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

 

Wicked Wicking

This PCB assembly challenge involved attaching a solar panel to one side of a pad using solder paste with a pass through an SMT reflow soldering oven.

Figure 1.

Solder wicking through the unmasked vias to the back side forms unacceptable “bumps” on top of the vias.

The attachment or bond itself wasn’t the issue; but after the first trial runs, it was clear that solder wicking through the unmasked vias was going to be. Solder would wick through the unmasked vias to the back side and form “bumps” on top of the vias.

These bumps made the surface nonplanar and of course were unacceptable. It wasn’t an issue of using excess solder paste. But the “wicked wicking” had to be stopped, or at least prevented.

Figure 2.

Kapton tape is applied to cover the unmasked vias; it will block the molten solder from leaking through.

But how? Clearly, to keep the solder where we wanted it to remain during reflow, we had to find a way to prevent it from wicking up, collecting at the opposite ends of the vias and forming bumps. We had to find a solution that was simple, temporary, and tolerant of reflow soldering temperatures. The answer was Kapton polyimide tape, a familiar product to PCB assemblers for many years, and a material that does not degrade at reflow temperatures.

Kapton tape was applied to cover the unmasked vias in order to block the molten solder from leaking through the vias to the back side during reflow. After reflow and cooling, it was a simple matter to peel off the tape. This temporary masking solution worked; there were no more solder bumps on the back side of the assembly, and the cost of the fix in terms of time and material was very low.

Figure 3.

Figure 3. This temporary masking solution worked; there are no more solder bumps on the back side of the assembly.

Roy Akber

www.rushpcb.com

 

Solder Defects Causes and Cures Webinar

If you missed the SMTA International preshow webinar supported by CIRCUITS ASSEMBLY you can view it online here.

Printing solder paste or other conductive material requires zero defects printing if a high first-pass yield is to be achieved when using fine-pitch components. Monitoring and control of paste height and volume are becoming the norm in many markets, but what capability can we expect?

Correct printer setup, good stencil design and manufacture plus consistent printing materials are key to successful manufacture but inspection and monitoring the performance makes a process more robust. The same three-dimensional inspections are required in other AOI applications like solder joint analysis. There are common process defects during printing and reflow, Willis says, and the webinar shows causes and cures to help yield improvement.

The webinar is presented by Bob Willis and covers:

  • Solder paste inspection standards
  • Soldering yield impact with poor printing
  • Common solder paste defects
  • Impact on reliability based on paste thickness
  • Solder joint inspection defects
  • Common process defects causes and cures

Results of survey of 98 engineers from last week’s webinar on process defects.

ProcessDefects
Print Defects
Inspection Location

Down the Drain

Figure 1 shows a closeup photo of a PCB assembly, it seems as though solder has flowed “down the drain” and away from the solder joint where it’s needed.

In fact it has, because the customer has inconveniently located a via right through the center of one of the two topside SMT pads for a surface mount component. When the assembled PCB is run through reflow, the molten solder drains away through the barrel of the via and out the other side of the PCB. There isn’t enough solder remaining post-reflow to create an acceptable solder joint per IPC-A-610. The joint is “starved”; this is unacceptable. What to do?

Figure 1. Insufficient solder, i.e., "starved" solder joint on an SMD pad.

Figure 1. Insufficient solder, i.e., “starved” solder joint on an SMD pad.

The via is there to stay, by virtue of the customer design. So, no matter how many times solder is added to the joint, every time the PCB is run through the reflow oven the solder is going to drain away because the PCB , including the via, is at reflow temperature.

Obviously, more than one run through the oven makes no sense. The only practical solution is to manually add solder to the individual solder joint, post-reflow, without running the entire PCB through another thermal cycle. It’s a touchup procedure that’s required to create a robust SMT solder joint that meets acceptability criteria. This is a manual PCB assembly soldering process that should be performed by a skilled hand-soldering or rework operator. Solder is added only to the joint, via cored wire solder or solid wire with flux, in order to build up the volume of solder at the solder joint to provide strength, connectivity, and an acceptable meniscus per IPC standards, covering the via drain-hole. The solder won’t flow through the via because only the surface joint area is heated.

Figure 2. The solution: Add solder to the joint manually via a touchup procedure.

Figure 2. The solution: Add solder to the joint manually via a touchup procedure.

It may seem tedious, but a skilled operator can touch up the joint in a few seconds, and if there is only one instance per assembly it won’t appreciably cause production delays.

Roy

rushpcb.com/rushblog

Using the Newest Gen Arm, Part II

I’m a bit behind in my blog work — well, way behind, actually. I started this series back in January with the intro post.

Here’s where I am right now:

  1. I have three different sets of PCBs.
  2. One set, I took home to see if it’s possible to solder a micro-BGA at home. (As someone working at a car manufacturer might want to see if they could balance a crankshaft at home, for fun)
  3. Two sets, from our partner, Sunstone Circuits, are here in my desk with parts, ready to go through our machines.

After I’ve got all three sets built, I’ll have them x-rayed to see how they look under the hood. Finally, I’ll solder through-hole headers on and fire up the chips to see if the shared escape system works.

Here’s one of the boards without access to the inner pads:

And, here’s the shared escape:

The main concern I have is that Reset is on one of the inside pins (B4). I’m not sure if I can get the chip to a state where it will operate properly without unobstructed access to reset.

The routing I’ve chosen is probably the only possible option for reset. Pin A4, right above, is used for the single-wire debug (SWD) clock. I’m assuming that can’t be shared. B5 is Vdd, so that’s out. It might be possible to go down. C4 defaults to one of the crystal pins, and D4 defaults to a disabled state.

In the route I’ve chosen, B3 is an ADC input, so it should start out high-impedance, and therefore not interfere. A3 defaults disabled, so it won’t get in the way.

Next step: solder time!

One other thing – The images above show non-solder mask defined (NSMD) pads. Those are standard for BGAs 0.5mm pitch and higher. This part is 0.4mm pitch. Some manufacturers recommend solder mask defined pads (SMD) for 0.4mm and smaller. I’m actually testing several pad styles: SMD, NSMD and solder mask opening = copper.

Duane Benson

Run it up the flag pole and see who solders

http://blog.screamingcircuits.com

Putting the ‘United’ in United States

In Ken Burns‘s excellent miniseries The Civil War, Shelby Foote states that the Civil War  made us a country by uniting North and South.  He argues that before the War, its citizens might say the United States are a good place to live, noting the feeling of separation. He points out that before the Civil War, most people had never traveled more than 30 miles from where they were born and therefore had only a theoretical notion of the US as a country. During the War, millions of men and women had walked its fields and hills and cast their eyes on her valleys. They came home with a solid feeling for what this great land is. So, today, everyone, North and South, would say the United States is a good place to live.

If the Civil War united us North and South, gold united us East and West. Shortly after the Louisiana Purchase  in 1803, President Thomas Jefferson commented that it would take 25 generations (a little more than 500 years) to settle the West. Most of us today would balk at this estimate, yet in 1803 it was very reasonable. Consider that by 1803, the US had been settled for 200 years and the vast majority of people lived with a few hundred miles of the Eastern seacoast. Yet by 1850 California had become a state and twenty years later the country was united East and West by the transcontinental railroad .  The sole driving force for these amazing events was gold. Gold was discovered in 1848 at Sutter’s Mill and within months the California gold rush  had begun.  By 1855 more than 300,000 people had come to California from all over the world.  It was the biggest gold find in the world up to that date, but, to put it in perspective, only 750 metric tons (MT) of gold were mined in 10 years of this Gold rush.  All of this gold would only be a cube only 3.4 meters (11.1 feet) on a side. This fascinating story is documented in The West, another documentary produced by the prolific Ken Burns.

Today over 2,000 MT of gold are mined each year, worldwide. Modern mechanized and automated mining techniques enable this tremendous increase. About 75% of all of the gold mined in the world has been mined in the last 100 years or so.

Today, about 50% of gold is used for jewelry, 40% for investments, and 10 % for industrial uses. Gold has one of the best surface electrical conductivities of any metal, making it a top choice for high-performance electrical contacts.  Its resistance to corrosion enables gold solders to be very robust in harsh environments.  Gold’s malleability and ductility also make it ideal for bonding wires  in semiconductor packages.  Gold is so ductile that a 0.5mm diameter ball can be pounded out into 0.5 square meters of gold leaf (see the image).  In electronics assembly, gold is used in Electroless Nickel Gold (ENIG) surface finishes for PWB pads and some corrosion resistant, mechanically-strong solders. Some gold solders have tensile strengths seven times greater than SAC305. With their 280C liquidus temperature, these robust solders can also be used in high temperature applications.

But remember, without the California Gold Rush of 1849, and the Alaskan Gold Rush of 1897, the United States would be a dramatically different country today indeed.

Cheers,

Dr. Ron

Musings on Metals: Copper

It could be argued that civilization began with the smelting of copper.  Although thousands of years before, humans fired clay to make figurines and containers, smelting required several non-obvious steps.  After all, the firing of clay, at some level, can be accomplished by simply dropping clay into a fire.

To smelt copper, our ancestors had to:

  1. Take malachite (see photo) or another copper ore, grind it up or break it into small pieces
  2. Mix the ground malachite with carbon
  3. Heat the mixture in a vessel to 1,085oC.

Malachite Ore

Achieving this temperature with a wood fire is, to me, astounding.  Think about those days when you are grilling some burgers.  You leave the grill on after the burgers are done, to burn off the grease.  You come back 20 minutes later and the grill is at 500oF.  You can feel the heat.  Even touching the knob to turn the gas off is intimidating, as the heat drives you back.  This temperature, 500oF, is only 260oC!  The ancients reaching 1,085oC with wood and bellows is, indeed, impressive. By the way, a good rule of thumb to convert degrees C to degrees F from 100oC to 1,5000C is that 2XC=F, this fast approximation is accurate to about 10% in this range.

The confluence of the three procedures is not only non-intuitive, but think how many times the smelter of old could only reach 900oC and failed.  I have argued that if copper melted at 1,200oC or so, civilization would have never gotten started.  This temperature is perhaps a little too high to reach with a wood fire.  The smelting of copper encouraged investigations into other metals, eventually resulting in the discovery of the processing of iron, an even less intuitive process than smelting copper.  So, I believe that the success with copper was necessary to the production of steel.

Copper smelting became an industry that encouraged permanent settlements and stimulated trade, which encouraged writing and ciphering.  An effective copper smelter would likely keep secret some of his craft as he wanted a competitive advantage.  He could make more by smelting copper than doing anything else, so he almost certainly was an early specialist.

Considering all of this, I believe that without the discovery of copper smelting, we might still be living in huts or teepees, using stone tools, and living a nomadic existence without commerce, writing, or mathematics.  Examples to support this thesis are the state of native peoples in the Americas in the 1400s.  These native peoples had never learned to smelt metals and hence also lacked the follow-on aspects of civilization mentioned above.

Today, copper is a foundation material for electronics, given its excellent electrical conductivity, second only to silver.  Copper’s ductility likely aids in the formation of PWB traces and plated through-holes in that it resists cracking.

Additionally, copper’s ability to form an electrical and mechanical bond with solder is another trait that makes it a winner as an electrically-conductive assembly material in modern electronics.

Copper has been used for more than 10 millennia, but, as with most metals, 90 to 95% of it has been mined since 1900.  About 15 million metric tons (MT) are used each year, third to aluminum’s  22 million MT and steel’s unequaled 1 billion MT.

In the next installment, we will discuss tin and how it forms an intermetallic with copper during soldering.  Thus making solder paste, solder wire, and solder preforms critical components of electronics assembly.

Cheers,

Dr. Ron

Service Life

A reader writes:

My company makes an electronic product that requires a 40- year shelf life. We assemble with tin-lead solder on FR-4 PWBs. The product is to replace older technology (i.e. 1960-70s), but has some newer components such as BGAs, SOICs, and PQFPs. The product will be stored in dry nitrogen at 70F.  We take great care in manufacturing, by cleaning, inspecting, and testing the end product.

My question is, Do you know of any studies that would discuss the reliability of products stored or in use for 40 years?

My sense is that our reader will be successful, but his question is profound and hard to answer with confidence. The military would like their electronics to perform for that long, but realistically much of it is replaced every 10 years or so. If you look at something like the B-52 bomber, which debuted in 1952, the electronics have been upgraded regularly. So there isn’t as much 40-year electronics experience as one might think. An exception being the IBM AP-101 computer. This computer was kept in service for over 30 years, because it served its function and had survived the rigorous and expensive military qualification testing.

However, anecdotal data might support optimism for 40-year shelf life. In a class I teach at Dartmouth, The Technology of Everyday Things, I have sought out some old transistor radios from the late 1960s and early 70s to show the class how this old technology works. Anytime I have every found an old device like this, they always work, unless the batteries have leaked inside the radio.

This question raises an interesting thought. Although those of us in electronics assembly are concerned with tin-lead and lead-free solder joint life, what about the modern devices inside the components? Forty years is a long time. How will the 3D-22 nanometer copper circuit lines in a modern microprocessor hold up over this amount of time? These circuit lines lines are so fine that the 22nm width is only about 70 atoms.  In addition, copper integrated circuits are still a relatively new technology. I’m sure much accelerated life testing has been done on such circuits, but would such testing confirm 40 years of shelf or service life?

I would appreciate any thoughts that readers have on these questions.

Cheers,

Dr. Ron

Revelations at ACI

Folks,

I’m taking a few moments from Wassail Weekend, held annually in my village, Woodstock, VT (“The prettiest small town in America”), to write a post about the recent workshops at ACI.

Indium colleague Ed Briggs and I gave a three-hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analyses of “graping” and the “head-in-pillow” defect are the best around.

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e., very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g., OSP, immersion silver, electroless nickel-gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of five years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years and under harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in Fred’s chart (see the pdf link). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many “gaps” that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately $100 million program in 2013.

For those that missed this free workshop, another is planned in about six months.

Cheers,

Dr. Ron

Via Shifting

Here’s an example of what via in pad can do for a small passive component. Other things can happen too, like tombstoning or twisting. But take a close look at this photo. In doing so, you’ll note that both sides of  Small fillet passive via in pad the part are soldered down. Sure, it’s shifted, but who really cares? It’s electrically connected. Right?

In this case, much of the solder on the lower pad flowed into the via. This led to an imbalance in surface tension between the two pads which shifted the part. Some logic might say that since both ends of the part are soldered in and there aren’t any shorts, it’s all cool.

It is all cool because it’s been out of the reflow oven for quite a while, but it’s not cool because it’s not good workmanship. The IPC created standard IPC-A-610 for just such an issue. Class I is the loosest. This might pass that. I’m not sure though because we don’t do anything with Class I here at Screaming Circuits except reject it. Class II is the typical commercial type standard and this shall not pass that standard. Nor would this pass Class III, an even tighter workmanship standard for higher-reliability requirements.

That’s the real issue: reliability. With a good, symmetrical solder joint, you not only have a good electrical connection, but you also have a reliable mechanical connection. It will resist flexing and thermal expansion stress. This one may not. Give it some good thermal cycles or bounce it around in a race car engine computer and you may find yourself sidelined.

The moral of the story is to keep those vias out of your pads; even with passive components. Or, put the vias there but fill and copper plate them at the board house.

Duane Benson
Balrogs in pad are bad too

http://blog.screamingcircuits.com/