Mario Scalzo’s Tech Support Blog
Recent Entries
-
Issues with using Indium for Wafer Bumping
Lets talk about some issues…
The first thing that I am worried about is the use of a small particle size of Indium-containing alloys. Indium is self-passivating, and will clump and cold weld to itself, even when stored as powder. For this reason we look at each individual case separately. Normally, we do not recommend the use of Indium alloys for solder powders that are smaller than Type 4 (20-38μm). For small aperture sizes, you would need a Type 5 (20-25μm). The smaller the powder size, the larger the surface area, so as the indium-containing powders get smaller, the more tendency to cold-weld in the packaging.Which leads us to my second concern, which is the higher metal percentage in wafer pastes. Usually, in order to print through the smaller apertures (and lower area ratios) for wafer bumping, the solder paste has a higher metal percentage. For these wafer pastes, the metal percents are usually >92%. Which makes them very prone to cold-welding.For example, the area ratio for an aperture opening of 140μm with a 90μm thick stencil is 0.39. Area ratios that are below 0.50 are not recommended.We can physically manufacture the paste, but whether it will be useable when you get it is the problems.More information may be found at IKB: Indium Knowledge Base.
-
Meet the Bloggers!
Indium Corporation’s industry leading electronics assembly bloggers are hosting a Meet the Bloggers session on Wednesday, August 20, 2008 at Indium Corporation’s SMTAI exhibit booth #517 at 11:00am EST.
The technology experts will lead discussions on topics including:
· Halogen-free
· RoHS or Greenpeace-Who’s driving our world?
· Pb-free compliance-What does it mean today?
· REACH
· High Ag vs. low Ag
· Head-in-pillow
· Next generation solder alloys
· Dopants
Indium personnel who will be discussing these topics include Dr. Ronald Lasky, Amanda Hartnett, Tim Jensen, Anny Zhang, and
All attendees are welcome to participate in, or observe, the session. Refreshments will be served.
Indium’s blogs can be seen at http://www.indium.com/blogs
Indium Corporation is a premiere materials supplier to the global electronics assembly, semiconductor fabrication and packaging, solar photovoltaic, and thermal management markets. Founded in 1934, the company offers a broad range of products, services, and technical support focused on advanced materials science. With facilities in the PRC,
For more information about Indium Corporation visit http://www.indium.com or email abrown@indium.com. -
Choosing Powder Size Part 2: Printing
Solder paste for printing follows the same guidelines as solder paste for dispensing. The good news about solder paste for printing is the apertures that are printed through are usually significantly larger than the needles used for dispensing. The BIG difference is that the paste is not as susceptible to air bubbles that would cause skips or clumping that would cause clogging.
Although stencils make a difference in the amount of paste applied, it is the paste itself that makes all the difference. Stencil release, often-called transfer efficiency or TE, can be tracked through a paste measurement system. By feeding the stencil details into the paste measurement system at onset, the system can calculate the theoretical amount of paste that should be deposited, and can create a percentage (efficiency) from measuring the amount of paste that was actually deposited.Transfer efficiency is just now becoming something that we are tracking scientifically (read statistically). Some variables that can affect transfer efficiency are stencil type, atmospheric conditions and the paste itself.For stencils, material makes the most difference. There are 3 types of stencils that we normally come across when visiting customers, they are Laser cut, laser cut with electro-polish, and electro-deposited (or e-fab). Also, the transfer efficiency commonly increases from laser cut, laser cut with polish and e-fab. The manufacturing cost usually increases across the three types, respectively.Room temperature, and sometimes humidity, also affects transfer efficiency as the viscosity usually drops when solder paste is warmer, as well as the paste also becomes less tacky at warmer temperatures. Humidity affect water washable paste in the same ways, so much so that cold slump may be induced.Most of the time, it is the paste itself, and the rosin, thickener or solvent constituents that affect the stencil release of the paste. As mentioned before, it is only in recent years that transfer efficiency is being statistically tracked to the point that the formulation may be tweaked to attain higher numbers.More information may be found at IKB: Indium Knowledge Base.
-
Choosing Powder Size Part 1: Dispensing
Again, there has been a trend in the past few days that shows me that there is something happening. People are looking towards a new application or project, and realizing that they need solder paste and don't know what size powder they need. As an engineer, we have graphs and posters of data on the walls at our desks for easy reference, but i think that there should be more reasoning behind what we recommend other than just cross-referencing.
Dispensing is a good place to start, because solder paste for dispensing is more subject to the process constraints than solder paste for printing. Making a recommendation for solder paste dispensing is fairly straight forward, and mostly (yes, mostly) depends on the size needle that is to be used. Case in point, the Solar Materials Manager came to my desk and explained that a customer was having serious issues dispensing. After some discussion, we found out that the customer was using a 20-gauge (0.023" ID) needle. this itself is nothing out of the ordinary. But, after digging, we found out that they were using a Type 2 (-200+325 mesh or 45-75um) powder and a metal percent of 87%. This would explain why they were seeing clogging and "skips".For future reference, I have attached a great picture of what we use to determine which powder size. In a nutshell, I would try to fit at least 7 spheres of powder across the ID of the needle, if the powder was all on the large side of the specification. For example, for a Type 3 mesh (24-45um), the smallest size diameter needle I would recommend is a 23-gauge (330um). This is because ~7 45um powder spheres would fit in the 330um diameter needle.Some people would ask why not just go to the smallest size powder, then you would not have any issue dispensing through any size needle greater than a 30-gauge. This is a no-no, as there are too many drawbacks to this approach. These include the high cost of the smaller powder sizes and the higher oxide content of the smaller particles, which may cause drying of the paste in the tubes.More information may be available at the IKB: Indium Knowledge Base. -
REFLOW: The secret to a high tensile strength! (Part 4)
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). -
REFLOW: The secret to a high tensile strength! (Part 3)
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.
-
REFLOW: The secret to a high tensile strength! (Part 2)
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.
-
REFLOW: The secret to a high tensile strength! (Part 1)
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. -
Universal Technology Update; Is your process biased?
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). -
BGA Red Dye Penetrant Testing
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.
Want to read more? Browse the archive of past entries.
