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All About Flex: Button Plating on a Flexible Circuit

Button plating describes a fabrication process widely used in the flex circuit industry to selectively electroplate copper to the vias and onto the pads capturing the vias. The rest of the copper traces do not have plating. Another industry term used to describe this feature is pads only plating. Producing a circuit with this processing methodology requires two photolithography steps. The initial imaging/developing process provides electroplating chemistry access to the copper vias and pads. The second process repeats the imaging to define the copper trace pattern. The term button is descriptive as the selectively plated pads are slightly raised from the surrounding copper regions and create a physical signature that looks like a button.

There are two primary applications where button plating is desirable.

1. Dynamic flex applications (when continuous or frequent bending of the flex circuit is required) and
2. Impedance control applications (often a requirement in high signal speed applications)

Dynamic Flex

Some applications for flexible circuits require specific areas of a circuit to be flexed during the operation of the consumer device. This could be thousands (opening and closing of a notebook computer) if not millions (interconnection to the read/write heads on a disc drive) of flex cycles required during the life of the product. Each dynamic flex application has a unique combination of motion, bend radius, cycle acceleration and proximate circuitry features that may affect flex life performance. Avoiding the addition of electroplated copper onto traces by button plating is a common practice for extending flex life. Since electroplated copper has a different grain structure vs. rolled annealed copper, the grain structure is less flexible and prone to fracture with repeated bending.

To further optimize flex life, additional design rules with specific material sets and features will often be adopted and/or recommended by the fabricator:

• Use of rolled annealed copper which has a grain direction parallel to the roll length
• Orienting the circuit on the panel to position the grain direction perpendicular to the flex plane
• Balancing the material thickness composition to locate the copper layer in the neutral axis of the flex circuit sandwich. This prevents the copper from being in either compression or tension when bent
• Avoiding any unusual shapes of traces or inclusion of features such as vias in the flexing region (parallel traces of equal width are optimum features through a dynamically flexing region)
• Thinner copper will generally perform better than thicker copper for extended flexing

These design practices, in combination with button plating, will help ensure a circuit design has been defined for optimum flexing. Ideally, a flexible circuit should undergo verification testing in a simulated end-product environment to confirm expected performance with some empirical data.

Impedance Control

Applications using high-speed electronics frequently require impedance control. As with dynamic flex applications, a specific structure and material set is ideal:

• Homogenous dielectric material (ER)
• Equivalent dielectric thickness spacing between top and bottom ground planes
• Consistent copper thickness and copper width
• Consistent copper spacing

Electroplated copper thicknesses are more variable with electroplated surfaces vs. non-plated copper. It is not uncommon to see a 10% variation in a circuit’s copper plating thickness as current densities cause features to plate at higher or lower rates. This variation can translate to a similar variation in the impedance. Pads only plating of circuitry requiring impedance control will result in less variability within an individual part and between multiple product builds.

Another critical aspect of impedance control is the relative dielectric (ER) constant of the dielectric stack-up. The dielectric stack-up can consist of one or more layers of dielectric film bonded together with adhesives. The adhesive has a different dielectric coefficient than the dielectric film. Using adhesiveless laminates reduces the variation caused by adhesive and will normally result in a more consistent impedance.

References

1. Processing options when plating vias on double-sided flexible circuitry have been discussed in previous columns publishing at flex.iconnect007.com: Plated Through-holes in Flexible Circuits  and Plating Process Options for Flexible Circuits.

2. Impedance calculator, www.ideaconsulting.com.

3. Considerations for Impedance Control in Flexible Circuits, flex.iconnec007.com.

4. Tradeoffs when implementing impedance control, www.allflexinc.com.

5. Impedance control using microstrip and strip line, www.allflexinc.com.

Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.