Indium Corporation Blogs

The development of SACM™ lead-free solder alloy by Indium Corporation’s R&D team was focused around improving the mechanical shock performance of low-Ag SAC (SnAgCu) alloys. We learned that the addition of Mn to create SACM™ also helps to improve the thermal cycling performance through refining the grain structure and slowing the intermetallic growth during aging.

The thermal cycling performance of SACM™ is superior to SAC305 after aging (see graph). 

To learn all of the performance attributes of SACM™, visit www.indium.com/SACM.

 

*This post is part of the Introducing SACM™ series.

Because of the volatility of the metals market, the price of traditional SAC solders has been somewhat unpredictable. The chart below plots the price of SAC305 metal from June 2009 to June 2012.

Traditionally, solder is selected based on whether it meets the temperature and reliability requirements.  As a result of recent price volatility, solder selection has now also been impacted by metals cost.  Today, companies are willing to sacrifice reliability for lower material cost.  As I noted in a previous post, low-Ag alloys are cheaper, but compromise thermal cycling performance. Indium Corporation’s SACM™ alloy provides a lower cost material without sacrificing  thermal cycling performance. If this sounds like an alloy that would be of interest to you, visit www.indium.com/SACM to download the technical paper, or contact a member of our technical team to review your application needs.

Tim

*This post is part of the Introducing SACM™ Series

I invite you to come to the Southeast Asia Technical Conference on Electronics Assembly in Penang, Malaysia where I will be giving a workshop on SACM™ .

My presentation examines the reliability of low-Ag SAC alloys doped with Mn (SACM) under JEDEC drop, dynamic bending, thermal cycling, and cyclic bending test conditions and compared to eutectic SnPb, SAC105, and SAC305 alloys.

 

For more information, or to register, visit http://www.smta.org/education/symposia/symposia.cfm#penang. 

Tim

As I discussed in my last post, the industry has found that reducing the silver content of SAC alloys helps to improve its mechanical shock performance.  However, low-Ag alloys such as SAC105 are still inferior to its Sn/Pb predecessors in some important ways.  The graph below shows this trend.

 

 

 

 

 

 

 

 

 

Indium Corporation spent several years developing an alloy that bridges the gap between SAC solders and Sn/Pb solders for mechanical shock resistance.  The culmination of this research has led to the development of SACM™.  SACM™ not only outperforms SAC105 but is superior to Sn/Pb when it comes to mechanical shock.  For complete test data on SACM™ visit www.indium.com/SACM.

 

*This post is part of the Introducing SAM™ series.

As the world began transitioning to Pb-Free solder in the early 2000’s, the electronics industry determined that SAC387 (95.5Sn/3.8Ag/0.7Cu) was the most appropriate alloy to replace eutectic SnPb.  While it did have a higher melting point than eutectic SnPb, SAC387 was seen as the best option relative to solderability and usability.  The industry quickly shifted to SAC305 (96.5Sn/3.0Ag/0.5Cu) because its lower Ag content resulted in a lower price.

At that time, the industry didn’t realize the performance impact of the Ag content.  We now know much more.  Ag has a significant impact on a solder alloy's reliability. When thermal cycling higher Ag content SAC alloys (3-4% Ag), the performance tends to be quite good (Ag adds creep resistance to the alloy).  However, because of the alloy's rigidity (more Ag - more rigid), it is more prone to brittle fractures during mechanical shock.  We achieved significantly improved mechanical shock resistance at the expense of sacrificed thermal cycling performance.  The diagram depicts the balance between mechanical shock resistance and thermal cycling performance.

Over the past several years, the Indium Corporation has developed an alloy that minimizes this SAC solder alloy composition compromise.  SACM™ is a low-Ag alloy that is doped with Mn.  This not only improves the mechanical shock resistance over other low-Ag alloys, it also enhances the thermal cycling performance, making it comparable to SAC305.  For more information about SACM™, check out our website at www.indium.com/SACM.

*This post is part of the Introducing SACM™ series.

 I would never have expected the sneaker manufacturer, Puma, to be the next in a long line of companies stepping into the mobile phone market.  I remember Puma from the '80s as a significant player in the sneaker market.  I haven't thought about them much until I fopund out about their pending mobile phone launch, in about a month.  Puma Phone looks to be branching out with some unique features separate it from the main players in the already over crowded market.  The question really is whether these differentiating features are more than novelties.

The first is the built-in solar cell on the back of the phone.  Initially, I felt this wasn't going to be of much real use.  However, as I look back on my travels there are many times I was in an airport or some country with a dead battery.  Having an alternative way to charge the phone may have helped me out of a jam or two.  What remains to be seen is how quickly that solar cell can charge and how direct the sunlight needs to be.

Clearly, the phone was designed to be sporty and fun.  I like their stopwatch, compass, and well integrated GPS.  This phone probably isn't ideal for my business needs, but has the features that appeal to the 18 year old in me.

Many consumer electronics transitioned to Pb-Free 4-5 years ago.  However, there are still a substantial number of electronics being built with Sn/Pb solder.  As some of these products begin to transition to Pb-Free, the decision on a bar solder alloy for the wave soldering process is a challenging one.  There are two widely recognized options to choose from:

• use SAC305 and keep the alloy the same as the SMT process
• choose a low silver (Ag)/Ag-free alloy which is substantially lower in cost

From a cost perspective the Ag-Free alloys are 40-50% cheaper than SAC305.  This is a substantial savings when you consider that solder pots hold around 1,000 lbs of solder.  In this arena, Indium offers our Sn995 which is a cobalt doped Sn/Cu alloy.  In addition to being lower in cost, Sn995 produces shinier solder joints than SAC305.

It would be great if that were the whole story, but there are cases when SAC305 should still be considered as a bar solder for wave soldering processes.  One advantage of SAC305 is that it has a faster wetting speed than any of the Ag-Free alternatives.  This can help when you have a board or process that is challenged with poor hole fill.  The chart below shows the difference between popular Pb-Free alloys for wave.

Another potential advantage of SAC305 is in thermal cycling reliability.  Because the typical wave solder joint is large and very robust, reliability may not be a concern for many consumer products.  However, as the reliability requirements increase for industries such as automotive, aerospace, and military, the stronger SAC305 solder joint may be what is needed to meet more stringent reliability requirements.

Both SAC305 and Ag-Free alloys, such as Sn995, have their place within the electronics manufacturing arena, but it is essential that the correct decision is made based on the product being assembled.

The US House of Representatives bill H.R 2420, affectionately known as US RoHS, was introduced in May of 2009.   From what I can tell, the goal of this bill is to create a uniform law across the USA that also matches the regulations in the EU.  On the surface, this seems like a reasonable idea.  However, the reality is a lot more sketchy.  Although the proposal restricts several materials, the restriction of Pb (lead) is the most significant.  If this legislation were to be passed into law, here are a couple of issues that I see:

1. Cost of implementation - There are a significant number of small electronics assemblers throughout the USA.  These "mom and pop shops" may only have one or two SMT lines that build for a very specific and niche application.  Many of these products never leave the United States and, therefore, the assembler never had to worry about the EU restrictions.  A quick Internet search can show the millions of dollars that major electronics manufacturers spent on the conversion from Sn/Pb solder to Pb-Free solder.  The cost of compliance will risk putting these small companies out of business.  The last thing the USA needs right now is fewer companies and fewer jobs!
2. Lack of long term global law uniformity - While this US RoHS bill is set up to match the current EU version, there is already a revision in the works in Europe.  Therefore, there will always be a struggle to globally match other laws.  To make it even more of a challenge, China is also working on their own RoHS law.
3. Unknown Reliability - The United States has already lost the majority of the high volume, low cost electronics assembly (such as cell phones and computers).  Now the US electronics manufacturing landscape is for predominately medical, military, and automotive applications.  These products usually have much more stringent reliability requirements.  At best, reliability experts will say that the we still don't have enough history to accurately predict the reliability of Pb-Free solders.  However, most also believe that Pb-Free will not be as reliable as Sn/Pb in many applications.

The good news is that it seems that this House Bill is stalled somewhere in our legislation system and there is no indication that it will be passed anytime soon.  

 Register at GlobalSpec to see Andy Mackie, Jim Hisert and me discuss various aspects of Green Electronics Manufacturing.  This live event will occur tomorrow (December 9, 2009) at 2 PM EST.

Andy will be discussing halogen-free and what it really means to semiconductor packaging and PCB Assemblers.

Jim will discuss solar photovoltaic cells and how material selection impacts their performance.

I will be discussing Pb-Free and some of the emerging legislation and manufacturing challenges.

Following the discussion, there will be an opportunity for attendees to submit additional questions on any of the topics.  The discussion will be full of technical information on all of the topics and, best of all, it is FREE!

 There is a proposal into the EU to add to the RoHS legislation by restricting all halogenated flame retardants.  You can find that proposal here.  This proposal has the support of a number of consumer electronics companies such as Apple.  Here are a couple of Pros and Cons for implementing these additional restrictions into RoHS:

PROS 

• Dramatically simplifies the testing and detection.  It is simple to test for Cl and Br, but much more difficult to determine which halogenated compound that halogen came from.  When restricting only 3-4 halogenated compounds, one would have to run the simple Cl/Br test and, if Cl or Br appeared, run costly tests to be certain they didn't come from one of the restricted compounds.
• Puts all electronics on an even playing field.  These companies are often using "green" marketing to promote their company and/or certain products.  There is no clear way to identify whether on mp3 player is more "green" than another.  It is all on how they spin their "green" efforts.  By having legislation, all companies will be required to take the same actions (at least relative to the restricted items).

CONS

• There are only a few halogenated compounds that have actually been tested and determined to have some health or environmental risk.  Therefore, a total ban would restrict materials that pose no risks.  Companies like Apple have proven that electronics can be manufactured without any halogens today.  However, as the technology advances and changes there may be needs or technological advantages to using certain halogenated compounds but if the law is in place it will be extremely difficult to change.
• The proposed regulation states to ban all halogenated flame retardants.  Exactly what is a halogenated flame retardant?  If there are 10,000 halogenated compounds, how do we determine which ones are a flame retardant and which ones are not?  Without spelling out specific compounds, individuals will be left to assess whether or not their halogenated compound could be considered a flame retardant.  This is a particular challenge for solder pastes and fluxes where halogenated compounds are used but are not intended as a flame retardant.  Would those be banned?

Restricting materials unnecessarily will limit innovation and possibly restrict the advancement of technology.  There is a very delicate balance between protecting the environment and preventing technology.  There are no easy answers.

 In a recent blog entry titled RoHS: 3 Years Later, Dr. Lasky comments on the overall success of the RoHS legislation.  Before implementing the Pb-Free portion of the legislation, companies were extremely concerned about its impact on performance and reliability.  Dr. Lasky reports that there have been relatively few issues.  However, there HAVE been some issues.  I will debate against myself on whether RoHS has been successful or destined for failure. 

Why RoHS is Destined to Fail (aka Against Lasky):

• Sn Whiskers - Sn whiskers are filament growth that protrudes from pure Sn surface coatings and are a result of the compressive stress inside of that Sn.  For an overview of Sn whiskers, check out the article titled Structure and Kinetics of Sn Whisker Growth on Pb-free Solder Finish.  When looking at Sn whisker mitigation, it turns out that Pb added to Sn is very effective.  That's why Sn/Pb components never show whisker growth.  There are other mitigation techniques such as Ni underplating and doping with Bi but they don't seem as effective a good old Sn/Pb.  Over time (often greater than 5 years), whiskers can grow large enough to form a short between adjacent components.  Whiskers may not be a big issue for cell phones (because of their short life) but is a major concern for military, medical, and aerospace electronics.
• Pb-Free Alloy Reliability - The two most common alloys used for Pb-Free soldering are SAC305 and SAC387.  When compared to Sn63Pb37, the SAC alloys are considerably more brittle.  This means that under low stress conditions, they actually may be more reliable than Sn/Pb.  However, under higher stress conditions, Sn/Pb can creep to absorb some of that stress while SAC alloys can simply fracture.  The reduced reliability of SAC can be seen under challenging thermal cycling and drop testing.  There are studies on doped SAC alloys that show promise in bridging the reliability gap, but more work is necessary in this area.
• Higher Reflow Temperatures - The peak reflow temperature for Sn/Pb assemblies was generally around 210-215 C.  For Pb-Free assembly, it tends to be around 240-250 C.  This increase of 30+ C can reap havoc on boards and components.  For components, higher temperatures increase their susceptibility to moisture.  The MSL levels are generally more stringent for Pb-Free.  For boards, you can get barrel cracking, delamination, and CAF growth.
• Proven Pb-Free Issues: There have been a number of reported issues that are likely related to Pb-Free.  Here are a couple: NASA and Sn whiskers; Pacemakers; X-Box RROD (Red Ring of Death)

RoHS and Pb-Free specifically is simply a ticking time bomb.  Just because there are only a few widely reported issues doesn't mean that nothing will occur in the future.  Anyone who thinks it is successful should be forced to fly only on Pb-Free airplanes.  

Why RoHS is a Success (aka Pro-Lasky):

• Recycling - As Dr. Lasky notes in his blog, there are a number of benefits to eliminating Pb from the recycling process.  Although, Pb contamination can easily be dealt with at state of the art recycling facilities, there are unfortunately too many uncontrolled reclaim situations in poor and developing countries.  The elimination of Pb makes those people safer.
• Technology Advancement - Consumer electronics are almost completely Pb-Free and have been since 2006.  Since 2006, we have seen a significant amount of advancement in the technology behind cell phones, laptop computers, and handheld GPS.  Had Pb-Free been such an impediment, there would have clearly been some stagnation in the advancement of those technologies.  In consumer electronics, there has been the implementation of 0.4 and 0.3 mm pitch CSP's, 0201's, package-on-package (PoP) to continue to improve the technologies.  Remember, the first iPhone was Pb-Free!  As the technology advances, there will always be challenges but they are not directly related to going Pb-Free.
• Whiskers (Non)Issue - It is absolutely proven that pure Sn can form whiskers that could be a long term reliability issue.  However, there are existing Pb-Free alternatives today and in many cases the standard mitigation techniques are good enough.  The real issue here is cost.  People want to use pure Sn (or as little mitigation adders as possible) to get the cheapest component.  However, if you eliminate the Sn, you can eliminate the whiskers.  Texas Instruments uses Ni/Pd/Au for many parts.  That is Pb-Free and contains no Sn.  Whisker free alternatives do exist!

The implementation of RoHS compliance has clearly been successful for consumer electronics.  Those products continue to advance in technology with little issues despite being Pb-Free.  While there are concerns relative to higher reliability assemblies, there are design techniques that exist to overcome those concerns.

Which side of the fence are you on?

 From the year 2000 to 2006, I spent a significant amount of time working with customers who were trying to make the conversion of their electronics assembly solder paste from Sn/Pb to Pb free.  Today, with much of the consumer electronics industry converted to Pb free, in some cases it can be debated that it is easier to "go with the flow" rather than resist the change.

A major challenge facing today's Sn/Pb assemblers is the limited availability of Sn/Pb components.  Why is this an issue?  The first is the concern of Sn whiskers when components are plated with pure Sn (the popular alternative to Sn/Pb).  This is a potential reliability issue specifically for assemblies that are required to last many years.  The second problem is with BGA components.  For Pb free, they are typically bumped with Sn/Ag/Cu solders.  These solders melt at 217 C which is typically higher than the peak reflow temperature of Sn/Pb.  If your BGA supplier no longer sells the Sn/Pb version, you could be forced into a mixed alloy system which is probably less reliable than all Sn/Pb or all Pb free.

The Sn whisker challenge can be addressed with a number of mitigation techniques, but if you are building something with extremely high reliability requirements, you are better off staying Sn/Pb.  From the BGA perspective, you are probably better off going to Pb free to avoid the mixed alloy system.  

I remember talking to companies that were ecstatic that they were building something that was exempt from RoHS legislation.  Unfortunately, today they face a new set of challenges that weren't there a couple of years ago.

In many halogen-free discussions, the primary emphasis is on Bromine (Br) and Chlorine(Cl).  However, the halogens also include Fluorine(F), Iodine(I), and Astatine(At).  Most of us know several uses of F and I, but what is this element At?  

As it turns out, Astatine is is the rarest of all naturally occurring element.  According to Wikipedia, there is less than an ounce in the entire Earth's crust.  I think that means it is safe to assume we don't have to worry about it showing up in printed circuit boards!

Since At is extremely radioactive, I highly recommend avoiding it if you see some on the sidewalk but don't worry too much about it in your halogen-free testing.

 Two years ago, electronics companies were talking about China RoHS becoming the next big set of legislation affecting the assembly of their products.  It was supposed to restrict all the materials that the EU RoHS does (Pb, Hg, Hex-Cr, and some halogenated materials) but have fewer exemptions and require more testing and documentation.  It didn't just go away, but it's implementation has been much slower than China initially reported.

The first phase of China RoHS is actually already in place.  On March 1, 2007 all electronics must have labels showing whether or not they contain one of the EU RoHS restricted materials.  Currently they are not actually resticting any of the materials.  If your product does not contain any of the 6 materials, you must use the green e-label shown to the right.  If it does contain one of those materials, you can still import it and sell it within China, but it must have the orange label shown to the right.  The number in the middle of the orange label is the number of years that you guarantee that product can be safely used (how do you figure that out?).

Phase two is the actual restriction of those 6 materials.  China will be creating a product catalog and all materials in that catalog must not contain any of the restricted materials.  Therefore, there will be not broad category exemptions.  To make it more challenging, they are expected to require products imported into China to have a testing report from one of a handful of China "approved" labs.  These labs are undoubtedly going to be government owned and simply a way to profit from the restriction.  Phase two continues to get pushed back and is now expected sometime in 2010.

Let's hope they continue to delay phase two!!  A good website on China RoHS is by Design Chain Associates and can be found by clicking here.

Ever since RoHS was implemented in 2006, people have been wondering what happens to the company that gets caught with Pb, Cadmium, Chromium, or certain halogens.  In the UK, they use the National Weights and Measures Laboratory (NWML) as the enforcement authority for RoHS.  Recently the NWML released their RoHS activities for April 2008 to March 2009.  This gives us insight into what at least on EU entity's RoHS efforts are.

NWML reported to have investigated 250 companies for RoHS compliance.  From that investigation, it is reported that they issued 14 improvement plans, 3 EU notifications, 4 product withdrawals, 8 compliance notices, and 8 warning letters. 

There are several things here that made me think.  First, RoHS compliance is a product based compliance and not a company based compliance.  Companies can't comply with RoHS but a certain product that they build may or may not comply.  Maybe the wording isn't the best or maybe they aren't investigating properly.  I assume that it isn't worded properly and that they are really saying that they took 250 products and worked with their producers to ensure complance. Of all the electronics being shipped into the UK, it really surprises me that they only looked at 250 products throughout an entire year.  Of the millions of electronic products flowing into the UK, they only looked at 250?  That doesn't seem like very effective screening.  What's even more disturbing though is that of the 250 they did look at, they found 37 with some issue.  That means there was 15% with some type of non-compliance.

In summary, they came up with a law that is not properly enforced AND not properly followed.  Who came up with this RoHS idea anyway? If you are interested in the complete NWML report, click here. 

Greenpeace continues to take unusual steps to push electronics companies to produce halogen-free products.  On Tuesday July 28th, Greenpeace activists climbed onto the roof of one of the HP buildings in Palo Alto to protest that HP is not moving fast enough to remove BFR's and PVC from their products.

It seems to me that HP is taking their time to figure out whether or not the alternatives are both safe, reliable, and cost effective.  It would make no sense to eliminate a hazardous material and replace it with something more hazardous.  The main reason for Greenpeace's protest is that Apple has already successfully eliminated virtually all of their use of BFR's and PVC.  However, the business model of HP and Apple are much different.  HP has to be very cost consious and has a much broader product range than Apple.  HP must be very careful in selecting alternatives that will work in all their products AND allow them to be competitive in the ultra-competitive computer market. 

 The humerous Dilbert cartoon reiterates an important fact about companies "going green."  While there are probably individual employees at the company who are passionate about helping the environment, the sole purpose of the company is to make money.  Companies use terms like "environmentally friendly," "reduced carbon footprint," and "green" to create a positive impression the the consumer which is designed to lead to more sales.

In many of the IPC Halogen-Free meetings, people would ask why go through such an undertaking.  Other people have asked  to wait for scientific methodologies for determining which halogens are actually bad.  Those questions ultimately don't matter.  The process is about developing a system that companies can refer to in order to make their claims of being better for the environment (and ultimately make more money).

Eliminating halogens may actually be better for the environment, but it is still too soon to tell.  We have no idea how good/bad the alternatives are.  Unfortunately, if switching to an unknown material allows the company to put a green sticker on it, then they don't have the incentive to carry out further testing on those alternatives.

In a follow-up to my previous posting, Greenpeace's rationale for giving Apple marginal rankings on BFR and PVC elimination seem to be somewhat misguided.  In the "Guide to Greener Electronics," they applaud Apple for making the move to almost completely eliminate BFR and PVC from their products.  However, Greenpeace added: "But Apple fails to score top marks in this criterion because it uses unreasonably high threshold limits for BFRs and PVC in products that are allegedly PVC-/BFR-free." 

If you go to the Apple and the Environment website, you can see that they are not only eliminating BFR's and PVC.  Apple is the only company that I am aware of that is specifically focusing on the complete elimination of Br and Cl.  This is much more stringent than the path of other electronics companies.  Apple is a part of the IPC committees in which halogen thresholds are determined.  They are following the same thresholds as everyone else: <900 ppm Cl, <900 ppm Br, and <1500 ppm Br+Cl.  Therefore, Apple uses the same thresholds as everyone else, but includes all forms of Br and Cl (not just BFR and PVC). 

Cl and Br show up as "contamination" in many chemicals, so guaranteeing that there is ZERO Br or Cl is impossible from a practical and cost perspective.  In addition, there is significant variability in the testing procedure.  Because of that, it is possible to test something with 100-200 ppm of Br and not actually detect it.   I wonder what threshold Greenpeace would find acceptable???  The acceptable level should be determined by hazard/risk assessments, availability of materials, and capability/repeatability of a test method for that material.  Based on my knowledge of soldering and flux chemistries, I don't think the 900, 900, 1500 limit is "unreasonably high."

Greenpeace has just updated their "Guide to Greener Electronics."  There are a couple of interesting tibdits that I took from their report:

1. They are really focusing on the phase out of BFR's and PVC from electronics.  They dropped HP and Dell down because they are loosening their timeline of BFR and PVC phase-out.  I find it interesting that they make no note of what the replacements should be.  This is concerning that the replacements could potentially be MORE toxic than what they are replacing.  It took years to fully characterize the situations where BFR's and PVC are of concern (dioxin formation and bioaccumulation).  In addition, all BFR's are not the same.  If companies are phasing out these materials, how can they do a full risk assessment of the replacements in one to two years?
2. They have added Antimony (Sb) to their list of materials that need to be phased out.  This can be challenging for a number of soldering applications.  Component manufacturers have been using Sn/Sb alloys inside their components.  Sn/Sb is the highest melting point Pb-Free alloy that actually solders reasonably well (other than Au/Sn which is 1000x as expensive).  The component guys are using this so that those alloys are not remelting when that component is assembled in a SAC SMT process.  Eliminating Sb will create a number of assembly challenges as well as potentially significant reliability issues.
3. By reading the summary of the report, they praise Apple for phasing out virtually all BFR's and PVC.  However, their ranking is still in the bottom half.  I will write more about this one in a future entry.

I am all for designing electronics for the environment, but I think there needs to be more focus on the consequences of making those design changes.  Are the alternatives actually any better?  What is the impact on product reliability and functionality?

Most of you have probably seen videos of countless Mentos/Diet Coke experiments all over the internet.  If you are one of the few who have not, this is a good site to teach you the basics: Steve Spangler Science.  Essentially what you get is an explosion of foam due to the rapid outgassing of the soda.  I am still not sure why diet soda works better than regular soda.  If you know why, please comment or email me at tjensen@indium.com.

What do Mentos and Diet Coke have to do with voiding you ask?  Truthfully, not much but it is just one example of things that shouldn't be mixed (unless you are intentionally looking for a mess).  Mixed alloys is another example.  This is particularly true when you mix Sn/Pb and Pb-free for BGA assemblies.  Today, the most common mixing is using Sn/Pb solder paste but using a Pb-Free bumped BGA.  There are concerns about overall reliability of these mixed alloys, but the most common problem people have encountered is high amounts of voiding.  This comes as a direct result of people trying to improve the reliability by raising the Sn/Pb reflow profile to around 225-230 C and allowing complete mixing of the two alloys.  This definitely gives a more homogeneous microstructure, but most Sn/Pb pastes weren't designed for that high of a reflow temperature.  High peak temperatures result in more flux outgassing and, therefore, more voiding.

To avoid the excessive outgassing, you could eliminate mixed alloys.  Everyone would like to do this, but it is often not possible.  Therefore, you best option is to select a solder paste that is more thermally stable and has a high oxidation barrier.  This will reduce the outgassing at the elevated peak temperatures and allow you to focus on diet coke outgassing rather than that of the solder paste.