This study was conducted by senior process engineers Lin Tianfu and Liu Zhonghui from ABOOSTY Antenna. The team is led by antenna experts with more than 20 years of terminal-industry experience and is dedicated to delivering reliability-proven solutions backed by data.

Introduction: A Common but Costly Failure Mode

In consumer electronics and IoT hardware assembly, soldering a coaxial cable onto an FPC antenna is a routine process. Yet one recurring failure continues to haunt manufacturers:

the solder pad lifts off together with the copper foil during cable routing or final assembly.

This failure mode not only forces scrapping of finished products but also poses serious long-term reliability risks. Field failures often lead to customer complaints and costly rework cycles.

So what exactly drives this pad-peeling phenomenon—material defects or process conditions?

To answer this, we conducted a focused data-driven experiment.

 1. Experimental Design: Identifying the Two Dominant Variables

We hypothesized that solder-pad adhesion is primarily influenced by:

a) Copper-foil type in the FPC substrate

• ED (Electro-Deposited Copper)
• RA (Rolled-Annealed Copper)

b) Soldering temperature

Three temperature points commonly used in FPC coax soldering were selected:

320°C, 340°C, 350°C

 Caption ：Measured soldering tip temperature at 345°C using the HAKKO FG-100B tester — one of the controlled process variables in the RA vs. ED copper FPC antenna soldering experiment.

HAKKO FG-100B measuring a soldering iron tip temperature of 345°C during FPC antenna soldering calibration.

 Test Method

Each material–temperature combination (6 total conditions) included five samples.

• After soldering, each sample was fixed on a jig, and a tensile tester applied a vertical pull at a constant speed until failure. We recorded:Maximum pull force (N)
• Failure mode (pad peeling, solder-joint break, cable fracture)

This allows us to quantify both reliability and process stability.

 

Caption ： Experiment samples of ED and RA copperFPC antennas soldered at three temperature levels, shown here before tensile testing, with ambient conditions recorded during sample preparation.

2.  What the Data Reveals: A Clear Gap in Process Stability

Pull-Force Results and Failure Records

Caption： Pull-force tensile test of FPC antenna samples using a digital force gauge to quantify pad adhesion strength.

 Key Insight #1: Rolled-Annealed (RA) Copper Delivers Superior Process Stability

Across all temperatures, RA copper demonstrated extremely low data variance—a hallmark of robust, predictable process behavior.

In contrast, ED copper shows high variability, confirming real-world feedback:

“Sometimes it peels, sometimes it doesn’t.”

This inconsistency reflects ED copper’s narrower process window. Slight fluctuations in soldering heat or pressure can shift the result from acceptable to catastrophic.

 Key Insight #2: The Best Performance Combination Is Clear

1. Temperature effect:

Higher temperatures generally improved adhesion for both materials.

2. Material effect:

Even at 320°C, RA copper already outperforms the best ED scenario, and never exhibited a single pad-peeling failure.

3. The optimal combination:

RA copper + 350°C soldering temperature

• This pairing showed:Highest average pull force
• Best consistency (std. dev. = 0.60)
• Zero pad-peeling failures

3. Failure Analysis: Understanding Why ED and RA Behave Differently

Microscopic observation after tensile testing revealed two distinct failure modes:

ED Copper FPC

• Pads detached as a brittle peel from the PI substrate
• The root cause lies in ED copper’s columnar grain structure, low ductility, and inherently weaker adhesion under thermal shock
• Thermal stress during soldering accelerates interfacial failure

RA Copper FPC

• Failures occurred at the solder joint or cable, not the pad
• Indicates strong copper-to-substrate adhesion
• RA copper’s dense grain structure and superior ductility better withstand thermal and mechanical loads

This fully aligns with literature and industry reliability data: RA copper is materially superior when mechanical or thermal stresses are involved.

 4. Practical Conclusions and Actionable Guidelines

Conclusion 1 — Material choice is the decisive factor

To prevent pad peeling in FPC antenna assemblies, the primary lever is switching from ED copper → RA copper.

This dramatically broadens the process window and fundamentally improves reliability.

 Conclusion 2 — Recommended process window

For RA-based FPC antennas:

Recommended soldering temperature: 340°C–350°C

This range balances adhesion strength and thermal stress tolerance.

 Conclusion 3 — For hardware developers and sourcing teams

When FPC antennas require soldering of coax cables, IPEX connectors, or discrete wires:

Specify RA copper as a mandatory material requirement in the FPC datasheet/BOM.

Although RA copper increases material cost slightly, it avoids:

• Mass-production yield loss
• Field return risks
• Customer complaint cycles
• Hidden reliability failures

This is a textbook example of Design for Reliability (DFR):

Preventing failures at the source is always cheaper—and always more effective—than correcting them afterward.