How Far Can We Go to Replace Lead?

The end is nigh for lead in solder, as our columnist Tim O’Neill writes this month in CIRCUITS ASSEMBLY.

Rules governing use of the materials — Directive 2015/863, aka RoHS 3 — are coming online and will be in full force by 2019.

Suppliers have until July 22, 2019 to meet the stricter provisions, which includes no more than 0.1% lead in medical devices, which are joining consumer, industrial and other electronics products on the effectively banned list.

The question Tim poses is, What comes next? Already, the future of commonplace unleaded alloys such as SAC is being questioned. As Tim writes, “It is even feasible SAC 305 will be dislodged by a new de facto alloy that better serves the needs of the market.”

A Norwegian scientist believes he may have the answer. As noted in Phys.org this week, Dr. Henrik Soensteby of the University of Oslo is working on an alternative alloy that contains nothing but common — and essentially benign — elements. In conjuring up his alloy, Soensteby is mixing sodium, potassium and oxygen with niobium, a very strong metal typically used in steel. While niobium dust is reported to cause eye and skin irritation, it reportedly is nontoxic, at least in the volumes used.

It’s not so clear yet how much niobium would be needed. Brazil is the biggest supplier of niobium, producing more than 85% of it each year. Other sources include Zaire, Russia, Nigeria and Canada. World production is relatively light: around 25,000 tonnes per year. Some scientists believe there are ample supplies still in the ground. There’d better be: Some 5 million tonnes a year of lead ores are mined each year, although obviously not all that goes into electronics.

Soensteby is optimistic he can use atomic layer deposition (ALD), a vapor phase method that uses gas at controlled temperatures to stimulate a reaction with the substrate; the output is thin films. It is an emerging technology in semiconductor manufacturing. There are many, many questions, of course. First and foremost, does the alloy actually, you know, work? Also, ALD typically involves higher temperatures that are used in electronics assembly: Would it work with today’s packaging? Will other technologies such as 3D printing or Joe Fjelstad’s solderless Occam process supplant the need for solder in any form?

Still, materials science is the most exciting area of electronics today. We may make fun of folks who walk around with smartphones seemingly permanently tethered to their ears, but we also have them to thank.

 

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Intermetallics and Kirkendall Voids Continue to Grow at Room Temperature

Folks,

In my last post, I discussed intermetallic compounds (IMCs) and what I referred to as the “miracle of soldering.” I also mentioned that research focused on the brittle nature of IMCs suggests that failures in stress tests are more likely due to failures between the interfaces of the IMCs and the solder, the IMCs and the copper, or the IMCs (Cu6Sn5 with Cu3Sn) themselves and are not related to any perceived brittle nature of the IMCs.

Another weakening mechanism in soldering and thermal aging of solder joints is Kirkendall voids. Kirkendall voids form when one metal diffuses more rapidly into another metal than vice versa. A copper-tin interface displays such a mechanism. Copper diffuses into the tin more rapidly than the tin into the copper. This mechanism can result in actual voids in the copper at the metal interface. See the image below. In addition to causing a possible weakness at the interface, the excess copper that diffuses into the tin creates compressive stresses than can result in tin whiskers.

Kirkendall voids

(Source: http://www.jfe-tec.co.jp/en/electronic-component/case/img/case_solder_02.png)

IMCs and Kirkendall voids are formed quite quickly at soldering temperatures. However, even at room temperature IMCs and Kirkendall voids continue to grow, albeit at a much reduced rate. The reason for this continued growth is that on the absolute temperature or Kelvin scale, room temperature is a considerable fraction of the melting temperature of solders. As an example, the melting temperature of SAC is about 219°C, this temperature is equal to 492K (219+273), whereas room temperature is 295°K, so room temperature is 60% of the way to the melting point of SAC solder (295/492 = 0.60). Compare this situation to steel, which melts at about 1480°C. The steel would be red hot at 60% (780°C) of its melting point on the absolute scale. So, since room temperature is 60% of the way to melting, the IMC and Kirkendall forming processes don’t stop at room temperature. Hence, IMCs and Kirkendall voids continue to grow, as do related effects such as tin whiskers.

Stay tuned. Next time we will discuss IMC growth rates and resulting effects in stress testing as we wrap up this series on IMCs.

Cheers,

Dr. Ron

 

So Long, Sola

I have to say, I didn’t think Jure Sola would or could last this long. The cofounder of Sanmina, Sola was one of the poster boys for wanton M&A excess, snatching up more than a dozen companies or OEM plants during the late 1990s and early 2000s. The spree culminated in the purchase of SCI Systems in mid 2001, a $6 billion deal that saddled the company with so much debt, when the ensuring tech collapse occurred, it was forced to take 20 straight quarters of “one-time” charges.

Most execs couldn’t have survived such a bloodletting. Sola wasn’t most execs, however. He continued to place his bets on fabricating in the US — in a memorable line, he told an IPC Printed Circuit Expo audience that “plating was in his blood” — and Sanmina remains the second (or third) largest board supplier in North America. Moreover, he correctly swung to the military and aerospace markets, eschewing the PCs that SCI was so dominant in.

Today the company is half the size in revenue of its peak, but consistently profitable.

Come October Sola will ride off into the sunset with his legacy intact, perhaps not the most beloved man to run a major PCB company, but a success nonetheless. In this era, that’s no small thing.

 

Green Herring

For those newbies, Bob Herring was the perfect example of good timing, building up and selling not one but two board shops. The first, Industrial Circuits, was sold in 1989 for $60 million. The latter one, Herco Technology (which we profiled multiple times in PC Fab), went for $122 million in 2000, just a year before the tech crash. (The buyer of Herco, Teradyne, closed it less than two years later. The former Industrial Circuits lasted less than one year longer before Toppan shut the doors.)

In case you were wondering where Bob went, well, he started his own network cable news channel. It now is televised in some 30 million homes.

Guess there is life after PCBs!

 

 

Ahead of Our Time?

Will Foxconn build in Wisconsin?

Its track record in India, Malaysia, and various other places says no, and according to this Washington Post reporter, “No one had gone back to see whether this had been carried through, and it hadn’t.”

Well, not no one

 

Copper-Tin Intermetallics: The Miracle of Soldering

Most articles discussing the copper-tin intermetallics that form during soldering refer to them as a necessary evil. The evil being the perception that intermetallics are brittle and can lead to failures in thermal cycling or drop shock.

I view the situation differently. From my perspective, the formation of copper-tin intermetallics is the miracle of soldering. Look at it this way, to assemble electronics, bonding copper to copper (the leads on the components to the pads on the PWB) in the presence of polymers (the PWB epoxies and the component cases) is required. These polymer materials can only take about 250°C for a few minutes. Copper melts at 1083°C, so bonding copper to copper in the presence of polymers would appear to be quite a challenge. Enter tin-based solder.

Lead-free (tin-based) solder, say SAC305, melts at about 219°C. So, with a peak temperature of about 245°C, in the reflow oven, solder can be melted and form an electrical and mechanical bond with the copper in the leads and pads. At 245°C, the many polymer materials are unharmed for the 90 seconds or so that soldering requires at this temperature.

But, what about the material properties of the intermetallics that are formed? Aren’t they too brittle? Lee et al* performed analyses that suggest that the intermetallics formed in soldering are not brittle. Their work also suggests that the failure modes are not in the intermetallics, but in the interfaces between the intermetallics and the solder, copper, or the different intermetallic compounds, Cu3Sn and Cu6Sn5. These two intermetallic compounds are shown in the figure below.

Copper tin intermetallics from Roubaud et al, “Impact of IM Growth on the Mech. Strength of Pb-Free Assemblies,” APEX 2001.

 

It has long been assumed that the thicker the intermetallics, the greater the risk of failure due to the intermetallic thickness. Lee’s work would appear to bring this concern into question.

Stay tuned for a continued discussion on intermetallics and their effect on reliability.

*Lee, C. C. et al, “Are Intermetallics Really Brittle,” IEEE Electronics Components and Technology Conference, 2007, pp. 648.

Cheers,
Dr. Ron

Jim Raby, RIP

I’m saddened to get the news this morning that Jim Raby has passed away. As longtime readers will know, Jim was one of my favorite persons, not just in the industry but in life. What a tremendous fighter he was for doing things right! I will always miss him. 

My sincere condolences to his wife Ellen, son David and everyone at STI on this sad day. We have lost a fine engineer, gentleman and human being.

Never Take Pin Numbering for Granted

Our all-things-about-electronics manufacturing standards body, the IPC, specifies the proper numbering order for most components. That’s a pretty nice thing that they do there, but it’s not always enough to prevent layout mishaps. Case in point a line of small PCB mount switches.

IPC calls out pin numbering for dual inline components, with pin one on the upper left (at zero degrees rotation), counting down, then over to the bottom right, and counting back up, as in the illustration below.

Given, that, it would be logical to assume that all dual inline components follow the same pattern. Logical, yes. Accurate, no. Multi-color LEDs, connectors and switches are some of the more common offenders.

In this particular switch, it’s not just a case of the numbering not following convention, it’s also different from one variant to another. I understand why. The switch isn’t changed between through-hole, top mount surface mount and side mount surface mount, but the leads have to be accessible from different parts of the package.

The following two footprints are from the same switch. One mounts on its side, and the other, standing up.

The pin one numbering doesn’t follow convention, nor does the numbering of pins 4 – 6. And, you may have also noticed that the two are top-to-bottom mirror images of each other. Ugh.

This is why my mantra is: Always check the datasheet. Always.

Duane Benson
Don’t take it for granite either, because granite is too heavy.

http://blog.screamingcircuits.com

Components So Fragile, They Break Before Arrival

There are a lot of components that require special handling. Some days, “special” requirements seem more the norm than the exception. But, every now and then, we see something that puts even those special components to shame.

Not long ago, we received a parts kit that contained a component so fragile, that most of them didn’t survive the trip with the shipper. It’s a 10 x 9mm (well, actually 9.68 +0.00/- 0.08mm x 8.64 +0.00/- 0.08mm, to be precise) sensor that’s only 0.05mm thick. That’s 1/4 as thick as the diameter of the solder balls connecting it to the PCB.

The part has solder balls on the silicon, with no other packaging. The dice has to be that thin, as the light-sensitive area is on the other side. That doesn’t make for a very robust component. It would require special handling all around. Unfortunately, no matter how careful we might be, if they’re broken when we receive them, there’s not much we can do (other than take pretty pictures).

In taking these closeups, I noticed that the registration in ball placement isn’t all that great. In the image below, take a look at the ball on the left, second from the bottom, and the ball on the far right.

The datasheets call out all non-specified tolerances as +/-0.001mm. With these being 0.2mm diameter solder balls, I’d have to say this is way outside of that tolerance. I’m sure the part would have adhered to a decent board just fine, but if the PCB were off a similar amount in the opposite direction, you may very well have a problem.

 

Duane Benson
You could make a very tiny sundial out of this.
But, could you use the shadow parallax to calculate the distance to the sun?

http://blog.screamingcircuits.com

Trolling NY

Apparently someone has decided to toy with New York state by assuming the role of “Foxconn US” and trolling a poor soul named Chris Souzzi, who works for Genesee County Economic Development Center.

I’m no fan of Foxconn, and I don’t think there’s a snowball’s chance in hell they put a plant in the Empire State, but stunts like these aren’t funny (even if that’s what’s intended) and simply go too far.