Absorption in Flexible Printed Circuit Boards
Printed circuit boards (PCBs) are sensitive to moisture. Moisture absorption in PCBs can lead to several issues, from delamination to solderability problems. The best way to prevent these issues is to understand the moisture absorption process in PCBs and take precautions when designing, fabricating, shipping, storing and using PCBs.
When it comes to flex and rigid-flexible printed circuit, the main causes of moisture absorption are poor design and handling practices. To minimize the effect of moisture on a flex or rigid-flex PCB, designers should follow proper bend guidelines and avoid placing copper on top of itself in the flex and/or bendable sections of the circuit board. This can cause stress points that are prone to cracking and fracturing.
A second major issue is exposing the flex or rigid-flex section of a circuit to direct sunlight. This can lead to the exposure of the conductive layers of the circuit to high temperatures and damage the underlying materials. The best way to protect a flexible or rigid-flex circuit from UV radiation is to cover the exposed area with a thin layer of epoxy, acrylic or hot-melt glue. This can be accomplished by dispensing the glue manually or through an automated fluid dispensing system. However, this adds additional steps to the production process and may increase cost.
How to Handle Moisture Absorption in Flexible Printed Circuit Boards
Another common problem is the failure to remove all absorbed moisture from flex and rigid-flex circuits prior to assembly. This is the primary cause of coverlay and layer to layer delamination as well as stiffener delamination. The absorbed moisture in these cases converts to steam at the assembly re-flow temperature, which causes the bonded parts to separate. This is also a common problem with the use of FR4 stiffeners and is especially problematic when RoHS temperature limits are imposed on the assembly.
It is critical to remember that a hydroscopic material, such as polyimide or polyester, used for insulating the conductors in a flex or rigid-flex circuit, is likely to contain moisture even if it has been vacuum packaged and stored under a desiccant. As such, these insulating materials require a pre-bake to remove all absorbed moisture prior to assembly. This is true regardless of the amount of time elapsed between the initial assembly and the subsequent pre-bake.
For this reason, it is always important to keep in mind that a hydroscopic material will need a pre-bake prior to each assembly cycle. This is true even if the product has recently been shipped from the supplier in a vacuum sealed package and has had time to dry under a desiccant. If this is not done, delamination and other defects will occur during the assembly process.