How to Prevent Adhesive Bubbling in Flex PCB Lamination?

Flex Plus Tech team
Jan 16
2 min read

Updated: Aug 26

In flexible PCB manufacturing, adhesives do a lot of heavy lifting. They bond the layers together, add durability, and directly affect how well the finished board performs. But when it comes to lamination, one problem can quickly spoil the process—adhesive bubbling. Even small bubbles can weaken bonds, create uneven surfaces, and, in some cases, lead to electrical issues.

So, what causes bubbling, and what can you do to stop it?

Why Does Adhesive Bubbling Happen in Flex PCBs?

Think of bubbling as trapped air or moisture expanding under heat. During lamination, adhesives are pressed between layers of material. If anything volatile gets stuck inside, it can turn into gas as the temperature rises and form bubbles.

The most common reasons for flex PCB adhesive bubbling include:

Trapped air – Small air pockets may slip in during the lay-up stage.
Adhesive outgassing – Some adhesives release gases when heated, which creates bubbles.
Moisture – Any leftover moisture in the material turns into steam under heat.
Uneven heat and pressure – If lamination isn’t applied evenly, weak spots can form.

Why Low-Outgassing Adhesives Make a Difference

One of the best ways to minimize bubbling is by using low-outgassing adhesives. Here’s why they help:

Fewer volatile components – Less gas is released during heating.
Better flow under heat – These adhesives spread more smoothly, forcing out trapped air.
More consistent curing – They harden evenly, reducing the risk of weak spots or delamination.

Choosing the Right Adhesive for Flex PCBs

When selecting adhesives, look at both performance and compatibility with your design:

Material compatibility – Make sure it bonds well with polyimide films, copper foils, or other materials in your stack-up.
Thermal stability – Pick adhesives that can handle your lamination temperature without breaking down.
Flexibility – Since flex PCB bend and twist, the adhesive needs to stay strong without cracking.
Low-outgassing rating – For critical applications, check that the adhesive meets standards for low outgassing.
Choosing the right adhesive is the first step toward eliminating flex PCB adhesive bubbling.

Process Tips to Reduce Bubbling

Even with the right adhesive, process control is just as important. A few best practices include:

Slow down the lamination press – More time allows adhesive to flow evenly and release trapped air.
Keep layers aligned – Misalignment can leave tiny gaps that turn into bubbles.
Use even heat and pressure – Consistency across the press surface is key to a smooth bond.
Choose the right thickness – Thinner adhesives often reduce outgassing and bond more consistently.

Why It Matters

Bubbling isn’t just cosmetic. It can cause delamination, weaken electrical performance, and shorten the lifespan of your flex PCB. By pairing low-outgassing adhesives with a carefully controlled lamination process, manufacturers can produce stronger, more reliable boards.

At FlexPlusFPC, we focus on the details that make the difference. From material selection to process optimization, our team helps customers build flexible PCB that meet the highest standards of quality and reliability.