Fire Damage and Repair Techniques for Flash Memory Modules: Implication for Post-Crash Investigations

Abstract

Abstract As part of the investigation of accidents in mass transit, air travel in particular, the National Transportation Safety Board (NTSB) retrieves electronic devices from the crash scene. Analysis of flash memory recovered from these devices can provide clues as to the events leading to the crash. Unfortunately, the flash memory is often damaged and unreadable. This study was undertaken in order to address two issues: (1) how much damage the device can sustain before it becomes completely unreadable, (2) what repair techniques can be employed in order to retrieve information from damaged memory units. The effect of thermal damage was investigated by subjecting flash memory modules to high temperature for three hours. Memory modules were found to be readable after exposure of up to 300°C for three hours. Small fissures evident in the overmold indicated thermal damage to the plastic, yet the chip retained the stored information. Above 300°C the plastic was completely consumed, leaving only the chip and wire bonds. Repair techniques were investigated by decapsulating the module, then purposely breaking the interconnecting wire bonds. The broken wire bonds were reconnected with metallic ink deposited by a precision printer. After the repairs, the modules were read with no errors, indicating a successful repair. Several methods of decapsulation were tested: acid etch, laser etch, plasma etch, and mechanical polish. Data integrity was retained through each of these decapsulation techniques, except for the acid etch. Plasma etch was not an effective method of decapsulation due to the low etch rate. Mechanical polish proved to be the simplest decapsulation technique. Bonded interconnect wires were first exposed by a grind/polish of the memory module surface. These wires were then “broken” by over polishing the sample. After confirming that the module could not be read, a repair was completed by printing the connections directly on the polished surface of the module. Subsequent re-reading of the stored data confirmed a successful repair. Of the decapsulation techniques employed, the most effective was laser etch. This method allowed targeted overmold removal at a specific location without disturbing the majority of the packaging - a desirable feature when relatively few circuit lines need to be repaired.