Tin Whiskers IV: Mitigation

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Folks,

In the last post on tin whiskers, we discussed detection. In this post, we will cover mitigation. Since compressive stresses are a primary cause of tin whiskers, minimizing these stresses will help to mitigate tin whisker formation. There are several approaches to accomplish this compressive stress reduction. The first is to establish a process that produces a matte finish as opposed to a bright tin finish. Experience has shown that a satin or matte tin finish, which has larger grain sizes, has lower internal compressive stresses than a bright tin finish. Studies have shown that avoiding a bright tin finish alone can reduce tin whisker formation by more than a factor of ten. Thicker tin layers will often reduce compressive stresses.

Since a major source of the compressive stresses in tin is due to copper diffusion into the tin, minimizing this diffusion will significantly reduce tin whisker formation. One proven approach to minimizing copper diffusion is to have a flash of nickel between the copper and the tin. Since nickel does not readily diffuse into the tin after initial intermetallic formation, tin whisker formation can be all but eliminated in many cases.

Adding bismuth to the tin, in small amounts, can also reduce tin whisker formation. The bismuth solid solution strengthens the tin. This strengthening will often reduce tin whisker formation.

Another mitigation approach is the use of coatings. Acrylics, epoxies, urethanes, alkali silicate glasses and parylene C have been used. Parylene C appears to be the most promising.

Often a tin whisker will penetrate the coating as seen in Figure 1. However, to be a reliability risk, it must penetrate a second coating.

Figure 1. A tin whisker about to penetrate a polymer coating. Source: Dr. Chris Hunt, NPL.

This situation is almost impossible as the tin whisker is fragile and will bend as it tries to penetrate the second layer of coating. See Figure 2. So, coatings can be a very effective tin whisker mitigation approach.

Figure 2. To be a reliability concern, a tin whisker must penetrate two protective coatings.

The next and last tin whisker post will be on using FMEA (failure modes and effects analysis) to develop a tin whisker reduction strategy.

Cheers,

Dr. Ron

Done Deal

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The Printed Circuit Engineering Association (PCEA) today announced it has closed the acquisition of the functional assets of UP Media Group Inc., including its industry leading publications and trade shows.

The deal, which was announced during the PCB West conference and exhibition last October, includes the annual PCB West and PCB East trade shows; PRINTED CIRCUIT DESIGN & FAB (PCD&F) and CIRCUITS ASSEMBLY magazine; the PCB UPdate digital newsletter; the PCB Chat podcast series; the PCB2Day workshops and webinars; and Printed Circuit University, the dedicated online training platform.

Printed Circuit Engineering Association (PCEA) (pcea.net) is a nonprofit association that promotes printed circuit engineering as a profession and encourages, facilitates, and promotes the exchange of information and integration of new design concepts through communications, seminars, workshops, and professional certification through a network of local and regional PCEA-affiliated groups. PCEA serves the global PCB community through print, digital and online products, as well as live and virtual events. Membership is free to individuals in the electronics industry.

Tin Whiskers 101: Part III: Detection

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Folks,

One of the great challenges of tin whiskers is detecting them. When one considers that their median thickness is in the 3 to 5 micron range (a human hair is about 75 microns,) they can be hard to see with direct lighting. Right angle lighting facilitates visual detection. See Figure 1. In this figure, Panashchenko shows that with direct light (left image), it is impossible to see the tin whisker, however with right angle light the tin whisker jumps out.

Figure 1.* It is not possible to see the tin whisker with direct lighting as in the left image. However, in the right image, right angle lighting makes it easy to see the tin whisker.

In her excellent presentation, “The Art of Metal Whisker Detection: A Practical Guide for Electronics Professionals,” Panashchenko offers these tips for identifying tin whiskers with a stereo optical microscope:

  • Use a 3x to 100x stereo microscope
  • Start with low magnification and work up to high magnification
  • Have the ability to tilt the sample in 3 axes
  • Use a flexible lamp that allows multiple angles of illumination, do not use a ring light
  • Use a LED or fiber optic lighting, not incandescent lights which can cause shadowing
  • Vary the brightness of the light source

The most important tip is to vary the angle of lighting while varying the magnification. Thus, analyzing a sample should take several minutes, at least. However, even the most thorough inspection may miss some tin whiskers. 

In the next post, I will discuss mitigation techniques.

Cheers,

Dr. Ron

*The image is from Lyudmila Panashchenko, “The Art of Metal Whisker Detection: A Practical Guide for Electronics Professionals,” IPC Tin Whisker Conference, April 2012.

If You Thought Foxconn Is Just a Manufacturer …

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… Think again.

Foxconn, the world’s largest EMS/ODM, with annual revenues now topping $200 billion (!), said this week it now has more than 54,000 invention patents worldwide.

Some 63% of them have been issued in the US (17,600 patents) and Japan (16,200).

Among the most common technology areas:

  • Computer accessories 17%
  • Semiconductors 14%
  • Processing and detection technologies 13%
  • Robots and optoelectronics equipment 12%
  • Display equipment 11%

If you wonder what the end-game is, think worldwide monopoly.

What’s Old is News

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God love the Internet.

Nothing ever ages. Or, better said, anything can be reborn in a moment.

Take for instance, today’s report in DigiTimes.

“India officials are allegedly subsidizing US$10 billion in semiconductor manufacturing, according to Reuters citing knowledgeable sources.”

A quick review of Reuters stories over the past 90 days show no such reporting, however.

Is DigiTimes wrong?

Nope. But one must go back to Mar. 31 to find the piece: “India is offering more than $1 billion in cash to each semiconductor company that sets up manufacturing units in the country as it seeks to build on its smartphone assembly industry and strengthen its electronics supply chain, two officials said.”

This happens a lot, actually. I got a kick out of a recent recycling by multiple industry news aggregators that claimed Epec has acquired NetVia.

“Hmmm,” I thought. “That’s weird.” Because I am pretty confident that already happened.

And sure enough, that deal dates to November 2020.

What happens is that aggregators use alerts to find news, and crawlers sometimes bring old information back to the surface. Unsuspecting or inattentive editors grab the “new story” and link to it for that day’s newsletter.

And everything old is new again.

Tin Whiskers 101: Part 2: What Causes Them

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Folks,

Continuing our series on tin whiskers. In the last post we discussed what they are. in this post we will discuss what causes them.

Tin whiskers are primarily caused by compressive stresses in tin. The most common cause of the stresses is copper diffusion into the tin as seen in Figure 1a. Such diffusion is common when tin is plated, melted or evaporated on copper. Copper preferentially diffuses into tin exacerbating tin whisker production.

Figure 1. Some causes of tin whiskers

 Another cause of tin whiskers can occur when the tin is plated, melted or evaporated on a material that has a lower coefficient of expansion than the tin, such as alloy 42 or ceramic. When temperature increases, the tin is constrained by the lower coefficient of expansion material. This constraint causes compressive stresses in the tin that can result in tin whiskers. See Figure 1b.

Less common causes are corrosion, as seen in Figure 1c and mechanical stresses as seen in Figure 1d.

Since copper diffusion is one of the most likely causes of tin whiskers, this mechanism deserves elaboration. The left image in Figure 2 depicts the mechanism of copper diffusion into tin. The mechanism is so strong that the diffusion of the copper often leaves voids in the copper. Such voids are called Kirkendall voids. The right image in Figure 2 is an x-ray map of copper (green) diffusing into the tin (black).

Figure 2. Copper diffusing into tin.

Clearly, one way to minimize this type of tin whisker growth is to prevent copper diffusing into tin. In a future post, we will discuss this and other tin whisker mitigation techniques. 

Cheers,

Dr. Ron

Best Wishes,

Tin Whiskers 101: What Are They?

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Folks,

Tin whiskers are very fine filaments or whiskers of tin that form out of the surface of the tin. See Figure 1. They are the result of stress release in the tin. Tin whiskers are a phenomenon that is surprising when first encountered, as their formation just doesn’t seem intuitive.

Figure 1. Note how thin a tin whisker can be compared to a human hair. The image is from the NASA Tin Whisker Website

They are a concern, as they can cause electrical short circuits or intermittent short circuits as a fusible link. Lead in tin-lead solder greatly suppresses tin whisker growth. Therefore, with the advent of lead-free solders there is a justifiable concern for decreasing reliability due to tin whisker growth in electronics.

Tin whiskers can vary in length and width, as is seen in Figure 2. Note that although only about 10% are as long a 1000 microns (1mm). That length and occurrence rate is such as to cause many reliability concerns.

Figure 2. The length and width of some tin whiskers. The source is also the NASA Tin Whisker Website.

Over the following weeks I plan to post how tin whiskers form and strategies to alleviate them. Most of the information I will post comes from a paper I presented with Annaka Balch at the SMTA PanPac 2019.

NASA has an excellent website that provides much information about tin whiskers and is a source for historic critical failures caused by tin whiskers.

Cheers,

Dr. Ron

Gray Mattered

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I am sad to report the passing of Foster Gray, the brilliant Texas Instruments engineer who over his 41 years earned eight patents, 27 technical publications, and four published papers.

I worked with him at IPC, where he participated or led dozens of standards and round robin studies, and he was always prepared and always a gentleman.

His obituary can be seen here.

The ‘Seers’

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It’s always interesting when the seers, also known as the industry’s journalists, get together for a chat.

Hosted by Mike Konrad, I joined Trevor Galbraith, Phil Stoten and Eric Miscoll to discuss post-pandemic production, innovations in our industry, supply chain and labor shortages, and of course, some predictions.

The podcast can be listened to or downloaded from PCB Chat here: https://upmg.podbean.com/e/rm-76-meet-the-press/

Or, if you are really brave, you can see us in action here:

Reducing the Environmental Impact of Electronics Manufacturing

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Manufacturers are increasingly evaluating the environmental effects of their practices as eco-consumerism becomes more widespread. Companies can increase their profits by making their product development more sustainable and supporting energy-efficient infrastructure development.

President Biden allocated trillions of dollars to the sustainability sector, promoting low emission production. Reaching carbon neutrality requires a restructuring of the electronic manufacturing sector. Before evaluating the impact of reduction methods, we must examine the degradation associated with commonly used devices.

Life Cycle Assessment

A significant portion of electronic development derives from material mining. Many devices contain lithium-ion batteries, linking manufacturing processes to ecological degradation. Inadequately maintained mining sites pollute local water supplies, like the Liqi River in Tibet. Here, a lithium mine generated a chemical leak, killing a significant quantity of marine life.

Mining also depletes local water sources in drought-ridden regions. Lithium derives from a saltwater brine, which workers extract. Over time, local farming operations suffer from low groundwater levels. The extraction process also pollutes the air, causing adverse health effects.

Inefficient manufacturing processes can also generate pollution by developing electronic waste. The U.S. produces the most e-waste in the world. Chemicals leach into the soil from electronic landfills, which degrades the environment and human health. These dumps contain lead, mercury, cadmium and more, polluting food sources and drinking water. They also contain persistent organic compounds from fire retardants. When consumed, the substances cause cognitive defects in children and behavior or motor skill challenges.

Another environmentally degrading factor of production derives from energy use. China manufactures the highest portion of electronics globally. Coal is China’s largest energy source, fueling many production facilities.

When products leave the center, they absorb a portion of the emissions generated. Fortunately, manufacturing facilities can increase the sustainability of their products by using renewable energy sources. Over time, their environmental impact will decrease.

Renewable Energy Sources

Large corporations recently adopted renewable energy systems in production, meeting eco-consumerism demands. Over the past year and a half, BMW used solar and wind power to decrease the emissions generated by its manufacturing facilities. It also increased the energy efficiency of their products, shrinking their carbon footprint throughout their life cycle.

If China’s electronic manufacturing facilities converted from coal-powered electricity toward renewable energy, they could significantly increase their practices’ sustainability. Reducing greenhouse gas emissions and adding energy-efficient appliances can decrease a company’s carbon footprint.

Some companies decrease their production facilities’ ecological impacts by swapping conventional lights with light-emitting diode (LED) bulbs. The bulbs absorb 75% less energy than incandescent lights and last 25 times longer.

Improve Product Longevity

The best way to target e-waste is through improving products’ longevity. Some companies utilize planned obsolescence to maintain a consistent revenue stream. The expiration date on electronics increases e-waste production and decreases their sustainability.

Some electronics companies source ceramic and glass for product development. The materials have a limited defense against electronic stressors and generate pollution over time. Replacing the materials with liquid silicone rubber can make a product last longer and eliminate the normalization of planned obsolescence.

Manufacturers can also sustain economic gains by increasing the price of long-lasting products. Customers are more likely to purchase sustainable goods over their less expensive counterparts. If we build products to last, it increases profitability while decreasing environmental impact.

Recycled Materials

Rather than mining lithium-ion battery elements each time we produce new electronics, we can utilize recycled materials. Environmental engineers and scientists are generating efficient lithium-ion recycling technology, extracting functional features from the devices. The Department of Energy (DOE) developed the first recycling center, increasing the industry’s profitability.

The DOE also developed a program influencing professionals to develop advanced lithium-ion recycling technology. It offered the winner a $5.5 million reward to expand the system’s efficiency. When using recycled materials, manufacturers can reduce their reliance on ecologically degrading mining practices.

Where to Start

Manufacturers can begin decreasing the environmental impact of their practices by leaving fossil fuel-derived electricity sources behind. Renewable energy is abundant and currently cost-effective, improving sustainability rates while reducing utility costs. It also helps companies immediately reduce greenhouse gas emissions, shrinking their carbon footprint and making them more appealing to a new generation of consumers.

Jane Marsh is the founder and editor-in-chief of Environment.co where she covers topics in green technology, energy and environmental sustainability.