Component Floor Life and Moisture Sensitivity Control

A moisture sensitivity level rating describes what happens to a plastic package when it absorbs water and is then heated. The absorbed water turns to steam inside the package, and if the pressure is high enough the package cracks, or the internal bond lifts, or the wire bonds break. The rating is not a property of the component in isolation; it is a statement about a combination of storage, floor life and reflow, and each of those is under the control of whoever is building the board.

What the MSL Rating Actually Says

The moisture sensitivity level is expressed as a number from one to six, with a lower number meaning a less sensitive part. It combines two pieces of information: the conditions the part must be stored under, and the time it may be exposed to ambient after the packaging is opened. An MSL 3 part, for example, may be exposed for 168 hours under the stated conditions after the packaging is opened, provided it was stored dry beforehand.

The exposure time is a budget, not a deadline, and the conditions matter as much as the hours. A part left at 30 °C and 60 % relative humidity absorbs water far faster than the same part at 22 °C and 30 % humidity, and the standard conditions under which the rating was measured are usually 30 °C and 60 % relative humidity. Where the shop is hotter or more humid than that, the effective floor life is shorter than the printed figure.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Three-green-PCBs-with-diverse-components-prominently-displaying-LED-technology.webp" alt="Humidity indicator card being read as a moisture barrier bag is opened” />

Floor Life, Reset and Accumulation

Floor life accumulates across the exposure periods a part experiences. A reel that is opened for two days, returned to a dry cabinet and opened again the next week has spent two days plus whatever the second period adds, and it does not start again from zero. This is the reason a simple rule of counting days since the bag was opened is not sufficient, and why a record of the exposure accumulation is worth keeping for parts that have been opened more than once.

The budget can be reset by a dry-storage period long enough to remove the absorbed moisture, or by baking, or by resealing the part with a fresh desiccant and re-reading the humidity indicator. Any of these can reset the clock where the process is managed; none of them resets it where the part has simply been left on the bench. The reset method should be defined in the procedure, and the trigger for it should be a humidity indicator reading rather than a calendar.

Dry Storage Cabinets and Their Limits

A dry storage cabinet holds the parts at a low relative humidity, typically below 5 % for a nitrogen cabinet and below 10 % for a desiccant type, which stops absorption and allows the stored parts to be counted as not on the floor. Its capacity is finite, and its failure mode is quiet: a cabinet whose door has been left ajar, or whose nitrogen supply has been interrupted, will hold a humidity reading that looks plausible while the parts are absorbing water.

The cabinet should therefore be monitored with a recorder or with a data log, and the log should be reviewed on a defined interval. The condition that matters is the humidity inside, not the fact that the cabinet is powered, and a cabinet without a recorder should have a mechanical or electronic indicator that is read at each shift. The parts inside should also be labelled with the date they were placed there, because a cabinet used as a general store becomes a place where nobody knows how long a reel has been sitting. The storage discipline that applies to paste applies to components for the same reason: the material changes with exposure and the change is invisible.

Baking: When It Helps and When It Damages

Baking removes absorbed moisture by holding the part at an elevated temperature long enough for the water to diffuse out. The schedule depends on the package thickness and on the body material, and the supplier’s figures should be used rather than a general rule. A thin package may need several hours at 125 °C, while a thick one needs far longer, or a lower temperature for a much longer time.

Baking is not harmless. The temperature is above the glass transition of many moulding compounds, and repeated bakes cause oxidation of the lead finish, which degrades solderability. Parts supplied on tape and reel usually cannot be baked at high temperature because the carrier tape deforms, so the parts have to be transferred to heat-resistant trays or baked at a lower temperature for longer. Each of these is a reason to keep the parts dry in the first place rather than to plan on baking them. The moisture sensitivity level baking guidance gives the schedules and the temperature limits for the common package types.

Humidity Indicators and Their Reading

A humidity indicator card changes colour at defined relative humidity thresholds, and it is the item that tells the user whether the parts inside the bag are still dry. The card has several spots, each with a threshold, and the reading is the highest threshold that has changed colour. A card read in a dark room or against a coloured background can be misread, and a card that has been left in the bag for years can change colour for reasons other than moisture.

The card must be read at the moment the bag is opened and before the parts are removed, and the reading should be recorded. Where the card shows that the threshold has been exceeded, the parts are either baked or placed in dry storage before use, according to the procedure. A bag that is opened and reclosed repeatedly should have a fresh card and fresh desiccant, because the original pair has already absorbed what they can. The reading is the trigger for every subsequent decision, so the record of it is the most important line in the component log.

Packaging, Desiccant and Re-bagging

The bag and the desiccant are part of the moisture barrier, and both have limits. A bag that is punctured or that has been heat-sealed badly will let moisture in, and the damage may not be visible. The desiccant has a capacity measured in grams of water, and once it is saturated it stops protecting the contents, which is why the amount of desiccant has to match the bag volume and the storage period rather than being a handful dropped in.

Re-bagging should use a moisture barrier bag, a quantity of desiccant appropriate to the bag size, and a fresh humidity indicator card, and the seal should be made with a heat sealer rather than with tape. The bag should be labelled with the part number, the quantity, the date of sealing and the exposure accumulated so far, because the next person to open it needs all three. A bag that is reclosed with tape and no card is a part whose history is unknown, and it will either be baked unnecessarily or used with too much moisture in it.

Line Handling Rules

The rules that keep floor life under control are simple and easy to break. Take only the reels needed for the current build out of the dry store. Return them at the end of the shift rather than leaving them on the machine overnight. Record the time out and the time back for each reel. Keep the feeders covered where the tape is exposed, because a reel on a feeder is exposed to the room for the whole of the run.

The last point is the one most often overlooked. A reel that is loaded on a feeder at the start of a long run is on the floor for the entire run, and the exposure accumulates whether or not the parts are being consumed. Where the run is longer than the floor life allows, the reel should be split, or an intermediate return to dry storage should be scheduled. The exposure record should follow the reel rather than the build, so that a reel used across several builds carries its accumulated time with it. The baking schedule for the shop should state when the accumulated time triggers a reset.

Records and Auditing

The record should be able to answer, for any assembly, which components were moisture sensitive, how long they were exposed, whether a reset was applied, and what the humidity indicator read when the bag was opened. That is the evidence that the parts were within their rating at the moment of reflow, and it is the evidence a customer audit will ask for when a package crack is reported.

Auditing the process periodically, by checking a small sample of reels against their records and by checking the dry cabinets against their logs, catches the drift that build pressure creates. The audit should also check the physical state of the bags and the desiccant, because a process can be documented perfectly and executed carelessly. Where the audit finds a reel without a record, the honest response is to treat the reel as unknown and to apply the reset procedure, rather than to assume that the elapsed time was short. The maintenance schedule should include the calibration of the cabinet monitors, since the whole system depends on their readings.

FAQ

Does a part in a dry cabinet count as being on the floor? No, provided the cabinet is within its humidity limit and the parts are in their original packaging. That is the purpose of the cabinet: it stops the clock. Where the cabinet is out of specification, the parts inside should be treated as though they had been exposed.

Can a part be baked more than once? It can, but each bake oxidises the finish and consumes some solderability. Where a part has been baked two or three times, its solderability should be checked before it is used, and a part that is repeatedly exceeding its floor life is a sign that the storage arrangement needs to change rather than the baking schedule.

Is the humidity indicator card sufficient on its own? It is the necessary trigger, but it describes the air in the bag rather than the water in the parts. A card that reads dry after a long period in a warm room may still be accompanied by parts that have absorbed some moisture, which is why the exposure record and the card are used together.