PCB Baking Schedule Before Assembly

Circuit board laminate absorbs water from the air, and the amount it holds depends on the material, on the humidity, and on how long the board has been exposed. That moisture becomes a problem at reflow temperature, because water turns to vapour and expands, and the pressure it generates inside a board or a plastic package is large enough to tear layers apart. Baking is the measure that removes the moisture before it can do that damage.

This article explains why a bake is needed, when the moisture sensitivity level or the storage history requires one, how the bake is specified for different materials, and what baking costs.

Why The Moisture Matters

The failure mechanism is the vapour pressure generated at temperature. Water absorbed in the resin turns to steam as the board passes through the preheat and soak stages, and the pressure rises inside any volume where the vapour is trapped. In a plastic package the result is popcorning, where the body cracks and the internal bond wires or the die attach are damaged. In the laminate the result is delamination between layers, blistering of the surface under the solder mask, and a measurable loss of insulation resistance between layers that are no longer bonded.

The severity depends on the amount of moisture and on how fast the board is heated. A slow profile allows some of the water to escape before the temperature becomes critical, while a fast ramp traps it, and that is one reason a profile that is used for a baked board may not be acceptable for one that has been stored in a humid room. The same mechanism explains why a board that has been through an aqueous cleaning step is often given a drying cycle before it is processed further.

When A Bake Is Required

The clearest trigger is an exceeded floor life. A moisture sensitive component is supplied with a level that states how long it may be exposed to ambient air after the bag is opened, and once that period has passed the part is no longer safe to reflow without a bake. The second trigger is storage: boards kept in a humid environment, in an opened bag, or in a workshop without humidity control absorb moisture even when no component is involved.

Other situations justify a bake as well. A board that has been washed and not fully dried, a board that has been handled extensively, and an assembly that needs a second or third reflow cycle after a long interval are all candidates. The decision is usually made from the storage record rather than from a measurement, because there is no simple in house test for the moisture content of a finished board, and the humidity indicator card in the packaging is the only direct evidence that the shipment was exposed.

How The Bake Is Specified

The classic schedule for a bare board is around one hundred and twenty to one hundred and twenty five degrees Celsius for several hours, with the time depending on the thickness and on the number of layers, and with boards standing in racks rather than stacked so that the air can circulate. Thin boards dry faster than thick ones, and a board with many layers of resin needs longer than a simple double sided board of the same thickness.

The temperature is limited by what the board can tolerate. A high temperature is efficient at removing moisture but it oxidises the surface finish, and the finishes differ in their tolerance: an organic finish degrades quickly, a hot air levelled surface changes appearance and solderability, and a gold or silver finish is more tolerant but not immune. Where a board carries a sensitive finish or has already been assembled with plastic parts, the bake is run at a lower temperature, often between ninety and one hundred degrees, for a longer period, and the schedule comes from the material supplier rather than from a general rule.

What Baking Costs

Baking is not free. The heat accelerates the growth of the intermetallic layer on the surface and in the plated holes, which reduces solderability, and it oxidises copper that is protected only by a thin organic film. Each bake is cumulative, so a board that has been baked twice is not equivalent to a board that has never been baked, and a schedule that is applied as a routine precaution rather than as a response to a real risk can reduce yield at the soldering step.

The mechanical handling is the other cost. Boards are moved into racks, loaded into ovens, and moved out again, and each transfer is an opportunity for damage to a fine pitch pad or for contamination from the racks themselves. The racks have to be clean and rated for the temperature, and the boards must be allowed to cool in a controlled way before they are packed, because a board that is bagged while still warm will condense moisture from the air inside the bag.

Managing Floor Life In Practice

Prevention is cheaper than baking. Components are kept in their sealed bags with desiccant until they are needed, boards are stored in a dry cabinet where the humidity is controlled, and the exposure time is recorded when a bag is opened so that the remaining floor life is known. The bags themselves carry a humidity indicator card, which changes colour when the contents have been exposed, and the card is the simplest check available at goods inwards.

The floor life clock is a process control rather than a paperwork exercise. Where an assembly is partially built and then paused, the parts that are already mounted have consumed some of their floor life, and the remaining time has to be tracked for the second pass. A simple board with a few moisture sensitive devices is easy to manage, while a board with several hundred requires a system, usually a label with the open date and the expiry date and a rule that any part past its date goes to a bake.

Deciding Instead Of Guessing

The practical rule is to bake when a documented trigger has been met and not otherwise. The triggers are an exceeded floor life, storage outside a controlled environment, a long delay between reflows, and an aqueous process that was not followed by a controlled dry. Where the board carries a moisture sensitive component, the schedule comes from the component datasheet. Where it does not, the schedule comes from the laminate supplier, and it is worth confirming with the fabricator, because the correct bake depends on the specific material and not on the board thickness alone.

The record matters as much as the bake. The time, the temperature, and the boards that were treated should be traceable, because a moisture related failure appears at the end of the line or in the field and the bake record is the first thing that will be examined. The related process decisions are described under lead free and leaded soldering, the fabrication sequence under PCB design and fabrication, and the wider quality framework under PCB design quality characteristics.

FAQ

How long should a board be baked? It depends on the material, the thickness, and the finish. A common starting point for a bare board is one hundred and twenty degrees for four hours, but the correct schedule should come from the laminate or component datasheet.

Does baking damage the board? It can. Heat oxidises the surface finish and grows the intermetallic layer, and the effect is cumulative, so a bake should be a response to a real risk rather than a routine step.

Can a board be baked after assembly? Only at a lower temperature, because the plastic packages and the solder joints impose their own limits. The schedule for an assembled board is set by the most sensitive component on it.