Dual-Side Copper Terminal Soldering on Flex PCB Gold Fingers: Process Challenges and Solutions

In high-reliability electronics such as medical devices, sensing modules, and compact interconnect systems, standard flexible PCB assembly is often not enough. Specialized structures—like dual-side copper terminals soldered on flex PCB gold fingers—require a much higher level of process control and engineering expertise.

At Flex Plus, we have developed a stable and repeatable solution for this challenging structure, ensuring precision alignment, solder reliability, and biocompatibility.

The Challenge: Dual-Side Copper Terminal Soldering on Flex PCB

Unlike conventional SMT assembly, this design involves soldering copper terminals on both sides of the flex PCB gold finger area, with strict structural constraints:

• Bottom side terminals: thin and elongated 

• Top side terminals: thicker and shorter 

• Both terminals must be perfectly overlapped 

• Zero positional offset is allowed 

This creates multiple technical challenges:

1. Sequential Reflow Constraint

Since both sides cannot be reflowed simultaneously, the process must be split into two stages. First reflow for one side, second reflow for the opposite side.

This introduces a critical risk:The second reflow can affect the already soldered terminals, leading to misalignment or re-melting.

2. Alignment Accuracy (No Offset Allowed)

Because the terminals must fully overlap, even a slight deviation can result in poor electrical contact, mechanical instability, and assembly failure during final integration.

Maintaining micron-level alignment across two reflow cycles is extremely challenging.

3. Solder Wicking Control

The gold finger area is highly prone to solder wicking, especially with:

• Long, thin bottom terminals

• Capillary effects along exposed copper

Uncontrolled solder flow can lead to:

• Inconsistent joint thickness

• Reduced contact reliability

• Cosmetic and functional defects

4. Biocompatibility Requirements

For applications such as medical devices, materials and processes must meet biocompatibility standards, including controlled surface finishes, clean soldering processes, and no harmful residues.

Our Engineering Solution

To address these challenges, Flex Plus applies a combination of precision tooling, process optimization, and material control.

Controlled Sequential Reflow Process

We optimize the thermal profile to:

• Protect the first-side solder joints during the second reflow

• Maintain structural integrity without reflow collapse

• Ensure consistent solder wetting on both sides

Custom fixtures are used to stabilize the flex PCB during both reflow stages.

High-Precision Alignment Technology

To guarantee perfect overlap:

• Dedicated alignment jigs are developed

• Fiducial-based positioning ensures repeatability

• Tolerance is controlled within extremely tight limits

This ensures the top and bottom terminals align exactly as required.

Anti-Wicking Design & Process Control

We implement multiple strategies to prevent solder wicking:

• Optimized pad and gold finger design

• Precise solder paste volume control

• Surface treatment adjustments to manage wetting behavior

This results in clean, well-defined solder joints without overflow.

Biocompatible Material & Process Management

For medical-grade applications:

• Biocompatible materials are used

• Strict cleaning processes eliminate residues

• Surface finishes are selected to meet application-specific requirements

Why This Matters

This type of structure is not just about assembly—it directly impacts:

• Electrical reliability

• Mechanical durability

• Product lifespan in critical environments

Many manufacturers avoid this design due to its complexity. At Flex Plus, we treat it as a core capability.

Typical Applications

This solution is ideal for:

• Medical sensors and wearable devices

• High-density interconnect modules

• Precision testing interfaces

• Compact electronic connectors

  

Looking for a Reliable Flex PCB Assembly Partner?

If your design involves complex terminal structures, dual-side soldering, or high-reliability requirements, our engineering team can support you from design review to mass production.

We don’t just build flex PCBs—we solve the problems others avoid.