Plasma Ashing as a Fire Investigative Tool

Mark Goodson PE
Lee Green PE
Michael Shuttlesworth PE
Goodson Engineering
Denton, Texas USA

Presented at International Symposium on Fire Investigation, 2014

One of the difficulties that has faced engineers who examine electrical or mechanical items and / or devices after a fire is that of cleaning the item. The key adage in cleaning is essentially a medical command – primum non nocere, or “do no harm.” The cleaning technique will preferably cause no damage to the artefact being examined. Successful cleaning allows for both microscopic, visual, and SEM / EDX analysis.

In a fire, it is not uncommon for fire artefacts (wires, as an example) to require cleaning. Historical cleaning techniques have relied upon ultrasonic cleaning as a means for debris removal. Ultrasonic cleaning makes use of mechanical (sonic) energy to cause debris to dislodge from artefacts. How successful this technique is depends (in part) upon the energy imparted, the solvent used, and the interface between the wire and the debris. In the case of partially pyrolyzed PVC insulation, there are conditions that occur (depending upon the state or extent of pyrolysis) where no amount of mechanical agitation will remove the fire debris.

Oxides can be removed from wires by the use of surfactants or cleaners, some of which can have an etching effect on the metal. Treatments such as Alconox  or Simple Green  sometimes work sufficiently, while a more aggressive oxide remover (Branson OR)  relies on citric acid to help clean the wires. With more aggressive reagents, the user runs the risk of etching the metal and ruining the surface finish.

The writers describe a technique for removing fire debris from metal objects (wire, CSST) for use in removing fire debris. The technique is referred to as plasma ashing . In plasma ashing, a vacuum is created around the artefact, and a carrier gas is introduced (such as O2). An RF field (13.56 MHz) is applied, and the oxygen takes on a monatomic state. Essentially, a plasma i s created, and the monatomic O is free to react with organics associated with the fire debris. This process is also referred to as a glow discharge . The end result is that organics are removed from the artefact, and the ashing takes place at low temperatures – sufficiently low such that grain structure of the metal is not changed. This technique is essentially what is used in one of the manufacturing steps for making integrated circuits (ICs). As such, it imparts sufficiently low energy such that crystalline semiconductor structures are not damaged.

We compare and contrast plasma ashing with other modalities of cleaning. More particularly, we note (through visual microscopy) the efficacy of ashing and ultrasonic cleaning, as well as material removal rates. We show that despite its relative expensive capital costs, ashing represents a cleaning technique that does what other modalities fail to do – consistent removal of organic debris with no damage to the underlying substrate.

Download the complete paper here