" Plastics used for insulating or mounting live components like connectors, must retain other key properties whilst meeting stringent flammability requirements aided by the application of fire retardants. "
Connectors or contactors are small plastic pieces linking printed circuit boards, wires, cables, or other parts of electronic devices, both internally and externally. From mobile phones, laptops or TV sets to refrigerators and washing machines, connectors are critical for the functioning of all electrical appliances.
From a manufacturing perspective, the main challenge lies in the downsizing of:
• external connectors (power, USB, HDMI, etc)
• internal connectors (between the printed circuit boards, HDD, or screens of E&Es).
As connectors get smaller and their designs more complex, the polymers injected into very small moulds with complex shapes must have low melt viscosity; be able to withstand processing temperatures of up to 300°C and have the required physical properties in the finished part.
Material challenges in connectors
UL 94 V-0 is the reference fire performance for electrical connectors. In addition, the IEC standards 60695- 2-11 to 13 (GWFI and GWIT) establish the requirements for glow wire resistance of the plastics used for insulating or mounting live components. The use of flame retardants is therefore essential.
While meeting such stringent flammability requirements, connectors must also retain other properties such as high melt flow, high tensile strength, elongation and impact strength, high comparative tracking index (CTI) and low corrosivity during the moulding process.
Engineering thermoplastics are the most commonly used polymers in connectors due to their high-flow and high thermal stability. They also have some of the highest processing temperatures of all polymers used in E&E devices. Therefore flame retardants used in engineering thermoplastics need to have excellent thermal stability. Just as for printed circuit board laminates, higher lead-free soldering temperatures mean an additional thermal stability challenge. These higher temperatures must not affect the physical properties of the connector or the flame retardant integrity during the soldering process.
Connector manufacturing involves very sophisticated material technologies from both a polymer and a flame retardant point of view, as the resulting material must ensure:
- Excellent electrical performance
- good signal integrity
- low conductivity
- Excellent physical performance
- high impact strength,
- high tensile, flexural strength,
- high heat deflection temperature (hdt)
- High fire-resistance
- UL94 V-1 or V-0
- IEC 60335 and 62368 specifications, typically power connectors, Cpu, ddr, large SATA (Serial Advanced Technology Attachment), large card sockets 
- Excellent thermal stability at high processing temperatures
- High precision moulding of large dimension connectors
- Good retention of properties after ageing at operating temperature, in view of the average lifetime of the devices, without losing flame retardancy, electrical or mechanical properties
- Good recyclability of scraps generated during the production of connectors.
In connectors, the physical characteristics of thermoplastics are often strengthened by the addition of glass fibre (loading: 20-40 % wt.). In certain cases, that addition can make the materials even more flammable than the virgin material. Flame retardants will, in addition, typically represent 10 to 25% of the connector weight. Consequently, in a typical glass-reinforced, flame retarded connector, the polymer itself only represents between 40 and 70% of the material.
Use of Flame Retardants
Polymeric brominated compounds
The use of high-temperature resistant brominated polymers, especially Brominated Polystyrene (BPS), Polybrominated Styrene polymers (PBS), Brominated Epoxy polymers (BEs), Brominated Carbonate Oligomers (BCOs) and brominated polyacrylate is widespread in engineering resins such as (glass filled) Polybutylene Terephthalate (PBT), Polyamides (PA6, PA6.6) and Polyethylene Terephthalate (PET).
For High-Temperature Polyamides (HTPAs) only Brominated polystyrene has the required thermal stability.
These resins are often considered as industry standard due to their tensile strength, elongation, impact strength, high GWIT, high CTI and low corrosivity. Flame retardants are typically added at 10-25% loading by weight, together with 4-7% synergist (typically antimony oxide or sodium antimonate), and demonstrate very good mechanical and electrical performance in those resins. Brominated polymers can also act as a processing aid for the manufacture of connectors, by increasing the fluidity and shock resistance of the final products.
Lower molecular weight brominated flame retardants can also be used in connectors. Ethane Bis (Pentabromophenyl) (EBP) or Ethylene 1,2 bis (tetrabromophthalimide) (EBTBP) display high efficiency both in filled and unfilled PBT and PET. EBP is also used in unfilled and filled PA6 and PA6.6.
Phosphorus- and nitrogen-based compounds
Nitrogen flame retardands are often used in polyamides in combination with phosphorous compounds (such as Red Phosphorus or metal phosphinates). Melamine Cyanurate is a widely used example, other nitrogen synergists such as melamine polyphosphates can also be used. These flame retardant systems usually require loadings of 20 to 30% by weight in order to achieve UL 94 V-0 or V-1 flammability performance.
When using nitrogen compounds in conjunction with phosphorous compounds in materials other than polyamides, mechanical properties such as tensile strength, elongation and impact strength need to be closely monitored in addition to colour and thermal stability. Also, the GWIT 775°C requirement can be more difficult to achieve. Attention should be paid to mould deposits and mould corrosion issues, since these can significantly reduce the life expectancy of the moulds and hence increase costs.
Organic metal phosphinates such as Diethylphosphinic acid, Aluminium salt (AlPi) or Zinc salt (ZnPi) with high phosphorus content are used in different engineering polymers like PBT, PET, PA6, PA6.6 and HTPA. They are used alone, or with different nitrogen synergists (typically Melamine Polyphosphate) and/or zinc borate.
An elemental source of phosphorus (Red Phosphorus) can also be used as a flame retardant. Its usage in glass-reinforced Polyamide 6.6 is well established for dark colour applications. In order to cope with the potential release of toxic phosphine, stabilised grades of red phosphorus have been developed and are also available in 50% active masterbatch in a polyamide carrier.
In certain applications, Magnesium Hydroxide (MDH) can be used as a flame retardant filler in glass reinforced polyamides. In this case, the flame retardant will represent up to 55% of the connector’s weight, and the mechanical and electrical properties must be carefully controlled. Also, the temperature stability of MDH tends to limit its use in glass fibre reinforced Polyamide 6 (~280°C processing). MDH can however be used at lower loadings in combination with brominated polymer solutions, with good mechanical and flammability performance, i.e. high GWIT, high CTI, low smoke levels, and low corrosivity.
Inherently flame resistant polymers
In addition to the most common solutions presented above, some relatively high cost polymer systems are inherently flame retardant. The most common examples are Liquid Crystal Polymer (LCP) and Polyphenylene Sulfide (PPS). While these are very valid solutions, they are relatively expensive, and may require post processing in connector applications.
Connectors and contactors: a challenging fire safety issue
Franck Poutch and Skander Khelifi, CREPIM
Connectors can potentially be the weak point of any electric device. They involve a complex association of dielectric (engineering thermoplastic resins for low voltage alternative/direct current) and metal parts. High voltage is not necessary to develop fire due to electric malfunctioning, but a local increase in resistance can create a source of ignition.
While the ignitable material is definitely plastic, the actual ignition cause can be linked to both metal and plastic parts. For metal parts, causes can include a copper/alloy oxidation that reinforces the resistance of the contact. That resistance can cause an arc creation during switching off, with risk of arc propagation, or an electric overload that melts the metal and the dielectric together and sticks the contactor.
Regarding the plastic, causes are numerous. They can include the (in)ability to bear short overload without melting or degrading. Heat exposure over time can also modify the dielectric strength of the resin and create new bypath ways for current. Water diffusion via cables can create an electrolytic effect and develop wet tracking. Constant switch on/off can lead to soot deposits over time, cause of surface arc tracking. Progressive off-matching of male and female parts (vibration, dilatation...) can finally create an increasing gap, enhancing resistance and heat dissipation.
All this holds true as long as the operating temperature remains below 960°C. During trials, we found traces of melt copper in the connectors due to high intrinsic self heating that finally causes ignition - copper melting temperature is 1083°C! Solutions currently available cannot guarantee a 0% fire risk, and the high temperature self heating cases certainly need to be understood. This is even more so as devices will increasingly need high value of current during short time periods - due to functions being activated - causing a specific but real fire risk.
1 IEC 60695-2-11 Fire hazard testing - Part 2-11: Glowing/hot-wire based test methods - Glow-wire flammability test method for end-products IEC 60695-2-12 Fire hazard testing - Part 2-12: Glowing/hot-wire based test methods - Glow-wire flammability index (GWFI) test method for materials IEC 60695-2-13 Fire hazard testing - Part 2-13: Glowing/hot-wire based test methods - Glow-wire ignition temperature (GWIT) test method for materials.
2 IEC 60335: Household and similar electrical appliances – Safety IEC 62368: Audio/video, information and communication technology equipment.
3 GWIT 775°C requirement as specified in IEC 60335-1 (2001).