Silver Solder Paste

Musings on Metals: Copper

It could be argued that civilization began with the smelting of copper.  Although thousands of years before, humans fired clay to make figurines and containers, smelting required several non-obvious steps.  After all, the firing of clay, at some level, can be accomplished by simply dropping clay into a fire.

To smelt copper, our ancestors had to:

1. Take malachite (see photo) or another copper ore, grind it up or break it into small pieces
2. Mix the ground malachite with carbon
3. Heat the mixture in a vessel to 1,085^oC. 

Malachite Ore

Achieving this temperature with a wood fire is, to me, astounding.  Think about those days when you are grilling some burgers.  You leave the grill on after the burgers are done, to burn off the grease.  You come back 20 minutes later and the grill is at 500^oF.  You can feel the heat.  Even touching the knob to turn the gas off is intimidating, as the heat drives you back.  This temperature, 500^oF, is only 260^oC!  The ancients reaching 1,085^oC with wood and bellows is, indeed, impressive. By the way, a good rule of thumb to convert degrees C to degrees F from 100^oC to 1,500⁰C is that 2XC=F, this fast approximation is accurate to about 10% in this range.

The confluence of the three procedures is not only non-intuitive, but think how many times the smelter of old could only reach 900^oC and failed.  I have argued that if copper melted at 1,200^oC or so, civilization would have never gotten started.  This temperature is perhaps a little too high to reach with a wood fire.  The smelting of copper encouraged investigations into other metals, eventually resulting in the discovery of the processing of iron, an even less intuitive process than smelting copper.  So, I believe that the success with copper was necessary to the production of steel. 

Copper smelting became an industry that encouraged permanent settlements and stimulated trade, which encouraged writing and ciphering.  An effective copper smelter would likely keep secret some of his craft as he wanted a competitive advantage.  He could make more by smelting copper than doing anything else, so he almost certainly was an early specialist.

Considering all of this, I believe that without the discovery of copper smelting, we might still be living in huts or teepees, using stone tools, and living a nomadic existence without commerce, writing, or mathematics.  Examples to support this thesis are the state of native peoples in the Americas in the 1400s.  These native peoples had never learned to smelt metals and hence also lacked the follow-on aspects of civilization mentioned above.

Today, copper is a foundation material for electronics, given its excellent electrical conductivity, second only to silver.  Copper’s ductility likely aids in the formation of PWB traces and plated through-holes in that it resists cracking.

Additionally, copper's ability to form an electrical and mechanical bond with solder is another trait that makes it a winner as an electrically-conductive assembly material in modern electronics.

Copper has been used for more than 10 millennia, but, as with most metals, 90 to 95% of it has been mined since 1900.  About 15,000,000 metric tons (MT) are used each year, third to aluminum’s   22,000,000 MT and steel’s unequaled 1,000,000,000 MT.

In the next installment, we will discuss tin and how it forms an intermetallic with copper during soldering.  Thus making solder paste, solder wire, and solder preforms critical components of electronics assembly.

 Cheers,

Dr. Ron

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

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

最近有一家供应商突然中断提供了银锡焊接材料，所以有些客户朋友们马上向Indium公司资讯。还好，Indium一直为大家提供银锡材料。

Sn96.5Ag3.5是221°C的共晶材料。这种银锡材料很早以前就被分层焊接(step soldering)和在汽车工业上使用(automotive industry ), 是最早的无铅材料之一。Dr. Ron Lasky的“Happy Birthday RoHS”博文中有更详细的介绍。

SnAg银锡合金有以下几种特点：

• 高热传导性：High thermal conductivity (33W/mK)
• 低的张应力（ tensile stress ）： 5800psi
• 良好的热循环性能thermal cycling：-55 到125 ⁰ C
  

Cheers,

Acknowledge: Dr. Andy Mackie with Indium Corporation; Dr. Mackie's blog post "Tin/Silver Solder Paste in Die-Attach (Sn/Ag)"

Pic: Google Image

• 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.

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.

Folks,

When the industry was preparing to transition to lead-free solders almost ten years ago (can it have been that long), tin-bismuth solders were serious candidates. Their low melting point, of about 138C, made these solders interesting candidates to replace tin-lead solder. However, if contaminated with lead, tin-bismuth solders can produce a eutectic phase that melts at 96C. In such situations the resulting solder joint exhibits poor performance in thermal cycle testing. Since early in the transition to lead-free solders it was expected that there would be numerous components and PWBs with lead-based surface finishes, this property made tin-bismuth solders unacceptable.

Another aspect of tin-bismuth solders is that they expand on cooling. This phenomenon can result in fillet lift in through-hole solder joints.

However, as we are now well into 2011, almost no components or PWBs have lead-containing finishes and many portable electronic devices have no through-hole components, so it may be time to reconsider tin-bismuth for some applications.

Some years ago, Hewlett Packard (HP) had performed work to show that adding 1% silver to tin-bismuth solder enabled this alloy to outperform eutectic tin-lead solder in 0 to 100C thermal cycle testing. Even at these low reflow temperatures, HP demonstrated solder joint strength with SAC BGA solder balls that was 65% that of tin-lead solder. Expanding on this work, Indium Corporation's Ed Briggs and Brook Sandy performed stencil printing and reflow experiments consistent with the requirements of current miniaturized components using this 57Bi-42Sn-1Ag solder. All of their results were promising. Ed presented a paper at SMTA Toronto,summarized the Hewlett Packard work, and reviewed the results of this new work.

So for applications consistent with 0-100C thermal cycling, 57Bi-42Sn-1Ag solder may be something to consider if the high temperature of SAC solder paste is an issue to components or PWBs in a product

Cheers,

Dr. Ron 

PS: Read my follow-on posting about bismuth.

从去年年中到现在，由于国际原油价格的连续上涨，带动了大宗商品包括金属价格的飙升。近期的利比亚动乱，使情况进一步“恶化”。

我们常有的锡铅SnPb63/37焊锡膏，和无铅锡银铜SAC305焊膏，都含有锡，SAC305还含有3%的银。根据下面的图片，不难看出锡的价格比去年年中涨了近两倍，银的价格更是涨了两倍多。这都使焊锡膏Solder Paste的金属成本上升了。随着原油和大宗商品的价格上升带来各国一定的“通货膨胀(inflation)”, 大家也应该发现，生活中各种商品的价格也有攀升。同理，焊接材料(solder materials)的运输成本，包装成本，劳动力成本，企业运营经费等都跟着上涨。





虽然Indium Corporation的主营焊接材料现在面临一定的成本挑战，但是我们还是采取积极的方法, 在保证品质和服务不变的情况下，给客户们提供物美价优的有竞争力的产品！

Cheers!

Pic:
1. www.lme.com
2&3. www.kitco.com

PS:国内的亲友们都“抱怨”物价的狂涨，美国这里又何尝不是呢。

Folks,

Many people have been infatuated by the price of gold in recent months, but the price of silver has also skyrocketed. In 2000 silver was about $3.00 per troy oz. In the eight years that followed, its price grew to $15/oz. Today it is trading at over $41/oz! This price is almost an all time high, except for the time when the Hunt brothers tried to corner the silver market in 1980. The aberration of their efforts jolted the silver price to just short of $50/oz, but it settled down to $11 or so after the Hunts came under margin call and other pressures.

Unfortunately, the dramatic price increase today, does not appear to be an aberration. Although we may hope that it will soon drop to more historic levels, we may not have reason to expect that it will.

Although not as dramatic, tin and copper have experienced significant prices increases as well. The price of tin has doubled in the last year to $15/pound and copper has increased from about $3/lb to $4.50.  These metals are obviously key ingredients in critical electronic materials such as solder pastes, solder bar, and solder preforms.

In addition, oil, which is used for most organic electronic materials such as PWB resins, flip chip underfill, and epoxy fluxes, has increased to $110/bbl - approaching its all time high of $145/bbl.

All of these price increases have a significant impact on the electronic materials supply chain. Although we are used to price decreases in the cost of our mobile phones and PCs, at this point in time, the price of the materials that go into these devices will be increasing.

As one materials supply chain executive commented at APEX, “It’s not like we can be clever and somehow work around the price increase of silver and these other materials, we have to pass it on to our customer, or go out of business.”

At the time of writing, the price of silver (Ag) was approaching the USD$50/tr.oz. (Troy ounce) level, and threatening to go higher. With 1 Troy ounce being 31.1grams, this makes the cost of pure silver ingot close to USD$1.60/gram.

Image from goldsilveroz.com

Materials costs are therefore a major consideration for anyone using silver in any form. Naturally, we are now seeing a few Power Semiconductor packaging houses evaluating the possibility of moving away from silver-filled epoxies for die-attach. The alternatives they are considering include the adoption of solder paste (or solder in some other form: wire / ribbon / preforms) versus a silver-filled epoxy.

Here are some thoughts on the Power Semiconductor assembly pros and cons, based on using solder paste as an alternative to silver-filled epoxies.

Good news (+)

+   Reduced materials costs
+   Improved pot-life / shelf-life *
+   Improved high temperature thermal-cycling
+   Strong, metallurgical joint formed between leadframe (substrate) / joining material / die
+   Improved thermal conductivity
+   Faster throughput (more units per hour, UPH)**
+   Easy clean-up ***
+   Does not wick onto NiPd surface to cause poor wire bondability

 * Although it is true that solder pastes are stored under refrigerated conditions, they do not require the -40C storage that is typical of silver-filled epoxies. 

 ** The dispense of solder paste is very rapid and can be done using multi-dot dispense heads. It undergoes rapid temperature reflow, versus the slow cure needed for metal-filled epoxies, which can be up to typically 1-3 hours, depending on the volume of silver epoxy.

 *** Because the solder paste flux does not cure like a polymeric material,  tubing and other conduits for the solder paste are easily cleaned out using common solvents, or can be simply purged with flux.


  ==================

Bad news (-)

-   Capital costs #
-   Adoption time / new process learning ##
-   Needs a solderable die surface
-   Voiding increase ####

 # The main cost-drivers here are:

- Reflow: Specialty reflow equipment is required for high temperature solders, such as
Heller or BTU reflow ovens

- Cleaning: If wirebonding is required after the reflow process, standard cleaning equipment and cleaning chemistry (aqueous or solvent-based) will be needed to remove flux residues

- Gas: Forming gas (H2/N2) or simple nitrogen may be needed to assist reflow.

Note that increasingly, for clip-bonding (non-wirebonding) applications using the new ultralow residue solder paste Indium9.32, even cleaning may not be needed, as the residue has been found to be compatible with compatible with a number of molding compounds in the industry.

 ## By partnering with a company like Indium Corporation with many years of experience in die-attach soldering, the ramp-up time can be significantly reduced.

 ### A solderable surface is usually a sequence of Ti / Ni / (Ag or Au) plated layers. The thickness of the silver (Ag) or gold (Au) precious metal layer is usually limited to 100nm (0.1microns). Compare this to a standard silver-epoxy bond line thickness (BLT) of 0.5-2mils (12-50microns).

 #### Acceptable voiding of less than 5% of the total die area is fairly easily achieved with good quality substrates and die-finishes.

  ==================

In closing, I am indebted to my friend and colleague Sehar Samiappan (Indium Corporation Area Technical Manager - South East Asia) for his insights.

Contact me to discuss this further.

Cheers!   Andy

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

Since I couldn’t find a good beginners guide to c-Si metallization paste, (not even from Wikipedia) I thought I’d provide an explanation of this important module assembly material:

The silicon solar cell has a low-temperature glass-frit paste applied to the active surface. This combination of glass, Ag, and other binder materials is printed onto the solar cell and fired around 850-1000degC to form the solderable metallization on the cell. This glass-silver mixture recombines during the firing process to break through the passivation/antireflective coating layer on the cell and form a strong bond to the cell. During firing the glass and silver are suspended in a mixture with silver forming an electrically conductive path from the top to the bottom of the deposit – and ideally a silver-rich layer is formed on top. This silver is the surface that tabbing ribbon is soldered onto when interconnecting cells.

Because the structure of the glass-silver is formed in the firing process, the firing can impact the solderability of the final metallization. That is the reason it is so important to determine the bond strength and diffusion/intermetallic formation of the interface between the cell metallization and tabbing ribbon solder coating.

Now here’s my challenge to you:

If you know of another good description, post a link to the document in the comments field below!

Thanks,

          ~Jim H.

You’re reading this because you want to know more about LTTF-6363 low temperature metallization paste right? Perfect.

I wanted to get your attention, because I want to talk to you about using this material. We have released a tech paper and (more recently) presented a poster at EU-PVSEC in Valencia earlier this year. With all the buzz about the “Novel Flexible Silver Paste”, I’ve had a chance to print, cure, and solder to LTTF-6363. I’ve had a chance to get my hands (well, gloves…) dirty like a good applications engineer should.

As you may already know from studying the other documents surrounding this material, it was designed for thin-film solar cell interconnection, as a solderable polymer met paste. It went a long way towards teaching us what properties we still need to work on. The material is currently not a product we actively promote, because we know there are properties that need to be improved to give you a wide process window for high volume manufacturing.

Soldering to an epoxy-based material has many intricacies, as I’ve learned. Maximum temperature, Ag dissolution, and a short pot life make processing challenging – honestly, it’s not good for everyone. Of course there’s no way I could have tested the material in every application, but I’m starting to get a good feel for the type of applications that it could work in. 

If you want to evaluate if this material will work for your application, please save yourself some time searching for the answer and just call me: (315) 853-4900 x7592. I can help make your decision easier. 

~Jim Hisert

Folks,

I was at SMTAI (Surface Mount Technology Association International) from September 24 and 27, 2010.   As I mentioned, I chaired a session on Alternative Alloys from 2:00-3:30PM on Tuesday 26^th.

At this session, Greg Henshall presented a paper on the Low Silver BGA Sphere Metallurgy Project. This paper was a collaborative effort of six companies. In addition, Richard Coyle presented an overview of the work of three companies entitled The Effect of Silver Content on the Solder Joint Reliability of a Pb-free PBGA Package. Both of these projects evaluated lead-free thermal cycle reliability as a function of silver content and compared the results to tin-lead reliability.

Both papers concluded that as far as thermal cycle reliability is concerned SnPb<SAC105<SAC305< SAC405. Coyle’s paper summed it up the best:

Each of the SAC alloys outperformed the SnPb eutectic alloy in every test including the long, 60 minute dwell time test. This tends to diminish the argument that SAC is less reliable than SnPb. (See Coyle’s figure. Data curves to the right are more reliable.)

Henshall’s paper also showed that the addition of dopants, to improve shock resistance, in SAC105 does not reduce thermal cycle life.

So, it appears, at this time, that, from a thermal cycle and drop shock perspective, it is looking more and more like SAC based solders out perform tin-lead solders in these two reliability arenas.

At the end of the session a noted lead-free curmudgeon came over to introduce himself.  We have had a jovial disagreement on several blogs etc. in the past re: lead-free status and issues, but had not met in person.   I should mention that this person is a college graduate, a former technical leader at several influential technological companies, and he owns a PE license. I asked him what he now thought about lead-free reliability after hearing the talks. He claimed that he is a little less likely to think that lead-free reliability is a disaster. He still refuses to purchase any lead-free products. He buys old units (pre-2006) on eBay.

I mentioned that over $2 trillion of electronics has been placed in the field since 2006 with no unusual reliability issues.   I then went on to say that a RoHS-compliant product is much more likely to fail due to a non-RoHS related issue. He did not disagree. So then I asked him why he won’t use RoHS compliant electronics. His answer: “I just don’t trust them.”

Cheers,

Folks,

Answers to the quiz of a few weeks back......

Phil and Rob had agreed to ask the GM if it was OK to ask the tech and engineers at some of their subcontractors to take the test anonymously. Over a period of two months Phil and Rob got 52 people to agree, almost all of them after Phil or Rob agreed to take them to lunch. They asked Patty to grade the “exams.” Today Patty would reveal the results.

“Phil, this is one of the best bets I have ever made,” teased Rob.

Everyone at the lunch table chuckled, but the look on Phil’s face said he expected to lose. Rob has said that he thought the average score would be less than 70%, Phil insisted that it would be greater than 85%. In asking the different folks to take the test, invariably Phil started asking questions not on the test. He was surprised that no one knew what tin pest was. He even asked how to time balance a chip shooter and flexible placer, only one in twenty knew.

As Patty approached the lunch table, the ensemble held their breath.

“OK, Patty, tell us the bad news,” Phil said in a resigned tone.

“Rob wins, the average score was 58%,” Patty said getting to the point. “Here are the answers and percentages on each problem,” she went on:

1.    What is the composition of SAC305?
96.5% tin, 3.0% silver, 0.5% copper. 60% got this right.

2.     What are tin whiskers?
Tin whiskers are metal whiskers that can “grow” from tin plating on component leads. They are mitigated by 2% bismuth in the tin, a nickel overplate of the lead copper, a matte tin finish, and a few other mitigation approaches. 40%.

3.     In a stencil aperture, what is the area ratio?
The ratio of the area of the aperture opening divided by the area of the side walls. This ratio is typically used for circular and square apertures. It is equal to D/4t, where D is the diameter of square side and t is the stencil thickness. 40%

4.    What is an approximate peak temperature for a reflow oven in lead-free assembly?
Any answer 235 to 250C accepted. 90%

5.     A board is inspected after wave soldering and one lead is not soldered to the board. The board is run through the wave solder machine again and has the same defect on the same lead. What is the most likely cause of the defect?

a.       The solder temperature is too low.

b.      The pad on the board is oxidized.

c.       The preheat temperature is too high.
b 70%

6.     What are local fiducials on a PWB for?
Local fiducials are located near the pads of a component with fine lead spacings to assure accurate placement. 70%

7.     What does "thixotropic" mean in regard to solder pastes?
The viscosity decreases with increasing shear stress. Hence, during printing the viscosity drops as the paste is forced through the aperture, aiding good aperture fill. It increases as the printed deposit rests, minimizing slump. 20%

8.     A chip shooter places passives at a rate of 36,000 per hour. It is placing 300 passives on a PWB, how many seconds will the chipshooter take to place the passives on one board?
300/36000 = 1/120 hr = 30 seconds. 90%

9.     A reflow oven belt speed is 100 cm/min. The PWB is 40 cm long. What is the minimum cycle time that the oven can support?
The amount of time that the belt needs to cover 40 cm is 40/100 = 0.4 minutes = 24 seconds. This is the minimum cycle time the oven can support. 40%

10.   What is "tombstoning"?
Tombstoning is observed when a passive component's terminations experience unequal wetting forces which are strong enough to lift one end of the passive so that it looks like a tombstone. 60%

Overall average score 58%.

“Wait a minute Patty, your answers are too demanding,” Phil shouted.

“Calm down Phil, I gave full credit for anything close,” Patty responded.

In unison, almost everyone at the table sighed “Yikes.”

Patty interjected, “One person who received a 70% commented after completing problem 9, ‘I didn’t think I would need a PhD in math to do this quiz.’ “

All agreed that organizations like the SMTA and IPC were more needed than ever.

Patty, Rob, and The Professor finished their tasks in Shenzen and were flying to Shanghai for their last set of challenges in electronics assembly.  Then they would head back to the US, Rob and Patty being only a week away from their wedding day.

As usual Rob, conked out as soon as the plane lifted off. Surprisingly, The Professor also drifted off to sleep. Patty was too excited to sleep. Rob’s mother had given her and Rob their wedding presents early … an iPad  for each. They decided to bring only one laptop and one iPad. Patty was a little nervous about using the iPad for presentations but it worked quite well. She was still surprised that the iPad did not have a USB port. The Professor also gave each of them an early wedding present, a Pickett slide rule for Rob and a K&E slide rule for her. She must be the only person in the world right now that was watching a movie on an iPad and solving a math problem with a slide rule!

True to form, The Professor was passionate about how learning to use a slide rule helped improve a person's innate math ability. He showed Patty and Rob how to use them and gave them several assignments. Rob was better with his slide rule than Patty due to the amount of “one on one” time he had with The Professor. She had to admit that using the “slip stick” gave one more of a feel for calculations and it was consistent with one of The Professor’s adages: “Always know approximately what the answer to a calculation should be…..it will help you to avoid errors."

In addition to the iPad and slide rule, Patty was excited to be going to Shanghai at the time of the World Expo 2010. Our trio had scheduled some time at the expo into their busy schedule.

Their plan was for Rob and The Professor to work on some productivity issues and for Patty to take on some of the process materials related problems. The three of them again met with the site GM for ACME’s newly acquired plant in Shanghai, a Mr. Wong. Wong was relieved to find that they all spoke Mandarin, as his English was a little rough. When The Professor addressed him in excellent Shanghainese, everyone was speechless. Patty was determined to ask him about this later. No American spoke Mandarin, Cantonese, and Shanghainese!

They again agreed to stick to Mandarin. Patty headed out to the line, accompanied by a young Chinese engineer, Zhou Chang, who seemed to be taking more interest in her than expected. She tried to make her engagement ring visible, but she wasn’t sure the he knew of the significance of it. When she got to the line that was experiencing yield problems, the Engineering Manager, Fei Ding, met her. He showed her some of the fails and she quickly identified the head-in-pillow (HIP) defect as the likely culprit. After investigating some more fails, looking at stencil printing, some of the BGA components, and component placement, she asked Zhou Chang what spec was used to thermal profile the line.

“I don’t understand what you mean,” Zhou said in Mandarin.

“How do you determine what the reflow profile should be?”  Patty responded.

With more discussion, Patty determined that they had one profile for all products! Fortunately most of the products were of similar, small thermal mass.

“What solder paste do you use for this line?", Patty asked.

The embarrassed silence suggested that Zhou did not know! They grabbed a tube and Patty was relieved to see that it was one of her favor solder pastes. Since profiling was so rarely performed, Patty and Zhou had to go to another part of the complex almost a mile away to find a reflow profiling unit. After taking the profile, the likely solution appeared. The 11 zone oven was very long and the reflow profile had a long thermal  “soak” before the temperature went above liquidus. This long soak probably exhausted the flux, so that when the PWB went above liquidus, there was little flux left, resulting in oxidation and poor reflow.

All during their time together she had mentioned that her fiancé Rob was here, with her on the trip. This information seemed to do the trick.

“Zhou, why don’t you look up the solder paste spec on the web and then set up the right type reflow profile,” Patty suggested.

It was clear that Zhou was troubled. It became obvious to Patty that Zhou did not know how to profile a reflow oven. Patty set about working with Zhou to accomplish this mission. Within an hour they had re-profiled the oven and, over the next two hours, 300 PCBs were manufactured with the yield improved to 95%.

Patty asked Fei if she could give a brief presentation on the head-in-pillow defect to his team and he cheerfully agreed. Fortunately for Patty, her friend Mario Scalzo had given her his presentation that he gave at APEX 2010 on HIP (head-in-pillow). Patty always enjoyed visiting Mario in Utica, NY, as he always knew the best restaurants in town.

Her major points were:

HIP is caused by the failure of the BGA sphere to reflow with the solder paste. There are 3 major reasons for HIP:

1.       Supplier Issues

a.       Solder BGA sphere oxidation

b.      Silver segregation to the BGA sphere surface

2.       Process Issues

a.       Stencil Printing

                                                               i.      Registration accuracy

                                                             ii.      Insufficient solder paste

b.      Component Placement

                                                               i.      Off pad

                                                             ii.      Out of plane

                                                            iii.      Non optimum pressure

c.       Reflow

                                                               i.      Inappropriate reflow profile

                                                             ii.      Flux exhaustion

                                                            iii.      PWB warpage

3.       Material Issues

a.       Poor solder paste transfer efficiency

b.      Insufficient solder flux oxidation barrier

c.      Solder paste slump

d.      PWB or BGA warpage

Patty went on to say that she had investigated all of these issues with Zhou, and that the reflow profile was not optimum as the very long soak time had exhausted the flux. The other possible issues in the list did not seem to be a concern.

At the end of the day Patty, Rob, and The Professor met at the GM’s office to leave together for dinner and the Expo. Patty had to ask, “Professor, how can you possible know Mandarin, Cantonese, and Shanghainese?”

“Actually I speak Min reasonably well too,” he replied.

“How can this be?", Rob inquired.

“Mother and father were missionaries with Wycliffe Bible Translators,” The Professor answered.

“I grew about around many languages during my youth. Mother and father speak more than I do,” he finished.

Patty went on to tell about the interest that Zhou Chang seemed to have in her, and how she had to discourage him.

“The burdens of being a beautiful young woman,” Rob teased.

Patty elbowed him, but they all left the taxi laughing as they headed for a restaurant near the Expo.

Best Wishes,

Dr. Ron 

The Shanghai, slide rule, and HIP images are from: 

http://pool14.files.wordpress.com/2008/12/shanghai_skyline_g.jpg

http://www.hpmuseum.org/powerlog.jpg

http://ppsimanufacturing.files.wordpress.com/2010/03/bga100.gif

This blog has been in existence for a little over two years now, and we would like to thank our readers for the feedback and inquiries you have provided. I welcome your comments on what you would like from us. Leave a comment below, or email me at jhisert@indium.com.

And now a look back on past topics of interest:
 

Grid Ink, Silver Ink, Conductive Ink

Bismuth/Tin Tabbing Ribbon, A Low Temperature Pb-Free Alternative

Plated Metallization for C-Si Solar Cells

Increase Packing Density for Evaporation Crucibles

Photon’s 5th PV Tech Show 2010 USA

IPC Solar Standards Update

Solder Shelf Life as Explained by Eric Bastow

Tips to Speed Your Solder and Flux Selection

What's Happening in the Technical Service Department 

A Day in the Life of a Tech Guy

A Clean Laboratory

CIGS for Beginners

3rd Renewable Energy Expo 2009 in New Delhi, India

Solar Products and Representatives

Kleenex®, Google™, FedX®, CIGs?

Indium Solar Products Reunited

Trade Show Visitors Love the Ground Floor

Solar Product Data Sheets

Intersolar 2009 – What Barrier to CIGS Technology?

Concentrator Photovoltaic Systems - Will they reach 50% Efficiency?

Standards for Solar Panel Manufacturing

Solar Panel Certification: “Barrier and Benefit” Reviewed by Eric Bastow

Low Temperature Metallization Paste

What Will Your Interest Be At InterSolar? Meet the Bloggers And Let Us Know.

Share Your Solar Images

SAC vs. Sn/Ag for Solar Soldering

Solder Thickness for PV Interconnect

What is Bus Ribbon?

Standard PV Interconnect Ribbon Sizes

No-Clean Flux

Photovoltaics in EMS Sector

PV Interconnect Products

Eric Bastow - East Coast Technical Support

Mario Scalzo - West Coast Technical Support

Au/Sn Sputtering Targets

SMT Goes Solar

A Trip Down Memory Lane 

More Information About Metallization Paste

A year in the Life of a Solar Blog

CIG Target

23rd European Photovoltaic Solar Energy Conference and Exhibition

TCO choices for CIGS manufacturing 

CIGS Absorber Layer Electroplating

No Slump Metallization Paste

Meet the Bloggers

CIGS - Can sputtering make a breakthrough?

Fluxes for Soldering Tabbing Ribbon

Computer Brain vs. Solar Photovoltaic

Beam it down from space

Selection of the Optimum Lead-Free Solder for Solar Tabbing Ribbon

Record Makes Thin-Film Solar Cell Competitive with Silicon Efficiency

Why Thin-Film Solar Cells are Here to Stay

Hot Rooftops to Flashy Digital Cameras

Synchronize Your Solar Cell

Solar Conversion Efficiencies  

Government Support is the Key

It's Just a Beginning ...

After a quick survey of a few of the technical service personnel in the office today, I put together a little snapshot of what is happening right now:

Amanda Hartnett and Ed Briggs are conferencing with an engineer that Amanda met at a local SMTA meeting.  This potential customer asked for product recommendations to improve their assembly process after they heard about Indium 8.9HF (a halogen free solder paste).

 


Brandon Judd is working on reducing voiding via profile modification with our flux coated preforms.

Mario Scalzo is tracking and organizing our technical team’s submissions to the Silver Quill program, where authors at Indium Corporation are recognized for technical papers and presentations.

Eric Bastow is helping a customer determine the best soldering materials for a medical application which involves soldering nitinol to nitinol. Common choices are using flux #2 or flux #3 with Indalloy 121 or Indalloy 182 – depending on the application.

And you’re reading what I’m doing right now. One interesting thing that I realized today is how we are working on very different things at the same time, both reactively and proactively. While some of us are fixing customer issues, others are helping to plan future processes to eliminate the need for a fix. 

Something that may not be apparent from this daily description is how this technical team works together. One of the nice things about being in the same office is that we can share ideas and learn from the experiences of our peers.  Even an office mate's tech call or brainstorm session can be a learning exercise. Working together gets us ready for your next call, so we can have the answer for you before it is even asked.

All the best
~Jim H

After just finishing her department’s monthly activity report, Patty took a break to stare out of her window, admiring the beauty of last night’s fresh snowfall. Her mind quickly went to the events of the past week. Rob had “popped the question” and Patty had quickly said yes. Her and Rob’s mothers were ecstatic. Both Patty and Rob liked and enjoyed each other’s parents. Patty recognized this as a blessed situation, but both mothers were now spending 10 hours each day planning the wedding. A result, Patty and Rob were both fielding 3 or 4 calls a day from each mom. Patty decided to go “with the flow” and count her blessings that both she and Rob had great parents.

She briefly looked down at the ring Rob had given her. It was a striking two carat emerald with 0.4 carat diamonds on either side. Rob was concerned that Patty might not like an emerald, but he explained that the price of diamonds is controlled and that “you could pave your driveway with diamonds for each equally good sapphire, ruby and especially emerald that exists in nature.” He went on to tell her that “all of the emerald mines of Colombia produce only one or two good 2 carat emeralds per year.”

Well one of them was right there on her finger. In addition to the uniqueness of emerald, the setting was in rhodium, the hardest and rarest of the precious metals. “Five hundred times more rare than gold,” Rob told her. She was especially impressed when she looked up rhodium on the internet and found this quote: “Rhodium has been used for honours, or to symbolize wealth, when more commonly used metals such as silver, gold, or platinum are deemed insufficient.” Gold and platinum insufficient!?

Rob was really secretive about how he found such an apparently rare ring. But it was consistent with his many other successes in life. She was thrilled to have him as a future hubby, even if she did beat him at golf. 

These happy and a little stressful thoughts were interrupted, by Pete coming to her door.

“Hey, kiddo, get packed, looks like will be going on another trip. Guadalajara, this time. Como es su espanol?” Pete said with gusto.

“Mi espanol es muy bueno. Why do you think we will go to Guadalajara?” Patty asked.

“Well, I just talked to Pedro and he said that they performed our productivity audit. Uptime is 29%, and all lines are time balanced to +/- 2%, about as best as could be hoped.”

Patty and her team developed a “Productivity Audit” from what they learned with The Professor in their recent adventures together.

“So then what is the problem?” Patty inquired.

Pete responded, “Jane, the finance exec we met on our trip to South Carolina, implemented a company-wide profitability software program. It was implemented and Guadalajara is 10% too low. No one can figure out why. I think we’ll want The Professor for this one.”

Patty called and was stunned that The Professor was again available. Apparently this was his off term teaching at Ivy University, as he teaches over the summer.  

When our trio arrived at ACME’s Guadalajara facility they all spoke in Spanish. Patty had taken Spanish starting in 4^th grade through high school, Spanish was one of the 7 or 8 languages The Professor spoke and Pete was second generation from Puerto Rico. They were surprised that the site GM, Harry Hopkins, asked them to speak in English.

“Give me a break, I grew up in Boston, I can barely speak English,” he joked in his heavy Boston accent. “We want you to help us find that lost 10%, we must be doing something wrong. Help us find it,” Harry implored. “One thing I can tell you is that I am really proud of my team, they are really working hard, you can tell by all of the product that is out there. It makes me proud just to walk out on the shop floor and see all of the product!”, he went on.

Patty was relieved that Harry was so supportive. Apparently Jane had sent the “good word” about how the trio had helped ACME’s South Carolina plant.

As the trio went on a tour, one thing immediately struck Patty, there was hardly room to walk around. There were partly assembled boards all over the place.

At the end of the tour Patty spoke up, “This facility is striking in how much partially completed product is on the shop floor.”

“And there-in lies the problem,”  responded The Professor.

How can profits be off when uptime and line balancing are so good? Could it be that Guadalajara uses poor performing solder paste, fluxes, or performs? Will our illustrious trio find the problem? Does Patty really like her emerald engagement ring? Stay tuned for the latest.