PB Free Solder

After the report by Isofoton regarding reliability testing of Bi-based alloys for tabbing ribbon, the world learned that Bi-based alloys could survive the lamination process and function in use. If you haven’t seen it yet, I consider this mandatory reading! Here is the info: B. Lalaguna, P.Sanchez-Friera, I.J. Bennett, L.J. Caballero, J. Alonso, “Evaluation of Bismuth-Based Solder Alloys for Low-Stress Interconnection of Industrial Crystalline Silicon PV Cells", 22^nd EU PVSEC, Milan, 2007Milan, 2007.

We all know the Bi based alloys like 57Bi/42Sn/1Ag and 58Bi/42Sn can be used in a standard module assembly process, but is there an advantage to using Bi/Sn or Bi/Sn/Ag when Sn/Pb and Sn/Pb/Ag alloys are so well known and trusted in the industry?

I’ll give you 3 benefits:

1)    1) Bi/Sn/Ag and Bi/Sn are Pb-Free

2)    2) Bi/Sn/Ag and Bi/Sn are low-temperature alloys, they allow you to lower your tabbing process temperatures

3)    3) When paired with the correct flux and metallization, these Bi alloys form a powerful bond without microcracks (due to the lower process temperature)

Below are results with SunTab^TM ribbon assembled on a Komax X series stringer and tested on a XYZTEC Condor 150-3 bond tester (provided by the respective companies).

You’ll probably notice the lack of y-axis scale – I’m not going to give away all the cool information that easily! Contact me at jhisert@indium.com to learn more.

Two categories of solder are available to choose from when the in-service environment for a device reaches above 125°C either in continuous operation or thermal cycling accelerated life testing. These categories are those comprised primarily of lead, and those of gold. From the electronics industry’s drive to eliminate lead, many manufacturers who have traditionally used lead solders are treading cautiously, looking now at the gold solders, primarily at Indalloy 182 (80Au20Sn).

The most common concern regarding this switch relates to the strength of AuSn, which is much higher than the lead solders. The degree that this should be of concern however, should be realized within the scope of the application.

For instance, review this case scenario:

Indalloy 159 (90Pb10Sn) was used in a device for years to adhere high temperature sensors to a calibration probe that is slowly cycled in operation from 350K (~75°C) to 500K (~225°C). The solder joins a nickel and gold plated Kovar™, or platinum or platinum coated, nickel lead to a tinned copper lead. The solder joint is not placed under tension or shocked.

Considering the high temperature solder options in this scenario, the AuSn would be mechanically preferred.

Why?

Well, tin-bearing soft solders will leach gold from gold metallizations during soldering, creating a brittle Au-Sn intermetallic layer within the solder joint. The more gold available, the more consumed, and the greater the thickness of the resultant intermetallic layer. The brittle nature of this layer, situated intimately next to the relatively soft PbSn solder layer, creates differential stresses that promote crack propagation upon thermal cycling.

AuSn was not considered previously because the engineers were familiar with its hardness and, given the cracking failure described using a softer solder, they did not anticipate improvement. It was a pleasant surprise to them to find that switching to a lead-free solder would not sacrifice the quality of their device. AuSn is a brittle alloy but, unlike the description above, no differential stresses are involved. 

Note: Eutectic gold solders have been used for many years to solder nickel plated Kovar™ lids to high reliability ceramic packages and have a good history of fatigue performance.

Folks,

I’m taking a few moments from Wassail Weekend , held annually in my village, Woodstock VT, “The prettiest small town in America”, to write a post about last week’s workshops at ACI.

Indium colleague Ed Briggs and I gave a 3 hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analysis of “graping” and the “head-in-pillow” defect is the best around. 

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e. very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g. OSP Immersion silver, electroless nickel gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of 5 years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years in harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in the Fred's chart (above). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being a confirmed one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many "gaps" that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately $100M program in 2013.

For those that missed this free workshop, ACI host Mike Prestoy is planning another one in 6 months.

Cheers,

Dr. Ron

Tombstoning

• the transition to Pb-Free (higher reflow temperatures, and related flux issues)
• miniaturization (0201s and 01005s)

• The reflow oven operator can slow down the ramp rate. A slower ramp rate allows for more uniform warming of the PCBA.
• Another technique is to employ a "soak" just below the melting temperature (solidus) of the alloy. For example, for a SAC305 profile (217°C solidus), one may implement a "soak" at 205 to 210°C for 30 to 120 seconds. This allows for the cooler parts of the PCBA to "catch up" to the warmer parts. After thermal equilibrium has been achieved, one can spike the temperature up to the appropriate peak temperature (i.e. 245°C). This technique (depicted in the reflow profile shown at the right) allows for all of the solder paste deposits to melt and wet the component terminations at roughly the same time; thereby, mitigating tombstoning.

Phone: +1.315.853.4900
E-mail: ebastow@indium.com

 Folks,

In gathering information on the status of lead-free soldering, some helpful friends pointed out two great sources of information: NASA and The Navy. NASA sponsored an impressive lead-free reliability investigation: "Lead-Free Solder Testing for High Reliability Project 1." This project is finished and the reports are online. There is a follow-on project: NASA DOD Lead-Free Electronics Project 2 which is currently underway. The Navy sponsored a project with ACI and the summary is here. I am currently studying these documents to help develop the consensus.  Some preliminary info follows:

Regarding -20^°C to +80^°C thermal cycling, NASA concluded:

“Under the conditions of this test, Sn3.9Ag0.6Cu (SAC) and Sn3.4Ag1.0Cu3.3Bi (SACB) were always more reliable than eutectic SnPb regardless of component type (CLCC, TSOP, BGA or TQFP).

It has been shown that conditions that highly stress the solder joints by maximizing the CTE difference between the PWB and the component will favor SnPb over SAC6. Conversely, conditions that minimize the stress put on the solder joints (e.g., compliant components such as BGA’s and/or a thermal cycle with a small delta T) will favor SAC over SnPb. The current test falls into the latter category and we can say with some confidence that the lead-free alloys tested will outperform eutectic SnPb under field conditions that are even less stressful than the -20 to +80^°C thermal cycle test conditions.”

For -55^°C to +125^°C thermal cycling, the conclusions were more cautious, likely because the data were mixed:

“The feasibility of using Pbfree solder alloys in place of SnPb solder alloys for new product designs was demonstrated under thermal cycle test conditions. Additional investigation and characterization of Pbfree solder alloys will be required as a segment of a Pbfree solder alloy implementation plan. The application/introduction of Pb-free soldering processes for legacy product designs is not recommended without extensive materials characterization and product design review.”

These results seem to be consistent with what others report, lead-free assembly produces good thermal cycle results for commercial-type thermal cycling, but the results are mixed for harsh environment thermal cycling.

Folks,

Recently I posted a note about a flurry of Technet posts in which I was misquoted regarding the status of lead-free electronics assembly.  Harvey Miller then weighed in.  I responded. And this in turn raised more comments.

All of this caused me to wonder, is it possible to achieve a consensus on the state of Pb-free assembly?  I think it might be and am going to try.  The main thing that I think is important in this quest is that any points for the consensus, or lack thereof, be supported by data and analysis, not emotion.

If you have a point to add, that is backed by data and analysis, please share it with me.  One of the things I hope to accomplish is to develop a list of references, that can be referred to to support the consensus.

Stay tuned for more info on this effort.

Cheers,

Dr. Ron

Folks,

An obvious disadvantage of lead-free electronics soldering assembly is that the oven must be hotter and therefore will use more electricity (versus 63Sn37Pb soldering). But is the extra amount of electricity significant? Bill O’’Leary claims that a typical SMT oven uses $7K of electricity a year at $0.072/Kilowatt hour (Kwh) or about 100,000 Kwh. That number strikes me as about right, as a household uses about 5-20,000 Kwh per year.

In the late 1990s there were 35,000 SMT lines in the world, at a 3% growth rate that would be about 50,000 lines now. So worldwide SMT reflow oven use would be about 5E9 KWhr (50,000 ovens x 100,000 Kwh/per year) world wide.  

With most heat loss be due to convection, the increase in energy use will be approximately proportional to the difference between the oven temperature and the room temperature (25C). An oven processing tin-lead solder would run at about 210C versus lead-free’s 250C. So the added energy for a lead-free oven would be about (250-25)/(210-25) or about 22% more. So if all assembly lines in the world are SMT the added energy use would be about 0.22x 5E9 Kwh = 1E9 Kwh. The cost of this extra electricity would be about $100 million (US) at $0.10/ Kwh. The electronics industry generates about $1.5 trillion in sales. So this added cost would be about 0.0067% of sales. Since world electrical use is about 150,000 E9 Kwhr per year, this increase is about 1/150,000 of all of the electrical use or 0.00067%.

So although more electricity is used, the increase is not significant to the value of the electronics sold or the total world use of electricity.

Thinking about higher temperatures reminds me that my Indium Corporation colleague Dr. Ning-Cheng Lee is presenting a paper this week on a high melting temperature lead-free solder based on a BiAgX alloy system. Higher melting temperature solders are often needed in what is referred to as a solder hierarchy. Solder hierarchies have solders that melt at decreasing temperatures in multiple soldering steps, starting with the highest melting solder.

Cheers,

Dr. Ron

Typical Tabbing Ribbon Solders

Only a few solder alloys have become common, industry-wide, among solar module assemblers, and those can be pared down into three categories:

• BiSn alloys (58Bi42Sn, 57Bi42Sn1Ag)
• SnPb alloys (63Sn37Pb, 62Sn36Pb2Ag)
• SnAg alloys (96Sn4Ag)

Tin-Silver Solder (SnAg)
SnAg has become the most widely used Pb-free solder alloy, particularly in tabbing ribbon designed for cell interconnection. Historically, its melting temperature (221°C) made it an obvious replacement for processes previously running SnPb solders.

In designs where step soldering is necessary (however uncommon in back end solar module assembly), SnAg can be used as the step previous to soldering with Sn63 or similar Pb-Free solder (albeit carefully since the second soldering temperature is quite near 221C). 

While SnAg eutectic solder is a desirable composition for electronic component soldering, for instance, power semiconductors, recent studies using this alloy for stringing solar modules have indicated that the other common alloys listed for this application are easier to work with and better designed to meet the needs of this solar assembly application.  SnAg does have a high melting temperature, and the preferred fluxes for module assembly are not yet optimized for this solder composition.     

Regardless, SnAg has its benefits.  When a solder that melts somewhat above the melting point of a “standard” solder alloy is needed, and it must be Pb-free, this is it!!  Check it out!

Happy Testing!!

Amanda

Folks,

It was five years ago today that RoHS was launched, amid concerns that the world of electronics would collapse due to the many challenges of lead-free (Pb-free) soldering. Well, we have five years of field data with no “the sky is falling” lead-free reliability events. But, has it been just five years?

No. As I mentioned in a recent post, Motorola implemented lead-free soldering around 2001 to take advantage of lead-free solder’s poorer spreading.  Hmmmm,  so it has been ten years! Not too bad!

Well it is actually better than that. SnAg3.5 solder has been used for decades in both:

1.     Step soldering:  with a eutectic temperature of 221C, SnAg3.5 can be used as the step previous to soldering with Sn63 or similar Pb-Free solder. The principle is to solder first with the SnAg3.5 and then with a lower melting temperature solder. The second soldering step is performed at a lower temperature, therefore not disturbing the SnAg3.5 solder joint or bond. 

2.      Mid-Temp Pb-Free alloy:  when a solder that melts somewhat above the melting point of a “standard” solder alloy is needed, and it must be Pb-free, SnAg3.5 is often the choice.  The automotive industry has used SnAg3.5 in these applications for decades.

While I still agree that lead-free solders need some time and experience, especially in harsh environments, to establish acceptable reliability for mission critical applications, the experience with SnAg3.5 is adding to lead-free solder’s reliability portfolio.

This information came to light with the recent announcement by a major solder materials supplier that they would no longer supply SnAg3.5. But take heart, Indium Corporation still supplies SnAg3.5.  

• High thermal conductivity (33W/mK)
• Higher melting point than SAC alloys (221C)
• Low tensile stress, so suitable for large die (5800psi)
• Excellent thermal cycling properties (-55 to 125C)

1.  Preform (a specially-shaped solder piece) with TACflux® used to hold the preform and die in place
2.  Solder paste, which holds the die in place with no extra materials added 
3.  Soft solder die-attach wire, a fluxless type of solder wire, which is melted onto the substrate metallization under an inert cover gas, and the die directly mounted onto the molten solder pool, then allowed to cool.

Folks,

Here is an interesting turn of events related to the reliability of lead-free (Pb-free) soldering reliability. 

I was reminded recently by something Carl Sagan said, or, actually, did not say: Billions and Billions Although this term is strongly associated with him, he never said it. Sagan believed that this term was connected to him because Johnny Carson mimicked him and used the term.

 
Although not even close to being in Sagan's league, I find that I am now equally unfairly associated with the term,  "lead-free solder is a grand success." This came about in an interview by Rob Speigel, which he summarized in a blog post.

In reading Speigel's post, you will see that,  "lead-free solder is a grand success," is Rob’s term, not mine. Well, Rob's post resulted in a string of postings on IPC’s Technet .

One person opined:

Irresponsible statements like "lead-free solder is a grand success" should NOT be ignored. Those who make such statements in the face of all of the contrary evidence should be noted, and treated as motivated only by greed. Lead-free soldering certainly has been known for many "thousand$" of successes.

I have learned that it is not even worth the bother to refute such statements with those who make them. It may be a "grand success" for PhDs who contract to solder paste companies, but it certainly has not been a "grand success" to literally thousands of companies dealing with the reliability elephant sitting in the room getting larger by the day, and the associated fallout as a result.

Ouch!

Another shared:

I disagree with the stated and implied affect of RoHS, on PWBs expressed in this article. Lead free assembly reduces reliability by 50%. There can be no doubt about that. There are too many studies that confirm lead free assembly significantly degrades reliability. There are so many studies that demonstrate a reduction in reliability that Rod's contention is almost laughable. We are now faced with increased failures of copper interconnections and dielectric material due to high assembly temperatures. There is an increase in crazing that can support CAF, significant copper dissolution, and cratering in assembly, Switching to lead free in most HDI applications is a significant challenge. Lead free assembly has a profound affect by degrading PWB's organic component (epoxy) due the temperature required and copper interconnection and also the exaggeration of the z-axis expansion of the dielectric.

I have asked for copies of the many reliability studies referred to. No response yet.

Finally someone hit the heart of the matter:

I'm curious if "grand success" were Dr.Lasky's words or Rob Spiegel's editorializing. Lasky does mention the lack of long term results, and Speigel, in the comments,  enumerates a number of reliability problems. ISTM that neither truly believes  those words.

Correct!, Thanks. 

Here was my response that I posted on Technet:

Folks,

Pete is correct. I never said lead-free implementation was a grand success. These were Rob's words in his blog post. 

I have said repeatedly that adequate lead-free reliability has been demonstrated for consumer products like mobile phones, PCs, portable electronics with service lives less than 5 years. This level of reliability has been demonstrated in numerous studies and more importantly with field data. Vahid Goudarzi, of Motorola, stated that field reliability of lead-free assembled mobile phones has been equal or better than leaded assembly units. His data go back to 2001 (not 2006. Motorola started early for reasons discussed below).

 The reason Motorola shipped early with lead-free products is due to the fact that lead-free solder does not spread as well. Because of this poorer spreading, Motorola was able to decrease lead spacings without getting shorts, thus increasing the amount of electrical function in a smaller space. Since increased function in a smaller space is the defining attribute of portable electronics, the importance of this lead-free advantage cannot be overstated. Admittedly, lead-free's poorer wetting is a challenge in other regards, especially hole fill in wave soldering, but the Motorola Droid X2 could not be assembled with leaded solder, there would be too many shorts. Since the packaging density of the iPhone and similar devices is on a par with the Droid X2, I suspect this statement is true for most mobile products.

I have also repeatedly stated that lead-free reliability for long term service, mission critical devices has not been demonstrated. As a result, these types of devices should not consider lead-free solder at this date.

I regularly discuss these topics in my blog (http://blogs.indium.com/blog/an-interview-with-the-professor). The most recent post shows a striking photo of leaded solders spreading -which is too "good" for portable electronics.

Cheers,

Dr, Ron

Folks,

I met Peter Borgesen back in the mid 1980s when he was a research scientist at Cornell working with Professor Che-Yu Li. Later we worked together at Universal Instruments. Currently Peter is a Professor a Binghamton University.  All during this time, Peter has been working on materials science-related topics in electronics packaging and assembly, most notably reliability. In addition to his many technical skills, he is a gifted linguist, speaking multiple European languages. Etched in my mind is Peter talking to several European graduate students in their native European languages in the space of 5 minutes, switching from one to the other effortlessly.

Few people know more about lead-free solder reliability than Peter. So I thought I would get his perspective on my recent post on lead-free field reliability data. His comments follow.

Hi Ron,

I agree that the sky is not falling. Also, we should be talking much more (only?) about life in service. I realize that we don't know enough about this (and our predictions based on test results are much more off than people want to recognize). The vast majority of practitioners focusing on 'engineering tests' are doing worse than wasting time and effort if comparisons of test results do not translate to relative performances in service. There is a lot of ‘sticking heads in the sand’ here.

I am not concerned about the long term life of cell phones, and not very worried about in which respect they do better or worse in service than with SnPb. Intermetallic bonds have generally gotten weaker and more prone to sporadic defects, and cratering is greatly enhanced for the devices Vahid Goudarzi mentions when discussing Motorola field data. I agree those are limited concerns, no sky falling indeed.

What still scares me (in the case of critical applications) or concerns me (in the case of expensive applications) is the naivete with which many seem to think we can learn much about sporadic disasters or long-term reliability of those from consumer electronics experiences.

I am not often interested in comparisons to actual life of SnPb either (any more). We face ever more applications (designs and service conditions) for which we don't have sufficiently accurate critical experience with SnPb either. The first challenge becomes not to be surprised by effects of long-term aging, combinations of loading, minor differences in pad finish, joint configuration, latent damage, process, .... and their interactions for the specific solder alloy used (!).

While I can't yet extrapolate test results to life in long-term service (I think we are close, but I really need an extra $1M to prove my hypothesis and turn it into a quantitative model) I can show how current models may easily be off by 2-3 orders of magnitude or more (worse, how they may screw up comparisons of alternatives). It obviously depends on the application whether this really matters (I side with companies who have cut drastically back on testing for many applications).

Keep up the good work.

Peter

I will keep in touch with Peter in the future for updates on his perspective.

Cheers,

Dr. Ron

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.

Power amplifiers and transistors come in many shapes and sizes. The performance requirements vary as well. Attaching them can be a critical aspect of your design.

Both Pb and Pb-free alloys can be manufactured as a solder preform with a flux coating.(Learn more)  Selecting the right alloy and flux coating can be crucial to meeting your void criteria.  

A high-tech SOLDERING solution might include NanoFoil®, which effects a solder joint while minimizing heat exposure to your components.

There are also thermal interface materials such as the HEAT-SPRING® which utilize the unique properties of indium to create a superior thermal connection, similar to a solder joint.

There are many different attachment methods available, contact me with your design parameters and we can find your solution.  

• 96.5%Sn 3.5%Ag (221C)
• 80%Au 20%Sn (280C).

It was a pleasure interviewing Pat Gallagher, who developed the first automated photovoltaic solar cell tabbing and stringing machine back in 1979. (Before I was even born!) Pat has seen the tabbing industry mature, and he was kind enough to help answer some questions about the process that I’ve grown to love. 

Jim: What were the initial design goals? How have they evolved over the years with customer’s needs?

Pat: Our primary goal was to replace variable hand labor in soldering with a machine and a process. That still holds today. Back then, solar cells were very expensive, thick, brittle, and not very efficient. So the biggest issue was to avoid breaking cells. Our first advice to the cell people was to turn the crystal 45 degrees to the bus bars so that the sides of the cell wouldn't break off along the solder joints. That little trick remains in place today.


Jim: Were the first machines designed to tab and string separately, or in a combined process?

Pat: Our first design was to make strings of cells in one shot. The two-step process, fronts then backs, was a holdover from hand soldering and there was no reason to do that anymore. Surprisingly, however, we ended up accidentally inventing the mechanized tabber on the way to creating a fully automated one-step stringer.

Jim: So that’s where the stand-alone tabber came from! I would have guessed it was the other way around. Have there been any changes to the heating method?

Pat: Oddly, the first thing we tried was induction heating. It was wonderful except that it took 5,000 watts to bring a small solar cell to temperature. It seemed rather wasteful, but that was the smallest industrial RF system available. Then we tried IR light, which also worked well. That's what we used in the first automated system.

Jim: Early tabbing ribbon must have been pretty crude. Have you noted anything that has changed with the copper or solder coating used over the years?

Pat: Basically, it's the same flat conductor that we started with in the 70's. The coating chemistry has changed dramatically. Taking cues from the electronics people, we started with lightly tinned copper and that was it. Solder was introduced on the cell so the ribbon did not need a heavy solder coating as is common now.

No-Lead (Pb-free) has been challenging mostly because the process window is smaller and simply hotter. The cells can be hurt if heat exposure is too long or too fast.

If you’d like to meet Pat (the President of Solar Automation) and learn more, you can email him by clicking here or visit the Solar Automation website.

Folks,

I thought I would take a stab at listing the minuses, pluses, and “it’s a wash” aspects of assembling with lead-free (LF) solder. Here are my first thoughts. Please tell me what I missed or disagree.

Cheers,

Dr. Ron

Minuses

1.    Pb-Free requires higher reflow temperatures
The Tm for LF solders, in the 217-229C range, has created numerous challenges:

a.      PWB warpage and damage

b.      Component damage

c.      New defect modes such as graping and head-in-pillow defects (although concurrent reduction in solder paste deposit sizes for 0201 and 01005 passives and 0.3 mm CSPs also exacerbate these defects)

d.      Defects related to increased oxidation

e.      Increases in voiding

f.       Increases in tombstoning

2.      The higher cost of LF solder, mostly for wave soldering

a.      It’s not just the silver, tin is much more expensive than lead

3.      Poorer wetting of LF solders, creating the most significant challenges in wave soldering

4.      More rapid copper pad dissolution on PWBs in wave soldering

5.      LF solder attack of wave solder machine components

6.      LF reliability in harsh thermal cycle testing appears poorer than tin-lead solders

7.      Tin Whiskers

It’s a Wash

1.      Short-term reliability in consumer product-type environments

2.      Protection of the environment if discarded products are improperly disposed of

a.      Lead in electronics has never been shown to cause a problem in land fills

3.      Since July 2006, about $3 trillion of products have been manufactured with LF solder, with no “the sky is falling”-type of problems

Pluses

1.      LF solder's poor wetting enables finer lead spacings (see photo Courtesy of Motorola)

a.      It may be argued that some modern electronic products (e.g. smartphones) could not be made with tin-lead solder

2.      It is safer to recycle LF solders, especially if performed in a non-controlled environment