FW: [sdiy] More about IC design

[Tonnie Wiegman]

How much tests are done depends also on the market type.

In the consumer market (MP3 players/cell phones), low test costs and fast
test (development) time are important factors, so not every parameter is
tested.  
In the automotive market (car radios/motor management) the goal is to have
"zero ppm", each separate part of the car should have no defects when the
car is assembled. This means that as much as possible parameters of a device
meant for the automotive market should be tested. The goal is when the
parameter is a spec item, it should be tested. 

The last five years of my career I worked for NXP as a test engineer in the
CAR-DSP group. 
During the development phase of a device, we had to test the matrix lot for
the characterization of the device. 
The matrix lot consist of some wafers, where four different production
parameters which influence the speed of the die are varied. 
The production parameters are still within the normal production process. 
So you got several corners of the matrix ranging from slow to fast
(ssss/ssns/nnns/ssfs/..../ffff). 
The final products of these wafers are tested at 5 temperatures
(-40/+25/+85/+105/+125) and with three voltages (Vmin/Vmom/Vmax).
All this test data gives a good idea how well the device is designed and
also how the test program is performing.        
(During the normal production runs the die is tested at 105 degrees and the
final product is tested at 25 degrees.)

About the IDDQ test: we do 30 IDDQ measurements with a special written IDDQ
pattern to increase the test coverage.
The challenge with IDDQ testing is setting the limits to determine if there
is a leaking gate or not.
It is not possible to set fixed limits, due to the manufacturing process
variations, the lowest IDDQ currents in "fast" devices are much higher than
the highest IDDQ currents from the "slow" devices. So we use some
normalization to detect IDDQ failures. 
First we measure the "ring oscillator" frequency of the device to determine
the speed of the device and then recalculate (=normalize) the measured IDDQ
values.

To increase the test coverage the device is in addition to the parametric
tests, the boundary scan tests and the BIST tests, also tested with an
"application test", where the device is setup as the customer would use it.
Sometimes extra tests are added to the testprogram if examining a defective
device from a customer showed that there are "test escapes". 
Writing these application and "extra" tests can be a real challenge!

Tonnie Wiegman