Moisture Sensitivity Level: Design Rules and Process Limits
A reel of components that has been open on the shop floor for a week is not the same material it was when the seal was broken. The plastic package has absorbed moisture from the air, and when the assembly enters reflow that moisture turns to steam inside the body faster than the plastic can release it.
What Moisture Sensitivity Level Means
The moisture sensitivity level, usually abbreviated as the MSL, describes how long a package can be exposed to factory ambient after the protective bag is opened before it must be dried again. The level runs from one to six, with a higher number indicating a more sensitive, more moisture-hungry package.
The classification is assigned to a package body, not to a part number, and the same die in a thinner or larger package can carry a different level. Industrial temperature parts and large ball grid arrays are typically the most sensitive because the body absorbs more moisture and the stresses at reflow are higher.
The exposure time measured against the level is called floor life, and it is cumulative. A reel opened on Monday, closed on Tuesday and opened again on Wednesday does not start the clock over.
Why the Moisture Causes Damage
During reflow the whole assembly is heated to well above the boiling point of water in a few minutes. Moisture in the mould compound vaporises, and the pressure it generates has nowhere to go until the temperature approaches the glass transition of the plastic and the material softens.
At that point the vapour expands and separates the plastic from the die paddle or from the die attach, producing the internal delamination known as popcorning. The audible click that gives the defect its name is heard only in the worst cases; most instances are silent.
The damage may not stop the part from working immediately. A delaminated package can pass functional test and fail later, when thermal cycling opens a bond wire or when moisture reaches the die surface through the separated interface.
Reading the Labels and the Bag
Moisture sensitive parts arrive in a sealed bag with a desiccant and a humidity indicator card. The card is the primary evidence: if the indicator has changed colour past the specified threshold, the bag was breached or the barrier was inadequate, and the parts require drying regardless of the clock.
The bag label carries the level, the floor life at that level, the maximum reflow temperature and often the date of sealing. The date of sealing matters because the shelf life in the unopened bag is also finite, typically twelve months.
Our design release checklist notes where these attributes should be recorded so that the assembly house receives them with the BOM rather than discovering them on the floor.
Baking Before Assembly
Baking drives the absorbed moisture out, and the schedule is a compromise between drying the part and damaging it. A typical schedule for a body that can tolerate the temperature is twenty-four hours at one hundred and twenty-five degrees Celsius, or a longer period at a lower temperature where the packaging cannot tolerate the higher one.
The lower temperature schedules take far longer because diffusion through the plastic slows exponentially with temperature. Forty degrees Celsius at five per cent relative humidity is gentler, and it can take weeks rather than days, which is rarely compatible with a production schedule.
The baking temperature must also respect the reel, the tape and the carrier. Many tapes and reels are rated only to sixty degrees, so parts on tape must be transferred to heat resistant trays before a high temperature bake unless the reel is rated for it.
Tracking Floor Life on the Line
The practical control is a log that records when each moisture sensitive reel is opened, how long it has been exposed, and when it was resealed with fresh desiccant. Without the log the level is unenforceable, because nobody on the floor can tell by looking whether a reel has been out for two hours or two weeks.
Where the accumulated exposure approaches the limit, the choices are to bake, to reseal and return the material to stock with the clock reset, or to consume it before the limit is reached by scheduling the sensitive parts early in the shift.
Resealing with a fresh desiccant and a humidity indicator card does reset the clock, and this is the normal remedy for material that was opened but not used. It requires the correct barrier bags and a sealer, and it requires the log entry that makes the reset visible.
Selecting Parts to Reduce the Risk
Sensitivity is a purchasing consideration as well as a floor consideration. Where two parts are functionally equivalent, the one in the more robust package reduces the amount of drying, the amount of logging and the number of opportunities for an escape.
Where a large fine pitch ball grid array is unavoidable, the design can reduce the consequence by avoiding it on the secondary side of the board, where it would be exposed to two reflow cycles, and by placing it where rework is possible.
Our component selection notes cover the package attributes that are worth checking before a part is released to the BOM.
The Assembly House Perspective
A contract manufacturer receives parts from many sources, and the material that arrives without the label data cannot be managed. Sending a moisture sensitive part in a plain bag with no indication of its level transfers a risk that the assembler has no way to control.
The information that matters is the level, the floor life, the maximum body temperature and the sealing date. A part number alone is not sufficient, because the same number can be supplied in different packaging with different ratings.
Where the assembly house has its own dry storage, the parts may never accumulate exposure at all. Dry cabinets hold the relative humidity low enough that the clock effectively stops, which is a simpler control than baking and logging.
Symptoms That Point Back to Moisture
The signature of a moisture problem is a defect that appears at first reflow in a pattern that follows the package rather than the board. Cracks along the package body, delamination between the die paddle and the mould compound, and voids in the die attach all point in the same direction.
Wire bond failures after thermal cycling, with no evidence of mechanical damage, are the delayed version of the same problem. Our solder defects and board failures notes describe how these appear alongside their more visible counterparts.
The differential diagnosis matters, because a cracked package is sometimes attributed to handling or to the reflow profile. The distinguishing evidence is the internal delamination, which is visible in an acoustic scan and is present even where the exterior of the package is intact.
Documenting the Control
The control should be written into the assembly documentation rather than left to local practice. A short specification covering bake schedules, drying temperature limits for tape and reel, the log format, and the rule for resetting the clock removes the ambiguity that otherwise produces a verbal decision at midnight.
Verification is a matter of auditing the log against the reels, which takes minutes and catches the case where a reel was returned to the shelf without an entry. The audit is worth more than any amount of exhortation about being careful.
Where parts are stored for long periods, adding a humidity indicator card to each storage bag makes the condition of the material visible without opening it, and it turns a storage question into an inspection.
FAQ
Does every part need to be baked? No. Parts at moisture sensitivity level one, and parts that have been stored sealed within their floor life, do not need drying before the first reflow.
Is a low temperature bake always safer? It is gentler on the packaging but far slower, and it still requires the parts to be removed from tape if the tape is not rated for the temperature used.
What does gopcb provide for moisture sensitive assemblies? We provide controlled storage with humidity monitoring, documented bake schedules matched to each package rating, exposure logging for every sensitive reel, and the reports that tie the assembly records to the parts consumed. Where a part is at the limit of its floor life we dry it rather than consume the margin.
