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The Complete Guide to Plating Types for Flexible Circuits

Introduction

Flexible circuit Plating directly contributes to the circuit’s performance by enhancing its electrical conductivity. A well-plated circuit ensures minimal signal loss and optimal performance. By providing a smooth and conductive path for electrical signals. This is crucial in high-frequency applications. where even slight increases in resistance can significantly impact performance.

flexible circuit plating

What is the plating in flexible circuit?

Plating is the technique that applies a metal layer to the conducting surfaces of flexible circuits. This accomplished via chemical or electrochemical means. The metal layer performs several functions. the main one of which is to improve the performance and longevity of the circuit.

Plating improves the electrical conductivity of the circuit. Metals with high conductivity, such as gold and copper. These are commonly used to minimize the resistance of conductive channels. This is critical for the circuit’s efficient operation. particularly in applications that need high-speed data transfer.

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What is the Purpose of Plating in Flexible Circuit ?

Increase Corrosion Resistance:

The metal coating functions as a barrier, shielding its core components from oxidation and corrosion. This is especially crucial in difficult situations where moisture, chemicals. or high temperatures might destroy the circuit’s components.

Improve Wear Resistance:

In applications where physical contact or wear is a concern (such as connectors). a plated layer can increase the surface durability, extending the lifespan of the circuit.

Enhance Solderability:

Plating materials like tin or gold improve the surface’s ability to soldered. facilitating more reliable connections between components and the flexible circuit.

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Types of Plating for Flexible Circuits

Plating TypeDescriptionAdvantagesDisadvantages
Panel PlatingDeposits copper on the entire panel, including vias, before any imaging. Minimizes current density variations; uses conventional fabrication techniques.Higher resource consumption; less flexible circuits; potential for thickness variation in impedance control applications.
Pattern PlatingDeposits copper only on selected areas defined by an imaged resist, followed by tin plating as an etch resist. LConsumes less plating resources; creates circuit pattern with one imaging operation.Added copper on traces can affect flexibility and impedance.
Bussed PlatingInvolves creating the copper trace pattern first, then plating up the patterned traces including vias. Only one imaging operation needed; used for specific surface requirements like keypad buttons.Requires physical connection of traces; uneven current density and distribution; issues with flexibility and impedance control; fine line traces limit current capacity.

Panel plating

Panel plating is a process that involves applying a layer of copper to the whole surface of a panel. including both sides and the holes, to enhance the entire surface of the substrate with metal. Typically, this process takes place before imaging. enabling imaging and etching using conventional circuit fabrication techniques. The primary advantage of panel plating is its capacity to equalize changes in current density. However, it needs more plating resources. due to the substantial amount of copper that’s etched away after the imaging process. Another disadvantage is the lack of stability and the higher likelihood of fracture. when electrolytic copper is added on top of rolled annealed copper.

Pattern plating

Pattern plating is a process that focuses on depositing copper in specific locations. By utilizing an image-resist coating to define the desired pattern. After creating the photoresist pattern, we proceed by applying a layer of tin on top of the exposed copper design to act as an etch resist. The procedure effectively preserves plating resources in comparison to conventional panel processes. During the process of etching, tin acts as a protective layer for the desired copper. Subsequently, the tin gets eliminated to expose the final plated trace. This approach not only conserves resources. but also introduces additional copper to the remaining pathways. which could potentially affect their flexibility.

Bus Plating

Initially, we generate copper trace patterns using the “print and etch” technique. After that, we deposit a layer of metal onto the patterned traces. which also covers the through-holes. The main advantage of this technology is that it requires only one imaging step. However, it has several drawbacks:

The-resist-stripped-away-leaving-the-final-circuit-pattern

For the plating process to be fully effective, all parts of the trace pattern have to be physically linked. Any interruption halts the process of plating on that particular surface. Alignment problems could lead to uneven current density and distribution. which can affect the uniformity of plating thickness. Similar to pattern plating, the process of plating copper on every trace can have an impact on both flexibility and impedance control. Coating slender pathways as a challenging endeavor. and can potentially decrease their ability to transport electrical current. Previously, it was typical to use bus plating for copper traces. which was not influenced by the width of the trace or the density of the plating current. In modern times, bus plating is mostly used for particular purposes. such as keypad buttons, numerous connector inserts, or gold ball wire splices.

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Process of Flexible Circuit Plating

The main steps in the electroplating process for flexible circuit boards are:

Surface Treatment:

First, clean the flexible substrate to remove dirt and oxides. ensuring a strong bond between the metal layer and the substrate. Common methods include chemical cleaning, mechanical polishing, and chemical etching.

Base Layer Plating:

Next, plate the base layer on the flexible substrate, typically using metals like copper or nickel. This step improves conductivity and adhesion.

Patterning:

After base layer plating, use photolithography to form circuit patterns on the substrate. This involves applying photoresist, exposing, and developing.

Mid-Layer Plating:

Then, perform mid-layer plating to enhance the circuit board’s conductivity and corrosion resistance. often with metals like copper or nickel.

Protective Layer Plating:

Finally, add a protective layer plating to shield the flexible circuit from environmental damage. using metals like gold or tin.

These steps outline the general electroplating process for flexible circuit boards. though specific procedures may vary based on different needs and materials.

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Tips for Plating in Flexible Circuit

plating process in FPC

Pre-treatment of FPC Electroplating:

After plating, the copper conductive surface exposed on the flexible circuit might get coated with glue or ink. Additionally, high-temperature operations cause oxidation and discoloration. To achieve a closely adherent plating layer. contaminants and oxidation need removed from the conductive surface and kept clean. However, some of these contaminants are strongly adhered to the copper conductors. and cannot be entirely removed with weak cleaning solutions. Stronger alkaline abrasives and brushing are frequently used for therapy. Most coating adhesives are epoxy resin-based and have low alkali resistance. resulting in reduced bonding strength. Although not outwardly obvious, during the FPC plating process. plating solution may seep in from the margins of the coating layer, causing delamination.

Thickness of FPC Electroplating:

Electroplating relies heavily on electric field intensity for metal deposition rates. This intensity changes based on the circuit pattern’s shape and the electrodes’ positioning. Specifically, narrower conductor widths, sharper terminal points. and proximity to the electrode boost electric field intensity. Consequently, this increases plating layer thickness in those areas. In FPC applications, which often feature significant variations in conductor widths within the same circuit. this can lead to uneven plating thickness. Implementing diversion cathode patterns around the circuit can mitigate this issue. These patterns effectively absorb the uneven currents on the electroplating pattern. ensuring a uniform plating thickness across all parts. Therefore, optimizing electrode structure becomes crucial. We recommend a balanced approach. enforcing strict standards for areas needing uniform plating thickness. and more lenient standards elsewhere. High standards apply to electroplated lead-tin for soldering and electroplated gold for wire bonding. whereas general corrosion resistance plating for lead-tin can follow less stringent thickness standards.

copper plating

Stains and Dirt in FPC Electroplating:

The electroplated layer may appear to have no issues. especially in terms of appearance. However, shortly afterward, some surfaces may exhibit stains, dirt, or discoloration. Especially in concave areas where various solutions tend to accumulate and react over time. This phenomenon arises due to insufficient rinsing, leaving residual plating solution on the surface. which undergoes chemical reactions slowly over time. Particularly with flexible circuits, due to their soft and not entirely smooth nature. various solutions tend to accumulate in the concave areas. leading to discoloration through subsequent reactions.

FPC Chemical Plating:

When a circuit conductor can’t act as an electrode and stands isolated. At that time chemical plating became necessary. Chemical plating, especially with gold, involves strong solutions. These solutions are highly alkaline with a high pH. Poor quality control in the covering layer lamination can lead to the solution seeping under the layer, weakening the bond.

Using displacement reactions for chemical plating increases the risk of solution penetration. due to the solution’s nature. Achieving perfect plating conditions with this method is tough.

FPC Hot Air Leveling:

Hot air leveling, initially designed for lead-tin coating on rigid PCBs. now also suits flexible PCBs (FPCs) for its simplicity. This method dips the board into molten lead-tin. and uses hot air to remove excess solder, demanding extra care for FPCs. FPCs need enclosing in titanium steel wire meshes for solder immersion. They must be clean and prepped with flux before dipping. Poor adhesion between the covering layer and copper foil can lead to solder seeping under. that is a common issue. Plus, polyimide films in FPCs absorb moisture. which can cause bubbling or delamination when heated during leveling. Thus, drying and moisture control are crucial steps before starting FPC hot air leveling.

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Conclusion

Gesflex has always adhered to the principle that no matter how big or small the order is, we will treat it with seriousness and rigor. What we offer is efficient service and a long-lasting win-win situation. Although the flexible circuit market is huge, but the quality of the supplier’s control of the product varies. we will strictly control the quality of the product for you!

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