Al Brown looks at the hazards associated with process liquid chemical heating and guidance that can help mitigate the risks
On a Saturday afternoon in December 2000, a fire started in a plating shop in Glasgow. Black smoke was seen issuing from a fume scrubber outside the plating factory and the smell of burning plastic could be detected in the air.
The fire was subsequently put out by the Strathclyde Fire Brigade, but not before smoke spread throughout the shop floor and office area. The fire started in an electrically heated plating bath following the failure of the automatic refill system, leaving the heating element uncovered. Although damage was limited, involvement of the polypropylene plating tank and other plastics, resulted in smoke contamination to equipment throughout the production area requiring extensive specialist cleaning. Apart from the fact that there had been a near miss the previous weekend on an adjacent bath, the incident was preventable. Fires in plating operations are a common feature of the industry despite the existence of systems and technology to prevent them. Immersion heaters for plating operations are normally submerged in the processing solution, but if the liquid level drops below the heater while still energized, ignition of the tank wall or liner can follow. Fires occur when heaters are left energized during idle periods or were activated by a timer several hours before the beginning of operations.
Advisory documents There are documents advising on safety interlock design for heated processes in plastic tanks. FM Global specifically addressed the issue in Loss Prevention Data Sheet 7-6, 'Heated Plastic and Plastic-Lined Tanks', in 1976. When it was recognised that the semiconductor industry had adopted similar processes in cleanrooms, similar recommendations appeared in FM Data Sheet 7-7 'Semiconductor Fabrication Facilities' and NFPA 318 'Cleanrooms' and in SEMI S3-91 'Safety Guidelines for Heated Chemical Baths'. There are slight differences between the standards but the fundamental principles remain the same. In particular, they call for hardware-based interlocks that are independent of the process control system. This ensures that should the process temperature system fail, then a separate system will detect process liquid over-temperature and de-energize the heating system. In addition the guidelines usually ask for at least one independent low liquid level interlock to be installed to detect a loss of liquid while heating is taking place. However, simply installing these safety features will not always prevent fires; they need to be maintained, as the investigation into a 1986 electronic industry plating fire in a facility in England, resulting in widespread smoke damage to the plating shop and adjacent areas, revealed. The smoke from the burning plastic plating tank spread through the gaps between the profiled roof sheeting and the walls around the plating shop. The fire was caused by a failure of the low liquid level device, allowing the electric heater to ignite the plastic tank. The contacts on the float switch had been affected by the process chemicals and prevented the signal being sent as the float switch fell. Had appropriate maintenance been carried out, the fire could have been prevented. Investigation of these and related fires have identified important factors in the design of process liquid heating systems that can prevent fires. This is emphasised by loss analysis from FM Global, which reinforces the view that losses can be reduced through suitable design using independent interlocks supplemented by planned inspection and maintenance routines. From 1985 until 1999, it recorded 52 losses involving plastic and plastic-lined tanks, at locations involved in metal plating, circuit board manufacturing and semiconductor manufacturing. Electrical immersion heaters were responsible for 61% of the losses and a major factor in most was the malfunctioning of low-liquid level interlocks and/or high temperature limit switches. These interlocks were often poorly maintained and as a result did not function properly when needed.
From PCB to silicon chip Production processes used in PCB plating shops are essentially the same as the processes carried out in the cleanrooms of semiconductor wafer fabs. Only size, purity and cleanliness has changed. Electric immersion heaters have, in most cases, been replaced with less hazardous heating systems. However, SEMI S14 equipment fire risk assessments continue to identify newly designed systems lacking independent interlocks for over-temperature and low liquid level, or where the heating element is not adequately separated from the combustible plastic bath or tank. In production cleanrooms, wet benches with electric hot plates are still being used, despite numerous fires over three decades associated with this method of heating. Inherently safe heating systems are available and in use by the industry, as is the use of firesafe plastics that do not propagate flame beyond the ignition zone. Yet equipment is still being made of combustible plastics and fitted with powerful heating systems. As a result the typical plastic wet bench fire still happens, despite available technology to prevent such fires. A wet bench fire in a Taiwanese wafer fab during 1996 showed the damage that can occur when sprinkler protection is not operational. Fire spread through combustible fume exhaust ductwork causing extensive damage throughout the cleanroom. Analysis by FM Global of incidents over a 30-year period has shown that fires in wet benches are almost invariably controlled by one sprinkler head, preventing fire spread to adjacent equipment. But physical damage is not the only issue, as a polypropylene wet bench fire in 1995 clearly shows. Following a fault in the electrically heated quartz bath process bath, the polypropylene wet bench structure was ignited and only controlled by a single ceiling mounted sprinkler head. Although production restarted about two weeks later, following extensive specialist cleaning, it was over 10 weeks before pre-incident production levels were achieved. Such incidents are often detected by cleanroom smoke detection systems in five to 10 minutes from ignition, with sprinkler activation following perhaps three to five minutes later, resulting in almost immediate extinction of a one megawatt fire.
Liquid chemical heating systems As process parameters become increasingly stringent requiring chemicals that are between 7 and 9 nines pure, concerns are growing about contamination of the process from 'out-gassing' and 'leaching' of chemicals from materials used for equipment construction, piping and ventilation. Of particular concern are fire retardant plastics made using elements that also occur in the manufacturing process, such as boron and tin. There are also concerns about using metal piping or containment systems within the process areas of some newer tools. To meet these process challenges equipment engineers are developing machines to heat flammable or combustible liquids in systems that are entirely constructed of non-fire retardant plastics to overcome these contamination issues. Existing safety guidelines usually require metallic containment or piping, so the new design needs a thorough hazard evaluation and risk assessment. In the latest draft of S3, the Task Force identified eight 'Safety Practices and Interlocks' (Fig. 1) that are seen to be essential when designing a safe yet practical process liquid heating system. The applicability of the individual features will vary depending on factors such as the method of heating, the heat transfer fluid properties and the properties of the process chemicals being heated. The draft guideline aims to be non-prescriptive and encourages designers to use risk assessment in the design process so that innovative designs can be achieved in the future to meet the increasingly demanding needs of the semiconductor industry. SEMI S3 will ask designers to prove designs using a SEMI S14 fire risk assessment, a process by which the whole machine and its intended use are subjected to detailed hazard identification and risk evaluation. This fire risk assessment process can be used as a design development tool and provides purchasers with a document confirming the hazards present and design features used to mitigate them. As equipment designs strive to meet the needs of future nano-electronic fabrication processes, this risk assessment approach allows fire engineers to work alongside equipment designers ensuring that designs are not only technologically advanced, but protect cleanroom workers as well as providing an acceptable level of risk for investors and insurers.