SMT & Inspection | July 10, 2009

Reconsidering flame retardants in electronics equipment

Accurate figures are difficult to obtain, but it has been estimated that fires kill up to 100 000 people annually worldwide, of which 10% are attributed to electrical faults, and account for 19% of injuries.
Flame retardants are used to prevent fires, reduce their seriousness and to delay onset and equipment manufacturers are required by law to use them.

The recent review of the RoHS Directive recommended the restriction of some of them and this would have an impact on design engineers. Flame retardants are used as additives to plastics in a wide variety of electronics and electrical equipment. Many types of plastics burn very easily and it has been estimated that the plastics in a typical TV set are equivalent to 6 litres of petrol!

What types are available?
There are over 100 different types of flame retardants available; the main types and their characteristics are:

- Brominated flame retardants: They are the only types that can be used with HIPS (high impact polystyrene) and ABS (acetonitrile Butadiene styrene). The family includes reactive flame retardants that react with other ingredients in plastics to create brominated polymers, which are stable and do not leach into the environment.
o Tetrabromobisphenol A (TBBP-A): is used in the largest quantities of the brominated flame retardants, with 90% being used relatively in FR4 epoxy resin PCB laminates. It is also under the spotlight of the RoHS Directive and may be restricted.
o Antimony oxide: Always used with either brominated flame retardant or chlorinated polymers and effective at low concentrations, so the amounts of brominated flame retardant used can be cut.

- Phosphorous: Used as replacements for brominated flame retardants and can be equally effective in some types of polymer, but are not suitable for HIPS or ABS. Most are new and have not been thoroughly tested.

- Chlorinated: Short chain are PBTs (persistent, bioaccumulate and toxic) and endocrine disrupters and are already restricted in the EU. Medium chain has similar toxicity and the EU plans to restrict this material. The risks from the long chain chlorinated paraffins are not yet know.

- Metal hydroxides: Aumina trihydrate (ATH) can be used in many types of plastic and is one of the cheapest options. To achieve UL94 Vo requires very high loadings so that ‘plastics’ are more ATH than polymer with 60% ATH typically being used.

Flame retardant selection & design implications
The flammability of plastics varies, depending on the amount of oxygen from the air that the plastic needs to burn, referred to as the ‘Limiting Oxygen Index’ or LOI. Air contains 21% oxygen and any plastic with a LOI value less than 21 will burn and may need flame retardants to meet safety standards. Plastics with higher values may still require flame retardants if they contain flammable additives that lower their LOI.

The types of plastics with high LOI may either not require flame retardant additives or require smaller amounts than low LOI plastics. Most high LOI plastics are more expensive than low LOI plastics, the exception being PVC. However, PVC is under pressure from environmental groups, because toxic dioxins and furans are produced when PVC is burning using unsuitable and uncontrolled recycling.

At the timing of this writing, no announcement around the scope of RoHS2 has been made, but it is anticipated that the restriction of certain flame retardants and plasticizers, both extensively used in plastics, will be included in the revised scope.

Author: Gary Nevison, Legislation & Environmental Affairs Manager, Premier Farnell
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