Flex PCB Solder Defects: Twist, Bending, and Weld Leg

Flex PCB solder defects are common in flexible circuits because the boards bend, twist, and warp more easily than rigid PCBs. These deformations can cause uneven soldering, misaligned components, and weak joints. The main causes are twist and bending of the flexible PCB and weld-leg (pin leg) issues. Understanding these factors is key to reliable assembly.

Twist and Bending 

Flexible PCBs are thin and soft, so small movements during handling, printing, or reflow can cause twist or bending. Even minor deformations can create problems:

• Pads may sit at different heights

• Component pin legs may not sit flat on solder paste

• Solder joints can have uneven thickness, leading to open or cold joints

• Optical components like LEDs or sensors can tilt

How to minimize twist and bending effects:

• Use rigid carriers, stiffeners, or vacuum plates to keep the board flat

• Check and maintain flatness before placement (<0.2 mm warpage)

• Avoid rapid temperature changes during reflow

• Support ultra-thin flexible PCBs with panel frames or edge rails

Twist and bending often lead to weld-leg problems in component placement.

Weld Leg — Pin Behavior on Uneven Surfaces

A weld leg is the metal terminal or pin that contacts solder paste. On a warped or uneven FPCB, different pin legs land at different heights, which can cause:

• High-sitting legs → no solder → open circuit

• Low-sitting legs → excess solder → bridging

• Components rocking during reflow → skew or tombstoning

• Fine-pitch ICs particularly sensitive to uneven pin placement

Preventing weld-leg defects:

• Keep the flexible PCB board flat during stencil printing and placement

• Check coplanarity of pin legs for sensitive components

• Adjust stencil paste distribution for outer pads

• Maintain uniform adhesive thickness in COB assembly

Factor	Risk if Ignored	Mitigation
Twist & Bending	Uneven pad contact, open joints, component tilt	Flat support, stiffeners, careful handling, controlled reflow
Weld Leg / Pin Leg	Open joints, bridging, skew, tombstoning	Coplanarity checks, stencil paste adjustment, adhesive height control

Common Flex PCB Solder Defects

Typical defects resulting from twist, bending, and weld-leg issues include:

• Pad lifting — pads detach from the PI substrate due to stress

• Cold solder joints — poor wetting from uneven paste or rapid cooling

• Solder bridging — caused by uneven pad heights

• Component shifting — soft FPC allows movement during reflow

• Cracked solder joints — bending during assembly or in-field use

Tips to Reduce Solder Defects

Design and Layout:

• Avoid placing solder joints in high-bending areas

• Add stiffeners under heavy or sensitive components

• Use reasonable pad sizes to reduce lifting

Handling and Placement:

• Keep flexible circuits flat during transport and assembly

• Use trays, plates, or support structures to stabilize boards

• Optimize solder paste printing and volume

Reflow and Assembly:

• Adjust reflow profiles for PI and copper stack-ups

• Ensure components are properly aligned and coplanar

• Monitor soldering quality and perform bend-cycle or pull tests as needed

  

Summary

Mechanical stability is critical for flex PCB solder quality. Controlling twist, bending, and weld-leg behavior ensures consistent solder joints, reduces rework, and improves long-term reliability for SMT and COB assemblies.