Moisture Sensitivity Level Management In Assembly

Plastic packages absorb water from the air. Once that water is inside the moulding compound and the reflow oven brings the whole package above the boiling point of water in a few seconds, the vapour has nowhere to go and the package can crack from the inside. The moisture sensitivity level is the classification that tells the assembly line how much drying a part needs and how long it may stay open on the floor, and managing it is one of the few places where a storage decision can destroy a board.

This article explains the classification, the physics behind the damage, how dry packing and floor life are controlled, when baking is allowed, and how the sequence is documented.

What The Classification Means

The moisture sensitivity level, usually written as MSL, is a number from one to six, and it describes how long a part can be exposed to a defined workshop environment after it is removed from its dry pack and before it is reflowed. Level one has no restriction; level three allows 168 hours at 30 degrees Celsius and 60 percent relative humidity; level five allows 48 hours; level six requires baking immediately before use. The level is not a measure of quality, it is a measure of how much handling the package can tolerate.

The level is assigned by the package manufacturer after testing, and it applies to the package and to the die attach, not to the board. A part with a low level is not inferior to one with a high level, but a design that is built with high level parts needs a process that can honour the restriction, which means either a dry store, a baking facility, or a sequence that consumes the parts within their floor life.

Moisture, Vapour Pressure And Popcorning

Water enters the moulding compound by diffusion, and the amount depends on the ambient humidity, the temperature and the time of exposure. When the part is heated rapidly, the water turns to steam and the pressure inside the package rises. The steam collects at the interface between the die paddle and the moulding compound, where the adhesion is weakest, and pushes the two apart. If the pressure exceeds the strength of the material, the package cracks with a sound like a cork leaving a bottle, which is where the name comes from.

The damage is not always visible. A delamination that has not broken the surface is detectable only by acoustic microscopy or by a cross section, and it shows itself later as a wire bond that lifts, a solder joint that opens after thermal cycling, or a leakage path across the package. That is why the requirement is written as a process limit rather than as an outgoing inspection, because a part that has absorbed moisture looks and measures perfectly.

Dry Pack, Floor Life And Indicators

Parts are packed with a desiccant and a humidity indicator card inside a moisture barrier bag, and the bag is sealed under vacuum. The card shows the humidity inside the bag as a set of spots that change colour, and the level of the spots is read against a limit printed on the card. A card that shows more moisture than the limit means the bag has leaked or was not sealed correctly, and the parts inside have to be treated as having been exposed.

Once the bag is opened, the floor life clock starts. The parts must be reflowed within the time allowed by their level, or they must be returned to a dry store whose humidity is low enough to stop further absorption. The time is cumulative, so several short exposures add up, and the only way to keep the sum under control is to record the time on and off the floor for every reel and tube rather than relying on memory.

Baking And What It Costs

Baking drives the moisture out by holding the parts at an elevated temperature for a defined time, and it is specified by the manufacturer in terms of temperature and hours, with different schedules for parts that are still in their shipping carriers and parts that have been soldered once. Baking is effective, but it is not free, because the same heat oxidises the terminations.

Oxidation is the reason baking is limited. After a total of about 96 hours at 125 degrees Celsius the terminations may no longer wet reliably, and the parts must be scrapped or reclaimed. A lower temperature of 40 to 45 degrees Celsius in a dry cabinet is kinder to the plating but takes far longer. The choice between the two depends on how much time is available and on how many reflow passes the parts still have to survive.

The Assembly Sequence

The practical control is to place the moisture sensitive parts early in the sequence, so that the second side reflow and any rework do not extend the exposure of a part that has already been through the oven once. Where a board is reflowed twice, a part that is board side one only sees one cycle, while a part that is on both sides is at risk in both, and the schedule has to be arranged around that fact.

Handling also matters. A part that is left in an open tube on the machine overnight absorbs moisture in the same way as one left in a tray, and a part that is misplaced and later found cannot be assumed to be dry. The full sequence, from solder paste printing to the final inspection, is described under PCBA development process, and the thermal profile that the parts must survive is discussed under lead free versus leaded solder.

Protection After Assembly

Moisture is a problem after assembly as well. A finished board that operates in a humid environment can suffer from electrochemical migration between fine features, and the remedy is a coating that keeps the water off the surface rather than a change of the parts. The materials and their application are described under conformal coating and board protection.

The two subjects are related because a coating does not prevent water from entering a plastic package, and a dry part does not protect a board from condensation. They address different surfaces, and a product that is intended for a wet or a marine environment usually needs both.

Documentation And Traceability

The record is what makes the control real. Each reel carries its date code, its level and the date the bag was opened, and each board can be traced back to the parts that were used on it. When a field failure is investigated two years later, that record is the only way to tell whether the part was inside its floor life or whether it was baked and for how long.

The same record supports the incoming inspection. A supplier that ships a part with a different level from the one on the drawing, or without a humidity indicator card, is a supplier whose parts cannot be used on the line as planned. The mismatch is cheaper to find at goods inwards than at a customer site.

Where The Requirement Comes From

The level is not chosen by the assembly house. It is fixed by the package and published by the part manufacturer, and it is carried into the design through the component specification and the approved vendor list. A designer who selects a part with a demanding level is choosing a constraint for the whole production line, and that choice is made at the schematic stage whether or not anyone notices it at the time.

Because the two travel together, the practical step is to list the MSL of the parts on the bill of materials and to check the list before the build starts. A single level five part is enough to require a dry store and a bake schedule for the whole product, and knowing that in advance is far cheaper than discovering it when the reels arrive on the day of the build.

FAQ

Does a low moisture sensitivity level mean the part is better? No. The level describes how the package tolerates moisture, and it is a process requirement rather than a quality grade. A high level part is simply more demanding to store and to schedule.

How long can parts stay out of the bag? That is exactly what the level states, from unlimited at level one to 48 hours at level five, and the time is cumulative over all the exposures between the bag being opened and the reflow.

Can a part be baked more than once? Repeated baking is limited by the total time the terminations spend at temperature. Once the cumulative exposure approaches the manufacturer limit, the solderability of the terminations has to be retested before the parts are used.