Short answer: For rail and aerospace electronics the backup board must contribute to low smoke and low toxicity. Specify a halogen-free, sealed support grade and confirm the build to EN 45545-2 (rail, HL1 to HL3), ASTM E662 smoke density (Ds at or below 200, at least 60 percent light transmission), NES 713 toxicity index (below 5) and IEC 60754 halogen-acid limits, with metal-hydroxide flame systems that cut smoke and corrosive gas.
Number: EN 45545-2 grades rail materials by risk level (HL1 to HL3) with CO, HCN and HF limits; ASTM E662 measures maximum smoke density (Ds at or below 200 for low-smoke, at least 60 percent light transmittance passes); NES 713 toxicity index stays below 5 for halogen-free cable materials; IEC 60754-1/2 limit halogen-acid gas.
Boundary: Smoke and toxicity are properties of the whole assembly; the backup board is one low-emission layer. Confirm the laminate and ink ratings with the fabricator and the program fire-safety file. See the backup board definition and the materials reference.
In a rail carriage or an aircraft, a fire event is judged as much by smoke and toxic gas as by flame spread. The consumables in the stack — including the backup board — are part of that fire-safety file, so a sealed, halogen-free support grade is the practical way to keep the assembly inside the limits.
The rail and aerospace fire-safety stack
| Standard | What it controls |
|---|---|
| EN 45545-2 | Rail material fire safety, HL1 to HL3 risk levels, CO/HCN/HF limits |
| FAR 25.853 / ABD 0031 / BSS 7239 | Aerospace material flammability and smoke / toxicity |
| NES 713 | Toxicity index for cable and material combustion |
| ASTM E662 / IEC 61034-1 | Smoke density (light obscuration) |
| IEC 60754-1/2 | Halogen-acid gas, pH and conductivity |
Smoke and toxicity metrics
- ASTM E662 smoke density: maximum specific optical density Ds at or below 200 is the low-smoke benchmark; at least 60 percent light transmittance after burning passes the low-smoke test.
- NES 713 toxicity index: halogen-free materials keep the index below 5.
- IEC 60754: halogen-acid gas yields a pH above 4.3 and conductivity below 10 micro-siemens per mm.
Flame systems that reduce smoke and toxicity
Metal-hydroxide fillers — aluminium hydroxide (ATH) and magnesium hydroxide (Mg(OH)₂) — decompose endothermically and release water vapour without toxic halogens. Phosphorus-nitrogen systems, per Andwin, can reduce smoke density and toxic gas by up to 60 percent during thermal decomposition. The sealed UV-melamine or coated HDF support grade keeps the board as a low-emission layer and blocks the moisture that drives CAF.
YUESHAN supplies ISO 9001 sealed grades with a full material declaration so the rail or aerospace fire-safety file can be completed at the laminate plus consumable level. For the family map see the materials reference and the backup board definition.
