Solder Evaluation

I was recently asked to gather some data comparing Indium Corporation’s tabbing fluxes and our largest competitor’s leading tabbing fluxes. Using a new method of solder spread testing found in an upcoming issue of Global Solar Technology, two Indium Corporation tabbing fluxes were directly compared to three of the leading competitor’s fluxes.

The test consists of these simple steps:

• Apply flux to cell
• Dry flux on cell
• Apply solder preforms on cell metallization
• Reflow on a hotplate
• Measure solder length

Finally, the measurements are plugged into the equation:

S = (L_f/L_i)100-100

                   Where:         S = Increase in preform length

                                      L_f= Final solder length

L_i= Initial length of preform

In the end, the Indium Corporation tabbing fluxes (GS-3434 and GS-5454) both caused the solder to spread ~44% further on a given cell – compared to only 13%, 15%, and 16% for the competitors' fluxes.

If you’d like to learn more about the test method or the results, or want help conducting your own evaluation, send me an email at solar@indium.com.

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)

每个客户在测试评估焊锡膏时，一般都有自己的一套办法。在通常的测试中，虽有许多相似的测试、方法，但也不尽相同。 最简单最基本的测试，一般有润湿测试wetting test, 锡球测试solder balling test， 坍塌测试slump test(hot slump & cold slump), 印刷测试printing test, 表面绝缘测试SIR test, 等等。 根据不同的侧重点，一般做完这些基本的测试后，客户们会做进一步不同的测试，比如有各种各样的printing test, 测试印刷的稳定性，一致性，和下锡量； X-ray空洞测试，BGA, QFN；热循环测试 temp cycle test；跌落测试drop test……

最近有一个客户和我们在交流slump test。 因为这个客户现在使用的锡膏有时候会有briding的现象，而他们的产品越来越微型化，pitch越来越小，所以工程师们就按照IPC slump test的指导对新的锡膏做测试。 我们了解了具体情况后，就建议客户工程师除了做一两个coupon的简单slumping测试，也做整板的印刷和slumping/briding测试；同时，我们建议客户提高印刷速度，从25mm/sec提高到50mm/sec或以上。现在很多免洗无铅锡膏，都是为高的印刷速度而设计的，因为这些产品的主要市场是在亚洲high volumn low mix的消费电子产品上，所以印刷速度要快，不能成为生产线的瓶颈(bottleneck)。 适当较快的印刷速度，能够使锡膏的总体最佳性能更好的表现出来。

客户按照我们的建议，得到了最好的测试结果。

Cheers!

Pic: Google Image 

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.

Recent test results show that solder fluxes handle high reflow temperatures (>450°C), providing better-than-expected visual results of flux residue!  

Solder fluxes have not traditionally been used with AuSn, AuGe, or AuSi eutectic solder, because their peak reflow temperatures were very close to or above the flux activation range. An average flux activates at approximately 125°C and is not recommended for temperatures in excess of 350°C. Although AuSn solder melts at 280°C, peak reflow temperatures are recommended to be >300°C, nearing the maximum suggested temperature of flux. AuGe and AuSi alloys melt at 356°C and 363°C respectively, which exceed the documented flux activation range.

Since these alloys contain ≥80% gold and are resistant to oxidation, flux is not always necessary. Other methods have conventionally replaced the flux function, such as mechanical scrubbing, or forming gas purging. If these technologies are not available, or assembly speed is priority though, a flux may be required.

So I, along with my fellow engineer, Brandon Judd, sought out to test some of our best fluxes with Au alloys at these high temperatures. The result: Not all, but a few of these fluxes work extremely well up to temperatures as high as 450°C!!!

The reflow profiles used tested the extreme abilities of our fluxes:

·         Peak temperature 410°C

·         Nitrogen Purge

·         Time above liquidus: 137 seconds

·         80AuSn solder preforms 0.249” square x 0.002”

Some fluxes did what we expected- they charred and burned. They simply were not designed for this environment. 

Others, such as our TacFlux010® were very resilient at these temperatures. 

For more information about these test results, please contact myself or Brandon.

Happy Testing!!

Amanda

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,

Back in October, I posted comments on lead-free reliability.   In this post, I mentioned that I chaired a session at SMTAI on “Alternate Alloys”. 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 titled “The Effect of Silver Content on the Solder Joint Reliability of a Pb-free PBGA Package.” Both projects evaluated lead-free thermal cycle reliability as a function of silver content and compared the results to SnPb reliability.

Both papers concluded that, as far as 0^oC to 100 ^oC thermal cycle reliability is concerned in their experiments, SnPb < SAC105 < SAC305 < SAC405

Coyle’s presentation summed it up best:

“Each of the SAC alloys outperformed the SnPb eutectic alloy in every test, including the long, 60 min. dwell time test. This tends to diminish the argument that SAC is less reliable than SnPb.”

To be clear, it was two papers by two different groups coming to the same conclusion. It would probably be a stretch to say that the conclusions of either group were “almost unique".

Denny Fritz responded to this blog post with this point:

“No one I know will dispute your ranking of SAC better than SnPb solder using the commercial temperature cycle Henshall uses – 0C to 100C. But, harsh environment electronics have to perform to either -40C or -55C, and most use a top end cycling temperature of 125C. IT IS IN THAT WIDE THERMAL CYCLE TESTING THAT SnPb outperforms SAC solders.”

Denny’s point is well taken. I believe it can be said that SAC alloys have demonstrated acceptable reliability in commercial, non harsh environments (i.e. mobile phones, PCs, consumer electronics, etc.) However, it cannot be said that acceptable reliability for SAC has been established for military (RoHS exempt) and harsh (i.e. automobile engine compartment) environments.

A short time ago, Werner Engelmaier wrote an article on this topic (Global SMT Vol 11, No. 1, Jan 2011, pp 38-40.), which among other things he said:

“Of course, ‘Dr. Ron’ selectively picks data agreeing with the point of view he held from the inception of the Pb-ban under RoHS on a plot with an expanded x-axis overemphasizing the differences and supporting a solder joint reliability ranking of SnPb < SAC105 < SAC305 < SAC405.”

Ouch! My motives were not quite so nefarious, I chaired a session and wanted to share the conclusions.

However, Werner makes good points in his article, data exist disagreeing with this reliability ranking and he suggests some good points on how to conduct reliability tests so that comparisons can be made between data sets.

In reading some of his other articles, I was delighted to find that we actually agree on the state of lead-free reliability in thermal cycle testing. Here is a statement of his circa 2008 (Global SMT, Vol 8., No. 8, Aug 2008 pp 46-48.):fting, and n

“It has been 2 years since the infamous ban of Pb-solders under RoHS. What have we learned? For solder joints, no dramatic differences in reliability are apparent. The data bases for LF-solders have grown, the favored LF-solders might be shifting, and no reliability model exists as of yet. Nevertheless, progress has been made.”

Cheers,

Dr. Ron

Patty had been working with engineering on a new product that needed a very precise and controlled volume of the stencil printed “brick” of solder paste on the PWB pads.  The product had many 01005 passives and CSPs with 30 mil spacings and the application was “mission critical.” So solder joint integrity was critical. The critical factor in obtaining this solder joint integrity was a consistent volume in the stencil printed brick. Her favorite solder paste gave a Cp and Cpk of 1.5 in 500 prints. The upper and lower spec limits were 60% and 140% of the aperture volume.

Purchasing called to tell her that XLK company just announced a solder paste with a Cp and Cpk greater than 3, under the same printing conditions that this product required. Needless to say Patty was skeptical. When she looked at the report, she groaned. The data were collected by Mort Bittler. She had seen him give several presentations and he always seemed to misrepresent the data to make his company’s solder paste look better than it was. She was on her way to a team meeting and expected that this new “break through” would be discussed.

As the meeting came to order, the VP of Engineering, Todd Hamilton, spoke.

“I saw this new data from XLK with a printing Cpk = of 3.72, we will use this paste,” Todd commanded.

“What a minute,” Patty responded, “the decision on which solder paste to use is with my group.”

“But you group has dropped the ball, how could you not know about this superior paste?” Hamilton challenged.

“We have evaluated their pastes continuously, they have always been second rate,” Patty shot back.

“Well things have changed, get with it Coleman, this project is too important,” Todd shouted.

Patty was really angry, technically Todd was her superior, but she found his attitude and words insulting. Using her last name was a bit unfriendly too.

“I’ll travel to XLK tomorrow and review their data,” Patty responded, her voice shaking more from anger than anything else.

She called Mort Bittler and he was available, so he agreed to meet with her the next day.

As she hung up, Pete showed up at the door, “Hey kiddo, how’s it going?” Pete asked.

“You were at the meeting, so what do you think? Hamilton impugned all of us,” Patty said flatly.

“Anyway I can help?” Pete asked.

“Why don’t you go with me to XLK tomorrow, it might be good to have two people check the data.

Fortunately XLK was only 120 miles south of their southern New Hampshire office.  

Pete had become one of her best friend’s in the past year. They spoke in Spanish the whole way to XLK to get their skill level up. Patty had also taught Pete some Mandarin, but it was slow going. After 120 minutes of discussing the PGA Tour vis a vis Tiger Woods, in Spanish, they arrived at XLK. Mort was waiting.

Mort was 45 years old, with a thick Boston accent. He came across as being knowledgeable ….. to someone who wasn’t knowledgeable.   

After brief pleasantries, Patty asked to see the raw data.

“Patty, I already made the calculations, why do you need to see the raw data?” Mort asked.

“The Professor always told me to ‘look at the raw data,’” often one can glean things that the final calculations don’t show,” Patty answered evenly.

“Well maybe later, let me show you how we took the data first,” replied Mort evasively.

Patty and Mort went to the printing lab and Patty noticed that Pete was not with them.

After verifying that the printing process was reasonable, Patty asked if she could have a little time with Pete……if she could find him.

Patty and Mort found Pete in the break room. 

“Pete let’s pow-wow for a while,” Patty said.

Mort said he would go answer some emails and they would meet in 30 minutes.

“Pete, where have you been? You’re not going to embarrass me again are you?” Patty pleaded.

“Me embarrass anyone?” Pete sheepishly replied.

“I found the person who took the data, Beth Thompson,” Pete went on, “she told me they average Cpks.”

“Not again, Patty groaned, we just went through that with a vendor last week, when will they learn that it’s wrong to average Cpks.

In 30 minutes they went to Mort’s office. All agreed to go lunch.

After ordering, Patty asked, “Mort what are your thoughts on averaging Cpks?”

Mort seemed defensive, and he squirmed a little, finally he said, “seems OK to me, it’s just like averaging golf scores.”

“What about the non linearity of the standard deviation in the Cpk equation?” Patty asked.

Mort was clearly not grasping the issue.

Patty continued, “If you have two sets of data and calculate the Cpk of each and average them, you will not get the same result as if you calculated the Cpk of the data added together. One of the reasons is that the standard deviation is non-linear. For the same reason it is wrong to add Cpks together”

Patty came right out and asked, “Did you average the Cpks?”

“Yes,” Mort said glumly.

“Let’s look at the data when we get back from lunch,” Patty insisted.

When they looked at the data it showed Patty’s point, four runs, of 100 samples each, had Cpk’s of around 1.2 to 1.3 and one run had a Cpk of 15.56. The average Cpk was 3.73, but if one takes the data all together, the Cpk is 1.58. Patty had calculated the total Cpk on the spot with Minitab.

The Cp and Cpk Data.  The correct results, calculated by Patty, are in the last row.

“But the 1.58 still is quite good,” Mort pleaded.

“But the data suggest, that run 5 is a fluke, it is clearly not from the same population as runs 1-4. Let’s go out to the lab and run another 100 data points to see if we can reproduce run 5,” Patty asked, while essentially insisted.

They ran another 100 and the Cpk was 1.28. On the way over Pete whispered in Patty’s ear that he had more vital intel to share with her on the way home.

With the end of the data collection, Patty and Pete were done, so they headed home.

“OK, what is the vital intel you need to share with me?  she asked Pete in Spanish?

“While you were collecting data with Mort, I visited Beth again. She told me that Mort had her collect 150 data points on run 5 and he threw out the 50 points furthest from the mean. You were right, run 5 was from another population, a cheating one,” Pete chuckled.

“Well, I guess we will still use our favorite solder paste,” Patty summed up.

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,

Patty and Rob return from their honeymoon.......

Patty had just finished some emails and was ready to head off to meet Rob and some of their buddies for lunch. When she and Rob returned from China a month ago, Sam, the site GM, told both of them he was giving them an extra week of vacation for their honeymoon. Their China trip had been an unqualified success in helping the China teams achieve more productivity and higher yields. Sam had received numerous positive reports from the Chinese managers involved. There were several requests to have Patty and Rob stay a year in China to help with the many process issues that the China team has. Fat chance of that happening, Sam needed Patty and Rob here! Sam also mentioned that he knew that the trip was a little stressful coming so close to their wedding, so the extra week was ACME’s gift to the young couple for their sacrifices.

The wedding went off without a hitch. Patty was touched at how choked up her dad was in “giving her away.” The weeding reminded Rob and Patty how close they were to their parents. They both agreed that the support of their parents was crucial in any success that they had in life.

For their honeymoon they decided to tour France, Italy, and Germany. Rob was really proud that he handled the languages a little better than she did. Of all the things that they saw, they were most impressed with Pompeii. Because the city was covered in hot ash in a matter of moments, it was as if Pompeii was frozen in 70AD.  Visiting Pompeii was like stepping back into the time of the Caesars.

Truth be told, Patty was happy things were back to “normal.” It was pleasant to have their working schedule and to go home to their apartment at night. A couple nights a week, and most Saturdays and Sundays, she and Rob played golf. He had improved somewhat and she was a little annoyed that so far this year he had beaten her more than half of the time….and yes, he was rubbing it in.

As Patty approached the cafeteria she heard a friendly but heated discussion.

“No way can you evaluate an assembly company with just 10 questions,” Phil Anderson stated emphatically.

“I’m really convinced we can, I’ve thought it through a lot,” responded Rob.

“What’s the spirited debate about?" asked Patty as she sat down.

“Rob thinks you can evaluate an assembly company by asking a lead process engineer only 10 questions. Phil thinks he’s nuts,” responded Patty’s best friend Jan Curtis.

“I’ve thought about this quite a bit,” said Rob. “I’ve just finished reading Malcolm Gladwell’s ‘Blink.’”  “Gladwell claims that often the best judgments can be made quickly with just a sampling of data,” Rob went on.

“Be specific,” challenged Phil.

“OK, I actually developed 10 proposed questions to evaluate a assembler, let me list them and then defend them. Maybe you guys have better ones,” said Rob. 

Patty thought, as she heard this, that it was good news that ACME was looking to buy more assembly companies to handle their ever increasing workload……not like AJAX that was laying folks off.

Rob had come prepared, he actually had some print outs. His ten questions were:

1.      What is the composition of SAC305?

2.      What are tin whiskers?

3.      In a stencil aperture, what is the area ratio?

4.      What is an approximate peak temperature for a reflow oven in lead-free assembly?

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.

6.       What are local fiducials on a PWB for?

7.       What does thixotropic mean in regard to solder pastes?

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?

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?

10.   What is tombstoning?

“You have got to be kidding,” shouted Phil, “everyone will score 100% on that test.”

Jan chimed in, “I’m not so sure. We hang around people all day who study this stuff. I’m not sure the typical process ‘engineers’ have enough time to study and learn new things…..Remember the 'water in the solder' and the 'isopropyl in solder paste' incidents?”

At this comment, Phil spit up his ice tea and started choking from laughter. One of their friends, Sally Herman, had been sent to a recently acquired company to help them with assembly process issues. One of the “process engineers” introduced himself by bragging that he was saving the company money by taking used, dried solder paste and mixing it with isopropyl alcohol so that the paste could be used again. Later in the day, the same chap shared that he thought he had a solution to the poor hole fill problem in lead-free wave soldering…….the solder was too thick, if it was mixed with water it would fill the holes better he opined.

Jan added, “As a minimum these questions act as a good screening process.”

Rob interjected, “That’s my point. I’m not saying this tells us everything, but you will agree that if a lead process engineer can’t handle these questions, it is unlikely he or she would be able to solve graping, or the head-in-pillow defect, right?"

All at the table murmured agreement.

“On second thought, maybe you have something here Rob," Phil said. “What do you propose as a passing score," he went on?

“Seventy percent,” Rob answered. 

Are Rob’s questions reasonable to evaluate an electronics assembler? What are the answers? Comment with your answers. Stay tuned to find out.

Cheers,

Dr. Ron

The image above is from: http://en.wikipedia.org/wiki/File:Blinkgla.jpg

Folks,

Tin Whiskers (TW) continue to generate considerable interest. People often suggest that their risk is great and yet unknowable. RPN may help to clarify the TW risk. What is RPN? It is the risk priority number from failure mode and effect analysis.  As this link tells us:

A failure modes and effects analysis (FMEA), is a procedure in product development and operations management for analysis of potential failure modes within a system for classification by the severity and likelihood of the failures. A successful FMEA activity helps a team to identify potential failure modes based on past experience with similar products or processes, enabling the team to design those failures out of the system with the minimum of effort and resource expenditure, thereby reducing development time and costs. It is widely used in manufacturing industries in various phases of the product life cycle and is now increasingly finding use in the service industry.

RPN is an important part of FMEA. It is the product of three numbers that range from 1 to 10. The first number is the severity (S) of a possible fail. A “10” would be given if the failure injured someone, “7” would be assigned if the failure caused a high degree of customer dissatisfaction, whereas a “2” would be given if the failure has only minor negative effects.

The second number is occurrence (O) of a fail. The highest rating is a “10,” which would be a failure every day (reminds me of Windows ME!) or one fail in 3 events, whereas a “7” would be a failure every month or one in 100 events. A “2” is a six sigma fail rate.

The last number is detection (D) of a potential fail. A”10” would suggest that the detection of a potential fail is either not performed or not possible. A “7” is a manual detection approach that may not be reliable, whereas a “2” is 100% effective potential failure inspection.

So obviously a product with a RPN of 10x10x10 = 1000 is a disaster, its failure is dangerous, frequent and incapable of being detected beforehand. Industry rules of thumb suggest that and RPN of 200 needs to be addressed and an RPN of 75 is usually considered acceptable.

Let’s look at a “ball park” RPN for tin whiskers (TW). We will assume the application is a critical IC in a PC.  Let’s assume that a severity rating of “S” of 8 (failure renders the unit unfit for use) is reasonable. TW are hard to inspect for future fails, so detection, “D,” could be as high as a 10. At this point we are at 8 times 10 equals 80 for both. A bad start.

Occurrence , “O” for TW failure modes is dramatically different. When trying to assess the occurrence of TW fails, one is often directed to NASA’s web page . Many reference this web site that lists a little more than a score of TW fails. What escapes me is that people don’t seem to appreciate the rarity of less than 100 fails in decades of data collection. Surely TW fails are not common. I could find no report of a failure of a RoHS compliant product anywhere on the internet. So it would be hard to rate “O” any higher than a “2.” I suspect that the reason few TW fails have apparently occurred is due to TW mitigation techniques that are widely practiced.

I would expect that “modern” process defects like the head-in-pillow or graping defects could have a much higher RPN than TW, if assembled without proper process controls and materials. However, there is little need to worry about these defects either, if you use the right solder paste and practice some assembly process precautions.

I’m really excited about a new option for those of us who are prototyping solar assemblies or evaluating new tabbing ribbon materials. I’ve been waiting for something like this – everything you need to solder solar cells together in one package. The turn around time is key too – you may recall an older post where I learned how quickly these materials shipped.

On the website where these kits are offered, the description reads:

“Tabbing ribbon kits come with everything you need to evaluate how Indium Corporation materials will work with your solar cells and assembly process. The kits can be used to:
- Evaluate which tabbing ribbon size is best for your design
- Determine which flux is best for your operation
- Experiment with new solder coating alloys
- Assemble a few solar panels”

The tabbing ribbon kits come in 3 flavors:

- Standard Sn/Pb/Ag (62Sn/36Pb/2Ag)

- Pb-Free (96Sn/4Ag)

- Low Temp Pb-Free (58Bi/42Sn)

I have a feeling the Low Temp Pb-Free kits are really going to be the most popular of the 3 that are offered though. Application temperature ranges will determine which kit to use, but all three versions of the kits are said to offer similar base copper sizes and tolerances:
“The ribbon itself is industry standard CDA 110 (99.9% Cu) core flat wire, coated with a precisely controlled layer of solder. Each ribbon is manufactured using our proprietary softening process so you can increase the yield of your stringing process.” Basically, this means that the softer tabbing ribbon will help eliminate the breakage of thinned cells during the heating/cooling cycle.

It also includes some matching bus ribbon to complete your panel build. If you’re trying to find the right flux, this kit serves dually as a flux evaluation kit as well. The kit is loaded with VOC-Free flux, rosin-based flux, and resin-based tabbing fluxes. I prefer GS-5454 as a flux for most tabbing operations, but you can see how the others stack up as well.

Let me know how you like the kit after you try it out!

~Jim

(jhisert@indium.com)

I love watching a good basketball game, and one of my favorite local teams is the Syracuse Orangemen. If you go to a Syracuse home game, notice the shot clock – it was made with Indium Corporation solder. There are a lot of places you can see our products in your everyday life. That smart phone in your pocket, the electrical components in your car, the thermal interface in the computer in front of you. That’s one of the things that makes this job rewarding, being part of so many various applications.

In addition to learning about these different applications, we also get a good reference for what assembly trends are developing, and which material technologies are becoming more popular. 

I’ve watched the halogen-free trend explode and fade, as it was adopted by some large OEMs and their contract manufacturers, but has not spread to most other companies. Another trend that is fading away from the spotlight is Pb-free die-attach solder, since the EU has not found a suitable replacement and has pushed back the exemption deadline. 

A long-existing topic that has had recent mention is solder jetting. The trend towards soldering smaller components is not new or surprising, but for smaller components (01005s and 0201s) we have seen a trend towards dispensing instead of jetting – which seems to suit those applications.

For small component printing, transfer efficiency is critical. Outside of solder paste optimization, “nano-stencil” technology is an upcoming technology that may take-off and improve paste release characteristics. Solder paste is being used in some other creative ways too, like low temperature alloy dipping paste for rework operations. Manycompanies are now using or evaluating specialized solder applications to replace components without fully reflowing the rest of the components on the board.

Integrated preforms are finding their way into more and more applications recently as well. These connected preforms are being used to reduce the need for component pallets and selective soldering operations.

All these applications are great ways that our customers are taking soldering technology to the next level, using materials and assembly methods that were not common before. I look forward to learning how you’d like to use solder in your application!

Folks,

Every so often a new electronics assembly technology comes along, and I am asked my opinion about it. The latest “new” technology for assembly is RF Activated “Green” Nano Solder.

My response when asked about this follows:

“I think Intel's caveat in the article tells it all:
 
‘Intel cautioned, however, that several engineering refinements need to be made before the new RF soldering method can be used commercially.’

 
Interpretation: This puppy needs $20 million of R&D before it is ready.
 
Nano solders have been studied for years.  They are interesting and have promise, but there are big hurdles.
 
People will say they want an exciting new technology like this, until that find that the soldering material costs much more than their current one, they need new equipment, etc.  All of the sudden today's process (disappointments included) doesn't look so bad.  It is hard to replace an incumbent process unless there is a strong need - and typically it must be at equal or lower cost.  These will be challenges for this proposed process.
 
So my take is, it is interesting process science, but let's wait awhile to see more data, prototypes and cost estimates before we get too excited.

Any new technology process must be evaluated under the following criteria:

1.     If “disruptive”, it must meet an overwhelming need. E.g.: If your process has a 95% first pass yield and the 5% of the product that is repaired only cost a small amount, you will be unlikely to want to take a chance on a new, unproven technology when the time comes to invest money in it.

2.     The new technology’s implementation must have a minimum of disruption, if implemented in a current process and the cost must be equal or less than today’s process. E.g.: You want to improve your process in #1, however if the new process requires radically new equipment and/or materials you will be hesitant to adopt it.

3.     The process will need several years to prove itself. You know the problems with today’s process, but what are the problems with the new process? You will likely want yield and reliability data. These requirements will take some time.

4.     You must consider the improvements in the old process. Often a new process will aim at where the old process is today, not recognizing that the old process is often improving by the time the new process is implemented.

Using these criteria, let’s look at the implementation of SMT technology in the age of through-hole (TH), circa 1980. How did it measure up to these four criteria?:

1.     SMT met an overwhelming need. One simply could not design a small, high performance personal product, like a mobile phone, with TH.

2.     SMT lines evolved from TH lines, sometimes with radical changes, but the need overwhelmed any disruption.

3.     Much work was performed on SMT products to demonstrate that reliability was acceptable.

4.     The need for SMT was so great that TH's "future" was not an issue.

Contrast this situation to the SMT process discussed above (that has 95% first pass yield) with the 5% fallout reworkable. It becomes difficult to envision making any “disruptive” change to a process like this…..it just won’t pay financially or in any other way.

Your comments?

Dr. Ron

Today I made my rounds in the office, collecting ideas for you from our tech guys  - ideas to help you speed the alloy and flux selection process.  The team gave me ideas from the start of the design process all the way up to speeding the order process, and all the steps in between.  These are solder basics, but they can help you get your process up on its feet quicker - if you put together a little information up-front:

1) Call a tech guy early, but be prepared by knowing the specifics of your material needs, like powder size, flux type, and any design requirements.

2) If you’re an engineer specializing in component attachment, get yourself involved with the component or board design team. It may mean extra meetings, but it will save many headaches in the long run after you help the team remember the meaning of “design for manufacture”.

3) Define the details of your application, equipment, and process before selecting a material. For instance, knowing the needle size that you will be utilizing in a dispense machine will speed the powder size selection for die-attach solder paste.

4) Be aware of cleaning requirements and your current in-house cleaning equipment and chemicals before choosing a flux or flux vehicle.

5) Understand the operational temperature of your assembly and the maximum processing temperatures of the components. This will make alloy selection much faster.

6) Don’t get hung up at the ordering process – know what size packaging you need. Do you have equipment that only fits a certain size syringe or cartridge?  Knowing this ahead of time will save you a second call to verify while talking with an Account Specialist.

7) For alloy compatibility and metallurgical considerations, be prepared to lets us know the composition and thickness of your surface finish. This will also save a second call, because it is required information in order for us to get you the right alloy and the perfect flux for your application.

8) For solder paste printing recommendations, know the specifications of the stencil you will be using. Aperture size, stencil thickness, and any other dimensions you can provide will help guide which flux vehicle and powder size we will recommend to you.

9) For preform selection, try using thinner preforms. For prototype situations you can stack the thinner preforms to build solder volume, and it is much quicker to order preforms in 1 thickness as opposed to many thicknesses.

10) Understand your process bottlenecks. By letting us know your material needs we can usually suggest a few materials, but perhaps one of those materials can help eliminate a problem that is slowing your process down.

11) Consider your company’s roadmap for the next 5 years. It doesn’t make sense to select a material and need to select a new one only a year later. Save yourself the time involved in a second solder evaluation and know what the future holds regarding safety/environmental concerns. Likewise, understand the roadmap of your supplier, their future materials, and how their current materials will fit your company’s future plans.

Source: Amkor



The new TVM™ PoP components have gotten a lot of press, and soon we should be able to get our hands on TMV PoP daisy chain parts for material testing.  I would love to evaluate next generation PoP solder pastes with these new components.  Lee Smith (Vice President of Business Development at Amkor) had this to say about obtaining parts for testing: 

LS: “We have significant direct and end customer demand for our TMV technology in next generation high density PoP applications.  Amkor has qualified the technology for high volume production and we are now completing customer ramp readiness and SMT validations.

In addition to our customer specific work, we have presented 3 TMV joint project studies on SMT stacking and board level reliability at industry conferences over the past 2 years.  We have demonstrated robust SMT stacking with standard dipping flux, paste and BGA underfill materials.  We plan on offering a 14x14mm daisy chain TMV PoP test vehicle through Practical Components sometime in Q2 2010.  Prior to that we will entertain joint projects with this test vehicle under non-disclosure agreements to validate compatibility with new SMT materials.”

Click here to learn more about the Amkor’s TMV and PoP package family.

For more information, Lee can be contacted at Lee.Smith@amkor.com

TMV is a trade mark of Amkor Technology, Inc.

Folks,

The adventures of Patty and The Professor continue.....

Patty and The Professor agreed to work with two of the local process engineers to develop a plan of attack to try and find the lost productivity. 

Patty spoke first, “Its tempting to look just at the new solder paste, but this approach wouldn’t be thorough.” 

The Professor and the two process engineers, Joe and Ann agreed. So they went ahead and developed a thorough productivity assessment plan, including uptime and line balancing measurements and evaluating changeover and assist times. Ann pointed out that one of the five lines was still using the old paste. All agreed that this situation was good news as they would have a new paste to old paste comparison. It was already lunch time and everyone was hungry, so off they went to a local Outback.

While riding in the car, Patty’s cell phone rang. It was Rob.

“Hey Patty,” Rob cheerfully started, Guess what I shot last night at the Golf Club of New England….a four under par 68! “The pro told me it was the best round this year at the course, from the back tees.” he finished.

“Rob, that’s great!” Patty cheerfully responded. Truth be told, she was really happy for Rob. He was the number two golfer on the men’s team at Tech a few years ago as a senior. She was a junior then and was the best women golfer in Tech’s history. The few times they played then, she beat him. Ever since her dinner date with him, after their success at AJAX, they had been going steady. At the time she had been thinking of breaking up with Jason and Rob’s invite to dinner was all the catalyst that she needed. In the last year or so, Jason would just watch sports on TV and drink beer. He didn’t have a fitness program or a real plan for his life. Rob was so much different. He worked out, mostly to improve his golf game and he was getting a Master’s degree part time.

After they started dating, Rob and Patty played golf together with some other guy friends from Tech. She usually shot the low score, but the three other guys were longer off the tee than she was. Her superior iron play and short game made the difference.

At lunch this working foursome talked about the audit they were about to perform.

“There is one comical thing we should tell you before we start,” Joe said with a twinkle in his eye. “Its about the “Saving a House Program,” he added.

At that, Ann started laughing and inadvertently started choking on her “sweet tea.” Patty was just about ready to perform the Heimlich maneuver when Ann revived.

With Ann still red in the face and laughing, The Professor requested, “Yes, please tell us.”         

Joe chimed in, “So that Ann doesn’t choke to death, let me take a stab at it. The new cheaper solder paste has not been very popular and has generated many complaints. The new COO, Fred decided he had to do something. He estimated that the new paste saves $100,000 a year on all five lines, that’s about what a modest house costs locally. So he tells all of the complainers that using the new paste saves enough money in a year to buy a new house. He even found a house for sale on the internet for $100,000 and had posters of it made with the words: “Saving Enough for a House.” It worked, people stopped complaining. “

“Joe, can you tell us what some of the complaints were about the solder paste?” asked The Professor.

“Well, for one thing, it is stiff coming out of the tubes or jars, we have to kneed it or it won’t print, Joe responded.

“Hmm,” both Patty and The Professor mused.

“Also, if we stop a line for a few minutes the paste stiffens up and we have to perform some “dummy” prints to kneed it,” chimed in Ann. “Sometimes even after this, the first print has to be discarded due to poor hole fill. It wastes time and solder paste,” Ann finished.

“Don’t forget the smell,” Joe teased.

At that, Ann just about spit up her sweet tea.

“The new paste literally stinks,” Joe added. “Fortunately, the vendor added some perfume recently.”

“What about reliability of the finished product?” The Professor asked evenly.

“That’s what is surprising. Its as good as the old paste.” Ann replied. “We performed some tests and asked around, the reliability is very good, Ann finished.

“A pleasant surprise indeed,” The Professor said.  

Our little group finished lunch and headed back to get to work on the audit. Ann and Patty and Joe and The Professor formed teams and went off into the factory. They performed detailed analysis of changeover times, assist times, line balancing, uptime, etc on the four lines using the new solder paste and the one line using the old solder paste.

As Patty approached one of the lines she saw a cheerful looking gent about 45 years old replenishing the solder on one of the stencil printers. Ann introduced her to Wilbur and asked if it was OK for Patty to ask him some questions.

“Darlin,” he said to Ann in his backwoods drawl, “Anything you gorgeous gals want to ask me is jus fine.”

“How does replenishing the new paste compare to the old paste?” Patty asked.

“Well, it takes a lot longer, stirring the paste and all, but to “Save a House” I’m willing to put up with it, sighed Wilbur.

After a total of a day and a half of work the team reassembled. The Professor suggested that Patty lead the discussion. Many calculations and comparisons were performed, finally after several hours they were ready to meet with Fred Perkins and Jane Wilson. Patty agreed to speak.

Patty, addressed the small gathering. She presented the approach they used to collect data, their analysis techniques and the fact that they had reached a consensus.

Patty went on the say, “The evidence is persuasive that:

1.       The site productivity is down about 8%, this will reduce profits about 12%

2.       The main culprit appears to be the new solder paste.”

At this Fred slammed his fist on the disk. His face a bright crimson, he shouted at Patty, “Liar, liar, you corporate types are all alike! You come here from your Ivory Tower and tell us how to assemble a product. You have never had to meet a payroll and make a profit in your life. I’ve been out on the line, it only takes two or three minutes longer per change over with the new paste and replenish times are even less.”

At these comments Jane rolled her eyes and glared at Fred. It was clear she wasn’t intimidated by him.

Patty shot back, “Fred you are correct, let’s look at the numbers. We measured your average uptime at about 25%, which is quite good. That means the lines are running two hours in an eight hour shift or 120 minutes. Eight percent of 120 minutes is about ten minutes a day. A typical line has two changeovers a day each requiring 2 extra minutes and 6 solder paste replenishments taking an extra 1 minute each with the new paste. This totals 10 minutes, hence cuts production by 8%.”

Fred screamed back, “This is mathematical gobblygook. I saved the company $100K a year.” 

At this Fred stormed out of the room. The remaining folks stared at each other.

Finally Jane broke the silence, “It never occurred to me how precious a few minutes here and there can be in how they affect profit. With the new paste, we will lose about 12% of our total profit of $10M or $1.2 M per year. It appears that while Fred was “saving a house,” we were “losing a mansion.””

Epilogue: Three weeks later Fred was “promoted” to Corporate Compliance Officer.   Jane became the new site CEO/COO. The old solder paste was re-instated a day after Fred left. A few of the old timers kept some of the “Saving a House” posters for future reminiscing.

 Cheers,

Dr. Ron