Moisture Bake: Preparation, Placement and Process Control

Laminate absorbs water from the air, and the amount it holds depends on the resin system, the storage conditions and the time since the board was fabricated. When the board is heated to reflow temperature, that water turns to steam, and the steam has to escape. If it escapes too quickly, or from a place where it cannot escape at all, it damages the board.

This article explains why a moisture bake is sometimes required, how the schedule is chosen, how the result is verified, and when baking does more harm than good.

Why Water In The Board Matters

Water diffuses into the resin over time, and the rate depends on the relative humidity and on the temperature of the store. A board in a humid room reaches a higher moisture content than the same board in a dry room, and a board that has been sitting in an open rack for weeks holds more than one taken straight out of sealed packaging. The absorbed water does not damage anything at room temperature; the problem appears only when the board is heated.

At reflow temperature the water vaporises, and the pressure it generates inside the laminate has to be released through the material. If the pressure builds faster than the resin can relieve it, a separation forms between the resin and the glass bundles or between the resin and the copper foil. The visible result is a blister or a bulge on the surface, and the hidden result is a delamination that reduces the dielectric strength and the mechanical strength of the board. The temperature behaviour of the laminate in general is discussed under PCB dimensional stability and expansion.

Types Of Moisture Damage

The industrial term for the most common failure is popcorn cracking, from the sound a blister makes when it opens. It appears as a local swelling of the surface, usually near a large copper area or at the edge of a dense pattern where the resin has the least room to move. Once the surface has opened, the board is scrap, and the damage often carries into the neighbouring conductors.

A second form has no visible surface effect at all. Water released inside a plated hole, or along the interface between an inner layer and the resin, separates the layers slightly. The board still passes a continuity test, and the defect is found later as a low insulation resistance or as a barrel crack after thermal cycling. This is the form that makes a moisture bake worth doing even when no blister has ever been seen, because the failures it prevents are the ones that escape the line.

When A Bake Is Required

A bake is required when the board’s moisture content exceeds the limit its material and thickness allow, and when the assembly process will heat it rapidly enough to trap the water. The relevant variables are the thickness of the board, the number of layers, the resin system and the thermal profile. A thin two layer board has a short diffusion path and tolerates more moisture than a thick multilayer board with large copper planes, which holds water deeper and releases it more slowly.

The manufacturer’s data sheet gives a moisture sensitivity level for the laminate, and the assembly specification converts that into a floor life and a bake condition. The floor life is the time the board may remain out of dry storage before a bake is required, and it is counted from the moment the packaging is opened. Keeping a log of when each lot was opened, and of the humidity in the area, is what makes the requirement practical rather than theoretical. The storage and handling conventions that go with it are described under design guidelines for manufacturability.

Choosing The Bake Schedule

A bake removes water by diffusion, so the schedule is a matter of temperature and time rather than of one number. A higher temperature removes water faster and risks oxidising the copper surface and the solderability of the pads; a lower temperature is safe for the surface and takes much longer. The usual compromise is a moderate temperature held for several hours, with the exact figures taken from the laminate data sheet for the board thickness in question.

Two practical points decide whether the bake works. The first is that the boards must be separated so that air can reach both faces; a stack of boards pressed together bakes only the outer two. The second is that the clock starts again the moment the boards come out of the oven, because they begin to reabsorb moisture immediately. The interval between the bake and the reflow must therefore be controlled, and where it cannot be, the boards are stored in dry packaging in between.

Verifying The Result

The verification of a bake is a measurement, not an assumption. The moisture content of the laminate can be measured by a weight loss method, in which a sample is weighed, dried and weighed again, and the loss is expressed as a percentage of the dry mass. The measurement is destructive and takes time, so it is used to qualify a schedule and then to audit the process periodically rather than on every lot.

The indirect check, and the one that runs continuously, is the profile. A board with a high moisture content takes longer to reach temperature because the evaporation absorbs heat, so the measured profile shifts. Where a process has a history of stable profiles, a shift at the soak stage is an early indication that the incoming boards are wetter than expected. The measurement of the profile is therefore a moisture check as well as a thermal one, provided the same assembly is used each time.

When Baking Does Harm

Baking is not free. Each bake cycle oxidises the copper surface a little and reduces the solderability of the pads, which eventually shows as poor wetting at the joints. Repeated bakes accumulate, so a board that has been baked several times may need a rework of its surface finish before it can be assembled. A better solution, where volumes allow, is to keep the boards in dry storage with desiccant and to assemble them within the floor life rather than to bake them repeatedly.

Baking also does nothing for a board that is already damaged. If a panel was exposed to a humid environment and then dried, the water has left, but any delamination that formed during an earlier thermal excursion remains. The bake prevents a defect from forming during the next heat cycle; it does not repair one that is already there, and it should not be used as a way of rescuing material whose history is unknown. Protecting a board once it has been dried and assembled is a separate question, treated under conformal coating as board protection.

FAQ

How long can a board stay out of dry storage? The answer comes from the moisture sensitivity level of the laminate and from the conditions in the assembly area. It is counted from the moment the packaging is opened, and it is shortened by high humidity in the room.

Does a bake need special equipment? A convection oven with controlled temperature and enough airflow between the boards is sufficient. The two requirements that matter are even temperature and separation of the boards so that both faces are exposed.

Can a baked board be baked again? It can, but each cycle further oxidises the surface and reduces solderability. Where several bakes are anticipated, the surface finish and the storage arrangements should be reviewed rather than the bake schedule extended.