Indium Corporation Blogs

There seems to be a growing trend to use a low-Ag or Ag-free solder alloy for Surface Mount Technology (SMT) electronics assembly, similar to what is commonly offered for bar solder, used in wave and selective soldering.

For through-hole performance, the strength and stability come from the entire barrel of solder, whereas it is usually the foot and heel fillets that give SMT solder joints their strength.

Lets talk about the other issue with using a eutectic solder alloy in SMT: tombstoning.  One of the benefits of using the SAC (tin-silver-copper) alloy for SMT and solder paste, is that it has a built-in plastic range, similar to that of Sn62 (62Sn 36Pb 2Ag).  It is this plastic range that prevents tombstoning, and takes into account the inconsistent heating of the solder across the part (which is the sole cause of tombstoning).  Switching to a eutectic alloy eliminates the plastic range and opens the door for tombstoning.

Any powder manufacturing issues, such as the inconsistent distribution of dopants throughout the alloy and powder matrix, takes a back seat to the surface mount reliability concerns. 

There are other alternatives, such as SAC0307 (99Sn 0.3Ag 0.7Cu)… But, with the price of Ag finally coming down, and a long history of SAC usage, we don’t think it’s going to be a major player.

Next time, we'll talk about the manufacturing and costs associated with low-Ag and Ag-free alloys. 

I hope this helps. Contact me with any questions.

The "wave soldering flux deactivation temperature question" arises every few weeks, and the easy answer is that wave solder fluxes are designed to see the temperature of the wave.

Now, in order to pass SIR testing, the test boards with the wave fluxes are sent through the wave pattern up, not in direct contact with the molten alloy. They still see the majority of the heat from the wave.

The “deactivation” temperature varies from process to process because of the differences in board complexity, flux application, and preheat, as it is really a matter of total energy input, rather than a specific temperature.

The potential for reduced SIR is the main issue, not necessarily ECM or corrosion.  With a rosin-free flux, there may be some visual clues, as well. If you see that the flux has dried to a white, powdery residue that may be a sign that it had not seen temperatures.

We get this question mostly from corporations that believe they can use the same flux formulation for wave soldering and hand assembly / rework. Wave fluxes are not designed for hand soldering, and will more than likely cause some sort of downstream issue if used as such.

Feel free to ask me any questions about your process!  From One Engineer To Another!

I just received a customer inquiry regarding a phenomenon that is little studied and even less quantified; “How many times can I reflow a solder alloy before damaging the solder joint?”

As you may already know, each time you bring a solder alloy above its liquidus temperature, it continues to dissolve the metallizations on the substrate to which you are soldering, as well as the metallizations on the leads of the component being attached. With modern processes, a 3-time “excursion” is common, especially with double-sided reflow and rework. Typically, the solder applied with paste is not reflowed again during the wave, because, through the use of pallets or selective soldering, it doesn't get quite hot enough to melt. That said, in such a case, the solder joint may become hot enough to receive some damage. To me, the most interesting thing with crystalline intermetallic layers, is that they don’t need to reach liquidus to form larger crystals. So a temperature excursion close to the liquidus may also increase the crystal structure size.

Another factor is surface metallizations, especially easy-to-solder surfaces such as gold or HASL. With gold, molten Sn/Pb solder at 200°C will dissolve at 35u-inch/s. So, a fine flash layer, such as 3-5u-inch, is gone within the first second, and the actual intermetallic is formed to the underlayment; most commonly nickel (Ni). This is similar with HASL, as the HASL layer is consumed into the solder joint at liquidus, and the intermetallic layer is formed with what is beneath the HASL.

The intermetallic layer will increase with time above liquidus (TAL) and also with temperature, with hotter dissolving more of the surface. This is why there are operating temperature limitations on the final solder joint, such as no more than 90% of the solidus of the alloy, in degrees Kelvin.

Another factor that affects grain structure, besides TAL and peak temperature, is cool-down rate. A faster cool-down rate will form a smaller crystalline grain structure, but keep in mind that a too-fast cool down rate may result in stresses being trapped in the grain structure from the CTE mismatch between the component and the substrate.

It has been our experience that 3 temperature excursions is the accepted limit (by most companies that I work). But, the only recommendation that we can offer is that you try “worst-case” scenarios, and have ALT testing and SEM cross-sections performed on real-world products in which 3, 4, and 5 excursions have taken place. Your particular case may be unique - it is well-worth determining your particular situation.

Tombstoning, simply, is the wetting of one side of a component before the other side, which causes the setting forces of the solder to lift the component like a drawbridge.  Sometimes, it even cause the component to complete stand, like a tombstone.

There are several ways to counteract whatever cause is making the defect occur.  All of them include either getting the component to come to the same temperature at the same time, or allow for flexibility in the melting point of the solder during reflow.

Below are some ideas on how tombstoning can be eliminated.

 

1.   Eliminate Nitrogen reflow - Nitrogen reflow prevents the additional build up of new oxides on surfaces and the solder alloy, and allows more activator to be used for wetting, increasing the wetting force.

2.   Lower the delta-T across the board and component to <10°C -  This allows for more stable temperatures through liquidus, which equalizes the wetting on both sides of the affected components.

3.   Slowing the ramp rate of the components through reflow to ~0.5°C/s - This allows both sides of the component to come to temperature simultaneously.

4.   Introduce an anti-tombstoning alloy, such as the Ind100 (62.6Sn 37Pb 0.4Ag, which has a 4-6°C plastic range.

5.   Increase placement pressure and depth, which uses the tackiness of the paste to hold the component in place.

6.   Proper placement ensures that the component is centered between the pads.

7.   Stencil design ideas, such as home plate or reverse home plate, takes advantage of the alloy’s wettability and uses it to your advantage to solder the part to board, rather than using the wetting of the alloy to lift the part.

8.  Ensuring proper board and Pad design makes sure that there is not solder-robbing, where the solder flows along a trace, and doesn’t leave enough for the component.

 

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.

Image from Metals Handbook", Volume 6 (1983)

When it comes to solidus and liquidus temperatures, things aren't just black and white – (except for the graph shown here). 

Let's start with some terms:

"Solidus" refers to the temperature that an alloy melts at.  "Liquidus" is the temperature that the alloy turns completely liquid.  Solids turn directly into liquids when they are heated, right?  Not exactly.

When formulating alloys, there are usually one or more points in the constituent ratio where the metal will be at a "Eutectic" ratio.  This ratio is usually the lowest melting point for the different combinations of those elements.  For example, if you mix 63%Sn with 37%Pb, it will have a single temperature (183ºC) for both its solidus and liquidus.  If you mix the alloy with a different amount of Sn or Pb, the solidus may remain while the liquidus increases.  The range between these temperatures is often called the "pasty range".  During heating between the solidus and liquidus, most metals act very much like a liquid.  The resulting mixture of liquid and solid material is able to wet to surfaces and form intermetallics, although it is recommended that soldering be done above the liquidus point.

More information on measuring temperatures from Differential Scanning Calorimetry, check out our Application Note on Determining Solidus and Liquidus points.

Team Indium: (LtoR) Mario Scalzo, Pat Ryan, Dana Ebensperger, Bill Manning, Greg Evans, Anita Brown and Ed Gudlauski

I would like to take a moment and talk about something that has nothing to do with Head-in-pillow defects, Halogen-free solder paste or Pb-Free solder reflow.

When I was in High School in 1993, an adorable little girl was abducted less than 3 miles from my home.  This was an outrageous crime in a sleepy little New York town!

To raise awareness of this heinous crime, a group of 7 courageous bicyclists rode to Washington DC to raise awareness and preach children's safety along the way, arriving in DC on May 25th, the first National Missing Children's Day.

12 years later, over 400 riders, like us on Indium's team, ride 100 miles every May to commemorate this ride and to raise funding for the National Center for Missing and Exploited Children, which works hand-in-hand with Law Enforcement to spread knowledge of and retrieval of missing kids.  We will always wear pink and blue in remembrance of that little girl, back in 1993 that was never found.

Our mission is "to make our children safer...one child at a time".  We have helped reunite 3000 of over 4600 missing children in 2008 with their parents and loved ones!

Donations are greatly appreciated and can be given on-line at Active.com/donate/RMFCCNY/MarioScalzo.  More information on our cause and our history can be found at RideForMissingChildren.com.

What would bring a person from outside the SMT world to a website like Indium.com?  Why would a marketing guru have interest in head-in-pillow defects?

Just ask Michael Fitzgerald of BtoB Magazine.

So, besides leaking to the general public about search engine optimization (SEO) and using Google to our advantage, it shows that the trials and tribulations that we fight in our world everyday effects the general public.  We know that the head-in-pillow defect on that last board we built will go into someones MP3 player.  And, as soon as that someone, we'll call him Bob, goes for a jog in the middle of the winter, or sits under the sun at the beach, that head-in-pillow defect will comeback to haunt him, and Bob's MP3 player will die.

So, we thank Michael for awakening the general public on head-in-pillow defects, and even though bob doesn't care that we're fighting defects everyday, he knows exactly how he feels when his MP3 player dies.

My paper may be downloaded at Indium.com.

This year we tried to plan something a little different for APEX.  Instead of the normal booth, we used the space as a meeting place for people to come and talk with other engineers (from Indium and other companies), charge their cell phone, and eat a healthy snack.  If you have been to any tradeshow in your life you'll understand – this was an oasis.

Many people that did not participate in APEX are asking what it was like, especially in light of the predicted drop in attendance.  We spoke with a few of the Indium members who made it out to Vegas this year to sum up APEX 2009:

Rick Short told us "…attendance was 25 to 30% lower than in 2008.  That said, the quality of the attendee was unusually high (KEY decision makers) and the number of really good leads that we captured was high.  We spent about 25% of what we spent in 2008 on the exhibit and did much better (leads)."

Dave Sbiroli mentioned "It's the same core group of industry experts that attend the show" in reference to the technical presentations and industry meetings.

Brandon Judd commented "Although we are in the middle of an economic downturn, there was definitely no lack of interest in Indium's solder products at this year's APEX EXPO in Las Vegas.  In fact, it was quite the contrary.  Several customers, both current and potential, approached our booth with new and exciting applications that show there just may be a light at the end of the tunnel for our industry."

Tim Jensen had this to say, "This year's APEX was probably the best in recent history.  While the attendance was down from last year, those who did attend came with a specific purpose: to educate themselves and solve their current issues.  At Indium Corporation, we were busy educating customers on the implications of going halogen-free and helping to address their current Pb-Free production challenges."

Paste deposits printed for a QFN footprint

In an article from Circuits Assembly Magazine, solder paste is determined to be the key factor in proper QFN assembly.  Joseph Ameen and Gilson Geralde mention that "If too much solder is applied to the ground plane, the part will float, resulting in poor connections to the I/Os.  If too little is applied, insufficient grounding will result." 

Although this may not be groundbreaking, it's the reason we shoot for 100% transfer efficiency with our solder pastes.  Many solder pastes vary between 60% to 110%, while the best solder pastes will see tighter than 80% to 105% transfer efficiencies. 

For my next trick, I  will be participating in a chat session as part of the Virtual PCB Show on February 25^th from 11:00AM to 11:45 AM. Our topic is "SMT Defects and Solutions".

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.

For a change of pace, again, I have asked another Technical Support Engineer, Chris Nash, to comment about powder sizes.  Chris is the Regional Technical Support Engineer for the Midwest region, and works from Indium Corporation HQ in Clinton, NY.

Next on the soldering to list is soldering to immersion silver (ImAg). ImAg is probably the only solder finish that is sensitive to the profile used for reflow. Usually, the reflow profile can be changed to optimize for voiding, wetting or component limitations. But, ImAg has a track record of causing "champagne" voids.
 
Champagne voiding is a term used to describe small voids (bubbles) that form along either the intermetallic layer or ImAg surface. These voids are known to be caused by the volatization (out gassing) of the organic co-deposit that is put down with the silver during the plating process, or the out gassing of the tarnish layer after the flux has cleaned it from the surface.
 
On the other hand, being a metal surface, the in-circuit testing (ICT) is facilitated by the lack of a non-conductive layer (like Organic Solderability Protectant (OSP) over copper). So this means that printing of solder paste is not needed to make the contact points able to be probed. Another advantage is that the shelf life of the bare boards is longer than that of OSP copper and immersion tin (ImSn).
 
But, there are some reflow profile tricks that can be used to get a good, solid solder joint without the champagne voids. Typically, shorter profile work better, as they prevent the tarnishing of the silver as well as artificially increase the slumping of the paste to increase the spread. The solder spread is usually reduced on ImAg, as well, so the slumping of the paste to increase the spread of the solder helps 2-fold.
 
Take it from experience; soaking the paste to remove the voiding on ImAg boards does not work. Short, fast and cool is the way to go.
 

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.

Soldering just plain copper has gone away. Now, there are many different surface finishes that we, as an industry, solder to. This includes Organic Solderability Preservative (OSP) copper, immersion silver (ImAg), immersion tin (ImSn) and Electroless Nickel / Immersion Gold (ENIG).

Each one of these surfaces has its own benefits and downfalls, as well as their own set of requirements to solder to them properly, with the best looking, highest tensile strength and lowest voiding solder joint possible. This list does not include the other surfaces, usually on components, that we must solder to. Such as bright tin, matte tin, Hot Air Solder Leveling (HASL), pure nickel, etc…

We will talk about OSP, ImAg, ImSn and ENIG, and discuss their strengths, weaknesses and how to solder to them. This includes discussions on relative shelf life, history of use, what to look for and special reflow needs.

Lets talk about some issues…

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.

More information may be found at IKB: Indium Knowledge Base.

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.