Paste Flux

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

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

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

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

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

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

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

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

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

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

 To learn more, please contact us.

 Cheers!  Andy

Folks,

Let’s look in on Patty and Pete and see how they are handling Rex “The Torrent.”

Patty wanted to give Pete a little more exposure so she nodded to him to chime in.

“It is true that Pinnacle’s line cost only 70% of Optoplace’s line and it does have a lower ‘cost of ownership’ in that it costs less to own, but we lose our shirt because of its 6 hours per week less uptime,” Pete began.

Torant stormed in, “There ain’t no way that 6 hours a week can make up for 30% savings in cost of ownership. We must be talking about over $600,000 dollars difference in capital cost.”

Patty heard this comment and wondered why people that make poor arguments need to add bad grammar, too.

“Torant makes a good point Pete,” Madigan quickly interjected.

“Actually it is $660K in additional initial capital investment per line, plus about $40K a year in service for the higher profit potential line,” Pete responded with a smile.

“I told you so,” Torant said excitedly.

At this comment Pete put up a PowerPoint® slide that showed the resulting comparison:

Pete explained, “The average of 6 hours/week of increased uptime in our typical 3 shift operation results in the additional production of more than 22,000 units per line per year for the higher profit potential line.  Each line producing on average more than $340,000 more profit.”

“But that’s not as much as the additional $660K cost of the line,” Torant countered.

“The extra capital cost is included in the calculation,” Pete calmly replied.

“Well, Torant, that’s one you lost,” Mike Madigan said in a way that indicated that discussion on this point was finished.

Torant looked temporarily defeated, but he recovered quickly. “What about the solder paste? Ultima costs $0.02/gram less than the ElectroMaterials paste,” Torant challenged.

“That’s true,” said Patty. “But we have to stir it out of the jar for it to print well, and it has poor response to pause.”

Torant wouldn’t let her finish, “But that can’t make up for two cents per gram,” he snarled.

“Not true,” Patty snapped back. “Every time the line is down for a short time we have to wipe the first print because the transfer efficiency is so poor.  We lose an hour a week of production time.  In addition, when we are printing a lot, the paste shear thins and we have to replace it with fresh paste.  We actually pay more for the Ultima paste because we scrap so much.  However, the lost time is what hurts the most financially.”

“Only one hour per week!" Torant screamed. “I spend more time than that on smoke breaks. One hour per week can’t possibly make a big difference.”

Patty rolled her eyes and then displayed another slide that showed the profit comparison.

“This slide shows that by using the Ultima paste we lose over 3,700 units of production and over $140K of profit per year per line in that 1 hour hour per week.  One hour per week is 52 hours per year, let's not forget” Patty responded.

At this, Torant slammed his fist on the table, packed up his briefcase, and literally left the room in, well.... a torrent.

Patty, Pete, Madigan, and Sam just looked at each other.

“Well, maybe we won’t have to put up with him for awhile,” Pete said smiling.

“Nice work Patty and Pete", Madigan said. "Let’s develop an implementation plan phasing everything in you recommended as soon as is practical.”

Patty was always surprised when Madigan showed a little warmth by calling her and Pete by their first names.

“Sure thing, Mike,” she answered.  It was the first time she ever called him by his given name.

“Oh, and I guess it was a good thing we didn’t get around to discussing solder preforms,” Patty teased. "The ones Torant sells have too much flux and they gum up the pick & place nozzles.”

With that comment, they all chuckled and took it as a key that the meeting was over.

Cheers,

Dr. Ron

Folks,

Let's look in on Patty and her colleagues......

Sam Watkins, ACME New Hampshire site GM, had just finished meeting with his boss, ACME CEO Mike Madigan. He was embarrassed that these meetings always stressed him; Mike was an intimidating character. Still, why should he be nervous? Things were going really well. Profits were up at all sites since NMAC/I/O was implemented as their new profitability metric. Patty Coleman, who suggested this metric, visited all of the ACME sites with weaker NMAC/I/O and profits, and, after performing process audits, helped these sites get their acts together. Oh, and we can’t forget Pete Ortiz, who works for Patty. They seemed to have a terrific synergistic relationship. He was an integral part of this success story.

Sam started writing an email to Patty. He and Mike concluded that, building on the recent NMAC/I/O success, they need to make ACME a “copy exactly” company. They agreed that if they were implementing a copy exactly strategy they should do it with the most cost effective assembly equipment and materials. It seemed to both of them that that the lowest “cost of ownership” should be the most important metric in this strategy. Sam finished his note to Patty asking (ordering) her to implement this strategy. She was to present a plan to achieve this goal to Sam and Mike in 6 weeks. Her presentation was to include the recommended equipment and materials, a phase-in plan, the budget needed to achieve the goal, and the projected ROI of the endeavor.

Patty was in her office having lunch while reading Golf Digest and USA Today. She looked up at her laptop screen and saw Sam's email. Reading it energized her. She was happiest when working on a significant project. After digesting the contents she thought she would call The Professor and ask his advice. Sam and Mike had insisted that she put The Professor on a retainer as he had added so much value to ACME. Patty had to chuckle, it was hard to get him to send in his bill; he seemed little motivated by money.

The Professor would never tell her how many languages he spoke, so she was going to try a little French on him.  She and Rob had been studying it at home.

“Bonjour Professeur, comment ca va?” Patty cheerfully said as The Professor answered the phone.

“Très bien Patty. Comment sont Rob et vos fils? Ma femme et moi avons été inquiets au sujet de Rob. Est-ce le dos guérit bien?” The Professor replied with a Parisian accent. (Very well Patty. How are Rob and your sons? My wife and I have been worried about Rob. Is his back healing well?)

Patty sighed and thought, “Well that makes about 10 languages I have verified so far.”

“Rob is doing quite well. Word got around and my Lean Six Sigma Green Belt instructor, Jim Hall called and shared his thoughts with me about over doing it in exercise programs. Jim is a fitness instructor and a big believer in moderate exercise. Rob has promised me to tone it down a lot,” Patty answered.

“I’m relieved,” said The Professor, “Rob needs to be healthy to keep up with your sons.”

“But, I imagine you have some business to discuss,” the Professor went right to the point.

“Yes, Sam and Mike want me to head up implementing a copy exactly program with equipment and materials, and they are strongly suggesting that the equipment and materials have the lowest cost of ownership,” Patty summarized.

“Copy exactly can be very beneficial, if the materials and equipment are good choices,” The Professor answered thoughtfully.

“But I have real problems with ‘Lowest Cost of Ownership.’ It is a good metric to compare something like automobiles, but to compare equipment or materials that are used to generate a profit it can be misused.” he replied.

Patty felt she understood where he was going, but wanted to hear it from him.

“Can you give an example?” she asked.

The Professor answered, “Let’s say a man mow lawns for a living. He considers two lawn mowers for his business, one is a push mower that cuts a 20 inch path and costs $300. Assume he takes 3 years to pay off the loan to buy it. Maintenance is $150 per year and fuel is $1200 for a 30 week season. The other is a sit down lawn mower that costs $3000, with $500 maintenance per year and it uses $3,000 in fuel per year. It cuts a 50 inch path. Which has the lower ‘Cost of Ownership?’”

“That’s easy,” Patty said, “the 20 inch push mower.” “But clearly the lowest cost of ownership is meaningless,” she went on.

“Explain,” replied the professor.

Patty answered, “Well, the man is in business to optimize profit. Clearly he can mow more laws with the sit down mower. Let’s say with the push mower he can do 4 lawns a day and with the sit down mower he can do 10 lawns a day. We can also assume he gets $35 per lawn. So, for a New Hampshire 30-week lawn mowing year, he earns 4x7x30x$35 = $29,400 with the push mower and 10x7x30x$35 = $73,500 with the sit down mower. Let me make a spreadsheet to determine the profit in each case.”

Patty was one of those young people who could type so fast that it made The Professor’s head spin. In seconds she had a spreadsheet developed.

“Wow, with the push mower he only makes $27,950 and with the riding mower he makes $69,000!” Patty exclaimed.

“And the same is true in electronics assembly. The best equipment, solder paste, solder preforms, underfill, cored solder wire, and solder fluxes are the ones that help your company make the most profit. Not the ones that have the ‘lowest cost of ownership,’” The Professor summed up.

We are frequently asked if it is possible to solder to aluminum. The answer is yes, if the following guidelines are followed: 

FLUXES:
Because it is difficult to solder to aluminum, Indium Corporation developed Indalloy Flux #3 (activation temperature is 96-343°C) to remove the tenacious oxides that prevent the solder from wetting to the surface. This flux is very corrosive and is not recommended for electronic applications because, if any of the post-reflow flux residue remains after a warm water rinse with mechanical scrubbing, the joint may be compromised. This flux is recommended for mechanical assembly joining applications only. 

Another alternate solution is to use a forming gas consisting of nitrogen and hydrogen. This method of oxide removal is generally used when the soldering temperature is greater than 350°C which is ideal for activating the hydrogen to reduce the oxides. With this method, there is no post-reflow flux residue to clean up.

METALLIZATIONS:
An alternate to corrosive fluxes is to nickel plate the aluminum so a weaker flux (RA, ROL1) can be used. These fluxes are less corrosive and can be easily removed with an appropriate solvent.   There are many solder alloys that will wet to nickel. Check out our solder alloy physical properties table.

SOLDER ALLOYS:
The solders that are normally recommended for joining aluminum are:

• Indalloy #201 (91Sn, 9Zn); 199°C E
• Indalloy #176 (95Zn, 5Al); 382°C E. 

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

Following on from our discussions of last time...

As you will recall from the previous post on this topic, My friend and colleague Chris Nash and I were discussing some puzzling results for low dip height found during testing of package-on-package (PoP) materials. The findings will be of interest to everyone who uses a dipping process in both SMT and flip-chip assembly.

Post II:
For greater solder paste and flux dipping heights it appears as though a linear doctor blade (back and forth) used in a dipping process running at high speed will allow dip heights close to those expected from the theoretical engineered limit, for 50 microns and greater dip height. The high speed shear-thins the flux, which has the effect of both reducing the thickness of the boundary layer, and also has the benefit of reducing the extensional (tack) viscosity, so components can be more easily released from the dip tray.

What if you want to go to lower dip depths?

As we move into the area of copper pillar flip-chip dipping, and even (we hear) some Japanese customers doing package-on-package assembly, the dip height (dip depth) can go down to as low as 10-20microns, and this where we are hearing that rotary dip trays are coming into their own. The diagram below shows a simplified version of a flux and solder paste dipping tray.


Rotary dip trays seem to have the following advantages:

- Height Setting: The dip height/depth is set using two micrometers, so is infinitely adjustable to a precise setting, although the dip height does have to be measured.

- Low Cost: They also add zero capital cost for a new dip depth setting, compared to specially-engineered dipping trays, which can be upwards of $2,000 each.

- Accuracy and Precision of Depth: From a more pragmatic viewpoint, however, the real reason for rotary trays being used with ultra-low dip heights is that the flux depth is actually measured: there is no tacit assumption of a given dip depth being correct and constant, based on the engineering of the dipping tray. As we saw last time, an error of 20 microns is possible, and with a dip height of 50 microns or less, this is a huge problem if you are using a 50 micron dip tray and assuming that you are getting exactly that dip depth.

However, rotary dip trays also have their share of potential problems compared to linear dipping systems: 

 - Larger Surface Area: Flux and solder paste may dry out faster, and a water soluble material will be more vulnerable to the humidity content of the air. It is also more wasteful of flux, since a larger surface area of flux is exposed than will ever be used, although this may also be true of some of the linear tray designs.
 
- Circular Tray: Materials will experience a higher shear rate at the outer edge than in the middle. If spun too fast, dipping materials may accumulate at the edges, thrown outwards by centripetal force.

- Lower Shear Rate: For the same flux or solder paste dip depth, the velocity of the doctor blade will be much lower with a rotary than a linear system. However, as you can see from the illustration below, for a doctor blade moving at 1/4 the speed and 1/4 the dip height, the shear rate is the same.

As always, please contact me if you need to learn any more.

Cheers!  Andy

Folks,

Rob heads to Guadalajara to solve the QFN voiding problem......

As Rob sat on the airplane, he was excited to go to GDL (Guadalajara, Mexico) to help solve the voiding problem. He knew Patty would be a little peeved that he asked for Pete to come along, but she was gracious, recognizing that Rob would benefit from a success in this effort.

As the plane circled for a landing, Rob was preparing for the somewhat comical trip through customs. He always thought that the red light/green light method of determining if they were going to search you bags was unusual. Oh well, go with the flow.

The ride from the airport was about 40 kilometers to the factory through GDL’s bustling traffic. After arriving at the plant Rob was relieved to see that Miguel Mendoza was there to meet him and Pete. Rob had worked with Miguel in the past and respected him as a process engineer. Miguel told them that a kick-off meeting was scheduled with the site GM, a fellow from the US named Grant Wilson.

As the meeting started, Rob introduced himself to Wilson in Spanish.

“Wow,” Wilson chuckled, “when asked if I am bilingual, trilingual or American, I have to say I am American.” “But, I am taking Spanish lessons,” he continued.

Rob looked at Miguel and saw him roll his eyes. But Rob thought it was at least a nice gesture that Wilson was taking Spanish lessons.

“Perhaps someone could share what actions have been taken and what the status is,” Rob suggested.

“Miguel, could you give Rob an overview of where we are” Wilson asked.

Miguel began, “The warranty send back rate is 5% on Druid phones. Almost all of these failures have been traced to high powered QFNs that have significant voiding under the thermal pad. The voiding percentage is about 50-70%. About a week ago we obtained Derrick Herron, Dr Yan Liu and Dr Ning-Cheng Lee’s recent paper, Voiding Control at QFN Assembly, at SMTAI 2011.  We changed our stencil design, as suggested in the paper, to allow for venting of the solder paste volatiles and voiding went down to 30 to 50%.”

“What level of voiding would be acceptable?” Rob asked.

“We’re not sure,” Miguel answered.

“So it seems we have two issues, one is to determine if 30 to 50% voiding is OK and the other is to see if we can reduce it further,” Grant Wilson reasonably commented.

“My sense is that we need to be in the less than 30% range,” Rob added. “This may require that we use solder preforms. Voiding is caused by outgassing but also by insufficient solder,” Rob finished.

“OK, you two go and solve the problem and get back to me. You have 3 days,” Wilson commanded.

Rob, Pete, and Miguel headed off to get started on their assignment. Rob was really glad Pete was there.  He was an expert in setting up and optimizing the component placement machines that were at this site.  Fortunately, Rob had also brought some solder preforms with him, expecting that they would be required. A call to the QFN vendor confirmed that less than 30% voiding should be the target. Rob looked at the data and x-ray images of the work that Miguel and his team did to reduce the voiding by improving the venting of the flux volatiles. He was impressed. But he didn’t think it would be enough.

(Dialogue translated from Spanish)

“Miguel, I’m almost certain that we will need to use solder preforms on the two most critical QFNs,” Rob began. “There are two major reasons for voiding, the first is flux volatiles forming voids, the second is solder starvation. Most people don’t realize that solder paste is only 50% by volume metal. In cases like this, where we really need low voiding, often the only path to success is to use solder preforms to add solder metal,” he finished.

Rob then showed Miguel Seth Homer’s SMTAI 2011 paper Minimizing Voiding in QFN Packages Using Solder Preforms. This paper describes the process steps needed to achieve a successful QFN solder preform process. Rob and Miguel spent the better part of a day setting up one assembly line to assemble with the solder preforms using this paper as a guide. They assembled 100 phones and the voiding level was 10.5%.

Early the next morning, they met with Grant Wilson.

“By the smiles on both of your faces, I gather you were successful?” Wilson asked.

Rob went on to explain how they determined that solder preforms were needed. He explained the process and waited for questions.

“What do solder preforms cost?” Grant asked.

“They are about $0.02 (US) in quantity, but understand that your warranty cost per $200 phone is at least $10 right now (0.05x200),” Rob answered.

“Did you have to slow the process done?” Wilson asked. “I have been a fan of the work that you and Patty Coleman have done with The Professor, you have convinced me of the importance of throughput,” he finished.

“The Professor has pointed out that almost never is a line completely balanced. Your flexible placers were waiting four seconds for the chip shooters. We put the preforms on the flexible placer and tuned up both machines by optimizing the feeder placement. The cycle time is now 1.25 seconds faster for the 3 phone per PCB card,” Rob answered.

“I’m curious, what was the greatest challenge?” Grant asked.

“Rob pointed out that the correct placement of the preform on the solder paste deposit for the heat sink part of the QFN is critical. We needed to assure that the preform was pushed into the paste far enough to leave a ring of paste around the preform to assure good mating with the QFN.  We couldn't have done this without Pete, he really knows the placement machines,” Miguel answered.

Miguel then showed Wilson an image from Seth Homer’s paper that displays this situation .

“Guys thanks for the great work. I have to admit that I didn’t really know anything about solder preforms, before today. In certain cases it is obvious that they can be lifesavers!” Grant summed up the situation.

“To celebrate your success, I’m treating for dinner tonight at the Santo Coyote, let’s meet there at 7PM,” Wilson suggested.

“Thanks,” Rob, Pete, and Miguel said in unison.

Santo Coyote was Rob’s favorite restaurant in Guadalajara, but it was Patty’s too. Rob was a little sad she couldn’t join them.

 Epilogue: Three months later it was confirmed that warranty send back rate was approaching zero.  Miguel was promoted to senior engineer for his part in the solution to this costly problem.

Cheers,

Dr. Ron

Hi there!

I am excited, this is my first blog post -ever. I am excited that it is a technical blog of Indium Corporation.

My story is very interesting; a common village boy has grown to become part of a BIG corporation in which everyone is obsessed with soldering! It was my passion to learn electronics assembly techniques 10 years ago. I strived and spent many sleepless nights on this – I would say on SMT.  When our Marcom Superstar Anita told me about the blogging opportunity I was really excited… how would I…? Anyway I am here!

So … soldering and solder paste is my passion. I have published two technical papers on solder paste and reflow. And you will see more thru this blog.

My two cents on soldering… although soldering process looks simple and any one can define with a single sentence; it is not a simple process. It is comprised of chemical, physical, and metallurgical process and deals with fluxing, melting of alloy, wetting, spreading, surface tension, coalescence, wicking, intermetallic growth/bonding, time above liquidus (TAL), cooling down for smooth grain structure etc.

We will have more discussions in upcoming post; stay hungry, stay foolish!

Best Regards
Liyakathali.K (Liya)
Sr.Technical Support Engineer - India
Based in Chennai, Tamil Nadu

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.

• exhibit reduced viscosity
• spatter during reflow
• produce excessive oxidation of the solder joint

Folks,

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

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

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

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

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

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

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

Image from goldsilveroz.com

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

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

Good news (+)

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

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

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

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


  ==================

Bad news (-)

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

 # The main cost-drivers here are:

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

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

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

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

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

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

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

  ==================

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

Contact me to discuss this further.

Cheers!   Andy

Folks,

It was Wednesday evening and I had just finished a brief pitch on applications of SPC  to a group of twenty. I was followed by Jim Hall,  he spoke of process mapping using SIPOC.  So did these folks have solder paste under their fingernails, or wave solder flux stains on their shirts, or, perhaps, a solder preform or two stuck in their pant leg cuff? None of these souls would have had any of this type of trace evidence of electronic assembly on their person. You see, they were all medical doctors and students at Harvard’s  famed medical school.   (I hope it is OK that the proud dad shares that my daughter Jessica is a colleague of these folks.)

Jim and I were presenting to the doctors, because they are interested in process optimization in the healthcare industry. The event was hosted by Dr. Andy Ellner.  He is a professor and doctor at the medical school and is a focal point for these process improvement efforts. I was introduced to him in the summer of 2009 by Dartmouth’s  new President Jim Kim. 

In November of 2009, Jim Hall, our colleague Larry Parah, and I facilitated Andy’s team in dramatically improving the prescription refill process in Brigham and Women’s Hospital Clinic.  Jim and I plan on working with Andy in similar efforts over the next year or two.

The most striking thing that Jim and I left with on Wednesday evening was how profoundly interested these doctors and students were in healthcare process optimization. The Q&A session lasted nearly an hour.

Ah, yes, would that our many colleagues in electronic assembly were as interested in optimizing their processes!

随着电子元器件组装微型化的趋势(miniaturization)，最近有越来越多的客户向我们咨询叠成封装的材料以及相关工艺(Package-on-Package；PoP)。

在向客户推荐PoP材料的时候，除非客户已经十分清楚自己要什么，我们一般会和他们详细介绍叠成封装焊锡膏PoP paste 和叠成封装助焊剂PoP flux具体是什么，分析各自的优缺点，然后让客户自己做决定。

Indium 公司的PoP paste (Indium9.88HF) 用的是5号金属粉，金属比重大概在80%-83%之间，根据是有铅还是无铅而定。 我们做过一系列的实验，和常规的SMT 3号粉和4号粉，各种金属比重的焊锡膏做比较，用5号粉在这个金属比重中做出来的PoP paste，各方面的性能最好。 Indium公司的PoP flux (Indium 89HF-LV) 也是根据各种实验结果都是最好的证实后， 才推出的。 通常检测PoP焊接材料， 可以做这三个实验： Transfer Test, Wetting Test, and Electrical Test. 具体的检测方法，Indium公司的Jim Hisert在他的论文中有详细描述。《Next Generation PoP Pastes for Electronics Assembly》

一般我个人比较喜欢推荐PoP paste，因为PoP paste能够提供extra solder。 PoP component本来就很薄，在焊接后回流的过程中十分容易“warpage 板翘”，那么component边缘部分就很有可能有一个上下之间很大的gap，导致根本无法形成良好的焊点。但是如果使用优良的PoP paste, paste中的extra solder metal 就能起到一个很好的“粘合剂”作用，即使有warage，也可以有一定的防御。 但是PoP flux在这方面就相对弱一点。

然而，PoP paste中的flux，因为要做很多功夫来清洗powder表面的氧化物，所以在回流过程中会有挺多的outgassing，这就很有可能导致空洞voiding 的产生。PoP flux相对而言，outgassing 就少很多，自然产生voiding的几率也小。

无论如何，优良的PoP paste and PoP flux，在防止wargage和voiding产生的defect方面，都是应该做得不错的。

Cheers!

Pic: Indium Corporation

Acknowledge to: Eric Bastow andJim Hisert with Indium Corporation