Polyimide Coverlay and Adhesive Squeezout

When a flexible circuit requires high dielectric or dynamic flexing, an adhesive coverlay film is often the best choice.

This coverlay film is traditionally a layer of adhesive bonded to a layer of polyimide. During processing, heat and pressure are applied to the stack up causing the adhesive to soften and flow.   The adhesive will flow (squeeze-out) slightly beyond the coverlay openings.

This process is necessary for complete encapsulation of the coverlay and to protect the edges of the film from chemicals or abrasion which might cause delamination.

Although this is a desirable result of bonding the coverlay, this “adhesive squeeze-out” also reduces the solderable area of the coverlay opening, and must be accounted for in the design stage.

We are often asked what an acceptable amount of adhesive squeeze-out is. According to IPC-A-600, the coverlay coverage shall have the same requirements as the soldermask coverage in rigid printed circuit boards. The acceptability requirements for coverlay coverage include both the coverlay and the squeeze out of adhesive and are different based on which Class is being built to.

For example, Class 3 requires 0.05 mm (0.00197”) solderable annular ring for 360 degrees of the circumference. Class 2 requires this same solderable annular ring for 270 degrees of the circumference and Class 1 requires a solderable annular ring for 270 degrees of the circumference.

We always recommend involving your supplier in the early stages of the flexible circuit design. An experienced flex circuit engineering will be able to guide you to the correct material stack up and tolerances needed to ensure you receive the product you require.

Please contact us for additional information.  Designing printed circuit boards should not be difficult! 

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Flexible Circuits: The Basics

Just getting started with flexible circuits?  Here are few “basics” to keep in mind.   Designing flexible circuits is not difficult, but does require adjustments to ensure that you achieve the flexibility and reliability that you are expecting.

Benefits of flexible circuits:

• Solution to a packaging problem
• Reduce assembly costs – elimination of connectors and solder joints
• Replacement for PCB and wires – simplify system design, reduces number of interconnections and eliminates human error.
• Reduce weight and space
• Dynamic flexing
• Thermal Management/ High Temp Applications
• Aesthetics

Design Considerations:  Conductor Pad Design and Filleting:

• Pads should have tie-downs (rabbit ears).  Tie-downs are captured by the coverlay to anchor the copper and prevent separation of copper and base materials.
• Filleting:  All pads should be filleted to reduce stress points during flexing.

Bending and Folding Considerations:

• Radiused Traces – help to alleviate breaking during folding and bending
• I-Beam  Effect- when conductors are routed directly on top of one another, stiffness is increased in fold areas.  A better alternative is to stagger conductors, alternating their location to retain maximum flexibility.
• Fold Lines – tick marks in copper or silkscreen help identify designated location for bending or folding.
• Circuit Trace Width – this should not change in bend areas and the transition should be at least .030” from fold line.
• Bend radius – very general guideline of 10 times material thickness will work with most applications.
• Button Plating – increased flexibility by plating thru holes but not traces.

 Shielding:

• Solid Copper – this is the most common method of shielding.  This can be done on one or both sides of the circuit or to cover selective conductors.
• Crosshatched Copper – crosshatching will help the circuit to retain its flexibility and can be done in selective “flexing” areas or entire layers.
• Conductive Silver – not recommended for dynamic- flexing applications, but can be substituted for copper shield in certain applications.

 Stiffeners:

• FR4 – normally used to give added rigidity under a connector area.  These can be bonded with pressure sensitive adhesive or thermal set adhesive.  Often used as a carrier panel for automated assembly.
• Polyimide – often used to give added thickness under conductor fingers to meet ZIF connector requirements.  Can be used to give added strength to high wear areas or to identify “fold” areas.  Outline can blanked or routed at the same time as the flex to meet tight tolerance requirements.
• Location:  Stiffener and coveraly termaination points should overlay a minimum of .030” to avoid stress points and to reduce the chance of traces breaking.

Please contact us if you have questions or if we can provide any additional information to assist with your flexible circuit needs.

www.omnipcb.com