Mario Scalzo's Tech Support Blog

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.

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).