 0.5Cu, 220°C liquidus).

Just as soldering to nickel, soldering to aluminum with a high Tin content alloy takes time to form a good Tin-Aluminum intermetallic. The longer the Time Above Liquidus (TAL) and higher peak temperature, the stronger the Tin-Aluminum intermetallic should be, hence the stronger the solder joint. A quick cool down also aids the solder joint strength by having a tighter grain structure in the final solidified joint.

Of course, aluminum soldering is not limited to electronics, and the procedure for soldering to aluminum may be used for other application as well. In fact, we normally do not recommend the use of Zinc-Chloride containing fluxes in electronics soldering, because they are very corrosive the residues are difficult to control on the final assembly.

More information may be found at our Online Help: Indium Knowledge Base or Soldering to Aluminum Application Note.

Soldering to gold may be both the easiest and most difficult type of soldering in our little SMT world. Gold; soft, warm, glowing, and insanely expensive right now is incredibly easy to solder to. Gold's natural ability to prevent the formation of oxidation makes it an ideal solderable surface, especially when matched with a strong metal such as nickel, but there are some drawbacks…

The first feature that makes gold so easily solderable is that it does not oxidize. There is no worry of surface contamination that is the bane of SMT engineers the world over. Whether soldering to Electroless Nickel / Immersion Gold (ENIG) or gold cable seals, it should remain solderable for years. Gold has remained a mainstay of the surface mount technology (SMT) world as the thin-film layer that keeps nickel solderable. Which is where the second feature of gold comes to light; that it is readily dissolvable in other metals. From my quick calculations, standard Tin-Lead eutectic solder dissolves gold at 35 micro inches (35μ") per second at 200°C. For comparison, molten tin dissolves gold faster, and Tin-Lead solder solders to nickel about 60 times slower. This is why when soldering to an ENIG circuit board, it is important to form the strong Tin-Nickel intermetallics with a relatively longer Time Above Liquidus (TAL) and higher Peak Temperature when reflowing when compared to copper.

Of course, there are some worries… I believe the biggest worry is the amount of gold that ends up in the final solder joint. If soldering to thick-film gold using a tin-based alloy, right around 10% gold in the final solder joint is where the brittle Gold-Tin intermetallics start to become a hindrance to reliability. Even worse, the tin may continue to dissolve the gold, even at room temperature, to form Kirkendall voids. Using thin-film gold may help in this area. Thin-film or "flash" of gold is typically 3-5μ". But, I have seen "thin-film" as thick as 15μ", which based on experience, I still consider a safe level of gold to be dissolved into the solder joint. On the other side, 50μ" and above is what I consider thick-film, where tin-based alloys are strictly prohibited. The gray area in between of 15-50μ" is when I normally recommend using Accelerated Life Testing (ALT) to determine whether issues will develop with the stated life-span of the product.

More information may be found at our Online Help: Indium Knowledge Base or Soldering to Gold Application Note.

We received a call today from a customer that had tested the halide content of their board and reflowed solder paste, without components, using Ion Chromatography. This was an attempt to get a "baseline" reading for halogen and halide content. Well, they got a suspicious reading and we got the urgent phone call. Halogen-free solder paste is by its very nature also halide-free solder paste, but not necessarily the opposite. Halogens are found everywhere; in some solder pastes, some fluxes, resins and solder masks, and even some surface finishes. A lot of it has to do with temperature and the curing of the boards. This shows up when testing for halogens. Try running a bare un-reflowed board, without solder along with a bare, reflowed board. Sometimes the differences are really concerning. Also, a little cleanliness goes a long way. Repeat the same test with boards that have been cleaned per the IPC recommendations for Surface Insulation Resistance (SIR), try the test again, cleaned versus un-cleaned test boards. Add in the solder paste factor, contamination from fingers, soiled gloves and even alcohol-based marking pens. Please let me know your results, Mario Scalzo. More information may be found at our Online Help: Indium Knowledge Base.

There has been a increase in the number of requests for reflow profiles for lead-free solders. This is a change from last year, where a majority of the requests had been for a low voiding reflow profile. Now, customers are just asking for reflow profiles, in general. Also, a majority of the question for these reflow profiles are coming from companies that are considered RoHS exempt. This trend over the past 2 months tells me that these customers, RoHS exempt companies for tele-communications, medical and military contractors, are now starting to look into lead-free solders.

There is one more surprising thing. Not all of the lead-free solders are the usual Tin / Silver / Copper (Sn/Ag/Cu or SAC). There are many requests for a low temperature lead-free solder, as well.

More information may be found at our Online Help: Indium Knowledge Base.

For my first entry, I'd like to say a word about the differences between Halogen-Free and Halide-Free, for both fluxes and solder pastes. The halogens are the group 7a elements on the periodic table. Common halogens are Chlorine and Iodine, both of which are used in our daily lives. The others are Fluorine, Bromine and Astatine. (Astatine, per Wikipedia is a halogen, but is a highly radioactive element has a half-life of 125 nanoseconds to 8.3 hours, depending on the isotope. It is the rarest naturally occurring element with less than one ounce available in the earth's crust at any given time. I don't think you will find this halide or halogen in solder paste paste.) What makes halogens so special is that in their ionic forms (halides), they are all missing a single electron, which it wants REALLY BAD. Making them very active, chemically. But, they form 2 types of compounds, covalently-bonded halogen and ionic-bonded halides. So, solder pastes that my be described as "halide-free" may not be completely devoid of Fluorine, Chlorine, Bromine, and Iodine. But, only free of ionically bonded halides. The other side of the coin, is Halogen-Free solder pastes and flux. These are "truly" halide-free, and halogen-free. Which means, that under chemical testing, there is NO Fluorine, Chlorine, Bromine, Iodine or Astatine. Of course there are different ways to test for each, halide-free and halogen-free solder paste and flux, which include Titration, Ion Chromatography, and Oxygen (Parr) Bomb/Ion Chromatography. Here are some details about each (Thank you to Indium's Advanced Assembly Materials Product Manager, Tim Jensen for the details): Titration: Titration can be an effective method for assessing ionic halides within a flux. However, if a solder paste manufacturer decides to use halide activators, they will typically use a covalently bonded halides (which are better for SIR and long term reliability) for no cleans which are not detected through titration. Therefore, a statement such as "halide-free by titration" simply means that there are no ionic halides. In addition, there are many chemicals used in fluxes that can interfere with the test to cause a false positive (i.e. appear as if they are a halide). Ion Chromatography (IC): This is the currently recommended test method used by the IPC J-STD-004. However, it suffers similar challenges to titration in that it is only good at detecting ionic halides. Ion chromatography is also prone to interference, which can cause false positives. Oxygen Bomb / IC: This test method actually burns off all of the organics and Hydrogen parts of the flux. The halides remain in the ash. This ash is dissolved and run through ion chromatography. This method breaks the covalent bonds and minimizes any potential interference to allow IC to give you a true halide reading. This is Indium Corporation's standard method for testing halide-free and halogen-free content. More information may be found at our Online Help: Indium Knowledge Base.