Guard Banding

The semiconductor manufacturing process is not perfect, and produces a percentage of defective parts. So, all parts are tested to ensure that they operate correctly and meet specifications. Testing the parts is expensive, and particularly so when the parts are tested over the specified temperature ranges. Testing parts at the high temperature is straight forward, requiring a handler with a heater. Cold testing is more difficult, because ice tends to form around the test socket, creating a mechanical obstruction that prevents further testing. It is common to see testers stopped, with test technicians using hair dryers to melt the ice. There must be a better way, and there is. And it is called guard banding.

Device parameters change with voltage and temperature according to the laws of physics. Parts get faster and stronger with higher voltages and lower temperatures. And they get slower and weaker with lower voltages and higher temperatures. Knowing this, we can pick a voltage for room temperature testing that causes the device parameters to behave the same as if we were using the specified voltage at an temperature extreme. We call this guard banding. While the theory sounds easy, real life requires more work. The question is where to set the limits. Most critical specifications involve maximum timing specifications, which corresponds to minimum voltage and maximum temperature. The first step is to measure the part performance at both room and elevated temperature at the minimum voltage, and determine how much the timing parameters change.

One option is to tighten up the timing measurement to reflect this shift. If a part has a specification of 50 ns, and shifts 5 ns with temperature, then it can be tested at 45 ns at room temperature to guarantee the temperature performance.

A second option is to lower the supply voltage until the delay has shifted by 5 ns. This new voltage can be used to test the part at the 50 ns specification.

And a third option is to use a combination of reduced voltage and tightened timing.

Every part is different, and may contain analog components or design techniques that restrict the voltage ranges of the part. It falls to Product Engineering to select the best approach for guard banding.

Guard banding can also be used to improve manufacturing yields. A part may go through multiple tests during its manufacture. There can be a probe test, a second probe test if the part contains flash, a final test, and occasionally temperature tests. If a part is at the very edge of a specification, it might fail a later test after it has accumulated more manufacturing cost. To prevent this, tighter voltage limits can be used at the earlier tests to fail these parts, resulting in higher manufacturing yields at the later steps.

Guard banding is one of the many techniques that Tekmos uses to provide high quality parts at reasonable costs to our customers.