Moisture Control in PCB Production

Moisture is present in every board shop and in every material the shop handles, and controlling it is a matter of knowing where it causes harm. Absorbed moisture in prepreg expands violently during lamination, moisture in a package turns to steam during reflow, and moisture on a panel surface interferes with adhesion and with solderability. Each of those is a different problem with a different control, and treating them as one subject is the reason moisture control is often superficial.

Where Moisture Causes Problems

In the lamination process, absorbed moisture in the prepreg is the classic problem. Heat drives it out as steam, and the resulting expansion appears as voids, measling or delamination. The cure is to keep the prepreg within its specified storage and working conditions and to respect the time it may be exposed to the ambient environment during lay up.

In assembly, moisture in the plastic body of an electronic package becomes steam during reflow and produces internal cracking or delamination, which is why components have a moisture sensitivity level and a floor life. A part that has exceeded its floor life must be dried before use, and using it anyway is a decision to accept a defect that will be difficult to trace later.

At the surface, moisture affects adhesion and solderability. A damp surface oxidises differently, a solder mask applied over residual moisture may blister during curing and a finish that has absorbed water may wet poorly. These effects are subtler than delamination and are often attributed to other causes, because the moisture is invisible and leaves no trace once it has evaporated.

Humidity in the Environment

Humidity in the production environment should be controlled where the process is sensitive, which in a board shop means the imaging area, the solder mask area and the assembly area. The typical range is around forty five to sixty percent relative humidity, which is high enough to avoid static problems and low enough to limit moisture absorption and condensation.

Control matters more than the exact value. A room that swings between thirty and seventy percent over a week causes materials to absorb and release moisture repeatedly, and the resulting dimensional movement is enough to affect registration on a fine line board. Stability is often more valuable than the absolute figure, and it is easier to achieve with a properly sized system than with periodic adjustment.

Condensation is the specific hazard to avoid. It occurs when a cold object meets warm humid air, which happens when material is brought from a cold store into a warm plant or when a cold water pipe passes through a humid area. Material should be allowed to equalise in its packaging before it is opened, which is the reason for the warning on many packaging labels.

<img src="https://www.gopcba.com/wp-content/uploads/2026/01/pcb.jpg" alt="Panels loaded into a baking oven before lamination” />

Baking and Drying

Baking is the standard method for removing absorbed moisture, and it is effective when the time and temperature are appropriate to the material and to the thickness. Too little and the moisture remains, too much and the laminate or the package is degraded, and the margin between the two narrows as the material becomes more heat sensitive.

The schedule should come from the material supplier rather than from a general rule, and it should specify the temperature, the time and the cooling method. Cooling matters: a panel removed from an oven and placed in a humid area will absorb moisture again as it cools, which undoes part of the work. Covered cooling or cooling in a dry environment preserves the benefit.

The baking schedule should also be recorded. A panel that has been baked and then left exposed for two days is no longer dry, and without a record there is no way to know. This is one of the cases where a simple time stamp has a direct effect on a defect rate.

Storage Conditions

Storage should match the sensitivity of the material. Laminate and prepreg are stored in a controlled environment with the rolls or panels sealed and dated, components are stored in their moisture barrier bags with a humidity indicator card inside, and finished boards are packed so that they are protected from both moisture and static.

The opening of a moisture barrier bag should be a deliberate act rather than an incidental one. The card inside indicates whether the material has been exposed to excessive humidity, and the date the bag was opened starts the floor life clock. Both pieces of information are only useful if someone looks at them and writes them down.

Stock rotation matters for the same reason. Material with a shelf life should be used in the order it was received, and the oldest stock should be at the front of the shelf. A store room where new material is placed in front of old material will produce a supply of expired material at the back that nobody notices until it is used.

Monitoring and Records

Monitoring humidity and temperature in the sensitive areas is straightforward and inexpensive. Data loggers placed at representative points record the conditions continuously, and the records can be reviewed against a defect pattern. Where a humidity excursion coincides with a batch of defects, the connection is visible and the cause can be confirmed.

The same applies to baking. Recording the oven, the schedule used, the load and the time gives a traceable history that can be compared with any subsequent problem. In a shop where baking is done occasionally and informally, this record is usually the missing link between a delamination defect and its cause.

Floor life tracking for moisture sensitive components is the third record. A simple label on the bag or the reel, showing the date of opening and the remaining floor life, is enough to prevent the majority of problems, and it is far easier than reconstructing the history after a failure.

Practical Rules

Separate the three problems in your own mind: moisture in laminate, moisture in packages and moisture at the surface. Control the environment where the process is sensitive, and keep it stable rather than merely within range.

Follow the supplier’s baking schedule, allow material to equalise before opening packaging, and record the times with the production records. Moisture control is invisible when it works and expensive when it fails, and reviewing the humidity records against the quality data is the quickest way to confirm that the controls are earning their cost.

Process Control and Verification

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Why bake PCB material? To remove absorbed moisture that would turn to steam during lamination or reflow and cause voids, delamination or internal cracking.

What humidity should a board shop target? Around forty five to sixty percent relative humidity in the sensitive areas, kept stable rather than merely inside the range.

Why let a bag equalise before opening? Because a cold item in warm humid air condenses moisture on its surface. Equalising in the sealed bag prevents it.