Laminate Moisture Absorption and Its Effects on Boards

Every organic laminate absorbs water from the air, and the amount it holds changes the properties that the design depends on. The board grows slightly, its dielectric behaviour shifts, its glass transition temperature falls and its ability to survive a thermal cycle without damage is reduced. Moisture absorption is therefore not a handling detail but a material property that has to be understood, measured and controlled from the laminate datasheet through to the assembly line.

How Laminate Absorbs Water

A laminate is a composite of glass fabric and cured resin, and both parts take up water in different ways. The resin absorbs moisture into its free volume and forms hydrogen bonds with the polymer network, while the glass fabric and the interface between the two provide paths for water to travel along and to collect. The result is a material that behaves like a sponge on a slow timescale, so a board that has been in a dry store for a week is not the same material as one that has been in a humid room for the same period.

The rate of uptake depends on the temperature and on the relative humidity of the surroundings. A board stored in a dry cabinet approaches equilibrium at a low moisture content, while one left in a humid workshop moves towards a much higher value, and the movement takes days rather than minutes because diffusion through the resin is slow. The surface reaches equilibrium quickly while the core lags behind, so the moisture content is not uniform through the thickness.

Measurement and Specification

Moisture content is usually quoted as a percentage of the dry weight, and it is measured by weighing a sample, drying it and weighing it again. The test is simple but it takes time, so the value on a datasheet is a steady state figure obtained under defined conditions rather than a number that describes a board on the shelf.

The specification should state the conditions under which the figure applies and the test method used, because a value obtained after a short immersion and a value obtained after long exposure to humid air are not comparable. For process purposes the more useful measurement is a moisture content measured on the actual material before assembly, using the same technique every time, because the trend of that measurement is what predicts a process problem.

Effects on Dimensional Stability

Water molecules occupy space within the resin, so a laminate that absorbs moisture expands. The expansion is small, often measured in fractions of a millimetre per metre per percent of moisture, but it is cumulative and it can exceed the tolerance on a long dimension or on a fine feature.

Dimensional changes of this kind matter most where the board is registered against tooling or where a dense array of features has to line up with a stencil or a component. The expansion is also anisotropic, because the glass fabric constrains the material in one direction more than the other, so a board can grow more in one axis than in the other and the effect is larger on a panel than on a small coupon.

Effects on Electrical Properties

Water has a high dielectric constant compared with the resin, so absorbed moisture raises the effective dielectric constant of the laminate and increases the dielectric loss, which shows up as extra attenuation in a long line. At low frequencies the effect is small, but at microwave frequencies it shifts the impedance of a controlled line and increases the attenuation.

The shift is one reason why high frequency designs use materials with low moisture uptake and why the assembly process should not expose the boards to unnecessary humidity. The same effect appears as a change in capacitance, which is measurable and which can be used as an indicator of moisture content in a test structure. The behaviour of these materials is described in the guide to high frequency laminates.

Effects on Thermal Properties

Absorbed water acts as a plasticiser and reduces the glass transition temperature of the resin. A board that measures one value in the dry state may measure a noticeably lower value after humid conditioning, and the reduction changes the mechanical behaviour of the material at the temperatures reached during assembly.

The consequence is that the material is softer and expands more at a given temperature than the datasheet suggests, which increases the stress on vias and on plated barrels. The measurement of these properties is normally made on conditioned samples, and the conditioning has to be understood before the results are compared.

Effects During Assembly

During reflow the absorbed water turns to steam, and the pressure it produces has to be accommodated. In a well made board the vapour diffuses out through the laminate, but in a board with a high moisture content and a rapid heating rate the pressure can delaminate the layers or blister the solder mask.

The damage is not always visible from the outside. Internal delamination reduces the mechanical strength of the board and can sever a connection without leaving an obvious mark, which is why the moisture content before reflow is treated as a process control subject rather than an inspection subject.

Drying and Baking

Baking removes moisture, and the schedule has to be chosen so that the board reaches the required dryness without being damaged. A higher temperature dries faster but can oxidise the copper surface and, on some finishes, degrade the solderability, so the temperature is limited by the finish as much as by the laminate.

The duration depends on the thickness of the board and on the moisture content at the start. A thin board dries in a fraction of the time needed for a thick one, and a board that has been stored badly needs longer than one that has been kept dry. The baking schedule should therefore be stated with the condition it assumes, and the result should be verified by weighing a sample rather than assumed from the time in the oven.

Storage and Packaging

Preventing the moisture from returning is as important as removing it. Boards that have been baked are normally packed in moisture barrier bags with a desiccant, and the bag protects the contents until it is opened and the floor life begins. A board left open in a humid room will return towards equilibrium within days.

The storage environment matters as much as the packaging. A dry cabinet with continuous monitoring is the most reliable option for material that has been prepared for assembly, and a passive bag is adequate for shorter periods provided the seal is intact and the humidity indicator is within its limit. A bag that has been opened should be resealed promptly rather than left open for the rest of the shift.

Design and Process Implications

The design should account for the fact that the laminate is not a stable material. Where a dimension is critical, the drawing should state the moisture condition at which it is measured, and where an electrical parameter is critical, the specification should define the conditioning before the measurement.

The process should control the exposure in the same way. A defined floor life, a drying step where it is needed and a record of both turn a variable material property into a controlled input. The structural properties that interact with moisture behaviour are described in the guide to laminate material properties, and the sequence that surrounds the assembly steps is set out in the guide to the PCB production process flow.

FAQ

How much moisture does a PCB laminate hold? The steady state value depends on the material and on the conditions, and typical figures are small percentages of the dry weight. The number that matters for a process is the moisture content of the board immediately before it is heated, which is why the storage history of the lot is recorded.

Is baking always necessary before assembly? Not always. A board that has been kept in dry packaging within its floor life usually needs no baking, while one that has been exposed for an unknown period should be dried or tested before it is used. The decision should be based on a record rather than on appearance.

Does moisture affect only high frequency designs? No, but the electrical effect is largest at high frequency. Moisture also reduces the glass transition temperature, causes expansion and creates the vapour pressure that drives delamination, so it matters on every board that will be heated.