Dry Cabinet Storage: 5 Rules for MSL Parts

A dry cabinet is a closed storage unit that holds relative humidity below the level at which a moisture sensitive component keeps absorbing water. It is not a convenient cupboard for open reels; it is process equipment with a set point, a record and loading rules, and it only does its job when those three things are respected.

The dry cabinet matters because floor life is the clock that decides whether a part has to be baked before reflow. Storage below the threshold pauses that clock, while storage in ordinary room air lets it run, and a part that is fitted after its floor life has expired will absorb enough moisture to damage itself in the oven.

What a Dry Cabinet Is For

The unit exists to break the link between the moisture barrier bag and the production line. A part is dry when the bag is sealed, and it begins to absorb water the moment the bag is opened. A dry cabinet gives the shop somewhere to put the opened reel where the absorption stops instead of continuing.

The second purpose is organisation. A cabinet holds the parts that are in use, keeps them visible and keeps the reels labelled. That visibility is what makes the floor life clock manageable on a shop floor where dozens of reels are open at the same time.

How Floor Life Works in a Dry Cabinet

Floor life is defined by the moisture sensitivity level of the part and by the conditions in the room. A level 3 part has 168 hours of exposure at 30 degrees Celsius and 60 percent relative humidity, while level 4 allows 72 hours and level 5 allows 48. The clock is cumulative rather than continuous.

Storing the part below the low humidity threshold suspends that clock, and the accumulated time is reset when the part is returned to the cabinet. The details are set out in the IPC/JEDEC J-STD-033 handling standard, which also defines the conditions under which the reset is allowed.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/High-Speed-Printed-Circuit-Board.jpg" alt="Humidity indicator card and desiccant beside a moisture barrier bag” />

Choosing and Setting the Cabinet

Size the cabinet from the number of reels that are open at once, not from the total inventory. A cabinet that is too small forces reels back into bags and out again, which defeats the purpose. A cabinet that is too large costs more to keep dry, especially where a nitrogen purge is used.

The set point depends on the level of the parts inside. A common working value is below 5 percent relative humidity, which is what the standard requires for the floor life reset. Cabinets that hold 10 or 20 percent are useful for slowing absorption, but they do not provide the same reset.

Loading Rules and What Not to Store

Load the cabinet with parts only. Cardboard, paper documents and wooden items release moisture and hold dust, and a box of paperwork inside a dry cabinet is a load on the unit and a source of particles. Tools and hand cream belong somewhere else.

Do not mix levels without labels. A reel from a level 5 part and a reel from a level 2 part look identical, and once the labels are covered nobody can tell which clock applies. Where a part must be traced, the floor life control procedure should require a label with the date the bag was opened.

Monitoring Relative Humidity and Records

The cabinet needs a relative humidity indication that is readable from outside, and a logged value is better than a display alone. A cabinet that is opened forty times a day may spend part of every hour above its set point, and only a log shows how long that lasts.

The sensor has to be calibrated on a schedule like any other instrument. A drifting sensor is worse than no sensor, because it gives the shop confidence in a number that is wrong. The bake requirement decision depends on that reading, so the calibration record belongs with the cabinet.

Desiccant, Indicator Cards and Bags

Desiccant keeps a sealed bag dry, and it does not dry a part that has already absorbed water. When a bag is opened, the indicator card tells the operator what happened during shipment and storage. A card that shows the sensitive spot darkened means the parts inside have been exposed to moisture and the bake decision has to be made.

Keep the bags and the desiccant with the reels. A reel that is returned to a dry cabinet does not need a bag, but a reel that leaves the building does, and the moisture barrier bag plus desiccant plus card is the only packaging that travels well. The desiccant storage rules cover resealing in more detail.

When a Part Must Be Baked

Baking is required when the floor life has expired, when the indicator card shows excess moisture, or when the storage history is unknown. The schedule comes from the standard and depends on the package thickness and the level, with a high temperature option that is faster and a low temperature option that is safer for the reel and the tape.

A dry cabinet is not a bake oven. Holding a part at 40 degrees in a dry cabinet removes surface moisture but does not drive water out of the mould compound, and the moisture sensitivity level tables assume a real bake. Where the bag is opened and the parts will not be used for a while, resealing is cheaper than baking later.

Common Mistakes on the Shop Floor

The most common mistake is treating the cabinet as a general store. Once wipes, documents and unlabelled bags are inside, the set point is being spent on material that does not need it and the parts that do need it are crowded out. The second mistake is leaving the door open while a panel is being set up.

The third is a lost clock. A reel that is taken out for two hours and returned without a note has lost the continuity of its floor life, and the next operator has to guess. A simple log at the cabinet door, with the reel number and the time out and in, is enough to keep the record honest.

Auditing the Dry Cabinet Every Week

A short weekly audit keeps the dry cabinet honest. Check the logged relative humidity for gaps and for peaks that follow busy periods, confirm the sensor against a reference, inspect the door seal, and read a few labels to see whether the dates are being written down.

The audit should also look at what is inside. Material that does not need dry storage is a load on the unit, and an unlabelled reel is a floor life clock that nobody can read. Both findings are quick to correct, and both stay invisible in the production record if nobody looks for them.

FAQ

Does a dry cabinet reset the floor life of every part? It resets the clock only when the humidity is low enough and the rules of the standard are met. Parts that were stored above the threshold before they went in keep the time they had already accumulated.

Can a parts bag be stored inside a dry cabinet? Yes, and a sealed bag inside a cabinet is well protected. What matters is that the bag is labelled with the part number, the level and the date, and that the cabinet is not loaded with material that does not need dry storage.

Is nitrogen required in a dry cabinet? Not for the floor life reset, which is defined by relative humidity, but a nitrogen purge makes the set point easier to hold when the door is opened often. The trade is gas cost against the recovery time of the cabinet.