Which humidity grade should I specify — A, B or C?

The YSM series comes in three grades, identified by the letter in the model number: Grade A — 20–60% RH. General component and stock storage. Prevents mould, condensation and the solderability loss that comes from sitting in humid air. Grade B — 10–20% RH. For parts with moderate moisture sensitivity: finished PCBs, connectors and optical components. Grade C — 1–10% RH. For the most sensitive work — IC trays, wafers, precision optics and lithium cells — where 10% RH is still too wet. Matching the grade to the part matters in both directions: over-drying wastes capacity on materials that do not need it. If a specification names a humidity window, we build to that window instead.