Moisture Bake and Popcorn Prevention for PCB Assembly

Plastic packages absorb water from the air, and that water turns to steam the moment the package reaches reflow temperature. The result is a package that splits from the inside, a bond wire that lifts, or a crack that never shows on the outside but fails in the field months later. Controlling moisture is one of the few assembly risks that can be managed completely with storage discipline, a documented floor life and a correctly executed moisture bake. This article explains the mechanism, the ratings that govern it and the practical rules that keep it under control.

Where the Moisture Comes From

Mould compounds and laminate materials are hygroscopic, which means they take up water from the atmosphere until they reach equilibrium with it. The amount absorbed depends on the material, the relative humidity and the time of exposure, so a part stored in an uncontrolled warehouse for a year can hold far more water than the same part stored in a dry cabinet for a week.

The board itself absorbs moisture too, and a laminate that has been exposed before assembly will outgas in the reflow oven in the same way. That is why the rule is not only about components: an unprotected panel sitting open in a humid room before the first reflow step carries its own risk.

What Happens During Reflow

Water that is trapped inside a package expands violently as the temperature passes the boiling point, and the pressure it generates has nowhere to go. It delaminates the interface between the mould compound and the die paddle, or between the mould compound and the lead frame, and in severe cases it cracks the package body.

The failure is not gradual. A part that has absorbed enough moisture can be sound in inspection before reflow and damaged immediately after it, which is why the controls have to operate before the oven rather than after it. Our laminate properties guide explains how board materials differ in how much moisture they take up and how they respond to heat.

MSL Ratings Explained

MSL is the moisture sensitivity level assigned to a package, and it states how long the part can be exposed to a defined ambient condition before it must be baked. The levels run from parts that are not moisture sensitive at all to parts that must be handled under controlled conditions from the moment the dry pack is opened.

The rating is specific to a package, a body size and a die attach, so two parts that look similar in a bill of materials can carry different levels. The rating only holds for the floor life stated, and the clock starts when the dry pack is opened, not when the parts are placed.

Floor Life and Dry Pack Storage

A dry pack is a moisture barrier bag with a desiccant and a humidity indicator card. The card tells the operator at a glance whether the bag has held, and a card that shows a breached condition means a bake is required regardless of how long the bag was open.

Once the bag is open, the parts are on the clock. The floor life should be tracked per package type, not per kit, because different packages in the same kit expire at different times. A simple log taped to the feeder bank, with the open date and time recorded, is usually more reliable than an electronic system that nobody updates.

When a Moisture Bake Is Needed

A bake is required when the floor life is exceeded, when the humidity indicator shows a breach, when a package has been stored outside a dry environment, or when the bag has been damaged. A bake is also worth considering after any operation that exposes the parts to humid air, such as a return to stores, a rejected lot that is reworked, or a shipment that sat on a humid dock.

The decision should be triggered by a rule rather than by judgement at the line. A written rule that says which conditions force a bake removes the argument and prevents the situation where a well meaning operator decides the parts are probably fine.

How to Bake Without Damaging the Parts

The bake temperature and time come from the part supplier, and both matter. A temperature high enough to drive the water out quickly can also degrade the plating, oxidise the leads or damage the tape and reel carrier, while a temperature too low takes so long that it disrupts the schedule.

Low temperature bakes in a dedicated oven are the usual choice for parts on tape and reel, and the carrier must be rated for the temperature. Parts removed from the carrier and baked loose need trays that will not contaminate them, and the bake should be followed by a cooling step in a dry environment so that the parts do not immediately reabsorb water.

Popcorn, Delamination and Internal Cracks

Popcorn is the name given to the internal separation that follows a violent moisture expansion, and it can occur at several interfaces inside the same package. Delamination is the more general term for separation between layers, whether it is triggered by moisture or by thermal stress, and it can reduce the thermal path and the mechanical strength even when the package still functions.

Once the package has delaminated, the outcome depends on where the separation sits. A delamination over the die can lead to wire damage, while one near the lead frame can lead to a crack that propagates during thermal cycling. Neither is necessarily visible on the outside, which is why process control before reflow is so much cheaper than detection after it.

Inspection Methods for Moisture Damage

The external appearance is a poor guide, because the visible crack is the extreme case. Scanning acoustic microscopy sees internal separation at the interfaces, and cross sectioning shows the damage directly at the cost of destroying the sample. For production control, the more useful tool is a temperature and humidity log on the storage area plus a floor life record that is actually filled in.

Our BGA inspection notes describe how X-ray and optical methods divide the work of finding damage in area array packages, and our solder defect guide groups the failures that show up after reflow.

Writing the Rule Into the Process

The controls that work are simple and written down: dry pack storage for every moisture sensitive part, a floor life label at the line, a humidity indicator check on opening, a defined bake procedure with the supplier temperature and time, and a record of every bake. Together they cost very little compared with a single lot of delaminated packages.

At gopcb the same logic is applied at the design stage, where the design release checklist asks for the moisture sensitive parts to be identified before the order is placed rather than discovered at the machine.

FAQ

Can a moisture bake be repeated indefinitely? In principle the part can be baked again, but every high temperature excursion uses up part of the life of the plating and the carrier. Repeated bakes should be a signal that the storage process needs fixing rather than a routine remedy.

Does a low temperature bake always work? It works if the time is long enough to reach the required dryness, which for a thick package can be many hours. Supplier data should give both the temperature and the time rather than just a suggested maximum.

Is a moisture bake needed for boards as well as components? It is worth considering for any laminate that has been stored in a humid environment or that has seen a previous thermal cycle, because the board outgasses just as a package does. The decision should follow the laminate supplier guidance and the history of the panel.