Electronic – How to decap a chip to expose the die

Flip chip is about a single die. On the circuitry side solder balls are placed and the thing is mounted upside down on the PCB, hence the name.

Package on package (\$PoP\$) goes one step further: you have a flip chip, with another BGA on top of that, where the lower die occupies the place between the top die's solder balls.
The top die is not a flip chip; note the bonding wires. The bottom die is a flip chip, though the soldered balls aren't visible; the die should have been 1/10mm or so above its substrate.

Like many \$PoP\$ illustrations, this one shows a stacked die at the top, but that's not a requirement to talk about \$PoP\$.

A typical \$PoP\$ application would be to have a microcontroller below and a Flash or RAM device on top, since they will have to be tightly connected anyway.

Very thin, ~700µm (0.7mm) is close to the upper limit. Around 100µm (0.1mm) is about as thin as they get. However the size varies a lot, depending on multiple things, like the package it's made for, quality, price, and the overall size of the wafer.

Update After further research, I found that for certain applications, the wafer may be as thin as 50µm.

guess the package top and bottom will also need a certain thickness to be useful, so how much is left for the die?

An incredibly small amount, take a look at this picture and the others at the bottom.

Yamaha YMF262 audio IC decapsulated

It varies with the size of the wafer, according to wiki,

• 2-inch (51 mm). Thickness 275 µm.
• 3-inch (76 mm). Thickness 375 µm.
• 4-inch (100 mm). Thickness 525 µm.
• 5-inch (130 mm) or 125 mm (4.9 inch). Thickness 625 µm.
• 150 mm (5.9 inch, usually referred to as "6 inch"). Thickness 675 µm.
• 200 mm (7.9 inch, usually referred to as "8 inch"). Thickness 725 µm.
• 300 mm (11.8 inch, usually referred to as "12 inch"). Thickness 775 µm.
• 450 mm (17.7 inch, usually referred to as "18 inch"). Thickness 925 µm.

Basically they take a slice of silicon that's about .6mm thick (on average,) grind it, smooth it, etch it, then grind the back side.

Here's a good video to watch, How Silicon Wafers are Made. And to see how a chip is decapsulated, watch Chris Tarnovsky's video How to Reverse-Engineer a Satellite TV Smart Card.

If your interested in decapsulating chips, and close up images and probing of the die, FlyLogic's blog has some awesome posts, and great pictures!

And a few pictures of decapsulated chips,

The following 2 images are of a ADXL345 3mm × 5mm × 1mm LGA package. The first is a side X-ray. The X-ray clearly shows the presence of a separate ASIC die and MEMS die, with a hermetic cap. The internal structure of the device is more clearly seen in the SEM micrograph of the decapsulated device, in the second image.