Moisture Sensitivity Levels and Bake Requirements for SMT

Plastic packages absorb water from the air, and that water turns to steam when the package passes through a reflow oven. The pressure has to go somewhere: it either escapes slowly through the moulding compound or it lifts the die attach, cracks the body or delaminates the interface between the lead frame and the plastic. The moisture sensitivity level printed on the reel label is a shorthand for how much of that risk a part carries, and the bake requirement is the instruction that follows from it.

What Absorbed Moisture Does at Reflow

Every organic material in a package takes up moisture until it reaches equilibrium with the surrounding air. The amount depends on the material, the temperature and the relative humidity, and it can be a fraction of a percent by weight, which does not sound like much until the water flashes to steam inside a sealed volume.

The damage that follows is known as popcorning. It ranges from an audible crack at the oven exit to internal delamination that leaves no external mark at all, and the parts that look perfect may still fail later, because the crack path runs through the wire bonds or the die attach rather than across the package surface.

How Levels Are Assigned

The levels are defined by the time a part may be exposed to a standard workshop atmosphere before it needs a bake, measured from the moment the dry pack is opened. A part with a short floor life is more sensitive to moisture and needs tighter handling, while a low absorption package may be rated for unlimited exposure once it has been through the oven.

The rating is established by the manufacturer with a soak and reflow test, and it is tied to a peak body temperature. A part qualified at a lower peak may fall into a worse level when the assembly profile changes, so the level belongs with the process window rather than only with the component datasheet.

Floor Life and the Clock

Floor life is a budget of exposure time, and it accumulates across every step instead of resetting when the parts move to another machine. Kitting, placement, a partial build that is set aside for a shift and a rework visit all draw from the same account.

The practical control is to record when the dry pack was opened and to keep that record with the kit. Without it, the only safe assumption at the end of a long build is that the budget has been spent, and the parts are baked even when they may not have needed it.

Dry Packing and Desiccant

A dry pack is a sealed barrier bag holding desiccant and a humidity indicator card. The card is part of the system rather than a decoration: the colour of the dots states whether the bag held its seal, and a card that has changed colour means the parts should be treated as though they have been exposed for the whole time.

Opening a bag does not start the damage, it starts the clock. The correct response is to plan the build so the parts are consumed inside the floor life, and to reseal what remains with fresh desiccant, because an open bag in a humid room is simply a slow bake in reverse.

When a Bake Is Required

A bake is required when the exposure budget is exhausted, when the humidity indicator shows that the bag failed, when parts have been stored in an uncontrolled atmosphere, and after any process that introduces moisture such as aqueous cleaning followed by a slow dry.

The temptation is to bake everything as a precaution. That is expensive and it is not free of risk, because a bake drives solderability backwards and consumes shelf life of the finish, so parts that did not need drying come out of the oven harder to solder than they went in.

Bake Conditions and Limits

The bake temperature and duration come from the supplier, and the usual options are a long, low temperature bake that suits most parts and a short, high temperature bake that puts the terminations at risk. The low temperature version takes many hours and requires the parts to be spread out rather than left in a tube.

Carriers and trays have their own temperature limits, and a tray rated for transport can deform in an oven. Reels are the classic case, since most are not rated for baking at all, so the tape has to be moved to a heat resistant carrier before the parts go in.

Rework, Hand Assembly and Partial Builds

Rework is where moisture control usually breaks down, because the board has already been through the oven and the remaining parts are assumed to be safe. Any component that has been in store, on a bench or in a feeder draws on its own budget, and a hand soldering operation adds local heating that no controlled profile would apply.

Partial builds are the other weak point. A board that is half populated and waiting for a shortage part sits in the open, and the clock runs while it waits. Either the board is stored dry, or the remaining components are baked before the second pass.

Reading the Labels and the Humidity Indicator

The label on the bag carries the level, the floor life, the peak temperature used for the rating and the bake conditions. Those four items are what the line needs, and a label that has been separated from the parts is worth nothing, which is why the information is normally copied onto the kit paperwork at receiving.

The humidity indicator card has three dots that change colour at different humidities. One changed dot may still be acceptable, while a fully changed card means the desiccant is spent and the parts must be treated as exposed, regardless of how long the bag sat on the shelf.

Controlling the Process on the Line

Control means a dry store with a log, a policy for opening bags, a clock that follows the kit and a defined action when the budget is used up. None of it requires special equipment beyond a dry cabinet and a calibrated oven with a record of every bake.

The payoff appears as an absence of problems: fewer cracked packages after reflow, fewer field failures traced to delamination, and fewer arguments about whether a batch of parts was handled correctly. The related defect patterns are collected in solder defects and board failures.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Can a part be baked more than once? It can, but each bake costs solderability and the supplier usually states a limit. Repeated bakes on the same parts should be read as a sign that the handling process needs fixing rather than as a routine remedy.

Does a low level part always need a dry cabinet? Not necessarily. A part with a long floor life can be handled in a controlled room without a cabinet, provided the exposure is recorded and the build finishes inside the budget. Cabinets are needed for the sensitive parts.

What happens if a part is reflowed wet? The water turns to steam and the pressure finds the weakest interface in the package. The visible result may be a crack in the body, but the more damaging outcome is internal delamination or a lifted die attach that only appears in later testing.