WOW!

 

I knew we were good, but I didn’t realize how good we really are.

We have a number of reports about our quality online.
(See:http://www.xilinx.com/publications/prod_mktg/Quality_brochure.pdf).

Besides the glossy pages, nice graphics, and our many standards certifications (certification is “we do” -- compliance is just “we try”) there is a much more compelling story (in my opinion): PPM.

PPM stands for parts per million. PPM defects are commonly used to describe how many parts are shipping that fail.

Here is where I was really impressed. In a recent period of review of our 65nm Virtex-5, FPGA family, quality, in shipments of 1.7 million parts, we had the following breakdown of real valid (actual) failures:

 

Assembly and Packaging:        1 PPM
Test Coverage:                           3 PPM
Random Defect:                         1 PPM

 

So what are these failures?

 

Packaging and assembly failures are commonly caused when a metal bridge or particle in the substrate that the die is mounted on causes the part to fail once it is soldered on to a customer board. Inspections and tests try to catch all problems before we ship, and the very few units that come back are used to improve those processes and improve the overall quality.

Test coverage is another area where we get failures: we just didn’t know how the part was going to be used so we did not test some tiny element. These are known as ‘test escapes’ in the business. One by one, we find the elements we did not test and add them into the test program. In doing so, the quality improves dramatically..

Random defects are just that: Random. They can be caused by some small particle on the die somewhere in the backend stack of metals and dielectrics, by a wire which has become too thin due to a particle that blocked the exposure for that mask, or by a particle landing across two wires, creating a short. At 65 nm, it is impossible to remove all particles from the process, and to the extent the fabricator misses a few particles and we don’t find them in test, they manifest themselves as failures as soon as the customer places the part on the board and loads their design.

 

 Wait a Second: You MUST Get More Returns?

 

According to my colleague, there were about 8 times as many parts returned which had nothing wrong with them when tested, and another 8 times as many parts returned which were completely “toasted” – what we refer to politely as “electrical overstress” or EOS. There were also about 4 times as many parts where the customer destroyed the part (like smashing it to pieces). Last, there were about 5 ppm where the application of the product was such that the product did not work, and once this was pointed out to the customer (the part is not being used within the data sheet specifications), the parts now work.

The parts that were EOS basically show whole sections of the die that are completely destroyed. It may have been an electrostatic discharge (ESD) event, or it may have been an over-voltage spike on the power or IO: we will never know. We can make a guess from the area of the crater, and we can engage with the customer to suggest what might have happened, but we will never know. We do know that these sorts of EOS events are not a particle or random defect, as that signature is very different (basically: no large crater!).

The engagement with the customer who returns the part is critically important:  they learn how to properly use the part, and their costs decrease because they are not throwing perfectly good parts away. Sometimes the solutions to these problems are as simple as “the assembly process is not meeting our requirements for the solder reflow profile” or some other easy-to-fix issue.

If a customer is stressing the parts and blowing them out, it is also important that we help them learn what ESD assembly rules they must follow as well as help them make sure their power supply systems are meeting our specifications.

 

Bottom line: quality is a feedback system. And when it is properly implemented, it drives the defects to 0. When it works, it is a beautiful process.

Austin Lesea