Tombstoning

Folks,

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

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

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

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

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

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

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

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

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

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

Cheers,

Dr. Ron

Tombstoning

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

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

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

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.

There seems to be a growing trend to use a low-Ag or Ag-free solder alloy for Surface Mount Technology (SMT) electronics assembly, similar to what is commonly offered for bar solder, used in wave and selective soldering.

For through-hole performance, the strength and stability come from the entire barrel of solder, whereas it is usually the foot and heel fillets that give SMT solder joints their strength.

Lets talk about the other issue with using a eutectic solder alloy in SMT: tombstoning.  One of the benefits of using the SAC (tin-silver-copper) alloy for SMT and solder paste, is that it has a built-in plastic range, similar to that of Sn62 (62Sn 36Pb 2Ag).  It is this plastic range that prevents tombstoning, and takes into account the inconsistent heating of the solder across the part (which is the sole cause of tombstoning).  Switching to a eutectic alloy eliminates the plastic range and opens the door for tombstoning.

Any powder manufacturing issues, such as the inconsistent distribution of dopants throughout the alloy and powder matrix, takes a back seat to the surface mount reliability concerns. 

There are other alternatives, such as SAC0307 (99Sn 0.3Ag 0.7Cu)… But, with the price of Ag finally coming down, and a long history of SAC usage, we don’t think it’s going to be a major player.

Next time, we'll talk about the manufacturing and costs associated with low-Ag and Ag-free alloys. 

I hope this helps. Contact me with any questions.

Folks,

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

Tombstoning, simply, is the wetting of one side of a component before the other side, which causes the setting forces of the solder to lift the component like a drawbridge.  Sometimes, it even cause the component to complete stand, like a tombstone.

There are several ways to counteract whatever cause is making the defect occur.  All of them include either getting the component to come to the same temperature at the same time, or allow for flexibility in the melting point of the solder during reflow.

Below are some ideas on how tombstoning can be eliminated.

 

1.   Eliminate Nitrogen reflow - Nitrogen reflow prevents the additional build up of new oxides on surfaces and the solder alloy, and allows more activator to be used for wetting, increasing the wetting force.

2.   Lower the delta-T across the board and component to <10°C -  This allows for more stable temperatures through liquidus, which equalizes the wetting on both sides of the affected components.

3.   Slowing the ramp rate of the components through reflow to ~0.5°C/s - This allows both sides of the component to come to temperature simultaneously.

4.   Introduce an anti-tombstoning alloy, such as the Ind100 (62.6Sn 37Pb 0.4Ag, which has a 4-6°C plastic range.

5.   Increase placement pressure and depth, which uses the tackiness of the paste to hold the component in place.

6.   Proper placement ensures that the component is centered between the pads.

7.   Stencil design ideas, such as home plate or reverse home plate, takes advantage of the alloy’s wettability and uses it to your advantage to solder the part to board, rather than using the wetting of the alloy to lift the part.

8.  Ensuring proper board and Pad design makes sure that there is not solder-robbing, where the solder flows along a trace, and doesn’t leave enough for the component.

 

Folks,

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

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

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

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

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

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

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

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

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

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

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

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

a.       The solder temperature is too low.

b.      The pad on the board is oxidized.

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

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

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

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

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

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

Overall average score 58%.

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

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

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

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

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

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,

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

上兩周在一個客戶端那裏， 他對焊接含有各種元件大電路板的爐溫曲綫有點困惑。 

通常的爐溫曲綫有兩种：綫性爐溫曲綫(Ramp To Spike---RTS)，含有恆溫區的爐溫曲綫(Ramp Soak Spike---RSS).   雖然常規來説，60%-70%不良焊接的根源在於印刷的過程中，但是有時候對爐溫曲綫的微調，也可以大大減少各種焊接的不良現象，提高產率。   

在那個客戶端，因爲板子有30cm x 40cm 那麽大，上面有各種常規和不常規大大小小的元件，有些還是十分熱敏感(thermal sensitive)的元件，所以這是一個thermal massive的板子，也就是說板子上各個焊接點同時達到熱平衡(同一個溫度的意思，thermal equilibrium) 比較難。所以我們根據客戶準備使用的合金(當時用SAC 305), 板子的凃層(surface finish/metallization)，板子上大概的元件情況，還有回流爐(reflow oven)的區間(當時用10 zones)，給客戶設計了一個適合那種產品的回流曲綫(reflow profile)。  這是一個有一小段恆溫區(shoulder soak time)的曲綫，就是在合金到達熔點前(solidus point)，來一兩段溫度保持平衡的區間。好在客戶是10個區的回流盧，設計起來條件也比較寬鬆。這樣設計可以大大減少熱不平衡(thermal gradient) 帶來的不良焊接，像unsoldered虛焊，立碑tombstoning, 等。

第二天早上，和客戶一起進行了三個不同的檢測后：visual inspection目測, microscope inspection顯微鏡觀測, and X-ray inspection, 一切良好！ Yeah Ha!! Cheers!   

Pic:   http://mayang.com/textures/Manmade/images/Plastics%20and%20Related/electronic_circuit_board_9131073.JPG

 PS:  西雅圖滿街粉紅色和白色的櫻花都很美。 一切安定好后，要開發美國大西北啦！ 最近在讀Randy Pausch 的書“The Last Lecture”，這是他回顧人生，面對死亡的一些平淡箴言。 這也有他著名的“Really Achieving Your Childhood Dreams”的錄像，英文的。  

 

In my twenty years in the electronics manufacturing industry, I have heard a lot of claims made about the use of nitrogen in inerted soldering processes: many of them completely wrong. In this discussion, I'll use the example of reflow in an enclosed oven, although many of these discussions may pertain to wave soldering and even vacuum soldering.

Let's start with the real reason an assembly engineer uses nitrogen in inert soldering: because it is the cheapest gas available that does not react with hot metal surfaces to form an unsolderable film. That's it. Period. People who use nitrogen for reflow are not using it because it has any wonderful properties, they are using it because it has low oxygen and moisture levels, and can purge (dilute) oxygen down to a low enough level to prevent or slow the oxidation of metal surfaces during heating.

To understand any process using inert (unreactive) gases, you have to understand the composition of air; the most abundant gas available to us. Air is around 78% nitrogen, 20.9% oxygen and 0.9% argon, with small amounts of other gases, carbon dioxide and so on, along with varying amounts of water vapor. Water vapor may go up to around 4%, and of course, at this level, it will dilute the other levels of gases by (96/100), just in case you think there as a problem with the math. The oxygen level (20.9%) equates to 209,000ppm (parts per miilion). The ppm unit is a much more useful measure when you are down at low percentage levels, for example 0.01% = 100ppm. It is also important to note that the fractional measure (ppm or %) correlates to the amount by volume and, from the ideal gas equation, also the molar percentage.

I'll cover half of the of the myths now, and half next time.

Myth 1: "Nitrogen removes oxides"
Fact: Nitrogen used at reflow temperatures has no fluxing (oxide-removing) properties whatsoever and does not chemically react with anything at these temperatures. Nitrogen prevents or slows oxidation or (in the case of a flux-cleaned surface) re-oxidation simply because it is not an oxidizing gas. Forming gas (a mixture of hydrogen and nitrogen) is very different, and I will discuss this in a subsequent note.

Myth 2: "Nitrogen improves heat-transfer"
Fact: It has no practical thermal effect on the soldering process. Heat transfer in gases at the same pressure and temperature is governed by the molecular weight of the gases: nothing else. Since nitrogen has a molecular weight of 28, and oxygen almost the same at 32, the difference in heat transfer properties between air and nitrogen is minimal whether you are talking about laminar or turbulent flow.

Myth 3: "If I measure the oxygen level in my incoming nitrogen, then I know the level in the oven"
Fact: Even an apparently well-sealed inerted reflow oven is actually mixing ambient air with your nitrogen to some degree. It does this through simple diffusion (driven by difference in partial pressure of oxygen in air versus inside the oven) or by turbulent mixing of nitrogen with air near an opening. What happens in a real oven is shown in the illustration (above). Putting a low flow rate of nitrogen into the oven will have little or no effect (a), then putting more in will reduce the level, but you will see large variations (b), then finally you will reach a plateau (c) where you have obtained the minimum oxygen level possible, but turbulent mixing is still introducing oxygen from the outside air. As you increase the nitrogen flow rate, you are simply increasing the turbulence, and hence the rate of mixing

Myth 4: "Purer nitrogen will give me better results"
Fact: Standard, cryogenic quality nitrogen has around 2-5ppm of oxygen in it. Even purging a well-sealed oven will not get you down to exactly the same level as the incoming gas. You will see no difference if you are using a nitrogen source at 10ppm or 10ppt (parts per trillion) oxygen. As you can see from the illustration above (c), the effect of the highly pure gas is completely negated by the mixing with air.

Myth 5: "Nitrogen reduces all soldering defects"
Fact: Nitrogen can help with some wetting-related defects, and can often turn a so-so (marginally acceptable) soldering process into an acceptable one. Other Fact: It may not only increase wetting ("wicking") uncontrollably onto leadframes or other surfaces, but may also cause solderspatter and contribute to die tilt (power semiconductor assembly) or tombstoning (SMT).

More next time. Part II of this post can be found here.

Cheers! Andy

For my next trick, I  will be participating in a chat session as part of the Virtual PCB Show on February 25^th from 11:00AM to 11:45 AM. Our topic is "SMT Defects and Solutions".

Folks,

Tim Jensen, Product Manager, PCB Assembly Materials, would surely be considered one of the most knowledgeable people in lead-free soldering and solder materials. I was able to catch up with him a few days ago for a few minutes and ask him some questions. Here are his responses:

Dr. Ron (DR): Tim, RoHS has been in effect now for almost 18 months. It appears that SAC 305 (96.5% Sn, 3.0% Ag, 0.5% Cu) appears to be the alloy of choice for solder paste. First, is this true from your perspective and if so, why is SAC 305 more dominant than the near eutectic SAC 387? Second, what about lead-free alloys that contain little or no silver (to save alloy cost). Do any of these alloys make sense for a solder paste, or just for solder for wave soldering? Or is the old adage still true that the major cost of solder paste is the processing, not so much the material?

Tim: SAC305 has become the alloy of choice for much of the industry. The reason was based primarily on the fact that SAC305 is a lower cost alloy than SAC387. Studies that have been conducted to this point don't show a negative impact on reliability. Therefore, a cheaper alloy with no downsides is an easy choice. A side benefit that has been realized with SAC305 is that because of the slight plastic range, it shows a lower occurrence of tombstoning defects than SAC387. However, the cost of Sn and Ag have steadily increased over the past couple of years. Because of this cost trend, some solder users are considered even lower Ag content alloys such as SAC105 (98.5Sn/1.0Ag/0.5Cu) and SAC0307 (99.0Sn/0.3Ag/0.7Cu). These alloys are lower cost, but have some significant processing disadvantages. They don't become fully liquid until 226-227 C which creates additional constraints on the already tight Pb-Free reflow process window. The low Ag should improve the alloy's drop test performance (for portable electronics) but it probably won't be as robust as SAC305 in typical thermal cycling environments.

Because of the tight margins on most consumer electronics, the assemblers have been working hard to reduce their costs through process improvements and through price pressures on their material suppliers. As the price of solder paste drops, the impact of metals costs becomes much more important. In addition, Ag is a high cost metal, so that also causes the SAC metal costs to have a greater impact that with the Sn/Pb alloy of the past. High volume producers of electronics will likely see a cost savings by using a low Ag alloy. However, the overall cost impact to process window and end product reliability is still not well known.

DR: From your perspective, what is the approximate percentages of lead-free vs tin-lead paste used in the US and around the world?

Tim: Depending on their customer base, every solder paste supplier's spread of Sn/Pb vs. Pb-Free will be different. Using general terms, the United States and Western Europe still use a significantly higher percentage of Sn/Pb than Pb-Free. On the surface, that may be surprising. Especially considering the drive for Pb-Free has come through the EU yet they are using a lot of Sn/Pb. If you dig a little deeper, it makes more sense. A significant portion of the manufacturing remaining in these locations is often higher reliability products that are currently exempt from the RoHS legislation. Some examples would include military, aerospace, medical, and automotive. In areas such as China, a much higher percentage of the solder paste used is Pb-Free. This also makes sense as much of the consumer electronics are manufactured in these regions. Worldwide, 30-60% of the paste made by most solder suppliers is probably Sn/Pb (depending on their customer base).

DR: Many people are concerned with voiding in solder joints, yet some reports from iNEMI and others suggest that voiding has not been shown to be a reliability issue. What are your thoughts on voiding and have there been recent advancements in low voiding solder pastes?

Tim: It is pretty well recognized that there isn't a direct correlation between percent voiding and solder joint reliability. Voids can impact the solder joint reliability, but typically only if it is in the location that sees the greatest stress (typically the ball to component interface or the ball to PCB pad interface). Voids in the bulk solder rarely have any impact on the solder joint reliability. The challenge is that most x-ray analysis provides only a 2 dimensional view of the solder joint and voiding. Therefore, you can see how much voiding is present, but not where (in the vertical direction) the void is located. Since this challenge exists, the most logical approach is to say that the less total voiding there is, the less likely that there will be a void at one of the interfaces. Many of the latest generation Pb-Free solder pastes, exhibit pretty low void performance on standard BGA's. The technology advancements that Indium Corporation has been developing are focused on providing low voiding in CSP's with via-in-pad technology and under QFN's on the large thermal pad.

DR: Reliability of lead-free solder joints continues to be a concern. What are your thoughts on this issue?

Tim: There is a significant level of concern surrounding the reliability of Pb-Free solder joints in high reliability electronics. Pb-Free has been used in consumer electronics for the past several years. While it is difficult to say whether or not Pb-Free solder joints have contributed to more field failures, this limited history does at least tell us that it doesn't cause dramatic fall out on these products (with expected life span ~5 years or less). For high reliability electronics, it is difficult to know for certain the impact of Pb-Free. A significant amount of work has been done on modeling to predict the life of the Pb-Free solder joints. However, until we have real data, it is impossible to know for certain how well the models correlate to real world applications.
In addition to the solder joint reliability, there are challenges with the other materials used which could also impact the end product reliability. For example, the circuit board used in a Pb-Free process may degrade more than through a Sn/Pb process. This will likely result in an end product that is less reliable. In cases like this, going Pb-Free isn't what hurts reliability, but rather it is the processing conditions that Pb-Free requires.

DR: Well that's all we have time for today. Stay tuned for further discussions with Tim on halide-free soldering.

Folks,

I was recently asked to give a presentation and audit an assembly line regarding minimizing "tombstoning" of passives at a major electronic assembler.
As my presentation brought out, tomstoning can be caused by many factors: the reflow profile, the solder metal composition (for lead-free SAC 387 tends to tombstone more than SAC 305), off-center placement, nitrogen reflow atmosphere, buried vias, etc. After giving a two hour presentation, I toured the line that "had a problem with tombstoning."

As I started asking, it became clear that no one knew the magnitude of the problem. "How many passives are on each board?" I asked. No one knew. "How many DPMO (defects per million opportunities) for tombstones have you had recently." This metric was also unknown. As people scurried around to get data, it started to become clear that tombstoning might not be as big an issue as everyone thought. It was more of a local legend.

Finally, we got some data. Each board had about 1000 passives, they had produced 100 boards with a total of two tombstones in the last two hours. Tombstones were the only defect. Hmmmmm two bad boards out of 100 = 98% first pass yield, not bad! From a DPMO perspective, they had 2 defects per 100,000 opportunities or 20 DPMO, which is world class. This level of DPMO would be very difficult to improve on without massive engineering investment. It is "in the noise" and it is likely caused by "common cause" variation. I then asked how much money it costs to repair a tombstone, as expected no one knew. My guess is less than $2. This situation is the rare case where yields are so good, it may not pay to make engineering investment to improve them.

This isn't the point of the story however, whatever is done in a case like this one, needs to be "data driven." Only with the proper failure rate data, plotted in a Pareto chart and understanding costs can the appropriate action plan be developed.

Always be data driven!

Cheers.
Dr. Ron

Folks,

I am giving an updated version of my one day workshop on WEEE/RoHS compliant assembly at The Thayer School of Enginnering at Dartmouth on Wednesday, April 12, 2006 from 10AM to 4PM. It will be held in the Jackson Conference Room in Cummings Hall. It is free (including lunch) and is sponsored by Indium Corp and the Cook Engineering Design Center at Dartmouth. If interested contact Judy Durell (judy.durell@dartmouth.edu.) She will give you directions and register you. There will also be a free, top quality handout of the slides.

In addition, there will be a brief presentation on the fascinating history of engineering at Dartmouth, the first professional engineering school in the US.

Please join us!

Download this PDF describing the event. (2.8 MB)

Here is the agenda:

- WEEE/RoHS Overview
. Trends
. EU regulations and Directives
. WEEE Overview
. RoHS Overview
. Educated Interpretation
. Member State Guidance Notes
. Other Impending Legislation
. Tear Down Analysis

- Selecting the Alloy
- Understand the Challenges
- Component Challenges
- PCB Surface Finishes
- PWB Concerns
- CAF Concerns
- Tombstoning of Pb-Free
- Pb-Free Voiding
- Inspecting Pb-Free Solder Joints
- Develop the SMT Process
- Wave Soldering and Alternatives
- Case Study: The Motorola Success
- Implementing Your WEEE/RoHS Plan

Cheers,

Dr. Ron

Haibing writes:

Question 1: My questions are centered around RoHS judgments. That is,
how I can judge whether a contract manufacturer is RoHS compliant or not?
Does a manufacturer need to have a certificate, like a UL certificate.

Answer 1: Since only the "producer" (the company that puts the product on the market) has to assure RoHS compliance there is no certification (at this point) for manufacturers. Even the producer does not need any certification paperwork to put products on the EU market. By the act of putting the products on the market, the producer declares them to be RoHS compliant. I do, however, expect the EU countries to quickly want "RoHS Compliancy Declaration" paper work to accompany the products. But at this point in time there is no such requirement.

Q2: RoHS compliance is not just Pb-free, for parts that do not
need to pass soldering processes, how can I judge whether they are
RoHS compliant?

A2: Your component vendor should give you some form that declares that the components are RoHS compliant. If you are working with a mainstream component supplier like TI, Amkor, or Motorola I would be quite comfortable that their components would be OK. However, this is not the time to be buying on the "grey" market. There will be tens of billions of dollars worth of non RoHS compliant components that people will want to sell.
Verifying RoHS compliance of a component requires a sophisticated laboratory with SEMs, UV-Vis Spectrophotometers, etc. to perform a "due diligence" RoHS verification. However, picking out a counterfeit may be easy. I unit like the Niton X-ray Fluorescence Analyzer , shown in the photo, could pick out a component with lead in its leads in seconds.

Q3: For SMT assembly processes, is SAC305 globally used?

A3: SAC305 (tin with 3.0% silver and 0.5% copper) appears to be universally accepted as an optimized alloy for tombstoning, voiding, silver plate, etc.

Q4: How can I know if all the components will withstand the higher soldering temperatures?

A4: Only your component vendor can help you here. The good news is that the best of breed lead-free solder pastes like Indium's 5.1 have been successfully reflowed at temperatures as low as 235 +/-5 C in air, with outstanding results.

Q5: If a manufacturer uses IPC 610D soldering standards, can we say they are
RoHS compliant?

A5: The use of IPC 610D is encouraging, because it shows that the manufacturer is "with it" from a lead-free inspection standpoint. This in no way assures RoHS compliance.....what if they were assembling with non RoHS compliant components?

Q6: Any special requirements for assembly equipment?

A6: Most modern reflow ovens can handle the high temperatures. However, for wave soldering, you will probably need a new unit. Work with your equipment suppliers like Speedline.

Some people think that placement tolerances are a little tighter, so you might want to discuss this with your placement vendor.

In summary, this is an important time to work with your component, materials, PWB and equipment suppliers. They can help you a great deal.

Cheers,

Dr. Ron