Graping

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

Solder powder particle size and shape impacts the functionality of solder paste in many ways: printing/dispensing/dipping; solderballing; graping; voiding; tack and so on.

For this reason, I just spent an interesting couple of months leading a cross-industry (two solder paste suppliers and two solder paste users) group to help my old friend Brian Toleno, chair of the IPC 5-24b (Solder Paste Task Group) put the finishing touches to the final version of the J-STD-005A. The concerns were with the definitions of powder size in paste: both the distribution and the “maximum allowable particle size”. We reached a nice pan-industry consensus, which should allow the J-STD-005A to see the light of day as a published document in 2012. I also saw some recent work by colleagues on the effect of particle size on surface area. I didn’t see the derivation of this work, so I want to show you how to calculate the surface area of solder powder in a paste.

Assume solder paste at a weight loading of x%. [Note that: As the solder powder size (diameter) decreases, the metal loading is usually also decreased by 0.5% or more to compensate for the boundary layer of thixotropic flux adhering to the particle surface, but let's make the first order assumption that x is independent of particle size]. So 1 gram of solder paste contains (x/100) grams of solder metal.

If the metal has a density of r (rho), then the volume of metal (v) per gram of solder paste:

               v = x / (r * 100)

Let’s assume that the metal particles are monodispersed (i.e.: all the same diameter (d)), so the number of particles per gram of paste (n) is then simply v (total volume of metal per gram) divided by the volume of one particle (v_p).

               n = v / v_p = x / (r * 100 * (4/3) * pi * (d/2)³ )

We can now also calculate the solder powder surface area (s) per gram of paste from our knowledge of n and the surface area per solder powder particle (s_p):

               s = n * s_p = n *4 * pi * (d/2)²

It is a simple matter of algebra to show that the ratio of surface area to volume is merely an inverse of the particle radius or diameter (I’ll leave that as homework for you):

A while back, I did a little Excel numerical integration to show the effect of powder type on the population distribution, and hence how powder “type” (2,3,4,5 and so on) affects the surface area, with some assumptions thrown in about the width of the distribution. The results are shown below, and are pretty much as you would expect. As you go from type 3 to type 6, you see about a 10 fold increase in the surface area.

最近已近有两个客户在询问Indium公司有没有Type 8 Solder Paste 8号粉焊锡膏，分别用在医疗器械上和wafer level方面。

在SMT中，我们通常使用的都是3号粉和4号粉。在IPC的规格中， 3号粉的规定是球半径在25-45micron之间；4号粉的规定是球半径在20-38micron之间。其实这中间也有很大一部分重合。 一般来说，随着锡粉球半径越小，相同重量锡粉中球的表面积就会越大，在焊接过程中锡球就越容易被氧化，那么对锡膏助焊剂(solder paste flux)的要求就越高，不然很容易出现solder balling/graping等现象。

IPC中是没有对7号粉或是8号粉的锡球定义，但是随着微型化，客户们确实是有这方面的需求了。

Cheers!

Example:

Figure 1: Example shown Indium8.9 flux with SAC lead-free alloy

The reason for approaching this subject is that often there has been some confusion in regards to the difference between max slope (a category reported on most profiling software) and the ramp rate listed on a data sheet.
























Figure 2: Max Slope

The max slope is very often attained in the first zone as the PCB moves from ambient temperature into the oven. In most cases the oven zone setting for the first zone is 100°C or better. The change in temperature between ambient and the first zone then is a minimum of 75°C (assuming 25°C as ambient) and so it’s easy to see that the greatest change in temperature (max slope) in most cases is typically found in the first zone

The focus of max slope is more from a component view point, to avoid thermal shock, usually 3°C/s is recommended as the upper limit

Figure 3: Ramp or Average Rate


The ramp rate may be better described as the rate (change in temperature over time) from ambient (room temperature) to peak. And is more practically used in a ramp to spike type profile

From the view point of the solder paste, the lower the ramp rate the better, usually 1-2°C/s. This is to drive off volatiles and help minimize solder defects such as solder balling, solder beading, and tombstoning. This rate becomes even more important as the solder paste deposit continually decreases in size, as we move to 0201’s and smaller discrete components and from 0.5mm pitch area array packages to 0.4mm and smaller. Due to this miniaturization, the observance of graping and head-in-pillow have become more common. The reflow process window is becoming very narrow and this attribute (ramp rate) has become as important as time above liquidus and peak temperature.

I'd love to discuss this with you, if this topic is affecting your SMT process. If you'd like, feel free to contact me.

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

• Oxidation Barrie能夠大大減少，甚至完全消除枕窩效應(Head-in-Pillow defect). 枕窩效應是因爲BGA球和solder paste在回流前的preheat or soak time階段分開了，在熔融過程中，表面被氧化；儅BGA球和paste再次接觸時，表面被氧化層太厚了，所以整個焊點沒有完全融合好。如果有良好的Oxidation Barrier, 那麽能大大減少BGA球或是paste在熔融過程中因爲分開而被氧化。
   
• Oxidation Barrie 能夠提高精密元器件，小開口印刷的焊點的結合。小開口的下錫中，4號錫粉的表面積(powders’ surface area)其實是比3號粉增加了，但是助焊劑(paste flux)沒怎麽增加，那麽在焊接過程中，有些錫粉表面的氧化物可能就沒有被完全清洗乾淨。被氧化的錫粉不能和整個焊點完全融合，形成良好的焊接點；而是在焊點附近出現一串像葡萄一樣的小珠子，我們也叫做graping defect。 有了好的Oxidation Barrier, 就能夠更有效地預防graping defect, 提高小開孔印刷焊點的融合。
   
• Oxidation Barrie能夠減少留在電路板上的活化劑activator，增強電性能的可靠性。Oxidation Barrie 能夠防止焊接表面被氧化，activator是清洗表面被氧化的部分。如果有了良好的Oxidation Barrier，那麽activator 就可以相對減少。回流后留在電路板上的activator 也少了，那麽也減少short cut 等現象，提到電性能可靠性。
   

Cheers!

Pic: Indium Corporation

Included in the Product Data Sheet, among other things, are parameters which guide the customer in designing an SMT reflow profile. The data sheet gives general recommendations, for time above liquidus, peak temperature, and ramp rate.

The reason for addressing this subject is that, often, there has been some confusion in regard to the difference between max slope (a category reported on most profiling software) and the ramp rate listed on a data sheet.

The max slope is very often attained in the first zone as the PCB moves from ambient temperature into the oven. In most cases the oven zone setting for the first zone is 100°C or better. The change in temperature between ambient and the first zone then is a minimum of 75°C (assuming 25°C as ambient) and so it’s easy to see that the greatest change in temperature (max slope) in most cases is typically found in the first zone

The focus of max slope is more from a component view point, to avoid thermal shock, usually 3°C/s is recommended as the upper limit

The ramp rate may be better described as the rate (change in temperature over time) from ambient (room temperature) to peak. And is more practically used in a ramp-to-spike type profile

From the view point of the solder paste, a low ramp rate is desired, usually 1-2°C/s. This  gently evaporates volatiles and helps minimize solder defects such as solder balling, solder beading, and tombstoning. This rate becomes even more important as the solder paste deposit continually decreases in size - as we move to 0201’s and smaller and from 0.5mm pitch BGA’s. Due to this miniaturization, the emergence of a defect known as "graping" has also become fairly well known. The reflow process window is becoming very narrow and this attribute (ramp rate) has become as important as time above liquidus and peak temperature.

Folks,

The adventures of Patty and Rob continue.......

Rob bolted upright in bed. He had that terrible feeling that he had overslept for an important appointment. His eyes quickly found the clock and it said 10:30! 

“Wait a minute!” he thought, “It’s Sunday.”

He looked a Patty peacefully sleeping and decided to let her sleep. They had had a tough two months. Ever since they proposed increasing uptime to greater than 60% on two “experimental lines”, they were working 90 hour weeks. They just felt they needed to constantly monitor two lines, to assure that things were going smoothly. They felt satisfaction that they achieved 68.8% uptime in a two month period, compared to the company average 30.4%, which is still very good.

The local newspaper got word of this effort and did a story on Rob and Patty’s work. The article was well written and very complimentary to both he and Patty, as well as ACME. Sam Watkins, the site general manager, was very pleased with the good PR. The accompanying photos were really nice too.

The big shocker came this past Tuesday. “Sixty Minutes” called and said they wanted to do a segment on “The US Competing with the Far East in Electronics Manufacturing.” In agreeing to be interviewed, Rob and Patty insisted that members of their ACME team be included. In addition, they felt it was only fair to include the efforts of Rita from their stencil printer and reflow oven supplier and States, their colleague from the component placement company. And they couldn’t forget Eric, from ACME’s prime solder paste supplier. These three folks helped Rob and Patty and their team to develop the plan to achieve the 60+% uptime.

An even bigger shocker came when the Sixty Minutes crew told them that Andy Grove would be in the segment because of his recent article in Business Week, How America Can Create Jobs

 Grove insisted that to participate in the piece, he wanted to visit ACME to see what Rob and Patty were doing. So the Sixty Minutes crew was visiting ACME’s plant this week as were Rita, States, Eric and now “Andy.”

“Maybe we should call him Mr. Grove,” Rob thought.

Rob had suggested that he and Patty go to Berdick’s in nearby Walpole, NH for Sunday brunch and then to play golf. Rob had to chuckle, it was mid July and he and Patty had played golf 27 times (she kept a spreadsheet), he had beaten her 14 times and she was miffed. Even during their 90 hour weeks they would take a break 3 times a week to play 9 holes.

On Monday they were meeting with site GM, Sam Watkins, to discuss what they would tell Sixty Minutes.

Rob and Patty’s Sunday was delightful. The brunch was delicious and relaxing and they both played golf well, Patty’s 68 beating Rob’s 69.

It seemed like no time at all and Rob and Patty were in Sam’s office.

“Just assure me that this Sixty Minutes thing is not some expose that will embarrass ACME or put me in jail,” he teased.

Patty took the lead and explained what they had done. They trained the operators on the importance of line uptime, they worked with Rita, States, and Eric to develop a plan to assure that there would be minimum unscheduled downtime. They had to order extra spare parts and solder paste to assure no stoppages due to parts or paste shortages. One obvious thing is that they would be using two times or more the normal amount of solder paste. The two lines in the high uptime experiment had an average of one change over per day, consistent with ACME’s business.

They also increased routine maintenance on all machines. Both this maintenance and added spares was an increased cost, but these costs were second order effects compared to the dramatic profit increases due to almost 70% uptime.

Preparation for the next three jobs for each line was meticulous, so that setup time was minimized.  Feeder racks were used extensively in minimizing setup time for changeovers. In addition tape splicing was employed to minimize any assist time for component placement. States’ help was crucial in the component placement part of their efforts, Rob pointed out.

Patty went on to describe how Rita helped them in their efforts to develop minimum assist times for the stencil printing process.  The reflow oven presented the least concerns in assist or unscheduled downtime.

The solder paste they selected was robust in that it had a very good response to pause, excellent tack, and minimal slump.  The paste also had the best track record for minimizing defects like Head-in-Pillow and Graping.  Eric also participated as an enthusiastic partner in the effort.

Patty mentioned that their colleague, Phil, had agreed to monitor uptime for two standard lines during the two month trial to compare downtime metrics to the high uptime experiment. These would be experimental “controls.”

She then showed the uptime data for the two high uptime lines and Phil’s control lines. The control lines had ACME’s respectable 30% uptime, but the high uptime lines had almost 70% uptime. Rob went on to explain all of the things the team did to minimize downtime, most of it was common sense. Sam was especially interested in one downtime category.

“What is floundering time?" Sam asked.

 “That is time when the line is not operating due to some unplanned error,” Rob answered.

“Can you give an example?” asked Sam.

“Sure, you know how we have a quite organized approach to setups?” Rob responded.

“You mean our use of white boards to write down all of the things needed for the next 3 jobs on each line?” Sam came back.

“Yes, that is one of our biggest sources of floundering time,” Rob replied. He went on, “Someone will write that they have the stencil for the next job, when they just think they know where it is. When it comes time for that job the stencil cannot be found and an hour is lost.”

“Phil also noted a case where a job was finished on a line at 11:15AM, since lunch was at 12 noon, the changeover for the new job was not started until after lunch. Forty five minutes was lost, forever,” Patty added.

Sam gulped.

“So we are losing more than 25% uptime to ‘floundering?’” Sam weakly asked.

“According to the Professor, it’s endemic in the industry,” Patty interjected. “He coined the term, ‘Floundering time’,” she went on.

Sam then mentioned how the “bean counters” at ACME we really impressed with the two high uptime lines. ACME’s CEO wants a concerted effort to transition all of ACME’s assembly lines in North America  to higher uptime performance. Manufacturing in North America would also mean no 2-4 weeks of transportation time from the Far East. Patty, Rob, and their “team” were to form a new group in ACME to do this. Patty would be the Director of the group.

As the meeting was about to close, Sam asked what surprises Patty and Rob had during this experiment.

Rob then shared, “It relates to floundering time.   We found that even among the engineers, no one appreciated the value of one hour of production time. We asked a group of operators what an hour of production was worth and the figures ranged from $50 to $500 dollars. ACME runs two shifts at 30% uptime, that’s about 1500 hrs per year. Our typical line produces $30 million per year, that’s $20,000 per production hour. When we told the operators this, floundering time dropped significantly.”

Patty added, “The other thing we saw is that a “watchdog” is needed. If someone isn’t constantly watching things, floundering and assist times will go up. Since productivity is doubled with a high uptime line, the added cost of a watchdog is insignificant.”

Epilogue: The Sixty Minutes Segment was a great success. Patty was made Director of Corporate Productivity, but was also asked to manage Pete, who would take over her old group. No one seemed to worry that Patty was Rob’s boss, except maybe Rob!

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.

Folks,

After a bit of a break, the adventures of Patty, Rob, and The Professor continue:

The plane droned on as it made its slow march from Detroit to Tokyo.   Patty looked down at Rob as he slumbered peacefully. She caught a glimpse of The Professor,  he looked at both of them from across the aisle with a satisfied smile. The proud mentor looking at his mentees. 

This was the first time in a while when Patty didn’t feel totally stressed. She had resisted going to China only three weeks before her wedding, but senior management insisted. She would arrive home only 5 days before the big day. She and Rob had their first real fight, she got angry with him because he wasn’t appreciating the pressure that she felt. However, with one long weekend with their moms, she was able to get most of the tasks done and finally felt relaxed that the wedding plans were in good shape.

She had to chuckle at Rob. He was all nervous being with The Professor by himself. The plans that they had made had Rob and The Professor focusing on productivity improvements at ACME’s new acquired plants in China. While they were working on these tasks, Patty would handle some process materials related issues. 

The rest on the trip went smoothly and after a night’s rest they were off to the first of ACME’s new factories. This one was located in Shenzhen. Our trio was ushered in to see the site GM, Peng Zhou, a native of the area. He addressed them in quite good English. When Rob and Patty answered in better Mandarin, he seemed shocked. When The Professor answered him in flawless Cantonese he and Patty and Rob were stunned. 

"要不咱们都讲中文吧，既然咱们中文都不错。" said Rob. ("Perhaps we should all speak in Mandarin, since we speak it well." For our non Mandarin speakers)

Rob and The Professor went off to audit a few assembly lines, while Peng accompanied Patty to visit an assembly line that was having a quality problem.

(Dialogue translated from Mandarin)

“I’m very impressed with how well you all speak Mandarin,” said Peng. “Where did you learn it?” he continued.

“Thank you,” replied Patty. "Both Rob and I studied Mandarin in college and we did an internship in China,” she went on.

“ Very impressive,” Peng commented. “But I have to tell you, I’ve never heard any American speak Cantonese at all, let alone as well as The Professor does. It’s like he was born here,” he went on.

“He never ceases to amaze me,” Patty responded.

Patty and Peng finally arrived at the assembly line. Patty was introduced to the line engineer, Elvis Chang. She chuckled inside, this was the third Asian person her age she had met that had chosen “Elvis” as an English nickname. Elvis was relieved that Patty spoke Mandarin. They went to a stereo microscope and looked at some of the assembled PCBs that had quality issues. Patty was quick to pick out the problem:  graping. She looked at the stencil and the pad sizes on the PCB. She performed a few calculations and appeared satisfied that she had the answer. Patty suggested that, if Elvis would like, she could give a brief presentation on what she thought the problem was.

“Patty, that’s a great idea, but it might be best to wait until after lunch,” Elvis suggested.

Elvis, Patty, and a few other young engineers went together for lunch. They seemed to be fascinated with Patty, especially her ability to speak Mandarin. They all spoke some English and were all studying it as they recognized that their ability to be promoted to a senior level required fluency in English. One of them pointed out that she had read that about 250 million Chinese people are studying English, while only 20,000 Americans are studying Chinese.

Patty enjoyed Chinese food and was happy to find Sea Cucumber on the menu. One of her friends said it was the only Chinese food he couldn’t eat. She tried it and liked it.

After lunch, Patty asked for a few hours to prepare her presentation. Her main points are summarized below:

1.        The aperture size for the pads that experience graping is 8 mils in diameter for the 4 mil thick stencil.

2.       The resulting area ratio (D/4t, D= diameter, t = stencil thickness) for this aperture is 0.50, less than the recommended 0.66.

3.       The very small solder paste deposit doesn’t not have enough flux to avoid oxidation of the solder particles in reflow. The resulting defect looks like a bunch of grapes so it is called graping.

4.       Likely solutions:

a.       Use a square aperture. An 8 mil square aperture provides 27.3% more volume, and it has better transfer efficiency. (Transfer efficiency is the volume of the solder paste deposit divided by the volume of the aperture times 100.) The result would be > 30% more solder paste. The more solder paste, the less likely to experience graping

b.      The solder paste they were using was not best of breed re: graping resistance. She recommended another one, which she knew performed well in all respects - and minimized graping. This solder paste’s flux was robust and designed to minimize defects like graping.

Her presentation was received very well. Fortunately some of this excellent solder paste she recommended was being used for another job in the plant. So with approval from Peng, the team switched to this paste.

After the meeting, Patty thought about how much one of the technical engineers from one of her favorite solder paste suppliers had helped her to understand graping and how to minimize it. His name is Ed Briggs and she had just attended SMTA Toronto where Ed gave a paper on graping. Much of the information in her presentation came from Ed’s paper. She had also learned from one of his blog posts on graping.

Epilogue: Three weeks later, the graping had disappeared from Elvis’s assembly line. They didn’t even need to adopt a stencil with square apertures, the solder paste change, itself, was enough.

Cheers,

Dr. Ron

Folks,

 Bob Landman’s comments to my tin whisker posts appear below. Friendly dialogue such as this helps us to all learn more and is appreciated. Thank you Bob, keep me honest!

However, Bob’s comments do not change my position, which is:

1.     Tin whiskers exist and can cause failures

2.     However, there is yet no data that suggest that there are numerous tin whisker failures, or that a significant reliability risk exists due to tin whiskers in RoHS-compliant products.  NASA's TW website notes only 26 fails.

3.     Although not completely understood, tin whiskers can be created in the lab, and mitigation (not elimination) and reliability test techniques exist and have been demonstrated.

4.    With well over $1 trillion in RoHS-compliant electronics manufactured since the early 2000s, there have been no significant reliability issues as compared to tin-lead solder.

5.     Long term lead-free reliability (> 5yrs) has not been demonstrated. Hence, mission critical products should not use lead-free electronics at this time.

6.     Tin-lead solder does not assure defect-free electronics with perfect reliability.

As I type this post, I am surrounded by more than 20 RoHS-compliant products, some dating from 2005. Outside my office, at Dartmouth’s Thayer School of Engineering, is our computer center. The thousands of RoHS-compliant products that the computer center buys each year (they get me my laptop, etc)  are almost all RoHS-compliant. No unusual reliability issues have been noted.

Bob mentions that CALCE reports that 31% of laptops fail in 3 years. This number actually seems low to me. Upon reading the paper, one finds that over 10% of the 31% is due to accidents. 
A study of 100,000 hard drives at Google suggests that hard drive fails are in the 5% range per year, which may account for much of the 20% of fails in 3 years. But what solid conclusion can be made from these data? Nothing, unless failure analysis is performed.

The sky is not falling. Lead-free has process-ability and reliability challenges, such as graping, head-in-pillow, voiding, etc. With data-driven process optimization at all steps in the manufacturing of the ICs, components and assemblies, good lead-free yield and reliability can be achieved.

Lead-free is here to stay. It is up to us to perform the experiments and develop the techniques to assure that RoHS compliant products have acceptable reliability.

Bob's comments follow:

My source for the dead vehicles that arrive at car dealers having whisker problems, comes from my former professor of physics, Dr. Henning Leidecker at NASA Goddard Space Flight Center in Greenbelt MD.   Dr. Leidecker said that in the last four years his office has been contacted by seven major suppliers of automotive electronics inquiring about failures in their products caused by tin whiskers. He said his office has contacted Toyota offering to help analyze its acceleration problem, but hasn't heard back. For full context, read the rest of the article [http://wtop.com/?nid=108&sid=1898265].

Ron Lasky confirms that parts plated in pure tin will grow tin whiskers "with a certain amount of aging". According to NASA, whiskers can grow in hours, days, weeks, months or years. It depends on at least six factors; the quality of the tin plating, the residual stress in the coating, was the coating annealed or not, grain uniformity, temperature, humidity, and unknown other factors we don’t yet understand which is what makes it so difficult to stop whiskers from growing and is why there are so many papers published on the subject (as you can clearly see at John Barnes website) yet we still do not understand why or how they grow.

So yes, is entirely within the realm of possibility that "new" products have failed due to tin whiskers or perhaps dendritic growth.

NASA cannot tell us who the manufacturers are who reported these events due to confidentiality agreements.  Dr. Leidecker says they get these calls from other industries as well and most request a non-disclosure agreement.  NASA feels it’s better to get some information rather than none, don't you agree?

Last  week at CALCE at UMd. it was reported that 31% of all laptops fail within 3 years. This is the link to the report http://www.squaretrade.com/pages/laptop-reliability-1109/  No information is given as to what has failed. Is it due to whiskers?  We do not know.

What we do know is that the laws of physics have not been repealed.  Tin will most certainly grow whiskers so using leadfree solder and tin plated components has to result in tin whiskers growing.

NASA continues to log failures.  NASA Goddard is now studying the Toyota incidents for NHTSA.  Again, a non-disclosure statement has been signed so they cannot comment on the study at this time.

Dr. David Gilbert of Southern Indiana University has demonstrated that a low resistance or shorted input between the wires from the pedal electronics to the electronics control module will cause Toyotas to open their throttles full.  Perhaps the problem is due to leadfree manufacturing (which Toyota admits it began in 2002-3)?  Perhaps it is software?  We don't yet know but we can be reasonably certain that not all the accidents are caused by the owners of the vehicles.  You can see pictures of the Toyota parts at my website [www.hlinstruments.com//RoHS_articles/Toyota/]   The pedal has a pc board layout that I would have been comfortable with.  In particular, the SOIC part that converts the signals from the Hall effect sensors (that sense pedal position) into 1-5Vdc signals sent to the electronic control module is very close to the edge of the board.  The board has serrated edges which indicates it was snapped out of a large panel of these boards after the parts were soldered to it.  It's possible a trace or lead has fractured or one of the capacitors or resistors.  We know that leadfree solder is more brittle than tin-lead. Perhaps a few boards are marginal and over time a lead opens or becomes intermittent?

The EU was warned that tin whiskers and brittle joints would result if lead was banned from electronic assemblies but went ahead and banned lead from tin-lead solder and platings on parts. They acknowledged the possibility of reduced reliability under intense pressure from hi reliability industries and did exempt some products (military, aerospace, etc...).  What difference did it make since the majority of component manufacturers refused to continue to offer tin-lead plated leads?  That is why NASA replates it's components with tin-lead at Corfin Industries and uses only tin-lead solder.


Bob Landman

http://www.hlinstruments.com//RoHS_articles/Toyota/Toyota%20Dr%20Gilbert%20Preliminary_Report022110.pdf

Graping is a phenomenon which appears as un-reflowed solder particles, typically seen on the surface of the solder joint.  



             Cross-section of “graped” solder joint





The graping phenomena has become more common due to some of the following issues:

1. Reduction of the stencil aperture to accommodate smaller and smaller discrete and passive components (i.e. move from 0603”s to 0402”s to 0201’s)
2. The use of finer particle size solder pastes to accommodate fine feature printing (move from Type 3 to Type 4 to now to Type 5)
3. Higher reflow characteristics for Pb-free soldering
4. The use of water-soluble vs. no-clean solder pastes. No-clean chemistries generally protect the solder powder particles and the metallized surfaces from oxidation during the heating process (after the activator package removes existing oxides). (so how does water-soluble fit into this?)

A combination of any of these factors may exhaust the capability of the solder paste flux to remove surface oxides. This depletes the flux and exposes solder paste particles to oxidation, which means the solder particles do not coalesce into the solder joint.

To avoid the graping phenomenon, use the following tips in setting up your reflow profile. The intent here is to decrease the amount of heat the solder paste experiences during the reflow process.

1. A ramp to peak profile is better than a soak profile   
2. Decrease total time in oven by adjusting the belt speed. A ramp rate of 1°C/ second from ambient to peak is recommended
3. Use a lower peak temperature - 235°-240°C
4. Shorten the TAL to 40-60seconds

Patty was checking her email. She noticed a note from someone who had attended last night’s SMTA meeting. Patty had just been elected chapter president, after giving a talk on the head-in-pillow defect. In her talk, she also shared how important it was to work closely with your materials and equipment suppliers. To her, it was obvious that her suppliers were interested in her success. If they were competent, why shouldn’t she rely on them for technical information and help. If she didn’t think they were competent, she should get new suppliers. She was surprised at how much “push back” she got from the attendees. Several people stated that they felt that suppliers where just out to make a sale and that a smart person just bought from the supplier with the cheapest price. Patty thought that this perspective was negative and self destructive. She was sure that 60% of all process knowledge was learned from her suppliers, either in person or at the technical shows like APEX and SMTAI. She felt the main reason to go to these shows was the technical program. And some of the best papers were presented by the better equipment and materials suppliers. 

One of their marketing VPs even told her, “We believe that the more technical help we give our customers, the more successful we will be.” 

“Well wasn’t that a condensation of what good business should be like?  He who helps his customer is the most successful,” she thought.

As she was thinking these thoughts a new email popped up on her PC. It was from Hal Lindsay, a noted curmudgeon.   Patty read on….

“ I heard you telling some of the people at the meeting last night that lead-free assembly had some process advantages: Hogwash. Lead-free has no process advantages, and it’s not needed,” he started.

After a few more complaints, he finished, “ It’s because of young tree huggers like you that never stood up and fought lead-free that we are in this mess to begin with.”

In preparing her response, Patty’s mind went back to some conversations about this she had had with The Professor. He had made two strong points:

1.       The first purpose of RoHS is to make recycling safer. So much recycling isperformed in poorer countries with chemically unsafe processes. RoHS compliant products will save the lives of the unfortunate people who have to perform this type of recycling to survive. 

2.       Lead-free soldering is challenging because the solder does not wet as well. This situation forced us to develop assembly processes with tighter process windows. However, an initially unseen benefit is that tighter lead spacings are possible with lead-free soldering because of this poore wetting. Many portable products such as mobile phones, could not be assembled with leaded solder. There would be too many shorts.

Patty was including this information in her response to grumpy Mr. Lindsay, when the phone rang. It was Rob.

He began, "我可能要出差去中国探访几家ACME在那里的新工厂。我想你可能也要在焊接工艺上做一些工作。"

(For our few readers that can’t read Mandarin: "It looks like I will be traveling to China to visit some of ACME's new factories there.   I think you will be going to work on some soldering issues too.")

“Whoa!,” Patty exclaimed, “Why would you be going to visit ACME’s factories in China?”

Rob went on, “You know things haven’t been going well here at AJAX, we never adopted “Lean Sigma” techniques like ACME did. Today, we had a layoff and I got hit.”

“Yikes!” screamed Patty. Her mind went through many scenarios with Rob being unemployed 5 weeks before their wedding.

“Easy girl,” Rob implored. “My GM called me in and said that he was sorry to see me go, but being a friend with your GM, he got me a job at ACME.   I am to be the liaison for the 3 factories that ACME has in China. My fleuncy in Mandarin made the difference,” he finished.

Patty and Rob were unusual for Americans, in that they both spoke Spanish and Mandarin. Both of their dads had encouraged them to take Mandarin at Tech as they had both taken many years of Spanish in high school. Both did a language study abroad (LSA) term and an internship in China. As their dads said, “If you can speak English, Spanish, and Mandarin, you can speak to almost any professional in the world.” Both Patty and Rob found that their language skills gave them a ready bond when they were abroad. One German colleague even told Patty that she was the only American he knew that would not fit the European view of Americans when they ask, “Are you bilingual, trilingual, or American?”

After Patty calmed down, she asked Rob why he thought she would be going.

He responded, “When your GM gave me the job offer on the phone, he alluded to a team visit to China, by me and this genius young woman that is a process expert. Apparently, they have some Head-in-Pillow, graping and productivity issues. I will be handling the business aspects, you the technical. He also mentioned he would like The Professor to go. I don’t think he knows we are engaged.”

Patty congratulated Rob and finished to conversation. She hoped that their being married wouldn’t create any issues in working together. She also was a little annoyed that she always seemed to be the last to know about trips that the executives were planning for her and her team. It was especially annoying that Pete seemed always know before her when they would need to go on one of their adventures. After all she was Pete’s boss. Well at least this time it was Rob, not Pete. There is now way Pete could know about this potential adventure.

She went back to finishing her note to cranky Hal Lindsay when she heard, “Pack your bags kiddo, it looks like China this time. Oh and Rob is going.”

 

Cheers,

Dr. Ron

I saw Patty at a recent SMTA meeting.  I mentioned that many of her fans would like to see a photo of her.  Surprisingly, neither of us had a camera.  As you remember she is also a self taught artist,  I asked if she would mind sketching herself.   Here tis.

 The reflow image of leaded and lead-free solder coutesy of Motorola.

Shhhh.  Don't tell anyone but I have a secret.  The activators used in halogen-free solder pastes aren't as effective as their halogen contained counterparts.  The real secret is coming up with a technology that overcomes the deficiency.  Indium Corporation's approach to this has been to focus on the oxidation barrier of the solder paste.  The theory here is that if you can prevent oxidation from occurring through the preheat and soak stages of the reflow process, then the activators have to work less to remove oxides.  Indium Corporation used this approach in developing the Indium8.9HF solder paste.  This solder paste clearly outperforms most solder pastes in reducing head-in-pillow and graping defects by preventing oxidation.

Indium's booth at Nepcon China 2009

Part of the Indium Team

这周Indium公司参加了在上海的我们这个行业的盛会，Nepcon China 2009。虽然全球还处于金融危机的深渊中，但是展会上各个参展商的热情，和到来的客户们对新产品和新技术的渴望，也有一种振奋人心的感觉。

Indium公司为大家推出了我们新一代的完全无卤配套产品（第四代），有Indium8.9HF焊锡膏，WF9945 & WF7745水基和酒精基的驻焊剂，CW802 & CW807焊线。 Indium 8.9HF除了有以往我们公司优异产品的所有特色外，还能很好地解决现在大家面临的葡萄现象(graping effect)，枕头效应(Head in pillow effect)，探针可探测性(ICT)等带来的挑战。

就像上文提到的迷你本(netbook)在中国的"崛起"，除了在SMT组装方面可以用到Indium系列的PCBA材料，其实从晶圆制造，芯片的组装封装，CPU/GPU的封装，Indium公司都提供一系列的解决方案---半导体材料(semiconductor materials)，散热界面材料(TIM, TIM1, TIM2, TIM1.5),等等。

……

久违了的祖国，久违了的城市，久违了的人们，久违了的展会，熟悉又亲切。

Cheers!!

Pic: Anny Zhang took the pics in the show.

Folks,

Recently I caught up with Ed Briggs, a member of Indium Corporation's technical staff.  I asked Ed if he would share with us his thoughts on "graping" and how to minimize it.  His comments follow:

Graping is a phenomenon which appears as un-reflowed solder particles atop the solder mass (see figure above). Graping occurrence has increased due to the higher lead-free reflow temperatures, the decrease in volume of the printed paste deposit, and finer powder particle sized solder pastes required in miniaturized electronics. The combination of these factors, puts a lot of "pressure" on the solder paste flux to remove the surface oxides. In addition, during the reflow process, the flux can "run-away" from the solder powder particles, spreading and pooling about the deposit. The exposed powder particles become oxidized. With no flux to protect or remove the oxide, these particles do not coalesce into the solder joint.