Outgassing: 5 Causes in Lamination and Assembly

Outgassing is the release of trapped gas from inside a board when it is heated. The gas comes from moisture absorbed by the laminate, from volatiles left in the resin and from air trapped in a via or under a plane, and it expands as the temperature rises.

Where it escapes freely the board survives, but where it is trapped under a surface it forms a blister that lifts the mask or separates the layers. The damage is done during assembly, which is why the cause is almost always upstream in fabrication or in storage.

What Outgassing Does to a Board

The visible result is a dome of solder mask or a separation between a copper plane and the laminate beneath it. A smaller, invisible result is a void in a solder joint where the gas passed through the pad while the alloy was molten.

The mechanical result is more serious. A blister breaks the bond between layers, and the board may still pass its electrical test while its long term reliability is already reduced, which is the profile of a defect that reaches the field.

Moisture in the Laminate and How It Arrives

Laminate absorbs water from the air continuously, and the amount rises with humidity and with time. Board storage in a humid room for a few weeks is enough to change the behaviour of a panel in a reflow oven, even when the board looks and measures the same as it did on delivery.

The moisture content is not visible, so it has to be managed by control rather than by inspection. Our shelf life notes describe how the storage window and the packaging work together to keep it inside limits.

Resin Chemistry and Volatile Content

Every resin system retains some volatile material after lamination, and the amount depends on the chemistry, the press cycle and the degree of cure. A stack that was pressed at the low end of its window holds more of it, and that material leaves during the next heating cycle.

Incompletely cured laminate is also softer, so it moves more under thermal load. The press cycle therefore affects both the amount of gas and the ability of the board to contain it, and the two effects point in the same direction.

Trapped Air and Plating Voids

Air can be trapped inside a drilled and plated via where the plating did not fill the barrel completely, and inside a lamination stack where a layer did not bond over its full area. Both create a reservoir that expands on heating.

Where the reservoir is connected to the surface by a small path, the gas pushes through and forms a void in the joint or a pit in the mask. Where it is sealed, the pressure builds until something gives, and the blister is the result.

Blister Formation During Reflow

A blister appears at the moment the board passes the glass transition temperature and the laminate softens. The gas pressure rises at the same time, and the softened resin offers less resistance, so the surface lifts.

The location is often beside a large copper area, because copper heats faster than the surrounding laminate and the bond line is stressed by the difference. Our coating notes describe what happens when a coated board is heated again in a later process.

Bake Schedules and Their Limits

A bake removes moisture before assembly, and its effectiveness depends on time, temperature and how the boards are stacked. Boards baked in a tight stack dry more slowly than boards supported on edge, and the difference is measurable between the outside and the middle of the pile.

The bake also has limits. A temperature high enough to drive the moisture out quickly can oxidise a finish or stress the laminate, so the schedule should come from the material supplier rather than from a shop habit. Our core baking notes cover the same principle for the inner layers.

Storage, Humidity and Floor Life

Once a board leaves the oven it starts absorbing moisture again, and the rate depends on the room. A floor life limit is the practical control, and it should be written with the packaging and the humidity it assumes.

Where a board exceeds its floor life it should be re baked rather than used, because the difference between a dry board and a damp one only appears at reflow. Our floor life notes describe how the same limit is applied to moisture sensitive components.

Vias, Planes and the Path to the Surface

A dense via field gives the gas many routes to the surface, which reduces the chance of a blister but increases the chance of a void in a joint. Sparse vias over a large plane do the opposite, holding the gas until the pressure finds a weak point.

The design response is to keep the copper distribution as even as possible and to avoid large unbroken planes next to areas with no copper at all. Where that is not possible, the bake and the storage controls carry more of the load.

Bake Equipment and Loading Practice

An oven used for drying boards is not the same as a reflow oven. It needs even air circulation, a controlled ramp and a way of holding the panels so that air reaches both faces, and a cabinet packed with boards on a shelf will dry unevenly whatever the timer says.

The load should be recorded as well, because a full oven and a half full one do not behave identically. Where the schedule was developed on a lightly loaded oven, the same time is too short for a full load, and the difference appears as blisters on the panels in the middle of the stack.

Coatings and Potting Over a Wet Board

A coating or a potting compound applied over a board that still holds moisture traps the gas more effectively than the mask did. The material then blisters in service or lifts from the surface, and the failure is attributed to the coating rather than to the board.

The sequence therefore matters: dry, assemble, clean, dry again if the cleaning step is wet, and then coat. Skipping the dry after a water based cleaning process is a common route to a coating defect that looks like poor adhesion.

Records and First Article Checks

The record should show the bake the boards received, the storage conditions and the time between the bake and the reflow. Those three items explain most blisters, and without them the investigation starts from nothing.

The first article should include a look at the board after reflow for pits, blisters and mask lifting, because these defects appear at that step and nowhere else. The IPC assembly standards give the acceptance criteria for the surface after soldering.

FAQ

Is a bake always necessary? It is necessary when the boards have been stored beyond their controlled window or in a humid environment. A board that has been kept sealed and dry may not need one, and the decision should follow the storage record rather than a general rule.

Can a blister appear after the board has passed inspection? It can, because the gas may need a second thermal cycle to build enough pressure. A board that survives assembly can still blister during a later rework or a coating cure.

Does a thicker board resist outgassing better? It resists the same way any thicker section resists pressure, but it also holds more moisture and takes longer to dry. The bake schedule has to account for the thickness rather than assume one time fits every panel.