4 ms·
> If you put WLCSP parts on a PCB that will be unprotected, and photosensitivity is not an acceptable design feature, you are either a hack, or making mistakes
by Kirby64 1y ago
> If you put WLCSP parts on a PCB that will be unprotected, and photosensitivity is not an acceptable design feature, you are either a hack, or making mistakes like a NOOB and should be supervised by a senior engineer.
Not all WLCSP parts have substantial or even noticeable photosensitivity issues. The vast majority of CSP devices have a backside coating which protects the top of the chip from most light, which leaves the majority of the photosensitivity to the edges of the device (or, reflections to the underside).
Some do, but in my view that’s largely a design flaw rather than an issue endemic of all WLCSP devices. Also depends on the type of device you’re building. Most basic digital logic, processors, and power parts shouldn’t have meaningful issues with light. The problem usually is band gap or oscillator light sensitivity, which can be alleviated by chip layout changes.
- K0balt 1y agoMaybe it’s just been the luck of the draw, but I’ve had the issue pop up almost every time I have built something with an WLCSP that I wasn’t sure would be potted. It’s usually NBD though, since normally things go in cases. In adversarial or critical situations a drop of (soft) potting glue does the trick.
- RetroTechie 1y ago> Most basic digital logic, processors, and power parts shouldn’t have meaningful issues with light. Depends... if you have an input that isn't driven externally (tri-state or nc), and there's just a weak pull-up or -down on the pin, you could have issues. An unexpected reset here, a bitflip there... Also there's high-impedance ADC inputs on some parts. Wouldn't take much to send readings waayy out of spec. Note that affected signal might be inside the IC (not going to an IO pin). More likely to be driven though (low-impedance).