Flexible circuits help us fit electronics in confined spaces better than conventional FR4 type PCBs. Along with the flexibility comes a challenge of reliably mounting chips on a flexible substrate. Generally, chips on flex use underfill to protect the component and interconnections, and add a level of reliability to the process. If a capillary underfill is in order, than the proper flux needs to be selected. I have 2 recommendations:
- NC-510 for no clean applications, it has an ultra-low post reflow residue level. This will help the underfill flow faster, and more completely.
- WS3622 for water soluble applications. This flux cleans easily, so the easily permeated flexible substrate does not need to spend excess time in contact with water.