Interconnection Reliability for Rigid-Flex PCB

Rigid-Flex PCB with multi-module interconnection structure has become a mainstream techincal option, since there are many modern electronic design is pursuing ultimate space efficiency.

In this recently completed Rigid-Flex PCB project by KnownPCB, we ensured stable layout connection of multiple independent functional rigid PCB modules through complex flexible circuits, and achieved high-reliability signal interconnection and all-in-one structural integration in limited complex space.

How to ensure the the long-term interconnection reliability in Rigid-Flex PCBs?

Since the FR4 rigid area and the PI flexible area have distinct physical characteristics including CTE and bending flexibility. The rigid-flex transition zone will easily suffer stress concentration. Therefore, improper manufacturing control may lead to risks such as circuit cracking, delamination, and via failure, which will directly compromise the service life of the rigid-flex board.

During the production of this multilayer rigid-flex PCB project, the following key manufacturing challenges are:

1. Bonding strength challenge:

After lamination of flexible substrate and rigid layers, how to prevent interlayer delamination under thermal shock and vibration?

2. Dynamic bending fatigue:

How to prevent mechanical stress from inducing microcracks on signal traces under a constrained bending radius?

3. Dimensional stability control:

Flexible materials can easily expand shrink in fabrication. So, How to maintain precise hole registration across the multi-module structure?

In order to solve manufacturing challenges of thie rigid-flex circuit board above, KnownPCB came to with these solutions.

Solutions for the Interconnection Reliability in Rigid-Flex Board Manufacturing

1. Interface optimization:

We introduced the Plasma surface treatment process to completely remove resin smear inside vias and micro-roughen the interface of rigid-flex area, these measure significantly improved the intermolecular bonding strength of rigid-flex transition zones.

2. Structural stress simulation:

We optimized the transitional stepped design at the DFM review stage for this rigid-flex PCB, and used teardrop rounded trace layout to disperse assembly stress effectively.

3. Material formulation optimization:

We used Adhesiveless base material to reduce overall board thickness and eliminate the high CTE expansion risk which caused by bonding adhesives.

4. High-precision alignment system:

LDI (Laser Direct Imaging) combined with an automatic optical alignment compensation algorithm is adopted by KnownPCB, it dynamically adjusts according to the real-time expansion and contraction of flexible materials to ensure interconnection accuracy.

Result

Through precise process control of KnownPCB, this multilayer rigid-flex PCB has successfully passed stringent thermal cycling tests and dynamic bending fatigue tests.

High consistency: Multi-module mechanical dimensions and electrical performance are highly consistent.

Superior reliability: The rigid-flex interface features a robust structure, meeting the long-term stability requirements under complex spatial layout conditions.