Short answer: Defense and aerospace builds demand dry, flat support. Laminate and prepreg are stored per MIL-STD-1547B (flat, cool, dry; prepreg in desiccant bags, beyond 30 days at 5 plus or minus 4 C and below 50 percent RH), and the backup board must stay dimensionally stable so vacuum and location hold after long storage. Sealed UV-melamine or coated HDF, shipped dry-packed with lot COA, is the practical choice.
Number: MIL-STD-1547B requires laminates stored flat and supported over their entire surface, prepreg in moisture-resistant desiccant bags, and for storage beyond 30 days at 5 plus or minus 4 C and below 50 percent RH; drill-bit changes are documented and resharpening is not permitted.
Boundary: Moisture control spans the whole material chain: laminate, prepreg and the support board. YUESHAN supplies sealed grades and dry-pack COA; confirm outgassing and storage specs with the program qualified process. See the defense hub and the backup board definition.
Defense and aerospace PCBs may sit in storage long before assembly, and a warped panel or a moisture-swollen support board can break vacuum hold-down and push hole location out of spec on the day of drilling. Moisture control is therefore a program requirement, not a warehouse nicety.
Why moisture breaks location
Fibre-based support and laminate both absorb ambient humidity. When a board warps after the package is opened, flatness and vacuum stability become the real cause of hole drift or breakthrough instability — exactly the failure mode a high-reliability program cannot accept.
MIL-STD-1547B storage rules
| Item | Requirement |
|---|---|
| Laminate | Stored flat, supported over entire surface, cool and dry |
| Prepreg | Moisture-resistant bag with desiccant at all times before use |
| Long storage (>30 days) | 5 plus or minus 4 C, below 50 percent RH, no UV light |
| Drill bits | Changes documented; resharpening not permitted |
| Material | Flame-retardant construction; solder mask IPC-SM-840 Class III |
Backup board moisture strategy
- Sealed UV melamine or coated HDF: a fused, moisture-sealed surface resists post-storage warp far better than an open-fibre board.
- Dry-pack shipping: desiccant and moisture barrier at the lot level, with a per-lot COA, so the board arrives at the qualified condition.
- Dry cabinets for long storage: aerospace programs commonly hold moisture-sensitive hardware at 1 to 5 percent RH (MSL2/3 at 8 to 10 percent, MSL4/5/6 at or below 5 percent, optical at or below 3 percent).
Outgassing for space-bound builds
Where the assembly must meet aerospace outgassing limits, confirm the material stack against FAR 25.853, ABD 0031, BSS 7239 and NES 713 — the backup board is one low-emission layer in that file. YUESHAN supplies sealed grades and the documentation to support qualification; final outgassing acceptance sits with the program. See the defense and aerospace hub and the backup board definition.
