Protect Rigid Flex From Moisture
Rigid-flex PCBs are a flexible solution for challenging systems with specialized requirements. However, the unique combination of rigid and flexible components can present some additional design challenges that must be addressed to ensure a high-quality product. In this blog post, we explore several common rigid-flex design issues and offer tips for preventing them. Provides a tin-lead coating but is less ideal for fine-pitch components.
One of the most common issues that arises in rigid-flex assembly is moisture delamination between the flex and the FR4 stiffener. This can occur for a number of reasons, but the most common cause is moisture absorption in the flex material prior to solder reflow during assembly. This causes the moisture to rapidly heat up and expand, causing delamination between the flex and the stiffener. To prevent this, it is essential to pre-bake the flex/rigid-flex before the reflow process.
Another common issue is excessive stress in the flex area due to improper stack-up. To avoid this, it is important to establish the minimum bend radius of your circuit early in the design phase and to keep it within this limit. This will help reduce stress and protect the copper during flexing, and it will also increase the reliability of your flex circuits. Surface finishes are applied to the exposed copper pads to prevent oxidation and ensure good solderability. Common finishes include:
How to Protect Rigid Flex From Moisture
To further protect your flex circuits from excessive stress, you should always use a high-quality flex material and stagger the traces. This will allow you to reduce the number of connections in your flex circuit, which can lower the overall impedance and improve signal integrity.
During the manufacturing process, it is also important to bake your boards before any assembly or rework takes place. This will help to remove any absorbed moisture and reduce the chances of oxidation on the pads and solder joints. It is essential to follow the recommended baking temperatures and times, which can vary depending on the specific flex/rigid-flex material used.
The polyimide materials used in rigid flex and rigid-flex PCBs are hydroscopic up to 2-3% by weight, meaning they can absorb moisture very quickly. This can lead to thermal defects like substrate blistering and barrel cracking during the soldering process. It can also cause thermal stresses on plated through holes, which can result in a loss of conductivity and poor connection. To prevent this, it is important to pre-bake flex/rigid-flex circuits prior to any assembly or rework processes.
Baking is typically done at 250degF (121degC) for two to ten hours. It is a necessary step to remove the moisture from the circuit board and ensure proper processing. This will eliminate the hazard of blistering, shrinkage and other defects associated with moisture absorption. It is particularly important for the high-reliability flex-rigid hybrid products that are manufactured using Pyralux, as it is a very sensitive material to moisture absorption. To further protect the circuits from moisture, it is a good idea to use a no-flow prepreg layer and a cut back coverlay or bondply.