Moisture Sensitive Devices: PCB Design and Handling Rules

Plastic packages absorb water from the air. When an assembled board enters a reflow oven, that water turns to steam inside the package and the pressure has to go somewhere. On a small part nothing visible happens; on a large ball grid array the result can be an internal crack, a lifted die or a delaminated substrate. Moisture sensitive devices are managed through a documented system of ratings, packing, floor time and baking, and the rules matter as much as the layout.

What the MSL Rating Means

Every plastic package carries a moisture sensitivity level, from MSL 1 to MSL 6. The level describes how long the part may be exposed to factory air after the dry pack is opened, and what bake is required if that time is exceeded. MSL 1 parts can be exposed indefinitely, while MSL 3 allows about 168 hours at 30 °C and 60 percent relative humidity, and MSL 5 allows only 48 hours.

The floor life clock starts when the sealed bag is opened and pauses whenever the parts are returned to a dry environment. Humidity indicator cards and desiccant in the bag show whether the packaging has remained intact, and a card that has changed colour is a warning that the parts need evaluation before use.

How Absorbed Moisture Causes Damage

The damage mechanism is vapour pressure. Absorbed water expands violently as the temperature passes 100 °C, and the weakest interface in the package, usually where the die attach or the substrate meets the moulding compound, separates first. This is the failure known as popcorning, and it can happen even when the outside of the package looks perfect.

Some damage is not visible at all. A delaminated interface changes the thermal path and the mechanical support of the die, so the part may pass functional test and fail months later under thermal cycling. Wire bond damage and bond pad cratering follow the same route, and both are invisible until destructive analysis is performed. The peak reflow temperature matters, which is one reason the movement toward higher melting point alloys described in the comparison of lead-free and leaded solder changed the moisture rules for many components.

Floor Life and Dry Packing

Floor life is managed by controlling the environment, not by hoping the schedule holds. A dry cabinet, a dry box or a nitrogen storage cabinet keeps parts below the humidity threshold, and a log records when each reel leaves and returns. Where a line runs several products in a week, the log is the only practical way to know which parts are still inside their allowance.

Reels and trays should be resealed with desiccant whenever a run is interrupted, and the seal should be checked before the next use. Parts returned to a sealed bag do not automatically reset the clock unless they are baked, so the safe assumption is that the floor time continues to accumulate until a documented bake resets it.

Baking Schedules

Baking removes absorbed moisture, and the schedule depends on the package thickness and the level. A thin body may need only a few hours at 125 °C, while a thick ball grid array can require 24 hours or more. Low temperature baking between 40 °C and 90 °C is used for parts that cannot tolerate the higher temperature, such as reels that would deform in a hot oven.

High temperature baking has its own cost. Repeated exposure to 125 °C oxidises solderable surfaces, and tin plated terminations may need to be used within a short window after baking. That is why the bake should be carried out only when the floor life has actually been exceeded, and not as a routine precaution before every build.

Design Choices That Reduce the Risk

Layout cannot eliminate moisture sensitivity, but it can limit the damage. Avoid placing a large moisture sensitive package where it must be reworked, because each additional reflow cycle adds another vapour pressure event. Leave enough clearance around the package for a rework nozzle and for thermocouple attachment, so that a repair does not require heating the whole area.

Where a design has a choice, prefer a package with a lower sensitivity level even at slightly higher cost. On boards with several large area array parts, sequence the assembly so that the most sensitive devices are placed last, and avoid a design that needs a second reflow after the first side is complete if that can be rearranged. Layout choices that also help are described in the notes on component shift causes, because a shifted part is often a reworked part.

Reflow Profile Control

The profile should follow the paste supplier’s recommendation and the component manufacturer’s limit, with the peak temperature as low as the alloy allows. A slow ramp through the preheat region gives absorbed water more time to diffuse out of a thin package, although it does nothing for a thick body that has already exceeded its floor life.

Time above liquidus is the parameter to watch. Extending it to improve wetting also extends the period during which internal pressure is highest, so the shortest profile that produces sound joints is the best one. Measuring the profile on the actual assembly, with thermocouples attached to the package body and to a nearby pad, is standard practice for any board with large area array devices. Where a large package is expected to see mechanical stress, dispensing a material around the perimeter helps distribute the load, and the options are described in the notes on potting and dispensing adhesives.

Storage, Handling and Traceability

Traceability turns a moisture problem from a mystery into a measurable event. Record the date code, the moisture sensitivity level, the date the bag was opened and the date the parts were placed. If a batch shows delamination or a bond failure after assembly, that record shows immediately whether the floor life was exceeded.

Handling rules matter too. Parts should not be left on an open feeder overnight, and a partially used tray should be resealed rather than left on the machine. Static control, clean gloves and a tidy bench all reduce the chance of contamination that would otherwise be blamed on moisture.

Rework and Repair Limits

Every rework operation is another thermal cycle. A moisture sensitive package that has already been through one reflow should be baked before a second heating, even if the floor time appears to be within limits, because the first cycle drove water deeper into the interfaces. Localised heating from a rework station is less uniform than an oven, so the risk of a partial delamination is higher.

Where a package must be replaced, the replacement part should come from a sealed bag or be baked to the correct schedule. The reworked area should then be inspected, and if the product is safety related, a sample should be cross sectioned to confirm that no delamination was introduced.

FAQ

How do I know the floor life of a part? Read the moisture sensitivity level from the bag label or the datasheet, then compare it with the time the bag has been open. The level sets the allowance, and the humidity indicator card shows whether the packing stayed intact.

Can a baked part be baked again? Yes, but each bake oxidises the terminations and consumes part of the component’s thermal life. Bake only when the floor life has been exceeded, and follow the schedule for the package thickness.

Does popcorning always break the part? No, and that is the problem. Internal delamination can leave a device that passes test and fails later, so a visible crack is only the most obvious outcome of absorbed moisture.