What is underfill?
Underfill is a type of liquid polymer applied to the printed circuit board (PCB) after it has been subjected to the reflow process. It will then be cured after the underfill is placed. In this process the underfill encapsulates the bottom side of the silicon chip, covering the fragile interconnected pads between the chip's bottom side and the PCB's top side.
Why is underfill applied?
Underfill provides a strong mechanical bond between the chip and the PCB's connection, protecting the solder joints from mechanical stress. It also helps transfer heat.
Underfill softens the mismatch between the coefficient of thermal expansion (CTE) between the chip and the board. CTE is a change in shape or volume because of heat. It also provides more protection from the elements.
How is underfill applied?
First, underfill is applied by dispensing underfill onto a corner or along the edge of the ball grid array (BGA).
The chip and PCB are then heated to the recommended temperature in the range of 125°C to 165°C to flow the underfill. The temperatures can vary depending on manufacturer. At the recommended temperature, the underfill turns more to a liquid and is absorbed under the chip.
What does this mean for digital forensics?
The use of underfill on a PCB can present a challenge in reaching data directly from an integrated circuit (IC or chip), especially if the device in question is damaged.
Whereas heat methods can be used to remove a chip that is connected to a PCB via only solder, the addition of underfill requires extra effort to safely remove from the PCB.
At VTO, we utilize tools such as a Lap & Polish or a mill to grind away the PCB, exposing the chip's BGA pads for reading with adapters designed to communicate with forensic software.