Hangie - it’s a related issue, but a separate nuance.

The original issue we had found earlier, the breakage at the coverlay, was addressed with uv cure passivation, and then retested with several thermal cycles of heat-freeze, as well as extreme flexing of the blade around a tight mandrel test fixture. We also did high-cycle automated thumping of the neighboring keys.

After the thermal and mechanical extreme tests, the data showed that units that had passed, and then were encapsulated, and then survived those tests - they performed well, and reliably over several weeks. This was a good indication that no latent defect remained.

We were able to grade our inventory before and after passivation with nondestructive tests, and those tests were good predictors of which units had been subject to fracture. So we tested, passivated, and tested again, and believed our graded inventory was good.

During TREG, despite all of that test regimen, a few that had passed eventually showed some intermittency. It was very hard to catch, because it was usually fleeting, but once we found one and kept on it, then we could see a progressive failure pattern over time.

This is despite having passed all the extreme initial test protocol. After studying the area microscopically, we concluded that this is a separate failure mode, and modeled as follows -

A partial fracture at initial install compromised a portion, but not all, of the trace.

It tests good, even at the extremes of mech and temp.

It remains good for weeks.

The section of annealed copper has however, had much of the metal crystal lattice parted.

The parted sections of the metal still contact and conduct, but are no longer a contiguous molecular lattice.

Over enough time, atmospheric Oxygen atoms migrate into the void in the lattice.

As the oxidized zone builds up over time, the connection gets weaker, and finally lets go.

Adjacent oxide growth can also force apart an intact portion of the neighboring lattice.

The oxidation process is driven mostly by time, but can be affected by temp and mechanical excitation that increases oxygen exposure. The encapsulation actually retards this, but is not perfectly gas-tight.

This was a somewhat hair-pulling level of subtlety, and was particularly nettlesome because of its delayed-action, progressive oxidation effect. We think it makes sense now.

All PCB copper traces, like the flexes in an iPhone, contain some level of imperfection, and redundant zones of copper generally provide some backup that eliminates this risk.

In this case, the fine pitch 4 mil (100 micron) traces of copper, the flexibility demands, and the high level of thermal and mechanical excitation for the usage profile all combine to make this assembly less forgiving of overstress during fab. So we have to be more diligent about prevention.

As you might imagine, it’s a bit difficult to give meaningful daily updates on subtle stuff like this. You have to take the time and care to characterize and understand it fully. We usually just hunker down and focus on getting the necessary data to know the physics that define it. Once you get that data, pretty much any problem will yield, and you summarily eliminate it. It’s also always better to address this stuff in your own inventory warehouse, rather than in the field.

Intermittency is all the more important to kill off in the context of a human interface device designed to detect very subtle human gestures and intent.

Hope this helps give some insights.