Several important process and equipment revelations were made at the annual Pan Pac symposium.

The 18th annual Pan Pacific Microelectronics Symposium took place Jan. 22-24 in Maui, HI. The conference brought together the industry’s top specialists from North America, Europe, Asia and Australia to share the most up-to-date science of electronics manufacturing and reliability. Technical papers and presentations covered a diverse range of electronics topics, including silicon interconnects and packaging, embedded devices, PCB assembly and cleaning, failure analysis and reliability models.

The Best Paper honor was awarded to Rolf Aschenbrenner, Ph.D., Fraunhofer Institute for Reliability and Microintegration, for his paper, “Panel Level Packaging – A Manufacturing Solution for Cost Effective Systems.” The paper highlighted some of the challenges in embedding high I/O power chips, including heat removal, electrical isolation and molding processes. This is a distinguished honor, as the award recipient is determined by the conference attendees.

Some of the SMT-related presentations of particular interest to Circuits Assembly readers included:

• Asymtek’s Horatio Quinones reviewed the latest advances in fluid jetting technology, highlighting the breakthrough of jetting Type 4 solder pastes. Until now, jetting has been performed with the finer powder Type 5 pastes, which bring reflow challenges like graping and balling. Larger particle size pastes were notorious for clogging jet seats after only a few shots. New jetting systems are able to produce very fine lines at speeds of up to 100mm/sec with no clogging. Applications include adding extra solder paste for RF shields, pin-in-paste reflow, BGA corner balls or SMT joints subject to frequent mechanical stresses, and dispensing paste for very fine feature components, MEMs lids or cavity PCBs.
• Kyzen’s Tom Forsythe presented new findings on cleaning that change the way we look at low-standoff components. Gap height is emerging as a significant factor affecting cleanability. As IC packages get miniaturized, gap height shrinks, creating new challenges for cleaning underneath the components – many of which were designed for no-clean soldering only. Forsythe presented the results of a sizable DoE to unlock the secrets of cleaning low-gap chip scale packages; it characterized the effectiveness of three cleaning solutions at five concentrations and four temperatures on two solder paste formulations. A lot of really good data to digest.
• Universal instruments’ Martin Anselm gave a fantastic overview of the PCBA failure analysis process, breaking it into four areas of investigation: material quality, assembly process, design and reliability. Anselm’s presentation was a summary of the three-hour class offered at the annual SMTAI conference, which contains valuable information for anyone in SMT production or new product engineering, as well as quality and SQA. The knowledge of how solder joints fail, what makes them fail, and the circumstances under which they fail is a powerful intellectual tool for people involved in PCB design, assembly or inspection processes.
• Northstar Imaging’s Wes Wren opened our eyes to state-of-the-art x-ray imaging. After explaining the difference between contrast and resolution, and the implications of tube-to-detector distance, he blew away the audience with images of new 4D tomography. The fourth dimension is the time domain, and the x-ray videos of real-time processes, like welding or the movement of a rack and pinion, are amazing. Go to www.xviewct.com/industrial-ct-news/news-items/taking-3d-x-ray-inspection-to-a-new-dimension-4d-computed-tomography to see some really cool videos.
• Tom Borkes gave us a lesson in economics that we all can use: realistic cost bases for PCB assembly in various world regions, and ideas on how to compete on a global scale. His analysis included labor, materials and a discussion of nations’ predispositions for off shore manufacturing. The paper introduced a noteworthy acronym: DF MATERS. That’s Design For: Manufacturing, Automation, Test, Environment, Reliability and Service.
• Nihon Superior’s Keith Sweatman introduced nanosilver particles that sinter themselves together at 150°C. In fact, they have such an affinity to self-sinter, the particles need to be protected by a passivation coating. They are less than 100nm or 300 atoms wide, and can be used in a variety of applications, including replacing high-lead solders in semiconductor die attach. They have good electrical and thermal conductivity and have been proven reliable, even in high-temperature operating environments. Solder pastes incorporating nanosilver have many different uses, are usually tailored to the application to address specific processing or performance characteristics.

In addition to the discussions of new technologies and methods, many general industry updates were provided, including Dieter Bergman’s review of IPC standards development, Chuck Bauer’s overview of new and novel interconnect technologies, Chuck Richardson’s iNEMI roadmap, and Wei Koh’s keynote address on China’s semiconductor development efforts.

The Pan Pacific Microelectronics Symposium is a three-day, single track conference that gives experts from across the industry and around the world the opportunity to learn from and inspire each other. It is made possible in large part by corporate sponsorships from Kyzen, Libra Industries, SonoScan, Indium, Nihon Superior and Nordson. The SMTA organizers and attendees sincerely appreciate their support.

Next year’s conference will be held Feb. 11-13, 2014 on the Big Island of Hawaii, and the call for papers is now open.

Chrys Shea is founder of Shea Engineering Services (sheaengineering.com); chrys@sheaengineering.com.

How to adjust an oven so a single recipe will work across multiple ovens for an individual product.

The ability of equipment to produce good product is fundamental to every manufacturing process. The ability to quickly setup and run a product on different process lines can make the difference in being able to satisfy a customer’s requirement, missing a delivery date, or making a profit. This process line flexibility requires good equipment with repeatability between lines to be a success. To do this, the process engineer needs to maintain their equipment to high standards and understand its capability.

In the 1950s, Wendell Abbott of GE wrote a detailed paper about the distribution of a measurement within a lot of material (or machine) and among lots of material that described the interactions. Although his paper is complicated, the concepts of variation within one oven and among different ovens are important when discussing flexibility across multiple SMT process lines.

Up until recently SMT process engineers with multiple process lines established a different recipe for each reflow oven because of variation between equipment. With modern technology and control systems, it is possible to adjust some reflow ovens so a single recipe will work “among” ovens for an individual product. Once this is done, repeatability among ovens becomes a process tool that saves time and makes production easier. To be comfortable with control among ovens, we first need to discuss the variation within an oven.

Repeatability within an Oven

Engineers are often asked how well their process meets a manufacturing specification. Phrases such as “nearly all the time” or “the majority of the time” are not acceptable in a technical world; thus establishing quantifiable data with measurable attributes is important. Placing a measurement or number on the ability of a process to meet a specification has evolved to what is known as process capability. This process capability measurement establishes a ratio of the process variation to a specification and is expressed as Cp or CpK.

It would be simple if we could make a single measurement of the process and compare it to the specification, but real life is not simple. Processes have variation that needs to be quantified before we can make a statement about how good they are. It takes multiple samples and math to quantify process variation. If we want to be 100% confident of the variation of a process, we would need to measure every piece. But that is impractical, so we use sampling and statistics to estimate the variation in a process. As you can guess, choosing the sample size becomes an issue of time and cost verses the accuracy of the data. We do not intend to turn this into a statistics course, but will give a simple overview of the concepts and how they can be applied to an SMT reflow oven.

Since process capability is a comparison of the process output to a specification, both need to be identified. In the case of a reflow oven, the specification is usually the reflow profile supplied by the paste manufacturer. It consists of ramp rates, soak time to traverse specific temperature ranges, time above the liquidus of the solder, and peak temperature.

The process output includes two pieces of process data for each specified attribute. The first is the average and the other is the standard deviation. The average is simple math (addition and division), while calculating the standard deviation is more complicated. Spreadsheets, statistical programs and some calculators can do the analysis without the need for manual calculations. (Consult statistical handbooks and texts for a detailed explanation of standard deviation.) What process engineers need to know is the standard deviation of sample data can predict the process variability.

The process to specification ratio can be expressed in two ways: One is called Cp and the other is CpK. The difference is that Cp focuses on machine capability, while CpK focuses on the process for an individual product. Cp uses the tolerance range and standard deviation without regard to the actual target to determine if the machine has the ability to make good product. The tolerance range is usually set to an industry standard. On the other hand, CpK uses Cp calculations and the actual target to ensure the process meets the specification for an individual product.

The calculations are:

Tolerance range = upper specification limit minus the lower specification limit

Cp = Tolerance range / 6 times the standard deviation

CpK = Cp times (1 minus K)
where K = the absolute value of the target minus the mean / by ½ the total tolerance range

Let’s say that we have the following peak temperature data:

The specification given by the paste manufacturer is 240 ± 10°C.

Therefore, the target is 240°C with a tolerance range of 20°C.

After running 10 profiles, we found that the average peak temperature
is 238.5°C.

And the spreadsheet calculations tell us that the standard deviation is 0.34.

Therefore, the Cp would be 20 / (6 X 0. 34) = 9.8

K would be (240.0 – 238.5) / 10 = 0.15

Then the CpK is 9.8 X (1-0.15) or 8.33.

Statisticians note that when the machine capability (Cp) is 2.0 or greater, we have a capable process, regardless of the process center. They also say that for a process to be in control, the CpK has to be 1.33 or greater, accounting for the process center. In this case the Cp and CpK are excellent.

To help make it clear, we can look at the following figures. Figure 1 depicts a perfectly centered process with the target and process average equal to each other. The statistical process variation is well within the specification limits and has a 1.33 CpK. This allows for some movement of the average, while the product remains within the specification.

Figure 2 shows a centered process, but the variation is larger than the specification limits. When this happens, the process is producing conditions that are out of the specification window. In this case, the CpK is about 0.8.

Figure 3 shows two processes that have variation less than the specification limits, but they are not centered. While the Cp (not considering the target) would be good, the processes are producing conditions that are out of the specification window. If they were centered, they would be OK. The CpK in these cases is about 0.85.

Now let’s look at the results from a study of an actual reflow process conducted over a five-day period. Profiles were taken each morning and afternoon on a 10-zone BTU Dynamo reflow oven running a Pb-free solder recipe. The product was a 350g board with six thermocouples attached to critical components.

The ramp, time above liquidus (TAL) and peak temperature data from each run were put in a spreadsheet to calculate the averages and standard deviations. The results are in Table 1.

Comparing results to the Pb-free solder specification, we see the averages are very close to the target (Table 2). This indicates the recipe is correct for this product, but does not indicate the process variation. Not knowing the variation reminds us of the guy who had one hand in boiling water and the other in an ice bath; he was uncomfortable, but on the average, the temperature was OK. Thus we need to know the variation of the process. Since we are taking a sample and not measuring the profile for every part, we can use the standard deviation of our sample and the formulas previously discussed to calculate the CpK.

This indicates that the process run on the third over 3 is in statistical control, because the CpKs are all above 1.33. If the process specification is correct, the reflow process is making good product. (Note: This study was conducted over a five-day period, but the same can be done with 10 consecutive runs or 10 runs in one day.)

Repeatability among Ovens

Now that we know one oven is in control, how do we show that the process is in control among multiple ovens with the same recipe?

We could obtain a single profile on a second oven to show that it is within the process widow identified by the paste specification. But that would not verify that the second oven had the same process variability as the first, and we could be deceiving ourselves into thinking the process was in control. The answer is again in statistics and CpK.

We could complicate our decision making with statistical evaluations such as the student t-test1 and Mr. Abbott’s calculations, but it is possible for the process engineer to keep it simple by comparing the results of CpK calculations on multiple ovens to show there is repeatability between them.

Before we start the 10 runs on the second oven, we need to fine-tune or adjust the profile by using thermocouple offset in the oven control program.

To obtain the offset, we run a few profiles on the second oven and compare it to the averages we produced on the first oven. We are tuning the oven, not calculating the CpK. Although we could get more accurate data with multiple runs on Oven 4, we chose three runs in an effort to save time (Table 4).

The peak temperature and TAL are slightly lower on Oven 4, so if we use an offset in the oven control of 3°C to increase the peak temperature, the TAL should be slightly longer.

Then obtaining profile data with the offsets from 10 runs and calculating the averages, we get the results in Table 5.

The small differences in the averages between Oven 3 and Oven 4 tell us that we chose the correct offsets.

Then, using the same process specification as Oven 3 and the standard deviation from Oven 4 data to calculate the CpK on Oven 4, we get the results in Table 6.
Both the comparison of the averages and CpK confirms that Oven 3 and Oven 4 are in control with the same recipe.

Table 7 shows the same for yet another oven (identical product, set points, offset procedures, and specification limits).

Therefore, we have three ovens using the same recipe that are all in control. This allows us to say we have repeatability among them.

Let’s do a quick review by comparing the data from the three ovens (Tables 8 and 9). The first thing we see is that the rising, falling, TAL and peak temperature averages are very close to each other; thus we have the correct tuning offsets. The second and most important is that every specified attribute is within control because the CpKs are more than 1.33.

We have proven that we have repeatability within each oven and among the three ovens, and the process engineer can be confident that they can produce good product in any of the three reflow ovens.

End Notes

1. A t-test is any statistical hypothesis test in which the test statistic follows a Student’s t distribution if the null hypothesis is supported. It can be used to determine if two sets of data are significantly different from each other, and is most commonly applied when the test statistic would follow a normal distribution if the value of a scaling term in the test statistic were known.

Fred Dimock is manager, process technology at BTU International (btu.com); fdimock@btu.com.

Next-generation materials make almost every stencil better.

Once in awhile, something really big comes along that makes a step function improvement in our PCB assembly processes. Stencil printing has enjoyed its share of milestone moments, and it’s currently on the brink of another. The science of surface energy modification is enabling a new level of print quality, and it is poised to make a significant impact on the process, on the same order of magnitude as electroformed stencils did in the 1990s or structured light SPI in the 2000s.

Commonly known as “nanocoating,” the process of applying a single-molecule layer of material to the stencil’s contact side dramatically lowers its surface energy and improves solder paste release. Several types of materials are classified as nanocoatings; the most popular is the two-part system introduced in 2010. Originally met with overt industry skepticism, the coating quickly proved itself in independent tests and developed a strong user base. I admit that I was one of the doubters who was quickly converted by the overwhelmingly positive data; the coating made almost every stencil tested perform better, and prompted an instant 5% overall print yield increase when implemented in production.^1,2

The original formulators have now introduced a next-generation nanocoating product that improves printing performance, along with a new business model that makes it more affordable and more available. Eric Hanson, Aculon’s vice president of technology, presented the science of how and why the technology works at the technical conference, while Edward Hughes, Aculon’s CEO, discussed supply-chain improvements on the show floor. I was able to speak with both, and they answered many of the questions that process engineers and printing specialists have been asking each other for the past two years.

What is it and how does it work? The coating is known as a Self-Assembling Monolayer of Phosphonates, or SAMP material (sidebar). These films are literally one molecule thick. They don’t change the surface roughness or topography; they change the surface energy. SAMP surface coatings are engineered to produce specific properties; they may be hydrophobic (water repelling), oleophobic (oil repelling) or fluxophobic (flux repelling).

The ultrathin (<5 x 10-9m) coating prevents flux from wetting out on the stencil. Poor wetting equals poor adhesion, so in the solder paste tug-of-war of stencil-PCB separation, the stencil quickly loses its grip on the paste. The minute “strings” of paste that would stretch between the stencil and the PCB as the two separate break free from the stencil, rather than snap back to it, keeping its underside cleaner. The flux-repellent surface also prevents solder paste from wicking out around the edges of the apertures during the print stroke,3 further limiting bottomside paste buildup. The cleaner contact surface results in better gasketing, less squeeze out, and crisper print definition.

How can a user tell if the coating has worn off? This has been a major concern since its introduction. Nobody wants to wait for performance drops to signal time for reapplication. The solution is a dyne pen: a felt tip marker filled with dyne fluid. It’s a basic go/no go gage; if you swipe the marker across the stencil and the fluid beads up, the coating is still robust. If the mark doesn’t bead up, it’s time to reapply.

In the laboratory, more quantitative indications of surface energies are acquired by measuring the contact angles of oil and/or water on surfaces. Higher surface energies produce good wetting and lower contact angles; lower surface energies produce poor wetting and higher contact angles. Untreated stencil foils exhibit oil contact angles of approximately 10˚, whereas treated foils exhibit oil contact angles of approximately 87˚.

How long does the coating last? The coating is designed for durability. Users of the original formulation cite reapplication intervals of 25,000 wipe cycles or more, a very long time for many print processes. Given that one of the prime benefits of this treatment is extending wipe frequencies, 25,000 wipe cycles could become a very, very long time.

At Apex, Hanson presented results of multiple durability tests, including one that demonstrated robust coating presence after 100,000 abrasion cycles, outlasting vacuum deposited nanocoatings as shown in Figure 1.⁴

Will stencil cleaning solvents attack the coating? No. The new coating has been compatibility tested with one major manufacturer of stencil cleaning chemistry and is currently undergoing testing with another. Single application coatings have withstood 160 15-min. cleaning cycles in typical stencil cleaning chemistries with no degradation in repellency. That’s equivalent to cleaning it once every day for almost six months, which is more cleaning than most stencils see in their lifetime. If stencil cleaning processes use solvents that have not yet been tested and approved, chemistries with a pH <9 are recommended.

What’s the new performance improvement? In addition to reaching new levels of durability for nanocoatings, the new formulation is reported to improve fluxophobicity, as it is 50% more concentrated than the original. The extra “slipperiness” of the stencil should extend underwipe frequency intervals even farther than its predecessor. Users testing the new SAMP reported incredible – almost unfathomable – results: 40 prints per wipe on 0.5mm µBGAs, as shown in Figure 2. Hanson comments on the performance feedback: “We understand PCB assemblers’ needs for better print quality, and are extremely pleased that this product makes such a remarkable improvement in the overall process.”

When testing the original formula with Vicor’s Ray Whittier in 2011, we didn’t test for wipe frequency, but found an enormous impact on print yields,⁵ as shown in Figure 3. We tested 13 pairs of stencils of varying composition and quality, and it boosted yields on all but two pairs. (Note that those were seemingly beyond help, originally producing only 0 and 20% yields.) The results were so dramatic that Whittier immediately implemented the coating as the Process of Record, applying it to all stencils in his operation. We have plans underway to test the new coating formulation on a similar test vehicle and quantify the performance differences.

Can it be used on any type of stencil? Yes, the coating bonds to metallic oxides, and is compatible with electroformed, electroplated or laser-cut nickel, as well as the many stainless steel alloys available. (As an aside, I just finished another big stencil study  in which laser-cut SS outperformed electroform for the third time in as many years.)

Does it actually coat the aperture walls, or just the bottom surface of the stencil? The product volume is formulated to treat both the underside and the aperture walls. The company developed an instructional video in which Hanson demonstrates the application process and offers tips for achieving optimum coverage (aculon.com/stencils.php).

Does it contain ionic materials that could pose reliability risks to solder joints? No. When properly applied, the cleaner/treatment chemistries will not contaminate downstream processes. While the cleaning solution does contain some ionic materials, they are removed during the rinsing step with water. The monolayer treatment step does not contain any ionic materials that could compromise subsequent processes.

What’s the cost per stencil? The cost of the two-packet system has been reduced from $40 per stencil treatment to $25. At $40, I thought it was a bargain because it was still less expensive than reworking a single BGA with a print-related solder defect. At $25, the deal just got sweeter and the ROI faster. One user quantified the payback period in wiper paper alone: about 170 wipes with standard paper, or one roll of the really good stuff. I want to apply it not only to my stencils, but also to my board support tooling and squeegee blade holders to make them easier to clean on changeovers.

The two-part coating system is available to stencil manufacturers and PCB assemblers, giving the latter the option to specify it on their stencils or apply it themselves. Because all parties have equal access and pricing, assemblers should expect a service fee if the stencil shop applies it. The application process is pretty straightforward; details are in the video.

With two years of limited distribution under its belt and a loyal following of users, SAMP-based stencil nanocoating is ready for prime time. It’s going to change the way most operations manage and control their printing processes over the next few years. But unlike some of the previous breakthrough technologies that advanced stencil printing, it doesn’t come with a hefty price tag or a steep learning curve. It’s $25, plug and play. This coating’s low price, widespread availability and ease of use may quickly make it the de facto standard for all SMT stencils.

What is SAMP Technology?

Self-Assembling Monolayer (SAM) materials are specially designed molecules that automatically organize themselves and bond to form engineered chemical structures, without requiring any catalysts or forming any byproducts. The molecules have head and tail groups; the head groups bond to a substrate, while the tail groups bond to each other and deliver the desired functionality.

Phosphonates (P) are organic compounds that bond readily with metal oxides to chemically “seal” surfaces, and are commonly used to inhibit corrosion or scale because they are extremely stable in harsh environments.

The SAMP molecule developed specifically for stencil nanocoating consists of a phosphonate head group that covalently bonds to the oxides on the stencil’s surface and a specially selected tail group designed to repel nearly all flux formulations. The molecules set up and form a single-molecule layer within seconds of their application. Familiar SAMP applications include surface treatments for sunglass and optical lenses, PDA screen cleaners, watch displays, nozzles and needles.

References
1. Chrys Shea, “SMT Stencils from a Production Perspective,” CIRCUITS ASSEMBLY, December 2011.
2. Chrys Shea, “What Used to be Old is New Again?” CIRCUITS ASSEMBLY, February 2012.
3. C. Ashmore, M. Whitmore and J. Schake, “Big Ideas on Miniaturization,” Proceedings of IPC Apex Expo, February, 2013.
4. R. Bennett, Ph.D., and E. Hanson, “Low Surface Energy Coatings, Rewrites the Area Ratio Rules,” Proceedings of IPC Apex Expo, February 2013.
5. Chrys Shea and Ray Whittier, “Evaluation of Stencil Materials, Suppliers and Coatings,” Proceedings of SMTA International Conference, Fort Worth, TX, October 2011.

Chrys Shea is founder of Shea Engineering Services (sheaengineering.com); chrys@sheaengineering.com.