Moisture Content Measurement in PCB Laminate Materials
Laminate absorbs water from the air, and the amount it holds changes everything that happens to the board in a hot process. A panel that has been stored in a humid room will outgas in the press, the drill or the reflow oven, and the symptoms range from a blister under a pad to a dielectric constant that no longer matches the stack up. Measuring moisture content is how that variable is moved from an assumption to a number.
Why Moisture in the Laminate Matters
Water in a laminate has two effects. It plasticises the resin, which lowers the glass transition temperature and softens the material, and it turns to steam at high temperature, which generates pressure inside the board. Both are worse in a thick panel, where the water has further to travel to escape.
The consequences are dimensional, electrical and mechanical at the same time. A board can measure correctly when it is dry and fail after a baking step, or pass a functional test and delaminate during the reflow of a second assembly operation.
Where the Water Sits
Absorbed water sits in the resin, in the interface between the resin and the glass fibres and in the voids and micro cracks that the process has created. The distribution matters, because water at an interface weakens the bond while water in the bulk resin mainly affects the dielectric and the transition temperature.
That distinction explains why two boards with the same measured moisture content can behave differently. The history of the panel, including the number of thermal cycles it has seen, determines how much of the water has reached the interfaces. Our laminate properties guide describes how the resin system absorbs and releases water.
<img src="https://www.gopcba.com/wp-content/uploads/2026/08/1-3-jpg.webp" alt="Laminate sample being weighed before and after a bake” />
Weight Loss Measurement
The simplest method is to weigh a sample, bake it under defined conditions and weigh it again. The difference, expressed as a percentage of the original mass, is the moisture content by this method. It is cheap and it needs no special equipment beyond a balance and an oven.
The result depends on the bake temperature and time, so the method is only comparable when both are fixed. It also includes any other volatile material that leaves the sample, which means the figure is an upper estimate rather than a strict water content.
Karl Fischer Titration
Karl Fischer titration measures water specifically by a chemical reaction, and it is the reference method when the water content itself has to be known. The sample is heated in a sealed vessel and the released water is carried to a titration cell, where the amount is measured coulometrically.
The method is accurate at low concentrations and it separates water from other volatiles, which the weight loss method cannot do. It needs controlled equipment and a defined heating programme, so it is used for qualification and for a reference measurement rather than for daily production control.
Dielectric and Thermal Methods
The dielectric constant and the loss factor both change with moisture, so a dielectric measurement can indicate the moisture state of a sample non destructively. The measurement needs a defined geometry and a reference, and it is affected by the copper distribution, so it is used as an indicator rather than as an absolute value.
Thermal methods, including differential scanning calorimetry, show the effect of moisture on the transition temperature and on the cure state. They answer a different question: not how much water is present but what it is doing to the material. Our thermal management notes describe how the material properties feed into the thermal design.
Sampling and Its Trap
A moisture measurement taken on the outside of a panel does not describe the inside. Water takes time to move through a laminate, so a panel that has been in a dry room for an hour has a dry surface and a wet core, and a sample cut from the edge will read differently from one cut from the centre.
The sample size and the location should therefore be stated, and the sample should be taken and sealed quickly so that it does not lose or gain water between the panel and the balance. A measurement that takes ten minutes to prepare is measuring the room, not the board.
Bake Schedules and Their Verification
A bake schedule states the temperature and the time needed to bring a panel or a component to an acceptable moisture level. The schedule should come from the material supplier, and it should be verified by measuring a sample before and after, rather than being assumed to work because it is common practice.
The verification matters most for thick panels, for high layer counts and for a material with a high absorption. Our moisture bake guide covers the practical rules for the bake and the storage that follows it.
Effect on Reflow and Delamination
During reflow the absorbed water turns to steam and the pressure inside the board rises. If the pressure exceeds the strength of the interface, the layers separate, which appears as a blister, a measling pattern or a delamination at the edge of a plane.
The effect is worse where the board has a large copper area, because the copper and the resin expand differently and the interface is already loaded. Our quality guide describes how a delamination found after assembly is classified at gopcb.
Specifying a Limit and Testing It
A specification should state the method, the sample size and location, the bake conditions and the acceptance limit, and it should state what the sample represents. A limit of a few tenths of a percent is meaningful only with the method attached, because the weight loss and the titration methods do not give the same number.
The most useful arrangement is a routine weight loss check on incoming panels, backed by an occasional titration measurement to confirm that the routine method is still tracking the water content. The pair gives a cheap daily figure and a periodic reference.
Process Control and Verification
On a design of this kind, moisture content is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
FAQ
What moisture content is acceptable in a laminate? It depends on the material and on the process, and the supplier data is the starting point. The limit should be set from the behaviour of the board in the actual process rather than from a general figure.
Is a weight loss measurement accurate enough? It is accurate enough to show a trend and to detect a panel that has absorbed a lot of water, and it is the practical choice for routine checking. For the water content itself, titration is the reference.
How long should a panel be baked? Long enough for the centre to reach the required level, which for a thick panel can be many hours. The time should be verified by measurement rather than copied from a thinner product.
