Testing One, Two, Three

The sheer number of off-the-shelf parts that are showing up in ICs these days—and that includes both hard and soft parts—means that to a large extent we are designing and manufacturing a series of interconnected black boxes.

Black boxes, at least in theory, are a major time saver. The idea that you can put together a series of well-designed, state-of-the-art Lego-like blocks that are proven in silicon, yield sufficiently, and which are characterized well enough so that the physical effects and proximity effects can be taken into account in a design can be a very good thing.

Reality is somewhat different, though, particularly at advanced process nodes and in stacked die. Black boxes can turn bad on their own, and they can start out good and perform miserably when combined with other black boxes. Known good die is the widely discussed problem, but even within die there may be known good blocks that don’t work together.

The problem is that you can’t characterize effectively if you don’t know everything else that’s going around a subsystem or IP or even a die in a stack. The creator of the black box has no idea how their technology will be used, what it will be next to, or even what process node it was created in. They can’t hope to understand the end usage model except as it’s defined by a bell curve, and if their parts are going into very advanced nodes they don’t know how process variability, stress and physical effects will work individually and in combination.

There’s no magic bullet here, but testing to the point of confidence is at least a step in the right direction. That test needs to be part of the initial design architecture, and it needs to follow right through to the manufacturing because there’s no such thing as a single test. Testing needs to be internal, external, and in stacked die it needs to be done before and after packaging. If there’s no way to test it after the package is sealed, then it’s a series of black boxes within a black box with no direct way back in.

Like any other business, it’s one thing to trust your business partners. It’s quite another to trust them blindly and not hold them accountable. The semiconductor business is no different, except that for years ensuring accountability was left to the manufacturers or the design teams. It now has to be part of every process and every facet of design through to final packaging—and parts within parts within other parts need to be included. Things have become that complicated, and test has to work across all of them.