Solder Reliability

Maria Durham, Indium’s new Technical Specialist in Semiconductor and Advanced Assembly Materials, has been doing some research on indium lead (In/Pb) solder alloys. We chatted about her findings this week. 

 [Andy C. Mackie: ACM] Which indium/lead solder alloys are most common, and what are their properties?

[Maria Durham: MD] Firstly, the use of lead-(Pb-)containing solders in some soldering applications is restricted due to local environmental and RoHS compliance, but there are still many applications where they are  allowed. Many military, aerospace, and industrial equipment uses, as well as many applications related to vehicles, are exempt. The table below shows the most common indium/lead (In/Pb) alloys (pink) and their properties, sorted by liquidus temperature; the higher of the two melting points (solidus and liquidus) seen for non-eutectic alloys. In blue are three comparison materials.

Indalloy 205 is the most commonly used, probably because it has the closest liquidus temperature to the tin/lead eutectic (183°C), 63Sn/37Pb (Indalloy 106). This means it can be reflowed using a standard Sn/Pb eutectic profile. The next most common alloys that are used are Indalloy7, 204, and 206.  Besides the melting range, indium has comparable thermal and electrical conductivity to standard materials.

[ACM] What makes indium-lead (In/Pb) solders so attractive, and why have we seen a recent resurgence in their usage?

 [MD] One main attraction to using indium/lead (In/Pb) solder alloys in soldering to precious metal surfaces is that, unlike tin-containing solders, they do not leach gold. That is, gold does not dissolve in them to any appreciable extent. During discussions at Semicon West in 2011, one of our California customers reported going through 8 simulated reflows with Indalloy 205 in contact with a gold surface with no loss of joint strength and no joint embrittlement. That is pretty impressive. Note that embrittlement is often caused by gold-intermetallic formation. It has been noted that even at 250°C, 50In/50Pb dissolves Au at a rate 13 times slower than it does into 63Sn/37Pb, although this, of course, is a kinetic, not a solubility limit, study.

The higher melting Indalloy 164 (92.5Pb/5In/2.5Ag) has the lowest coefficient of thermal expansion (CTE) of all of the In/Pb solders and is able to withstand the higher temperature excursions that can be seen in step-soldering type applications (where a very high melting solder is used to form the first joint, followed by a next lowest melting alloy, and so on). This is seen in applications such as power electronics assembly, where the first step solder is often used for die-attach either as a solder paste, wire, or preform. The high melting point helps the solder withstand the operational temperatures associated with under-the-hood electronics, in applications such as engine control modules, where Indalloy 151 (92.5Pb/5Sn/2.5Ag) or Indalloy 163 (95.5Pb/2Sn/2.5Ag) are most commonly used. In/Pb solder is excellent on very rigid structures such as ceramic-to-metal or ceramic-to-ceramic. The desired solidus / liquidus temperature range can be adjusted by changing the indium:lead ratio, making it very easy to “dial in” the alloy to a specific reflow process.

Another attraction to using In/Pb solders is that they exhibit good fatigue resistance in thermal cycling from -55°C to 125°C.  In testing, the 50In50Pb solder joint fatigue life is about 100 times greater than that for 63Sn/37Pb.

 [ACM] What fluxes are used in these applications, and how are they formulated differently?

 [MD] The fluxes most compatible with the lower melting point (<200°C) indium-containing solders are NC-SMQ-80 (solder paste) or the lower-tack TacFlux® 012 (suitable for use with wire, preforms, and spheres). These are no-clean fluxes, specifically formulated for lower temperature reflow.  Under appropriate low temperature reflow these fluxes leave behind benign residues that do not need to be cleaned off (“no-clean” flux), although they are often cleaned off in most practical applications, usually to ensure reliable wirebonds absent of flux spatter.

 To learn more, please contact us.

 Cheers!  Andy

Reducing the surface oxides of Nitinol is just the first step in getting a good solder joint with this versatile medical assembly material.

Next you have to choose the right solder alloy.  You will probably want to stay away from anything containing lead, cadmium, or antimony, particularly in medical applications.  And you will want something with a high tensile strength.

The best choice is Indalloy #121 (96.5Sn 3.5Ag).  It has a tensile strength of 5,620 PSI and a melting temperature of 221C and is obviously lead-free.  It wets well to the cleaned Nitinol.

If you need a higher melting temperature solder (one that can withstand autoclave temperatures for example) you should consider Indalloy #182 (80Au 20Sn) which melts at 280C, has a tensile strength of 40,000 PSI, and has long been considered a highly reliable solder.  Additionally, this alloy is available in very fine diameter solder wires to minimize waste.

Soldering temperatures should be 25C to 50C above the liquidus temperature of whichever solder you use and proper cleaning should be always be performed afterwards.

Contact us at medical@indium.com for more information about soldering for medical devices or visit our web site at www.indium.com/medical

Carol

Folks,

Patty is getting ready for her meeting on "Copy Exactly" with Mike Madigan.......

It was after 6:30 PM and Patty was just arriving home.  Since Patty was working late, Rob had agreed to make his signature dish, crispy macaroni and cheese.  Patty and Pete had just finished their project to develop a copy exactly strategy for ACME.  They would present it tomorrow to CEO Mike Madigan.  The local GM, Sam Watkins, would be there too.  Technically Mike was her boss in her Senior VP position, but since she had an office at the ACME facility in Exeter, NH, she reported to Sam - “dotted line.”  Patty had been working late for weeks on this project and was glad that the greatest portion of the work was over.

As she opened the door to her house, her twin 2 year old boys ran up to her in their excitement to see their mom and nearly knocked her over.  She tussled with them for a few minutes and then went to give Rob a hug.  He had the dinner on the table and they all quickly sat down.  Rob and Patty had a "no technology" rule at meals…..no mobile phones, iPads etc.  Meal time was family time.  After discussing the events of the day, Rob’s face lit up.

“I found out today that there is something we look at more than anything else,” Rob stated.

“OK, OK, let me guess,” Patty replied.

After a number of tries, she hadn’t gotten it.

Alright, I give up, Patty said with playful exasperation.

“Indium, or really Indium Tin Oxide (ITO), it is a transparent conductor of electricity.  We look through it when we look at our computer, tablet or mobile phone screens.  Think about it, for most of us we probably look through ITO for 8 to 10 hours a day.  It’s like we have a love affair with the stuff,” Rob explained.

Patty almost choked on some of the mac and cheese on the last comment.

“Why have you become such an expert on this stuff?” Patty asked.

“Well, you remember that ACME may go into component assembly? Sam asked me to look into indium thermal interface material (TIM)  for some of the component packages that need to dissipate a lot of heat,” Rob answered.

Patty knew a little bit about TIMs, but not about ITO.

“But why did you learn about ITO?” she asked.

“Sam is worried that Indium supplies may not be enough to satisfy TIM requirements, so he asked me to look into it,” Rob answered.

“What is the conclusion? Patty asked.

“Well, Indium is about as common in the earth’s crust as silver, but a little more difficult to extract.  This probably gives it the reputation of being rare.  Fortunately for me a recent analysis was performed that showed that the indium supply will be more than adequate for the next 75 years ,” Rob said.

Rob went on, “Indium is a very interesting material, it is one of the few materials that wets glass, so it enables metal sealing to glass.  It was only discovered in 1863 and it wasn’t until the 1930s that the first practical use for indium was discovered: aircraft bearing lubrication.  In a sense, it could be argued that it is one of the materials of the future, as we are just now learning about its potential.”

While he was talking, Rob reached into his backpack and took something out.

“Look at this, or rather listen,” Rob said.

With that, he took a thin bar of metal and bent it. A crackling sound came from the metal.  Patty was fascinated.

“What was that?" she asked.

“When a thin bar of indium is bent, it gives off a sound.  It is called “Indium Cry.”  The salesman for the TIMs we are using let me borrow it for a presentation I am giving to Sam Watkins next week,” Rob answered.

Dinner was soon finished and Patty had to get the boys to bed after playing with them for awhile.  Today was Spanish day and all of their discussions were in that language.  Another day was Mandarin Chinese day.  The boys already understood the three languages spoken at home.

A few hours later, Patty lay in bed - energized by the thought of her meeting tomorrow.

When she woke up the next day, she exercised at home, ate breakfast, and took the boys to day care.  See arrived at the office 30 minutes before the big meeting.  After checking emails, she went to the conference room where the meeting would be held, to set up her computer.  At precisely 8AM, Mike Madigan and Sam Watkins arrived.

“OK Coleman, let’s get this show on the road,” Madigan commanded.

“Since our last meeting we have analyzed assembly equipment and materials to determine which ones would be best for a copy exactly strategy,” Patty began.

She then showed her third slide and spoke to it.

“The winner for component placement equipment is Optoplace, as are their stencil printer and reflow ovens.  Exactotest makes the winning testers and ElectoMaterials the best solder paste and solder preforms,” Patty went on.

“Can you explain your methodology?” Sam asked.

“We looked at what The Professor calls ‘Profit Potential,’ simply the equipment and material that gives the most profit, assuming you are running a well tuned organization.  Fortunately, since ACME has 80 assembly lines we were able to get real process performance data on all of the major machines available, ” Patty answered.

“You answer seems a little evasive, why didn’t you use ‘Cost of Ownership?’” Madigan challenged.

“Some machines cost less to own, but they are down more for assists and when they need repair, we have to wait longer for the repair man.  From what The Professor taught us, uptime is very important. Anything that hurts uptime, like a late repairman or a machine that needs more assist time, will hurt profits.  The same is true for materials like solder paste.  If they cost less, but result in line downtime for response to pause issues or some other fault, they hurt profitability.” Patty responded.

Just then Sam’s administrative assistant, Clare Perkins opened the door.

“As you requested Mr. Madigan, your guest is joining the meeting,” Clare said.

“Well Torant, looks like Coleman said you lost,” Madigan said to the new arrival.

Upon seeing Rex Torant, Patty became a little unsettled and Pete turned his famous crimson red.  Patty and Pete called him “Rex the Torrent” as he spoke so rapidly when trying to sell them something.  Both found this manufacturer’s “rep” annoying.

“Everyone, I invited Rex to the meeting.  We met at the airport last night and started chatting.  He assured me that his Pinnacle equipment line and Ultima solder paste would be the winners today since they have the lowest cost of ownership,” Madigan explained.

Torant saw the slide announce Optoplace, Exactotest and ElectoMaterials as the winners.

“My products are just as reliable and cost 30% a year less to own,” Torant fumed at Patty.

Patty had not anticipated Torant’s attendance at the meeting but had prepared for this type of question.

“Mr. Torant is correct, however Pinnacle’s component placement machines have more downtime for machine assists and, when the equipment does malfunction, it is down for repairs on average for 28 hrs, whereas Optoplace is only down for 14 hrs.  All in all, Optoplace machines are up 6 hrs more a week in a two shift operation,” Patty calmly responded.

Will Patty’s arguments win the day?  Can a 30% more expensive machine really have more “Profit Potential?”  And what about the solder paste and materials?  Stay tuned.

Cheers,

Dr. Ron

image

I recently had a chance to catch up with a friend and colleague, Jasbir Bath. If you’ve spent time in the electronics assembly industry you have most likely met him, heard of him, or used an industry standard that he has helped create. Jasbir is a founding member of The Solar Engineering & Manufacturing Association,  SEMA. Who better to talk to about a new association than a founding member?

Jim: The Solar Engineering and Manufacturing Association (SEMA) is a relatively new association for engineers in the solar industry. Can you tell me a little about why it was created?

Jasbir: It was created about 2 years ago based on a need by the solar engineering/manufacturing base to address issues in the industry. There are many organizations in the solar industry but none are wholly dedicated to the engineering/manufacturing profession. SEMA was formed to address this need. We are working to address a number of gaps in the industry highlighted by the SEMA membership which include Education, Training, Standards, Reliability, Cost Reduction and Technology Gaps.

SEMA is a group of engineers, manufacturers and related professionals in the solar manufacturing and related disciplines who volunteer to conduct activities in the organization. The projects/programs we work on are driven by the active involvement of the membership.

Further details on SEMA and what we do can be found on the SEMA website at www.solar-ema.org

Our membership costs are low as we are not an organization looking to make a profit but to encourage participation and work to advance the solar industry as well as advancing education, training and collaboration within the solar manufacturing industries.

Jim: I heard there’s a new solar conference coming up? Can you tell me what makes this one different than all the other solar conferences we go to throughout the year?

Jasbir:  SEMA is collaborating with SMTA (Surface Mount Technology Association) to develop a conference meant for engineers and managers in the field to look at the areas of concern in the industry and develop ways to address them. We don’t see a similar conference to this which covers such a broad range of subjects which is specifically focused to address the needs of the industry. The program will consist of presentations and discussion covering the reliability testing of PV Modules covering gaps and where future work needs to be done. It will highlight various reliability programs being done in the industry with an assessment of current and evolving standards in manufacturing and reliability.

We are pleased to have a great line up of speakers and presentations. SEMA will present its reliability report assessing the reliability of PV modules at the conference. We will also have speakers from UL, IPC and NREL to discuss international solar standards together with a discussion of the work of the PV QA Task Force forum from leaders in that Task Force group. Areas covered will include temperature, humidity, voltage, mechanical and UV testing of PV modules and diode testing.

We will also have presentations on the reliability of microinverters/inverters and future trends from organizations including Sandia. PV Manufacturing Issues will be discussed by companies including Flextronics. The Global Solar Outlook will be reviewed by companies including Navigant, Custer Consulting and Prismark. Finally we will review general PV Module hazardous issues such as Electrical and Fire Concerns and well as Module Warranty/ Traceability Issues.

In addition we have industry leading training courses at the event on PV Module Manufacturing and Troubleshooting and PV Standards in addition to exhibitions.

The SEMA/SMTA Conference, Training Courses and Exhibition are from March 21^st to 23^rd at the Fairmont Hotel in San Jose. Further details on the program and sign up can be found at http://www.smta.org/solar/

Jim: One more question for you Jasbir. I know from working with you in different associations, that you are personally invested and involved in the future of module assembly. What attracted you to this field, and what keeps you interested in it?

Jasbir: I have been involved previously in the electronics manufacturing industry during the transition from tin-lead to lead-free soldering due to environmental legislation requirements. This was a challenge being involved in both from a technical and logistics perspective, but it was also fun as you saw the rewards of your efforts when the transition occurred successfully.

The solar/PV industry has challenges in addressing how to produce good quality and reliable products at lower cost, and it gives me the opportunity to try to make a positive contribution in an evolving expanding industry.

Jasbir and I look forward to seeing you in San Jose!

~Jim

Readers have asked how to visually assess a tabbing ribbon interconnection after a bond test.

This image is a cell that has been bond tested after soldering.

The first indication that you have a good bond is the physical resistance during the bond test. Even if you are peeling the ribbon off by hand, you will still notice if the ribbon jerks as it tears away from the cell. Fluctuation of bond strength may be caused by insufficient or inconsistent tabbing parameters, incomplete fluxing, or even contamination on the tabbing ribbon. If the resistance varies rapidly across the length of the bond, there could be an issue with microcracks. Microcracking of the underlying silicon is usually caused by built-up CTE (Coefficient of Thermal Expansion) stresses from tabbing. The ideal bond will peel apart where the tabbing ribbon meets the metallization, and it will be uniform. It should look like the image seen here.

There are some things you can do before, during, and after tabbing to get a better looking, and higher reliability, tabbing bond.

Before

Consider using alternative tabbing alloys and fluxes. Using Bi-based alloys at lower temperatures will lower the stresses caused by CTE mismatch and help eliminate microcracking. Softer tabbing ribbon can help keep stresses to a minimum as well.

During

Cell tabbing/stringing machines have many adjustable parameters. You owe it to your customers to explore the effects of parameter changes so you know you are building the best modules possible. (If I have time I’ll probably come to your facility to help – all you have to do is ask.)

After

Not everyone has time to wait, but if you have the luxury to let the tabbed cells sit for a day you should notice much better test results. Stresses built up in the silicon are partially relieved after 24-48 hours, which will result in less microcracking.

Let me know if I can help you make some beautiful cell interconnections!

~Jim (jhisert@indium.com)

Folks,

A reader writes:

My company makes an electronic product that requires a 40 year shelf life. We assemble with tin-lead solder on FR-4 PWBs. The product is to replace older technology (i.e. 1960-70s), but has some newer components such as BGAs, SOICs, and PQFPs. The product will be stored in dry nitrogen at 70F.  We take great care in manufacturing, by cleaning, inspecting, and testing the end product.

My question is, do you know of any studies that would discuss the reliability of products stored or in use for 40 years?

My sense is that our reader will be successful, but his question is profound and hard to answer with confidence. The military would like their electronics to perform for that long, but realistically much of it is replaced every ten years or so. If you look at something like the B-52 bomber, which debuted in 1952, the electronics have been upgraded regularly. So there isn’t as much 40 year electronics experience as one might think. An exception being the IBM AP-101 computer. This computer was kept in service for over 30 years, because it served its function and had survived the rigorous and expensive military qualification testing.

However, anecdotal data might support optimism for 40 year shelf life. In a class I teach at Dartmouth, The Technology of Everyday Things, I have sought out some old transistor radios from the late 1960s and early 70s to show the class how this old technology works. Anytime I have every found an old device like this, they always work, unless the batteries have leaked inside the radio.

This question raises an interesting thought. Although those of us in electronic assembly are concerned with tin-lead and lead-free solder joint life, what about the modern devices inside the components? Forty years is a long time. How will the 3D-22 nanometer copper circuit lines in a modern microprocessor hold up over this amount of time? These circuit lines lines are so fine that the 22 nanometer width is only about 70 atoms.  In addition, copper integrated circuits are still a relatively new technology. I’m sure much accelerated life testing has been done on such circuits, but would such testing confirm 40 years of shelf or service life?

I would appreciate any thoughts that readers have on these questions.

Cheers,

Dr. Ron

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 thought I would post a few short thoughts as the new year begins. Here it goes:

1.    A billion hours ago the stone-age was the future, a billion minutes ago Caesar ruled Rome, a billion seconds ago Jimmy Carter was President, a billion passives ago you took your last break (about 4 hours ago). As exciting as the latest quad core microprocessor is, the largest number of components that we assemble is passives, approaching two trillion per year. That is about 6 billion a day. If you lined up all of the 7 billion people in the world, each year you could give every man, woman and child several hundred passives from all of the passives that are produced. If two trillion passives (assume 0402s) were lined up end to end they would circle the earth 50 times!

2.    Schools in Indiana are no longer required to teach cursive writing. Key board skills are considered more important.  Yikes! I’m all for keyboard skills, but I want my grandkids to be able to write in cursive. If not, how do they write their names? Are we less than a generation away from people writing their names as an “X?”

3.    Thoughts on lead-free solder reliability in long term mission critical environments from a NASA study:

        “Test vehicles assembled with lead-free materials (notably tin-silver-copper) exhibited lower reliability under some test conditions.”

Some people would respond to this statement by saying, “I told you that lead-free solder was no good.” However, another way of stating the results would be, “Lead-free solder performed better in more tests than tin-lead solder did.” The ratio, by my count, was about 5 to 3 in favor of lead-free. However, I agree that lead-free is not ready for mission critical (>20-year) service life. The main reason being that, in some cases, when lead-free solder joints failed in these types of studies, the results were much, much worse than tin-lead solder joints. These failure modes need to be understood and addressed. In addition, tin whiskers and pad cratering are looming problems in these, mission critical, long service life quadrant D applications as discussed in the Navy's Manhattan Project (http://www.navyb2pcoe.org/pdf/LFEMP_book.pdf).

4.    SACM has arrived. SACM is a SAC105 alloy that is doped with manganese. Work performed on SACM by Liu, Lee, et al was reported in a May 2009 ECTC paper, Achieving High Reliability Low Cost Lead-Free SAC Solder Joints Via Mn Or Ce Doping. The thorough testing reported in this paper suggests that SACM has promise as a material candidate for quadrant D applications mentioned in #3. In explaining the superior performance of this material the authors state:

“The mechanism for high drop performance and high thermal cycling reliability can be attributed to a stabilized microstructure, with uniform distribution of fine IMC particles, presumably through the inclusion of Mn or Ce in the IMC.”

We have had to wait awhile for this material to become commercially available as it is a challenge to manufacture doped solders like this in large quantities.  I think this paper should be on you "must read" list.

5.   I had not planned on reading Steve Job’s biography , as I thought I knew quite a bit about him from reading recent articles in Forbes, Fortune and Business Week. But I went ahead and downloaded it to my Kindle anyway. This work by Walter Isaacson is a masterpiece. To share one tidbit from it that relates to those of us in electronic assembly: 

"In almost all cases electrical engineers first design the circuits that perform the functions of some device like a mobile phone or tablet. Mechanical Engineers are then left to fit the circuits into the “box.” (Hence MEs are often called “box stuffers” by EEs). Jobs completely changed this approach. He told the engineering team how he wanted the product to look and function first, then they had to determine how to make it work that way. I’m convinced that only through this approach are the revolutionary design concepts that Jobs and Apple came up with possible."

The book also points out his many flaws (e.g. Jobs would regularly park in handicap spots, the author reports several times that Jobs just didn’t think the rules applied to him, etc.). Another interesting thought (read it and see if you agree with me) that if Steve was not Paul Jobs' adopted son, Apple would have never happened.

Cheers,

Dr. Ron

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

Folks,

The Guadalajara (GDL), MX Chapter of the SMTA held their first meeting on November 9 and 10 at CETI in GDL.
Approximately 70 engineers from local companies attended. It was a great success. 

The agenda was:

November 9^th, 2011

0830-0900am    Registration and Exhibits Open

0900-0915am    Welcoming Remarks and Exhibition starts

0915-1045am    Inventec: “Reliability Assessment regarding Flux residues"

1045-1215pm    Sanmina-SCI: “Capabilities of a Failure Analysis Lab”

1215-1315pm    LUNCH

1315-1445pm    DEK: "Optimizing the Print Process for Mixed Technology"

1445 -1615pm   Vitronics Soltec: “How to Choose a Robust Configuration for Equipment
                          for Defect Free Soldering - Reflow, Wave and Selective”

1615-1745pm    KIC: “Fixing Reflow and Wave Related Defects as Well as How to Avoid 
                          Them in the First Place”

 November 10^th, 2011

0900-1030am    Sanmina-SCI: “Process development of 01005 components”

1030-1200pm    Indium: "Lead-Free Assembly for High Yields and Reliability."

1200-1300pm    LUNCH

1300-1430pm    Universal Instruments: "Tutorial on Failure Analysis"

1430-1600pm    Zestron: “PCB cleaning before conformal coating”

1600-1730pm    Kester: “Understanding Soldering Chemistries - Reducing Costly
                          Defects, Increasing Yields and Reliability.

I spoke on “Lead-Free Assembly for High Yields and Reliability." We had several raffles and gave away autographed copies of my book “The Adventures of Patty and the Professor,” which has just recently been formally published. 

As usual, I had dinner at Santo Coyote, one of my favorite restaurants, however my Mexican friends also took me to Sacromonte, claiming it had better food. They were correct. I was convinced to try chicken mole which I liked. It is tough to beat Santo Coyote’s ambiance, however.

I can’t cite data for this, but I am quite sure that GDL has the largest number of workers in electronics assembly outside of Asia. It is great news that they now have an SMTA chapter to help the local engineers network and continue to grow in their skills.  It was great to play a small part in this success, but most of the credit must go to Indium Corporation’s Ivan Castellanos who is chapter president and Kester’s Miguel Vazquez, chapter vice president.

Cheers,

Dr. Ron

Folks,

Let's look in on Rob:

Rob looked at the new photo of Patty and their twin sons Peter and Michael. What a handful those two 18 month-olds were. Just like their mother! Rob was still pinching himself that he was lucky enough to have Patty as his wife. Rumors were that she would make VP soon, and a few of his buddies asked him if her success bothered him. 

He would always respond, “Let’s see: beautiful, successful, athletic, fun to be with, great mother, and most of all she loves me. What’s not to like?”

Rob really meant it. He felt Patty deserved her success. One of her great assets was her high energy level. She went to bed at 11:30PM and was up at 5AM to run two miles, lift weights, shower, and then take care of the kids. Rob just couldn’t keep up on less than seven and a half hours of sleep. So he got up at 7AM. Rob had to insist that they have some quiet time each night after the boys were in bed, to talk and maybe even watch some mindless TV. But Patty would often sneak her laptop out to work while NCIS was on. Patty, the workaholic!

Rob and Patty spoke Mandarin at home one day each week and Spanish another night. The boys were picking up all three languages. It was amazing to both Patty and Rob as they watched this miracle.

Well anyway, Rob did have one thing up on Patty: math. Rob was close to a math genius and also good at writing software. He was the “go to” guy for math modeling and writing software for the math models. He even helped the Professor improve ProfitPro. Rob also wrote a program that could be used to design an SMT line for maximum throughput. The software could do what Arena did in hours of simulation, in seconds.

Rob was startled from his daydreaming by the phone ringing. It was Sam the site GM.

“Hey Rob, we need your help in our plant in Guadalajara. Can you come right down to discuss it?” Sam asked.

“I’ll be right there,” Rob replied.

Rob walked to Sam’s office with a feeling of exhilaration. It was always fun and exciting to be sent on a trouble shooting mission.

“Rob, thanks for coming right down. This issue is QFN reliability. About 5% of the Druid mobiles phones in our Guadalajara plant are coming back with some of the QFNs burned out,” Sam began.

“Sounds like a voiding issue under the QFN thermal pad,” Rob interrupted.

“Wow, you seem to know quite a bit about this type of problem,” Sam remarked.

“Remember how I pleaded with you to go to SMTAI,” Rob teased.

“Yep,” Sam replied.

“Seth Homer gave a talk on this issue at the show last week. It was a terrific overview of the problem. From what you described the connection may need more solder. We may have to use solder fortification preforms to solve this. Optimizing the solder paste printing process may not be enough,” Rob summarized.

“Well, go there and solve the problem. The warranty issues are costing us a fortune,” Sam commanded.

After a moment of contemplative silence, Sam asked, “Do you need anything?”

“It would be nice to have Pete come. He knows the people there and is well connected. His Spanish is also terrific,” Rob said.

“OK, no problem. Since you sleep with Pete’s boss, you can work out the details with her. I need you to go this week,” Sam said.

“No problem,” Rob said.

As Rob left the office, he was elated with his new assignment. He had to admit though, he thought it was unprofessional and a little annoying of Sam to say, “since you sleep with Pete’s boss, you can work out the details with her,” but it wasn’t the first time someone said this. Truth be told, Patty might be a little annoyed. She really depended on Pete.

Will Patty be angry at Rob for taking Pete to Guadalajara? 
Will Rob solve the QFN problem?
How does Patty get by on only 5.5 hrs of sleep each night?
Stay tuned for the exciting conclusion.

Folks,

Some time ago, I mentioned that I was working on a consensus of the status of lead-free/RoHS compliant assembly. My hope is to find data and facts that will support the consensus. I am making progress, but at this time I would like to share the subtopics in the consensus. Look them over and see what you think:

1.       Was/Is lead-free electronics/RoHS needed to protect the environment?

2.      Is lead-free solder easier and safer to recycle than lead-containing solder?

3.      How has the increased use of tin and silver affected their supply and price?

4.      How much did it cost to implement lead-free/RoHS compliant electronics?

a.      What is the cost adder to a typical lead-free product?

5.      What are the process challenges of lead-free assembly?

a.      Are these challenges being addressed?

b.      If so, how?

6.        What is the reliability of lead-free vs leaded electronics for commercial applications?

a.      E.g. 0C to 100C thermal cycle, drop shock

7.        What is the reliability of lead-free vs leaded electronics for harsh environment/military applications?

a.      E.g. -55C to 125C thermal cycle, other Mil stress tests

8.      What is the threat of tin whiskers, tin pest and other similar lead-free related reliability phenomena?

9.      What is the status and need for halogen-free assembly?

Help me by suggesting topics that I have left out.     
Contact info here.
Cheers,
Dr. Ron

 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.

Last week I spent some time in the simulation lab with Eric Bastow, verifying the printing characteristics of our newest low temp metallization paste LT-918. Due to its current success with a variety of customers, we needed to take production capacity to the next level. New equipment was purchased to keep up with the demand, but there is always the chance that material may not perform the same when it is made in substantially larger batches. Our testing confirmed the printing characteristics of the material made on the new equipment surpassed that of previous batches. That’s good news for everybody.

As you can see from the picture, we used a standard printer designed for stencil printing solder paste onto electronic circuit boards. The printer was not the only similarity to solder paste printing though. An interesting characteristic of LT-918 is that it has a higher viscosity than most metallization pastes, which helps with print definition. The high viscosity of LT-918 helps it print like a solder paste, this is great for solder paste printers (like Eric and I, and many of you for that matter) from the SMT and semiconductor assembly industries.

In my opinion, LT-918 is the best metallization paste currently available for interconnecting thin-film cells. It has not only excelled in printing, it also has industry-leading resistivity scores, and has passed customer reliability testing including thermal cycling, damp heat stability, and accelerated UV tests. Much of the data that we can share will be available soon as a product brochure that we hope to have ready for you at EU-PVSEC in September.

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