Low-temperature low-cost 58 Bismuth – 42 Tin alloy forensic chip re-balling and re-soldering
Résumé
The re-soldering of electronic components is often necessary during forensic investigations. Such re-soldering usually occurs in two scenarios. In the first in vivo scenario, a component is extracted from the exhibit board and analysed (or unlocked) externally before being re-implanted in the exhibit board. In the second in vitro scenario, the extracted component is implanted in an external test board or in an unlocked device of the same brand and model. We call such manipulations chip-off/chip-on procedures.
In some cases, data manipulation, performed during chip-off and chip-on, may also help to recover forensically significant data such as emails, text messages, contacts, photos or videos….
Chip-off/chip-on techniques involve two risky steps during which forensic data can be irreversibly lost, given that a chip must be de-soldered and re-soldered again. During both operations temperatures beyond the normal operating range are applied to a chip containing valuable information. In addition, for cost reasons, chips are usually not designed to withstand repeated soldering during their life-cycle.
A recent publication by the Netherlands Forensic Institute (Jongh, 2014) proposes to minimise thermal stress by using low temperature alloys during the re-soldering phase. The necessary low temperature soldering 42Sn/58Bi 300 μm balls have a burdensome cost (several thousands of euros per ball jar) if a mass production is not considered. Given that the forensic analysis of mobiles telephones has become a standard requirement in most criminal cases, unitary forensic analysis cost reduction is currently a necessity. Luckily, it is possible to find cheap soldering pastes composed of inhomogeneous balls of 42Sn/58Bi (25–45 μm) mixed in a solvent. How to use such pastes for forensic re-soldering proves to be a nontrivial laboratory exercise, on which we focus in this paper.
This work introduces a method called reballing that will produce 300 microns beads from 25 to 45 microns balls. The proposed process is based on the use of a reballing stencil. We analyse the influence of the temperature descent curve during welding, and explore its effects on the final weld's quality. Finally, we will verify the compliance of our low-cost (less than €40), low temperature (138 °C), curve-optimised reworking process on micro-BGA components.