Who’s afraid of Dodd-Frank? How a coalition of companies and NGOs are opening up the Congolese tin trade.

Ever since Joseph Conrad set the stage in Heart of Darkness, the Congo has been seen as a reductionist land, one at various turns dark, distorted, mystifying, terrifying. Rarely are its complexities even noted, let alone depicted comprehensively.

So when news of warlord-ruled militias enslaving children to mine the Congolese mountains permeated the West, it reaffirmed Conrad’s metaphoric view of the region. Social and economic differences, and sheer distance, made it more comfortable for the world to just turn its back to the horrors, and, in the US, even enact rules banning trade with the country. As if a 15-years-long civil war could be legislated away.

Tucked away in Article XV, Section 1502 of the Dodd-Frank Act of 2010 was an amendment to the 1934 Securities Exchange Act mandating new rules that put the onus on buyers to audit whether materials originated in the Democratic Republic of the Congo or an adjoining country and to “stop commercial activities … that contribute to the activities of armed groups and human rights violations.”

The outcome of that was an SEC rule, adopted in August 2012, requiring companies to publicly disclose their use of so-called conflict minerals. While the rule only applied to firms publicly traded on US stock exchanges, it had a chilling and immediate effect on any company doing business with affected firms, since they too would be subject to additional tracking and disclosures to ensure their supply chain partners’ compliance. Indeed, the impact began even earlier, as companies raced to sever their ties to the DRC ahead of the anticipated SEC rule. The mindset: It’s simpler to avoid all business with the DRC than it is to validate, track and report that nation’s products.

This is a story of how a Dutch prince and one of the world’s largest electronics manufacturers decided to change the arc of history. In doing so, they put new and potentially daunting responsibilities on the supply chain, and certain leading suppliers have stepped up to the task.

Jaime de Bourbon Parme is a special envoy at the Natural Resources Ministry of Foreign Affairs of the Netherlands. He is also the child of Princess Irene of the Netherlands and the late Carlos Hugo, Duke of Parma, which makes him a prince and royal heir in two nations. When it comes to electronics materials, he is not the first person you might think of.

Prince Jaime is serious about Africa. He was the lead interviewer in a 2006 documentary on that continent’s war economies called Africa: War is Business. That film, he explained to Circuits Assembly, asserted that the supply chain was indirectly adding to the cost of warfare. “Warlords take over the mine, sell to traders, who sell to other traders, who sell to smelters who sell it all over the world, and from there, no one could tell the origins of the minerals,” he says.

But making the film did not satisfy Prince Jaime’s need to find an answer to the problem. “The frustration I had in making that documentary was I was telling the story, but I wasn’t delivering any solution.” Many of those he interviewed asserted that the never-ending wars provided convenient cover for unethical and inhumane profiteering. According to Prince Jaime, Dodd-Frank shut down around 90% of trade in the DRC. Local workers were forced to move from a cash system to a barter economy.

Fast forward to November 2011. As part of his role at the Natural Resources Ministry, Prince Jaime was asked to chair a meeting in Paris to discuss guidelines on how to accommodate the newly minted Dodd-Frank Act. Included were various representatives from business and non-governmental organizations, and from the region itself. In Prince Jaime’s words, “The meeting went very well. The guidelines were adopted. Then the representative from the Congo said, ‘Now who is going to do business with us? And we all looked around the room.’ ”

It was during a coffee break that a representative from Philips stepped forward. The Dutch OEM suggested a new pilot program in a conflict area, based on the Solutions for Hope Initiative, a Motorola project designed to use a closed supply chain to obtain conflict-free material from the DRC for eventual use in cellphones. While SfH focused on tantalum, Prince Jaime’s group chose tin because of its wide use in electronics and cans.

A plan was conceived and set in motion. Biweekly conference calls commenced, and by May 2012, the group was ready to move on its plan. Funding was secured for bagging and tagging minerals. In September, the Conflict-Free Tin Initiative formally launched the plan at a joint meeting involving the Global Sustainability Initiative and the Electronic Industry Citizenship Coalition.

With Philips willing to lead the way, its supply chain was left to choose how to proceed. One of the major suppliers of solders and alloys to Philips is AIM. As a private company, AIM is not directly responsible for complying with Dodd-Frank, but many of its larger customers are. Up to that point, AIM had never sourced material from the Congo. As David Suraski, executive vice president of Assembly Materials, summed up, “We’ve always tried to avoid potential hotspots.”

But the opportunity to work more closely with a significant customer was the key. As AIM president Ricky Black says, “We don’t have a business without our customers. If something is important to them, it’s got to be important to us. If a customer says, ‘We would like you to join forces with us,’ it’s a great opportunity. It shows we are committed and serious and not just ‘putting metal in a box.’ ”

For AIM, agreeing to help a customer was one matter. Ensuring that the minerals were of the proper purity, that the mines were valid, that the ores remained segregated, and that the cost to establish a new supply chain was within reason was something else.

Like all electronics metals companies, the RoHS Directive gave AIM added experience in maintaining and tracking individual material supplies. The risk of lead contamination provided tremendous motivation to develop new processes. In AIM’s case, that meant storing product in different places. “With lead-free, we’ve had to divide out buildings so we keep them separate,” Suraski said. “Any intermixing can cause huge problems.”

Conflict-Free tin, on the other hand, poses no technical risk, making it in one sense easier to handle. “If we mix [tin] materials, there’s no technical problem. And customers would generally be OK with it,” Suraski says. “Where there’s an issue is if CF tin is mixed with other tin. Our job has always been to isolate materials from each other. It’s not just lead and lead-free; it’s all alloys.”

“We’ve developed the quality assurance system over several years,” AIM environmental director Mathieu Germain elaborates. “Everything is identified by lot as soon as the material comes in. We can follow it by its exact weight and location of manufacturing and ensure where it is used. We are using the skills developed over years of handling metals.” AIM also maintains two labs to ensure the integrity of the respective environments.

Then there was the matter of the mine itself. The Kalimbi mine, in Kalehe, South Kivu, on the Congo’s eastern border, was selected, as it had remained open and secure throughout the Congolese war. The effort was supported by the provincial leaders, including South Kivu Province Governor Marcellin Cishambo. A contingency plan was drawn up whereby tagging of the material would be suspended if there were any direct risk that would compromise the integrity of the CFTI system.

Documentation is vital to ensure compliance with corporate rules ranging from internal to ISO to Dodd-Frank. The process goes like this: AIM obtains documents from the mine that the ore is mined per the standards they’ve negotiated. A Certificate of Conformity is then issued. The ore is bagged and tagged, and shipped to a smelter, always remaining segregated from non-Congolese material. Once smelted, the bars are labeled and shipped to AIM in Montreal.

Upon arrival, the CF material is stored in a specific section of the plant. A lot number is attached, and AIM traces the path as each lot number is integrated in production. Once readied for shipment to Philips, the material is again labeled. At no point will CFTI material be mixed with other products within AIM.

“Philips was very, very specific about the products they want to obtain,” Germain said. “They were specific about the packaging and the products.”

All parties agree that Dodd-Frank is clear as to what companies can and cannot do. The CFTI model, as executed by Philips, MSC and AIM, traces material from cradle to final production.

However, Dodd-Frank covers only freshly mined ores. It does not extend to recycled materials, so AIM put recycling procedures in place. “The US and Canadian Armies have extreme tracking procedures,” Germain noted wryly. “I’m sure our CFTI system is on par.”

It takes a long time to prove Germain right. Late last October, the Kalimbi mine began producing material for the CFTI program. In December, the first container, holding some 24 tons of tin, was shipped to the trader. A month later, the material was on its way to Malaysia Smelting Corp. The smelter then purified the ore, molded the bars and transported them via ship to AIM in Montreal, where they arrived at midday on Aug. 20. After AIM processes the solder, it will be shipped to Philips in Mexico. The company plans to manufacture bar and wire alloys to start.

There is a cost to the CFTI program, but it’s not seen as a significant premium over traditional suppliers. From Black down the line within AIM, all those we spoke with downplayed the financial expenses to start up another supply line. Unanimously, they agree there are bigger issues in play, and the benefits far outweigh the risks.

“We’ve thought this through,” Suraski says. “It wasn’t a quick decision. We analyzed the situation quite well.”

“You buy a hybrid [car] not for the cost but for the sustainability,” adds Germain. [Likewise,] “we’re trying to do the right thing.”

“Strategically, it’s important to be involved in this because if it does turn out this country becomes a significant contributor to the industry, we’ve been there from the beginning,” Black notes. The global raw materials marketplace is dynamic, he points out. “It’s interesting how commodity markets work. When people think tin, they think China and Indonesia. [But] there could be local considerations, geopolitical considerations. A lot of that is unpredictable and out of our control. You don’t want to be that guy who has nowhere to turn. You want to leave yourself some flexibility, keep your options open. If you are in early on a trend, you always stand to benefit from it.”

He also allows there’s a marketing advantage: “It’s ethical; it’s conscious. These are so much more important than they ever were. I hope things like this help us stand out from the market.”

Although the first shipment of CF tin product is still in process at the Montreal site, AIM feels it is already reaping the benefits. “This has strengthened our relationship with Philips,” Suraski said. “We are in frequent communication with them, and we now speak to people at different levels than we once did.” AIM’s participation was also a factor behind Philips Lighting in China citing the solder company as its Green Supplier of the Year, he adds.

Germain feels the CFTI is helping suppliers pursue sustainability. “We are working with regions affected by slavery. No one is buying tin from them because of Dodd-Frank. So sustainability in this case is helping customers buy a product and ensuring that all aspects are controlled. That’s what we are doing: making sure every single aspect is checked.”

Back in the DRC, the workforce at the Kalimbi mine has jumped from under 100 to over 1,200 diggers. The income stream has been rising too, more than doubling to $4 to $6 per kilo. It’s hard work. Much of it is performed manually with crude tools. But an entire local economy has sprung up, and the residents have migrated from the barter economy to a cash-based system.

No one expects the amount of ore being shipped to change the war-torn country’s near-term future. But revolutions start one person – or company – at a time. Says Prince Jaime: “This won’t be a silver bullet. Resources are not the cause of conflict, but the fuel of conflict. The Dodd-Frank Act stimulates a war economy. We need to stimulate a peace economy, where the solution can be political. There is opportunity and hope for the local population. This is their ticket out of absolute poverty. It’s very tough conditions, but they can work for something there.

“It’s like a dry economic plant. With a little bit of water, it starts to grow by itself. We finally have a formal mine, not a mess of smuggled minerals. Our supply chain has proven you can source from the region.”

The CFTI is onto something. Such efforts come about to build a company’s brand, not to sell a product. And there are ancillary benefits. With the electronics industry engaged, now the tin can industry’s interest is piqued as well.

The fruitlessness of trying to legislate away slave labor in lands thousands of miles away, not to mention the unintended yet painful consequences of such endeavors, is enough to drive one as mad as Kurtz in Conrad’s novel. Heroes often come from unexpected quarters. A Dutch prince. A major OEM. A Malaysian smelter. A solder supplier with a conscious. Together, with insight and heart, they are making an African nation’s future a little less dark.

A Closed-Loop Supply Chain

This piece focuses on a few of the key companies involved in the Conflict-Free Tin Initiative (http://solutions-network.org/site-cfti). There are others, however. Motorola, BlackBerry and Alpha are among them, and the CFTI welcomes mines, smelters, component manufacturers and end-users to join, adding that success will “be largely measured by the industry participation in the closed-pipe supply system.”

Most of the CFTI information is in the public domain. AIM indicated it hasn’t been approached by competitors, but it holds out hope the program will grow for the broader industry. Already, interest is rising.

“As soon as we announced our participation, a lot of companies have been asking us for information and to get batches of these products,” David Suraski said. “And it’s not only major companies; it’s companies that want to sell product with a label that says CFTI.”

Mike Buetow is editor in chief of Circuits Assembly; mbuetow@upmediagroup.com.

Devices are known to pass qualification testing, then fail in the field. Does that suggest the test specifications are inadequate?

“Experiment without a theory is blind. Theory without an experiment is dead.” – Unknown reliability physicist

Shortening a product’s design and development time in today’s industrial environment typically precludes time-consuming reliability investigations. To get maximum reliability information in minimum time and at minimum cost is the major goal of an IC package manufacturer. On the other hand, it is impractical to wait for failures, when the lifetime of a typical electronic package today is hundreds of thousands of hours. Accelerated testing is therefore both a must and a powerful means in electronics manufacturing.¹

The major AT types are summarized in Table 1. Product development testing (PDT) is a crucial part of design for reliability (DfR). A typical example is shear-off testing, when there is a need to determine the most feasible bonding material and its thickness.

[Ed.: To enlarge the figure, right-click on it, then click View Image, then left-click on the figure.]

Highly accelerated life testing (HALT) (see, e.g., Suhir et al²) is widely employed, in different modifications, with an intent to determine the product’s design and reliability weaknesses, to assess its reliability limits, to ruggedize the product by applying stresses (not necessarily mechanical and not necessarily limited to the anticipated field stresses) that could cause field failures, large (although, actually, unknown) safety margins over expected in-use conditions. HALT is a “discovery” test, while it is the qualification testing (QT) (see, e.g., Suhir³) that is the “pass/fail” one and, as such, is the major means for making a promising and viable device (package) into a reliable and marketable product.

QT brings to a “common denominator” different manufacturers and different products. When it comes to manufacturing, however, mass fabrication generates, in addition to desirable-and-robust (“strong”) products, also some amount of undesirable-and-unreliable (“weak”) devices (“freaks”), which, if shipped to the customer, will most likely fail in the field.

Burn-in testing (BIT) is supposed to detect and eliminate such “freaks,” so that the final bathtub curve of a product that underwent BIT does not contain the infant mortality portion. In today’s practice, BIT, which is a destructive test for the “freaks” and a nondestructive test for healthy devices, is often run within the framework of and concurrently with HALT.

Despite all the above AT effort, devices that passed the existing QT often fail in the field. Are these QT specifications and practices adequate? If not, could they be improved to an extent that for a product that passed the QT and survived the appropriate BIT, there is a quantifiable and sustainable way to assure that it will perform in a failure-free fashion in the field? It has been suggested4 that probabilistic design for reliability (PDfR) concept is used to create a “genetically healthy” product. The concept is based on recognition that reliability starts at the design stage, that nothing is perfect, and that the difference between a highly reliable and an insufficiently robust product is “merely” in the level of the probability of its failure. If one assesses, even tentatively, the probability of failure in the field and makes this probability sufficiently low, then there will be a reason to believe that a failure-free operation of the device will be likely (“principle of practical confidence”). With this in mind, a highly focused and highly cost-effective failure-oriented-accelerated testing (FOAT), which is the heart of the PDfR concept, should be conducted in addition to and, in some cases, even instead of HALT. FOAT is a solid experimental foundation of the PDfR approach. The prediction might not be perfect, but it is still better to pursue it than to turn a blind eye to the fact that there is always a non-zero probability of the device failure. 

Understanding the underlying reliability physics is critical. If one sets out to understand the physics of failure in an attempt to create a failure-free product, conducting FOAT should be imperative. Accordingly, FOAT’s objective is to confirm usage of a particular more or less established predictive model (PM), to confirm (say, after HALT is conducted) the underlying physics of failure, to establish the numerical characteristics (activation energy, time constant, exponents, if any, etc.) of the particular reliability model of interest.

Here are some well known FOAT models:

Arrhenius’ equation and its numerous extensions and modifications used when there is evidence that the elevated temperature is the major cause of the material or the device degradation (lifetime of electrical insulations and dielectrics, solid state and semiconductor devices, inter-metallic diffusion, batteries and solar cells, lubricants and greases, thermal interface materials, plastics, etc., as well as reliability characteristics other than lifetime, such as, e.g., leakage current or light output).
Boltzmann-Arrhenius-Zhurkov’s (BAZ)⁵ can be used when the material or a device experience combined action of elevated temperature and external loading; Crack growth models are used to assess the fracture toughness of materials in the brittle state. Inverse power law is used in numerous modifications of the Coffin-Manson’s semi-empirical relationships aimed at the prediction of the low cycle fatigue lifetime of solders that operate above the yield limit. Miner-Palmgren’s rule is used to address fatigue when the elastic limit is not exceeded. Weakest link models are used to evaluate the lifetime in extremely brittle materials, like Si, with highly localized defects. Stress-strength interference models are widely used in various problems of structural (physical) design in many areas of engineering, including microelectronics. Eyring-Polanyi’s equation is used to evaluate the lifetime of capacitors and electromigration in aluminum conductors. Peck’s equation is used to evaluate the lifetime of polymeric materials and the effect of corrosion. Black’s equation is used to quantify the reliability in electromigration problems, to evaluate the lifetime of hetero-junction bipolar transistors and the role of humidity. It is important to emphasize that all these models can be interpreted in terms of the probability of failure under the given loading conditions and after the given time in operation. A bathtub curve is a good example of a FOAT. If this curve is available, then many useful quantitative predictions could be made (see, e.g., Suhir⁶). As another example, let us consider an IC package whose steady-state operation is determined by the Boltzmann-Arrhenuis law Here τ is the lifetime; τ₀ is the time constant; U is the activation energy; T is the absolute temperature and k is Boltzmann’s constant. The probability of the package non-failure can be found, using an exponential law of reliability, as Solving this equation for the T value, we have:  . Addressing, e.g., surface charge accumulation failure, for which  assuming that the FOAT predicted time factor τ₀ is τ₀ = 2x10^-5 hours, that the probability of failure at the end of the device’s service time of τ = 40,000 hours should not exceed Q = 10^-5, the above formula yields: T = 352.3⁰K = 79.3⁰C. Thus, the heatsink should be designed accordingly. More complicated examples of FOAT and design decisions based on it can be found in Suhir^3-8.

An extension of HALT. FOAT could be viewed as an extension of HALT. It should be employed when reliability is imperative, and therefore, the ability to quantify it is highly desirable. HALT is, to a great extent, a “black box”, i.e., a methodology that can be perceived in terms of its inputs and outputs without a clear knowledge of the underlying physics and the likelihood of failure. FOAT, on the other hand, is a “white box,” whose main objective
is to confirm usage of a particular predictive model that reflects a specific anticipated failure mode. The major assumption is, of course, that this model is valid in both AT and in actual operation conditions. HALT does not measure (quantify) reliability; FOAT does. HALT can be used, therefore, for rough tuning of the product’s reliability, while FOAT should be employed when fine tuning is needed, i.e., when there is a need to quantify, ensure and, if possible and appropriate, even specify the operational reliability of the device. HALT tries, quite often rather successfully, to kill many unknown birds with one stone. HALT has demonstrated over the years its ability to improve robustness through a “test-fail-fix” process, in which the applied stresses are somewhat above the specified operating limits. By doing that, HALT might be able to quickly precipitate and identify failures of different origins. HALT often involves step-wise stressing, rapid thermal transitions, etc. Since the principle of superposition does not work in reliability engineering, both HALT and FOAT use, when appropriate, combined stressing under various stimuli. FOAT and HALT could be carried out separately, or might be partially combined in a particular AT effort. New products present natural reliability concerns, as well as significant challenges at all the stages of their design, manufacture and use.  HALT and FOAT could be especially useful for ruggedizing and quantifying reliability of such products. It is always necessary to correctly identify the expected failure modes and mechanisms, and to establish the appropriate stress limits of HALTs and FOATs to prevent “shifts” in the dominant failure mechanisms. There are many ways this can be done (see, e.g., Suhir⁹).

Conclusion

The FOAT-based approach, which is, in effect, a “quantified and reliability physics oriented HALT,” should be implemented whenever feasible and appropriate, in addition to the currently widely employed various types and modifications of the forty-years-old HALT. In many cases the FOAT-based effort can and should be employed, even instead of HALT, especially for new products, whose operational reliability is unclear and for which no experience is accumulated and no best practices exist. The approach should be geared to a particular technology and application.¹⁰

References

1. E. Suhir, “Reliability and Accelerated Life Testing,” Semiconductor International, Feb. 1, 2005.
2. Intertek company website, www.intertek.com/performance-testing/halt-and-hass/.
3. E. Suhir, R. Mahajan, A. Lucero and L. Bechou, “Probabilistic Design for Reliability (PDfR) and a Novel Approach to Qualification Testing (QT),” 2012 IEEE/AIAA Aerospace Conf., 2012.
4. E. Suhir, “Probabilistic Design for Reliability,” Chip Scale Review, vol. 14, no. 6, 2010.
5. E. Suhir, “Assuring Aerospace Electronics and Photonics Reliability: What Could and Should Be Done Differently,” 2013 IEEE Aerospace Conference, March 2013.
6. E. Suhir, “Remaining Useful Lifetime (RUL): Probabilistic Predictive Model,” International Journal of PHM, vol. 2(2), 2011.
7. E. Suhir, “Predictive Modeling is a Powerful Means to Prevent Thermal Stress Failures in Electronics and Photonics,” Chip Scale Review, vol. 15, no. 4, July-August 2011.
8. E. Suhir, “Applied Probability for Engineers and Scientists,” McGraw-Hill, New York, 1997.
9. E. Suhir, “Analysis of a Pre-Stressed Bi-Material Accelerated Life Test (ALT) Specimen,” ZAMM, vol. 91, no. 5, 2011.
10. E. Suhir, “Considering Electronic Product’s Quality Specifications by Application(s),” Chip Scale Review, vol. 16, no. 4, 2012.

Ephraim Suhir, Ph.D., is Distinguished Member of Technical Staff (retired), Bell Laboratories’ Physical Sciences and Engineering Research Division, and is a professor with the University of California, Santa Cruz, University of Maryland, and ERS Co.; suhire@aol.com.

Foil tension variations may create distortion and unpredictable print deposition. A novel method for foil characterization and verification is proposed.

Typical foil mounting methods involve stretching a woven fabric, typically polyester, across an aluminum frame, creating a taut screen (Figure 1). There appears to be no standard for mesh tensioning values, but different sources report a range of 25 to 35 N/cm¹. The thin metal foil is placed and bonded on the bottom of the frame, and the mesh covering the squeegee side is cut away. The mesh tension will suspend the bonded metal foil, holding it taut. IPC-7525B is informative regarding stencil design guidelines and various terminology. In this mounted state the tension of the actual foil is rarely verified due to lengthy verification and specialized tools.

Densely populated printed circuit boards and fine printing features make planarity and consistency from frame to frame important. Variation negates any precision established in the stencil printer design and manufacture.

Among the concerns for mesh mounted stencils:

• Aluminum frame: Planarity variability, bow under tension (different wall thickness).
• Mesh to frame: Variable tension, gluing variability.
• Foil to mesh: Foil flatness, gluing variability.
• Hand-assembled: Operator-dependent.

Alternative stencil mounting technology involves a reusable metal foil mounted to a reusable master frame, tensioning by either pneumatic bladder or a multitude of levers and springs. This technology is targeted at addressing the space and planarity drawbacks of mesh-mounted stencils. Tensioning of the foil in a master frame is created by either air bladders or springs attempting to emulate the mesh mount tensioning characteristics. This technology has been touted as having better tensioning characteristics than mesh-mounted stencils. However, as seen in Figure 2, a different tensioning trait can be seen.

Tension versus elasticity. Within this specific research, we found it difficult to comprehend the use of the term of “tension” in a system that now uses a steel plate or steel membrane for print deposition. Therefore, following definitions are highlighted for clarity and understanding of the correct terms to be used and how this paper will refer to them:

• Tension. Tension is the pulling force exerted by a string, cable, chain, or similar solid object on another object. In the case of today’s stencils it is the lateral force pulling on the foil by either the tension of the mesh in a mesh mount stencil frame or the spring/pneumatic force derived by reusable tensioning frames.
• Elasticity. Elasticity is a physical property of materials that return to their original shape after they are deformed. How this applies to stencil systems of today is it is the characteristic of the foil property returning to its original position after a force has been applied and then removed. The tensioning system applied to the foil/steel plate maintains the plate in a state of planarity. Any force applied will deform the plate by a proportionate amount. This is explained by Hooke’s law, a principle of physics that states the force needed to extend or compress a spring by some distance is proportional to that distance.

From these definitions, it is clear that the true property and characteristic of a stencil system should be measured by the spring constant k. This is a measure of elasticity of a spring that in our specific stencil systems is the stencil foil.

k = ΔF/Δx

where

ΔF = Force applied to the foil to displace
Δx = The distance moved by the force applied.

Area mapping of elasticity across the foil. To understand the characteristics of elasticity across the various stencil mounting technologies, a measurement methodology must be employed. A novel, patent-pending, fast real-time mapping system for foil characteristics utilizes complex resonance algorithms to determine regional characteristics of the foil. The tool used for our initial characterization is an RST Gage SMT Foil Analyzer1. This system consists of a known weight, a floating dial indicator and a gantry mechanism. The gantry system rests on the outer frame, suspending the dial indicator in the selected region. The dial indicator is set to touch the foil and zeroed. Applying the known weight on to the dial indicator will deflect the foil; therefore, a deflection measurement will be displayed.
Each displacement reading is converted into N/cm by the formula k = ΔF/Δx.

Nine points are taken across the foil area² (Figure 3), and for graph smoothing, additional data points are interpolated from the nine point reading. The types of stencils characterized are:

• Standard mesh mounted from different suppliers.
• Space saver mesh mounted frames.
• Mesh-less foil mounted system from manufacturer A.
• Mesh-less foil mounted system from manufacturer B.

Test Readings

The following surface plots represent the plotted regions of each stencil system. These graphs are a visual representation of how the foil is behaving in its relative mounting system. The narrower the lines, the greater the range/deviation.

Mesh mount – standard 29” x 29”. The following outlines the format and characteristic of the mesh mounted stencils under test (Figure 4).

• Vendor A – Range across the foil = 10 N/cm, foil center = 40 N/cm.
• Vendor B – Range across the foil = 5 N/cm, foil center = 38 N/cm.
• Vendor C – Range across the foil = 9 N/cm, foil center = 39 N/cm.

As can be seen by the surface plots, patterns are starting to appear. It is clear that the corners of the foils are undergoing higher stress than their center points, resulting in a dishing effect.

Mesh mount – space saver 29” x 29”. Figure 5 shows surface plots of production stencils from the same vendor.

• Vendor D-a – Range across the foil = 15 N/cm, foil center = 41 N/cm.
• Vendor D-b – Range across the foil = 11 N/cm, foil center = 49 N/cm.
• Vendor D-c – Range across the foil = 22 N/cm, foil center = 23 N/cm.

Once again the dish-shaped patterns are occurring. Surface plot “D-c” was of a stencil that showed delamination around the foil. The data show clear indication of non-normal distribution and need for replacement.

Mesh-less tensioning systems. This test is directed at characterizing the two most popular tensioning systems. One test is to use one master frame with different foils, and one test with two master frames is from the same supplier with one foil. This was based on availability of master frames and foils at the time of the test.

Test 1: Same frame, different foils. With the mesh-less surface plots, pattern characteristics are appearing similar to the mesh-mounted stencils, but clearly the stress intensity is far greater on these tensioning devices (Figure 6). These patterns still reflect the foil corners undergoing higher stress than their center points, and the dishing effect is greatly exaggerated.

• Vendor E-a – Range across the foil = 40 N/cm, foil center = 47 N/cm.
• Vendor E-b – Range across the foil = 16 N/cm, foil center = 28 N/cm.
• Vendor E-c – Range across the foil = 24 N/cm, foil center = 51 N/cm.

Test 2: Same foil, two different master frames by same supplier. See Figure 7.

• Vendor F-a – Range across the foil = 21 N/cm, Foil Center = 47 N/cm
• Vendor F-b – Range across the foil = 24 N/cm, Foil Center = 44 N/cm
• Vendor F-c – Range across the foil = 25 N/cm, Foil Center = 45 N/cm

Summary of Elasticity Results

Although the patterns of both types of mounting technology have similar traits – higher in the corners, lower in the center – the spring/pressure tensioned systems display higher center elastic force and greater range across the whole foil area compared with the mesh-mounted stencils.

The typical stencil central point value is around 40 N/cm of a newer mesh-mounted stencil. The deviation from this value across the foil area is +10 N/cm (25% deviation).

For the mesh-less systems, the typical central point value is 46 N/cm, with a typical range deviation from this value of +20 N/cm (43.5% deviation).

As both systems (mesh mount and tensioning frames) have been in full production worldwide for years, there has been no specific evidence that one works better than the other with regard to print deposition performance. For discussion: If the perceived tension of a stencil is that the higher the better, then would that imply that the corners of each foil system print better than the center? Maybe edge-justified stencils print better than center-justified stencils? Either way, it would make better sense if these values were tightly controlled frame to frame, foil to foil where this consistency would remove variation and doubt from this argument.

The concentrated effort toward higher performance stencils for improving the transfer efficiency on smaller features is attempting to use the higher tension model to achieve better print performance. Given the effects of surface energies of stencil apertures, variations of solder paste chemistries and PCB surfaces/topography, it would also stand to reason that each individual image in the stencil will bring about infinite variables, due to positioning and sizes of the printing features within this image. On a side note, if the foil is to be held in a planar state during board separation for increased paste release performance, the mass of solder paste resting on the top side will clearly deform the stencil during the separation sequence.

In Figure 8, a represents the idealistic yet unrealistic stencil to board during the print separation process; b represents a slack foil and how it will sag based on little to no applied tension, and c represents a taut foil with a paste mass on the squeegee side. As the board separates, the unsupported paste mass will deflect the foil, causing a sag and therefore potentially uncontrolled separation. As the paste mass/bead deteriorates in size, so does its mass; therefore, the effect is lesser on the separation process.

As an exercise using the Catenary curve equation and a paste mass of 500g, it would take a tension force of over 300,000 N/cm to overcome the sag caused by the effect of the paste mass.

Area mapping of resonance across the foil. The Equi-Tone handheld device has been developed as a quick and objective measurement method/device. First phase development results are very encouraging.

Harmonic oscillation. Given Hooke’s law and our perspective that a stencil be viewed as a spring, then pulling the foil and releasing would set it into oscillation about its equilibrium position. However, due to the various mounting system techniques, the foil as a spring will be affected at a location across its area, bringing about different oscillations within these locations.

To test this theory, all the above tested stencils were compared with measuring the frequency responses in each of the nine locations previously selected and tested.

Frequency response across the foil. Moving a frequency recording device around each of the previously selected nine points highlighted not only patterns, but uncovered unwanted overtones. Split frequencies, beat frequencies and other mixed frequencies are noise to our signal. To isolate the primary frequency at each location, the use of Fast Fourier Transform (FFT) Spectrum Analyzer (Figure 9) and sophisticated filtering was required. These techniques would eliminate any false readings and overtones, and a stable primary frequency response could be isolated.

It is clear that the frequencies generated were complex and required various filtering to identify the primary frequency. Once this was achieved, the novel device was able to accurately and repeatedly identify the specific tone.

The testing and comparison study could then take place, with the results plotted below.

Novel device v. displacement. Using the novel device, each of the previously measured stencils was tested for comparison. The frequency data collected were paired and plotted with their respective stencils. As seen below, the patterns generated resemble the N/cm plots fairly closely.

Summary

It is clear from these findings that the steel used in stencils will deflect based on its own elastic properties. The mesh-mounted stencil and newer direct-tensioned frames are shown to have similar patterns. The mesh-mounted stencils have two modes of elasticity: one being the mesh, the other the foil. The lower elastic properties of the mesh permit the foil to be more coplanar under stress than are direct-tensioned foils. Direct-tensioned systems clearly distort the foil and create larger deviations across the foil area. This phenomenon is counterintuitive when it comes to the idea of uniform separation between foil and board post-print. Further increasing the tension applied to the foil in these direct-tensioned systems may create further distortion and is therefore likely to create unpredictable print depositions. This research also points out that there are clear differences not only among vendor-to-vendor master frame technologies, but also different master frames from the same supplier.

Compared to other analyzers, the novel device clearly shows it can plot similar characteristics when area mapping the foil. Therefore, it is a viable alternative tool for fast and convenient foil characterization and verification.

Subsequent research will look at how these technologies and their elastic properties affect print deposition.

References

1. RST Gauge, company website.
2. Murakami Screen, company website.

Ricky Bennett, Ph.D., is founder of Lu-Con Technologies; rbennett@lu-con.com. Richard Lieske is director of applied product development at Lu-Con Technologies.