Folks,
In the last posting, we saw how Weibull analysis helped us to determine that SACM lead-free solder (SAC105 with about 0.1% manganese) has comparable (actually better) thermal cycle performance versus SAC305 solder. Software like Minitab will give us even more detailed information about the performance of the solder joints in stress testing as we see in Figure 1, above.
In addition to the Weibull plot, we also have the Probability Density Function (PDF), the Survival Function and the Hazard Function. The PDF tells us when it is most likely that a test board will fail in a test population, as shown by the inserted red line. We see that it is a little less than 2,000 cycles. The Survival Function shows the percent of surviving test boards. We observe that the expected life (the 50% point) is quite close to the maximum of the PDF. The Hazard Function tells us the rate at which the test boards are dropping out. It increases with time, but there are few boars left so the PDF drops down at the end of the test, even though the fall out rate is the highest.
It is interesting (and perhaps appropriate as Halloween approaches) to consider if human mortality follows a Weibull distribution. I used some data for the Centers for Disease Control that are a little over ten years old, for males in the US. So, the mean life expectancy is a little low at 72 years. (I was a little lazy, the old data were a little easier to work with than new data, some conversions are needed to make it work.) The data appear above in Figure 2.
As you can see, just like a solder joint, your life expectancy can be modeled quite well by the Weibull distribution.
Cheers,
Dr. Ron
Calculating the exact amount of solder paste needed for 
ented 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
Tombstoning (also known as the Manhattan effect, drawbridge effect, or Stonehenge effect) is described (in the simplest, and most common, sense) as occurring when one end of a passive device, such as a resistor or capacitor, rises up out of the solder and breaks contact with the circuit. But it is not limited to passive devices. Other surface mount devices can tombstone as well (see the tombstoning diode image - top). Tombstoning is a "fatal" defect because it produces an open circuit.
the transition to Pb-Free (higher reflow temperatures, and related flux issues)
Thermal gradients are usually easily remedied with minor adjustments to the reflow profile:
The pad design and lay-out can also affect tombstoning. Usually pads that are located mostly beyond the terminations or have large pad areas beyond the terminations can contribute to tombstoning. To the left is an image of a cross section of a soldered passive component. Notice how the solder fillet reaches to the top of the termination. Solder paste deposits that extend well beyond the component cause a lot of wetting force and leverage to be applied to the extreme ends and tops of the component. This wetting force, if not evenly applied to both terminations, can cause the component to tombstone. 
Similar to the placement of the solder paste deposit (pad design), solder volume can also impact tombstoning. It is very simple. More solder equates to more wetting force and vice versa. To the right is an image that has an extremely reduced amount of paste volume (not recommended to this degree). If one could imagine that this component had indeed properly soldered to the pads, one could see how it would be nearly impossible for the component to tombstone. There is simply not enough solder to wet the entire end of the termination. Solder deposit volumes that restrict the solder from being able to wet up to the top of the component greatly reduce the wetting force and leverage that the solder can apply to the component. Depending on the class of workmanship that one is building to, it may not be practical to reduce the solder volume. The product class may require fully wetted terminations. 
Content on the Solder Joint Reliability of a Pb-free PBGA Package.” Both projects evaluated lead-free thermal cycle reliability as a function of silver content and compared the results to SnPb reliability.
Now I can't talk about the great things about wikipedia without talking about the concerns. Being an open forum type encyclopedia, and with thousands of posts being updated daily, there is a lot of room for error. Many of us young enough to have wikipedia available to us in College remember being told that we could, under no circumstances, use wikipedia as a reference. But now, with Wikipedia's stricter policies on articles, this may soon change, if it hasn't already.
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