Why do some chips come as bare silicon ?

[Beamin]

That seems like a terrible idea. It's like the chip is naked. And in the case of the raspberry pi delicate even to light. It seems like the black plastic would be the easy part. Why do they do this? At least put it in a blob of black stuff or something! I'm always super cautions about even toughing it.

[lgbeno]

It is the smallest possible package and also the least expensive.  The light sensitivity of the Raspberry PI regulator is a little of an anomaly, most electronics are enclosed, also it was mostly susceptible to a Xenon Flash, not regular light levels


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[Twoflower]

It depends: One option is going cheap: Glue the die on the PBC band wire-bond the connections. The other is size does matter in some cases (like the one you mentioned on the RasPi).

Some designs are so dense you can't fit any housing or protection-coating in it. Look at (Micro-)SD cards for example or Smart-Phones and Smart watches. The manufacturer fight for each fraction of a mm. Sometimes it has also some electrical reasons. As routing high speed signals through a housing will reduce the signal quality which can be avoided if you mount the die on the PCB.

You should keep in mind that in a end-user product you usually don't have access to this parts. So why bother placing them under a blob? And the people handling these bare die packages are aware what they do. Or at least they should. The many destroyed AMD processors some time back show this exactly: If handled with care you're safe. But if you're a bit too careless mounting the heat sink or bumpy transport with a heavy heat sink and you created a expensive paperweight.

To my understanding the coating is has to be used for wire bonded chip on board solutions as the wire-bonds are very fragile. See cheap multimeters or displays.

And the light sensitivity is not a problem for end user products. As they are usually build in into housings that keeps light from these devices. But actually some SanDisk seem to have a semi-transparent housing: Here I really wonder why as some card readers have half of the memory card exposed. And light could potentially create some charges in an un-powered card that has some influence on the memory cells.

Fun fact: The 200MByte SanDisk MiroSD card is not even painted because it would exceed the allowed dimensions.