Revelations at ACI


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.


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

Little Chippy Challenges

And “chippy,” in this context, refers to chip caps and any other tiny two-connector components. When considering surface mount, most people think of the many-connector parts, like BGAs and QFNs as the challenging components. That’s mostly true. However, the little passives can be big bears too if not treated properly.

Two part tombstone You could have tombstoning problems. This can be caused by unequal sized pads, unequal sized traces going to the pads or inequality in copper plane in a different layer. A big part on one side can cause tombstoning too — the big part’s thermal mass may slow the solder paste melt on one side of the part, leading to tombstoning.H Skewed passive via in pad

Via-in-pad is still a problem too. Open vias can lead to unreliable connections, tombstoning or crooked  parts.

Soldermask tombstoning for blog Solder mask can cause problems too. Too thick a solder mask can prevent the part from reaching the solder and can cause tombstoning. Too think a solder mask can also interfere with outgassing in the reflow oven which can cause solder ball splatter. (A = okay, B = not okay).

Duane Benson
It just goes to show you…
It’s always something.

Let the Data Be Your Driver

I was recently asked to give a presentation and audit an assembly line regarding minimizing “tombstoning” of passives at a major electronics assembler. As my presentation brought out, tombstoning can be caused by many factors: the reflow profile, the solder metal composition (for lead-free applications, SAC 387 tends to tombstone more than SAC 305), off-center placement, nitrogen reflow atmosphere, buried vias, etc. After two hours of talking, I walked the line that “had a problem with tombstoning.”
Tombstoned component
As I started asking, it became clear that no one knew the magnitude of the problem.

“How many passives are on each board?” I asked. No one knew.

“How many DPMO (defects per million opportunities) for tombstones have you had recently?” Also unknown.

As people scurried to get the data, it dawned on us that tombstoning might not be as big an issue as was thought. It was more of a local legend.

Finally, we got some data. Each board had about 1000 passives, and the company had produced 100 boards with a total of two tombstones in the past two hours. Tombstones were the only defect. Hmmmmm, two bad boards out of 100 = 98% first-pass yield, not bad! From a DPMO perspective, they had two defects per 200,000 (two defect opportunities per passive) opportunities or 10 DPMO, which is beyond world-class. This level of DPMO would be very difficult to improve on without massive engineering investment. It is “in the noise” and it is likely caused by “common cause” variation.

I then asked how much money it costs to repair a tombstone; as expected, no one knew. My guess was less than $2. This situation is the rare case where yields are so good, it may not pay to make engineering investment to improve them.

This isn’t the point of the story, however. In a case like this, the response — whatever it is — must be data driven. Only with the proper failure rate data, plotted in a Pareto chart, and a complete understanding of all costs, can the appropriate action plan be developed.

Always be data driven!