Design Guidelines for Flexible Printed Circuits

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An important but sometimes overlooked aspect of flex and rigid-flex fabrication and assembly is the flex circuit tail, which is attached to a rigid PCB with pressure-sensitive conductive adhesives. This sub-assembly is becoming very common. We often see this applied to glass displays and microelectronic applications.

But this method of attachment without connectors is not as straightforward as one may think. The wide varieties of circuitry to be bonded create many attachment challenges. The best track configuration is not always possible, so one must be familiar with the optimum layouts as well as what is not recommended when considering these trade-offs. Less than optimum trace and pad layouts result in the need for more customization and smaller processing window variations for the bonding process.

Here are some common configurations that flex assemblers have developed, and the preferred approach for the method of bonding.

Interposer Effect and Co-planarity Adjustment

Bonding imbalance results from either bad co-planarity between the bonder head and stage or a thickness deviation of the flex and PCB materials. The interposer will absorb these discrepancies to some extent and help bring the bonding results in parallel. 

To read this entire article, which appeared in the February 2024 issue of Design007 Magazine, click here.