Flexible Printed Circuits

Whether it is a flexible printed circuit or a rigid PCB, electronic components are subjected to harsh environmental testing. Thermal cycling and random vibration are common tests conducted on these components to ensure they can withstand the rigors of everyday use in the field. These testing conditions can cause solder joint fatigue. Fortunately, a few simple preventative measures can help mitigate this potential problem and extend the lifespan of the solder joints in your final product.

The key to preventing solder fatigue in your flexible printed circuit is the selection of the proper solder alloy and copper plating process. Different solder alloys have a different response to mechanical loading and temperature changes. Some of them have superior shock/vibration and thermal cycling performance, while others have poorer performances in these areas. Using the correct reflow profile for your specific alloy can significantly improve both these performance metrics.

The coefficient of thermal expansion (CTE) is another important factor to consider when designing your flex PCB. This property defines how much a material will expand or contract at a given temperature. CTE mismatch is a common issue that causes solder joint failure in flex PCBs. It occurs when the board is bolted into a rigid metal housing with a different CTE than that of the board itself. The housing expands and contracts faster than the board, causing it to bend excessively during thermal excursions, and stressing the solder joints.

Solder Joint Fatigue in Flexible Printed Circuits

Solder joint overstress failure usually manifests as a pad crater in the laminate layer beneath the copper pad of the solder joint. It may also appear as a joint fracture along the intermetallic connection (IMC) region, where the solder and copper combine to form Cu3Sn or Cu6Sn5. The IMC is the most brittle part of the solder joint, which makes it particularly susceptible to overstress.

Using an accurate finite element modeling technique can also help you predict solder joint fatigue in your flex PCB. This method combines both creep strain and matrix-creep model to determine the fatigue life of a solder joint. Flexible printed circuits (FPCs) represent a pivotal advancement in electronics manufacturing, revolutionizing the design and functionality of various electronic devices. These thin, lightweight circuits offer unparalleled flexibility, enabling them to conform to complex shapes and fit into tight spaces, which is often unattainable with traditional rigid PCBs (Printed Circuit Boards).

Another important prevention measure is to ensure that your flex PCB has a well-designed layout. Make sure that any slots, slits or inside corners are terminated properly to avoid tearing the flex substrate material. IPC recommends terminating these areas with tangential curves with radii of 1.5 mm or more to minimize the risk of this problem.

Finally, it is important to avoid applying excessive solder iron pressure during hand soldering of your flex PCB. This is especially important when you are soldering to a flex area, as the adhesive system can soften when exposed to high temperatures. Therefore, it is crucial to avoid using too much pressure and to keep the solder iron tip as close to the surface of the pads as possible.