Component Baking Guide for Assembly

Plastic packaged components absorb moisture from the air. When they are heated to reflow temperature the water turns to steam, and the pressure inside the package can crack the body, delaminate the die attach or lift the wire bonds. Component baking is the step that removes the absorbed water before the part is exposed to heat, and the rules that govern it come from the moisture sensitivity level of the part rather than from a general procedure.

Why Moisture Matters

A mould compound is not a barrier. It absorbs water slowly through the bulk and quickly through the interface with the lead frame, and the amount that is absorbed depends on the material, the storage humidity and the time.

During reflow the absorbed water expands. The vapour pressure inside a delaminated area can reach a level that is high enough to crack the package, and the damage is described as popcorning because the sound and the appearance resemble the food.

The damage is not always visible. An internal delamination that leaves the exterior intact will pass an optical inspection and fail later, either as an electrical open or as a reliability failure under thermal cycling.

Moisture Sensitivity Level

The moisture sensitivity level is a classification that states how long a package can be exposed to a defined environment before it needs to be dried. A low number means a more sensitive part with a shorter floor life.

The exposure period begins when the original packaging is opened and it stops when the part is reflowed or when it is returned to a dry environment. A part that is taken out, used for a few hours and stored in an open bag has consumed part of its allowance, and the remainder is not reset by putting it back in the bag.

The level is stated on the reel or the tray label and it should be recorded with the receipt. A part whose label has been lost cannot be classified from its appearance, and the conservative response is a full bake using the schedule for the most sensitive level.

Floor Life and Its Control

Floor life is managed with a log. The time out of the dry environment is written down when the part is removed and again when it is returned, and the accumulated time is compared with the allowance. On a production floor this is a simple but easily broken discipline.

Dry storage is provided by a dry cabinet or by a bag with a desiccant. A dry cabinet with a controlled humidity holds the parts in a state that stops the clock, while a moisture barrier bag with a humidity indicator card protects the parts during transport and storage.

A humidity indicator card that has changed colour shows that the bag has been compromised. The parts inside should then be treated as though they had been exposed for a full floor life rather than for the transport time, because the actual exposure is unknown.

The Bake Schedule

The bake schedule is defined by the package thickness and by the body material. A thinner package needs less time, and a thicker one needs more, and the supplier datasheet gives the combination as a table of temperature and duration.

The bake temperature has an upper limit set by the packaging. A part supplied on a reel cannot be baked at the temperature used for a tray, because the carrier tape and the reel will deform, and the low temperature bake takes much longer as a result.

A part that has been baked must cool before it is sealed. Sealing a warm part in a bag traps moisture that condenses inside, and the part then begins the next storage period with water already present, which is worse than the state it was in before the bake.

Baking in Practice

A convection oven with a controlled temperature and a defined airflow is the usual equipment. The parts should be spread so that air can circulate around them, and trays that stack should be separated so that the parts in the middle are not shielded.

The oven should be dedicated or at least verified to be free of contamination. A part baked in an oven that has been used for other purposes may pick up a residue that affects its solderability.

The time should be counted from the moment the parts reach temperature rather than from the moment the oven is switched on. The difference matters for a thick package and for a full load, and a bake that is timed from switch on can be short by a significant fraction.

Records and Traceability

Every bake should be recorded with the part number, the quantity, the level, the temperature, the duration and the operator. The record is the evidence that the part was dried, and it is what allows a batch to be released when a reflow defect appears.

The record should also show the floor life consumed before the bake. A part that was baked after an unknown exposure cannot be shown to be dry, even though the bake was performed correctly.

Where the parts are placed back into a moisture barrier bag, the bag should be sealed with the desiccant replaced and the indicator card checked. A used bag that is resealed without a desiccant will not hold the parts dry.

What Happens When It Goes Wrong

The failure appears at reflow, or soon after. A crack at the package corner, a delamination visible on a scanning acoustic microscope image and a lifted die are the classic outcomes, and each of them corresponds to a different internal path for the water.

The defect rate is not uniform across a panel. The parts that absorb the most moisture, that are the thickest or that see the highest temperature are the ones that fail, which is why an assembly can lose a few parts on every board rather than failing completely.

The diagnosis begins with the bake record. Where the record is missing, the response is to verify the storage condition, to recheck the floor life rules with the operators and to introduce a log that is signed rather than a practice that is assumed.

Practical Rules

Read the level from the label, control the floor life with a log, store the parts in a dry environment and bake to the schedule for the package thickness. Cool the parts before sealing them.

Record the bake and the floor life with the build records and the defect history, and review the moisture control rules and the shelf life management whenever a reflow defect pattern suggests absorbed water.

FAQ

Why does an absorbed moisture part crack during reflow? The water turns to steam and the pressure inside the package rises faster than it can escape. The result is a crack, a delamination or a lifted die, described as popcorning.

Can floor life be reset by putting a part back in its bag? No. The exposure accumulates until the part is baked or reflowed. Returning it to the bag only stops the clock, it does not rewind it.

Why must baked parts cool before sealing? Sealing a warm part traps moisture that condenses inside, so the next storage period begins with water already present in the bag.