Technology: Crystalline Dust Growth

FCL Components research sheds light on crystalline dust growth in relay housings

Reliability is one of the key attributes of relays used in automotive applications. One phenomena that compromises reliability is the growth of needle-shaped dust crystals that form due to the sublimation and deposition of flame retardants used in the relay housings. Although the accumulation of dust and its negative impact on electrical performance has been known for quite some time, no in-depth research had been conducted so far. In a recently published whitepaper, FCL Components now shares its findings in how crystals form, how they impact contact behavior, and which measures can be taken in future designs to reduce their effects.

Due to the electrification of cars, one of the main trends in automotive relay design is an increase in electrical load conditions. This rise in voltage and current requires more robust designs that withstand stronger arcing effects without compromising reliability. Add to this other challenging specifications such as miniaturization and a steep increase in required switching cycles, and it becomes clear that each improvement matters. For FCL Components, this was the reason to finally get to the bottom of a promising (advance in) quality enhancement: eliminating degradation due to dust collected on the relay contact points. In order to achieve this, a study on how dust is formed and distributed, as well as its effects on electrical performance, had to be carried out first.

Needle growth and impact

From the start of the study, it was clear that the dust collected between anode and cathode of relays was caused by the sublimation of the flame retarder used in the relay housing. Under the influence of temperature fluctuations, the flame retarder Polybutylene terephthalate (PBT) forms solid Pentabromo benzyl alcohol (PBBA) particles of a crystalline structure. The mechanism behind the distribution was however unclear. For this, the density in gaseous state had to be compared to that of air, and the effect of temperature on growth direction needed to be examined. Only then could the impact of the dust deposition be measured and understood, and new design principles be proposed.

Conclusions

The way in which all of these things were researched and the outcome of several laboratory experiments can be found in FCL Components' recently published and peer-reviewed whitepaper ‘A Study on Growth of Dust of Needle-like Crystal from a Flame Retarder of Electromagnetic Relay Housing’. The most important findings however are that the vapor from the flame retarder is actually heavier than air, resulting in an accumulation of dust in the lower regions of the relay housing. This means that by paying close attention to contact positions and directions in the design phase, substantial contamination can be prevented. Also, under the effect of PBBA needles, the duration of arcing increased, having a negative effect on the durability of the contact points due to increased erosion. As expected, PBBA also proved to have a negative effect on the contact resistance. Although PBBA-covered contact points will still act as a conductor, the contact resistance becomes more unstable with the accumulation of more dust.

For more information:

A Study on Growth of Dust of Needle-like Crystal from a Flame Retarder of Electromagnetic Relay Housing (Open IEEE website)