Indium Corporation Blogs

We’ve heard about the solder paste “graping” defect, but the same oxidation challenge occurs in other solder forms as well, such as solder washer preforms used to attach small connectors. 

Solder graping is a defect that arose as a result of small solder paste deposits and higher (than were used in the past) reflow temperatures.  The peak reflow temperature for SnPb alloys were just over 200°C, but. with SnAgCu alloys, that peak is as high as 260°C.  New solder pastes have been developed that address this defect by utilizing flux chemistries that function as an oxidation barrier.

But what about other solder forms? 

I was recently working with a customer who was evaluating solder preforms for a connector application.  These solder washers (outside diameter of 0.025”), like the small solder paste deposits, had a large surface area with a tendency to oxidize on their heated journey to melting.  These washers were made of SnAgCu, so the peak temperature required of them was no different than it would be for Pb-Free solder paste.

I found that the same 1°C ramp that causes graping of solder paste can do so for preforms as well.  The difference though, is that the non-molten portion of solder takes the shape of its un-reflowed form, that of a small washer in this case, as opposed to the “grapes” of aggregated solder powder.  

Fortunately, the issue can be addressed in 3 ways, and I encouraged my customer in this case to process their assembly using the combination:

1. Adjust the reflow profile. Ed Briggs' recommendations work well here.  Further, use a fast ramp to peak. I tested the reflow on a hot plate set to 250°C; this improved the wetting.  Preforms are unique from solder paste in that they are solid metal, as opposed to a mixture of metal and flux.  They do not benefit from the same heating controls that solder paste requires. 
2. Reflow in nitrogen.  By purging out the environmental oxygen during the reflow process, the solder preform will not oxidize.
3. Apply a tacky RMA flux, such as Tac007.  RMA fluxes provide stronger oxidation barriers than other non-rosin flux types. 

In the second image here, it is evident the improvement these changes made in terms of the spread and coalescence of the solder preforms. Note that the addition of tacky flux left an amber-colored no-clean residue, however, this can easily be washed away using a mild solvent.

Any questions?  AskUs@indium.com!

Eric Bastow recently wrote about using our Indalloy Flux #2 for soldering to Nitinol.  He did many tests and wrote an Application Note called Soldering to Nitinol.

Fort Wayne Metals, a leading supplier of medical wire (including Nitinol) also did a test on various fluxes as they relate to break load (maximum load before the joint breaks.

The fluxes tested included:

• Indalloy Flux #2 and Flux #3
• Indalloy Flux #5RMA; #5R; #5RA
• Indalloy Flux #4R
• Flux #400 (no longer commercially available)

The #5 series and the #4R were found to not be strong enough to clean off the tenacious oxides formed on Nitinol. Therefore, they didn't enable the solder to wet the surface properly.

Flux #2 and Flux#3 gave the best results (of the fluxes tested for break load) since they removed more of the oxides and allowed for a stronger solder bond.

Want to know more about soldering to this important medical material?  You can contact Eric Bastow directly at ebastow@indium.com or email us at medical@indium.com. 

Carol Gowans

cgowans@indium.com

 

Solder paste is comprised of powdered solder alloy suspended in a flux vehicle. There is a group of flux ingredients that is generically identified as "activators". It is the activators whose primary function is to remove oxides not only on the surfaces that are being soldered but any oxides that may be present on the solder powder, itself. These activators are generally "activated" by heat. The flux chemist knowingly selects activators that are relatively dormant at room temperature but become very active at soldering temperatures. Their level of activity is often directly related to temperature.

Given that the flux is in direct contact with the solder powder, this allows for the flux activators to interact with the solder powder even while the solder paste sits on the shelf. Those activators can begin to "react" with the powder, and, given enough time, can "clean" the powder surface to the point where the solder particles will actually "weld" together. So, now instead of the paste containing free-flowing powder, it contains clumps of welded together solder particles. Those clumps often increase the viscosity and can clog stencil apertures and dispensing needles. For these reasons, the paste manufacturer will require refrigerated storage of the paste in order to realize the optimum shelf life.


As a rule water-washable solder pastes often include activators that are more aggressive than the activators found in no-clean and RMA type solder pastes. This is because water-washable flux residues are designed to be washed off. So, there is no concern about the flux causing corrosion over the life of the product. On the other hand, a no-clean flux generally has milder activators, because the flux residue may remain on the device indefinitely; where corrosion would be detrimental to the performance and life of the device. As a result, no-clean type solder pastes typically have a longer shelf life and are more tolerant to higher storage temperatures than water-soluble/washable solder pastes.

A solder paste typically has a shelf life of 6 months when refrigerated. One may ask what happens if the paste has been refrigerated for 2 months, then thawed to room temperature, remains at room temperature for 12 hours and is then re-refrigerated....Will it still have a 6 month shelf life? That is a very difficult question to answer. The same situation could arise with a perishable food item that requires refrigeration, such as milk. Lets say that one buys a container of milk at the store and it has an expiration date that is 5 days away. After having it home, properly refrigerated, for 2 days, one of the kids leaves the milk on the counter for 3 hours before anybody notices it and puts it back in the refrigerator. Can one expect the milk to stay good for the remaining 3 days? What about if it is left out of the refrigerator for 1 hour? or 5 hours? You can see how difficult the questions become to answer. What is the impact if a solder paste is exposed to elevated temperatures when it is 3 days old or 3 weeks old or 3 months old or with 3 days left to expiration????? The answer is not fully known. It is impossible for the solder paste manufacturer to study every possible scenario for its impact on the shelf life of the paste.

The best and only sure approach is to refrigerate solder paste immediately upon receipt and only thaw when needed, in amounts that will be completely consumed. Avoid thawing and re-refrigerating pastes as much as possible, in order to take advantage of the full shelf life.

The particle (mesh) size of the solder powder can also impact shelf life. As the powder size decreases, the surface area per volume or mass of powder drastically increases. More powder surface area means more area for the flux to react with, and more surface area for welding to occur. Therefore, a type 3 solder paste that has a shelf life of 6 months may not provide a full 6 months of shelf life with a type 6 solder powder, all other things being equal.

For the most part, solder paste manufacturers are conservative in assigning shelf life. It is highly unlikely that a properly stored solder paste's performance is going to collapse 1 day after the expiration date. In fact, depending on the paste, it may still be good for months beyond the expiration date.

How does one know if their solder paste is still usable? This can be determined rather easily. As mentioned earlier, one artifact of a degrading paste is a rise in viscosity. So one can perform a simple printing or dispensing test to see if it still performs adequately in that regard. Another aspect that often suffers is coalescence. As the flux degrades it looses its ability to adequately remove oxides on the solder powder. In order to gauge the degradation, it is best to put a small amount of paste on a non-wettable substrate, like a piece of ceramic. Reflow the paste and see how well it coalesces. If coalescence is good, the solder paste will reflow into a ball, surrounded by a flux pool that is relatively free of uncoalesced solder particles. If the paste has significantly degraded, the paste will not coalesce well and there will be a significant amount of uncoalesced solder particles in the flux pool.

Please see this IPC test method for determining the coalescent properties of a solder paste.

This post was prompted by a Korean customer, who this month asked what the limitations are for aqueous (water-based) cleaning for fluxes used in copper-pillar flip-chip applications. How low can the pitch get before aqueous cleaning becomes unfeasible / impractical?: 40microns? 20microns? 5microns?

Good question. I don’t have a definitive answer, but I do now have an industry consensus that seems to be consistent. I’ll try to answer the query in a little while, but firstly, you’ve got to ask, "What is 'aqueous' cleaning?" Is it:

-        Deionized (DI) water?

-        Water plus a surfactant?

-        Water with a saponifier?

-        Mixed phase (oil /aqueous phase)?

-        All of the above?

DI water alone may be an ideal cleaning fluid from both a reliability and a “green” perspective, but its poor wetting onto even mildly hydrophobic surfaces, high viscosity compared to many low molecular weight organic solvents, and poor solvency for many molecules used in fluxes (resins are a good example) make it a poor choice as a pure solvent. 

It is also important to distinguish between saponifiers and surfactants:

- Surfactants: Usually a nonionic surfactant: most commonly a hydrophilic polyethylene glycol moiety with a hydrophobic carbon chain attached
-  Saponifiers: Usually basic chemistry that chemically reacts with high molecular weight acids in RMA and no-clean fluxes

Adding surface-active agents (surfactants) to water will help it to wet to less hydrophilic surfaces, and so help it move into confined spaces. The advantage of a nonionic surfactant over an ionic one is two-fold; a) there are no potentially ionic residues to cause electrical problems if improperly rinsed off b) the optimum surface-wetting enhancement (so-called “CMC”) is at a much lower surfactant concentration, but note that this also makes it more difficult to rinse off completely.

The general structure of the most common nonionic surfactant is:

               CxHy-(OC₂H₄)_n-OH

On the other hand, the way the saponifier works is a simple acid/base reaction, usually using amine chemistry:

               R’R”NR’” + RCO₂H ---> R’R”NR’”H+ + RCO₂-

The purpose of the basic saponifier is to massively increase the solubility of resins in aqueous solutions, while the surfactant is simply a means of enhancing wetting onto hydrophobic surfaces. Where it gets complicated, is when you realize that the saponified resin is now also capable of acting as a surfactant.

The reason I bring this up is that for smaller pitch flip-chip applications, we are now predominantly seeing the use of small amounts of nonionic surfactants with deionized water, along with other tricks to get the aqueous solution under the chip.

I think I’ll stop there. More next time. Meanwhile, feel free to comment or to email me on this.

Cheers! Andy

After a long and involved conversation with colleagues regarding attaching heat pipes using a low temperature solder, such as Indalloy 290 (97In 3Ag; 143 C) or Ind281 (58Bi 42Sn; 139 C) , here are a few ideas that we've come up with to make your decision easier.

You can use either Ind290 (97In 3Ag) or Ind281 (58Bi 42Sn) in your process for bonding the copper heat pipes.  But, if you are going to solder to bare copper, we do not recommend the use of an In-containing alloy.

HARD:
When soldering to hard-to-wet materials like aluminum, you can use the Indalloy Flux #3 to bond directly to an aluminum heat-sink. The very strong Flux #3 would have to be removed after soldering, but would provide the best solder joint.

EASIER:
If you use bare Ni on the surfaces you can use either alloy. More good news, you can also use a less aggressive flux. In this case, consider using a flux such as 4-OA, which is easier to use than the Flux #3, but would still have to be cleaned after soldering.

EVEN EASIER:
If you plate a solderable surface over the Ni, such as Ag, Sn, or Au, then you can use an even less aggressive flux than that, such as a No-Clean type, like 5-RMA.

In the end, Indalloy 281 (58Bi 42Sn) was chosen for the bare copper heat pipe. 
 

Should you have any further questions, please don’t hesitate to ask.  Click LEARN MORE - EMAIL AUTHOR below. Or feel free to make a comment - below.

The second main ingredient in solder paste is the flux (vehicle). Flux is a very complex group of chemicals/materials that must be able to do a number of things, some of which must happen simultaneously (a partial list is below). This requires the knowledge of experienced chemists and material scientists.

 

1)     The flux must not react with the powdered solder while in storage (shelf life). This is aggravated by the wide variety of solder alloys available in paste form. Each alloy family typically requires it own unique flux formulation.

2)     The flux must effectively remove any surface oxides present on the solder powder itself and mating surfaces prior to and during the melting of the solder.

3)     It must effectively prevent re-oxidation of the solder powder and mating surfaces at the elevated temperatures associated with reflow soldering, especially when performed in an air environment (air is ~21% oxygen).

4)     The performance of the flux must be unaffected by a wide range of temperature and humidity conditions.

5)     If the flux is “no-clean”, the residue must be non-corrosive and non-conductive per J-STD-004.

6)     If the flux is RMA, the residue must be easily cleaned with commercially available chemicals.

7)     If the flux is water soluble/washable, the residue must clean thoroughly with heated and pressurized deionized water.

8)     In a stencil printing application the flux must adequately fill and release from the apertures of a wide variety of sizes, shapes and stencil types, not stick to the squeegee, not dry out too quickly on the stencil, retain a brick like shape both at room temperature and elevated reflow soldering temperatures (minimize slumping), provide sufficient tack to hold components in place, not spatter during soldering (boiling of flux solvents), not outgas excessively (voiding)  and provide effective wetting of the solder to a wide variety of board metallizations, component lead platings and package bumps.

9)     In dispensing applications, the paste must dispense smoothly and consistently through a variety of needles sizes either through manual application or a variety of dispensing equipment types and technologies without clogging (and do many of the things listed in item 8).

10) Provide a cosmetically appealing solder joint and flux residue (if “no-clean”).

Eric rounds out the week with this post comparing the 2 major classifications of solder paste:

 

(If you missed the first 2, click here)

“Water cleanable solder pastes have advantages and disadvantages when compared to their RMA and “No-Clean” kin.

 

1)Higher Activity – Knowing that the flux residues will be removed from the assembly, the solder paste manufacturer has a certain amount of liberty to include aggressive ingredients that are more effective at removing oxides from the surfaces that are being soldered. A “No-Clean” paste does not have this luxury because its residues are designed to be left on the assembly, without cleaning. Therefore the ingredients in a “No-Clean” solder paste must be relatively benign. “Dirty” (heavily oxidized) surfaces require a higher level of flux activity as compared to fresh, clean, relatively oxide-free surfaces.

Even among water soluble pastes there are different levels of activity. An industry standard used to classify solder pastes, IPC J-STD-004, breaks them down into these categories: ORL0, ORL1, ORM0, ORM1, ORH0 and ORH1. The “OR” at the beginning of this four character code identifies it as a water cleanable flux. Literally, it stands for “ORganic acid”; organic acids being a family of chemicals that can be included in fluxes to help them remove oxides from the surfaces being soldered. The third character in the four character code, L (low), M (moderate), or H (high), refers to level of flux activity, specifically the level of corrosiveness of the flux as observed when left on a copper metallized glass slide. Generally, the more corrosive the flux, the more effective it is at removing oxides. The last character, which is either the number 0 or 1, indicates the presence and/or level of halides in the solder paste flux. “0” indicates a halide free or low halide flux; whereas “1” indicates the “presence” of halides. Halides typically improve the fluxes ability to remove oxides. There is a legacy fear of halides in solder paste fluxes. The fear relates to a belief that a halide containing flux residue, left on the board or not entirely removed with cleaning, is more prone to corrosion. However, with adequate cleaning and removal of a solder paste flux residue, there should be little, if any, halides remaining on the assembly.

 

2) Board Cleanliness – Due to the simple fact that the solder paste flux residue will be removed with cleaning adds to the cosmetically “clean” appearance of the board as compared to solder pastes where the flux residue is left on the assembly. It also may improve the reliability of the assembly because flux residue corrosion and leakage current concerns are virtually eliminated with proper cleaning.

 

3) Moisture “Sensitive” – One of the most distinct disadvantages of water cleanable solder pastes is that they are sensitive to the level of humidity in the working environment. They can “extract” moisture from room air causing them to become soupy in humid environments. Water cleanable solder pastes are best suited to situations where the humidity is controlled and exposure to air is minimized.

 

4) Shelf Life – The shelf life of water cleanable solder pastes is often less than comparable “No Clean” or RMA formulations. As mentioned earlier, water cleanable solder paste fluxes are typically more active than the other varieties. In solder paste, powdered metal (solder alloy) is suspended in the flux. That means that the flux is direct contact with the powdered metal. The “active” ingredients in the flux can react with the powdered metal even while the product is sitting on the shelf in a sealed container at room temperature. Therefore, refrigerated storage of water cleanable solder pastes is necessary to insure the optimum shelf life and condition of the solder paste.” –Eric Bastow 

 

Don’t’ expect a simple answer – that depends on your application.  

 

If you are looking for a solder paste, NC-SMQ80 is the flux vehicle of choice.  For spray applications 5RMA has a strong following on Au substrate applications.  For niche ball attach or specialty processes needing a tacky flux, WS-363 delivers the best wetting on ENIG (see attached chart), but WS-575 has a more useable rheology for pin-transfer.  PoP Flux 030B is your ticket in no-clean applications at these low temperatures.

 

Did I mention all these options are Halogen-Free?

 

For our second guest blogger i would like to introduce Dr. Harald Wack, President of Zestron.  Zestron is manufacturer of flux residue cleaners and removers for our industry, and several other cleaning products for many other non-related industries.  Dr. Wack, take it away...

 

Should you have any question, please don't hesitate to contact me.  More information may be found at our Indium Knowledge Base (IKB) and Zestron.