SIwave is a power integrity and signals integrity tool. MTTF is one of the tools in SIwave and will be discussed in this blog.
MTTF stands for median time to failure. MTTF is a necessary tool to understand the lifetime of the PCB based on the DC current density distribution. It uses Black's equation to calculate the median time to failure of a semiconductor circuit due to electromigration: a maintenance metric that measures the average amount of time a non-repairable component or structure operates before it fails.
Figure 1: MTTF solver after the DC solver
Another blog talks about the DCIR solver for a PCB. Once a DCIR solution is available, one can perform an MTTF solution. After selecting the MTTF solver, SIwave displays a dialog box and fills it with as much information as it knows. SIwave expects the user to verify the contents. If satisfied, then launch the solver.
Figure 2: MTTF dialog box in SIwave
The definitions of all the parameters in the equation are shown in the dialog box: J is the current density, Ea is the Activation Energy factor, k is Boltzmann's constant, and T is the temperature. The user can modify any number to his liking. The warning and error numbers are just thresholds to be used in the postprocessing to identify the bad sections.
- Examining the solution:
- Plot the lifetime on the top/bottom surface.
Display the MTTF on the top or bottom surface. If you want to know the MTTF with any component on the PCB, identify its location, then read the map. Move the mouse over the plot to read the MTTF at any location.
Figure 3: MTTF on the top layer
Figure 4: MTTF on the bottom layer
Double-click here to change the scale from log to linear or to log in dB. This is maybe useful to highlight issues in the model. But usually, the log, the default one, is a more powerful presentation.
- Plot Warnings and Violations
So in the setup, the user specifies the warning and the violation threshold. In the postprocessing, one can display the regions that cross the warning threshold and the regions that cross the error threshold.
Figure 5: MTTF violations and warnings
It is all green simply because all regions passed all the thresholds.
- Examining vias
With MTTF, even vias are examined. One can display them, but it is hard to see them. The best way is to display them in a table. That is available from SIwave. The vias are arranged from the least MTTF to the highest. The user can check the numbers and make sure that they are OK.
Figure 6: MTTF in Vias
- Examining the metals in another way
Also, click on the metals to see the location of any metal that may fail before the product's lifetime.
Figure 7: MTTF in Metals
So from these plots, one can tell the weakest sections in the design and try to fix them, especially on the top and bottom layers.
