FPC hipot testing: Causes of Arc Discharge and Leakage Voltage Failures

FPC hipot testing is one of the most important electrical reliability inspections in flexible PCB manufacturing. While continuity testing verifies that circuits are connected correctly, it does not reveal insulation weaknesses that may lead to electrical failures in actual applications.

During hipot testing, manufacturers occasionally encounter arc discharge, leakage voltage, or insulation breakdown. These failures can occur even when the FPC passes all previous electrical inspections.

Understanding why these failures occur is critical for improving product reliability and ensuring long-term field performance.

What Is FPC Hipot Testing?

FPC hipot testing, also known as high-potential testing, is a method used to verify the insulation integrity of a flexible printed circuit. As part of a comprehensive flex PCB testing program, hipot testing helps manufacturers identify insulation weaknesses that may not be detected through standard continuity inspections.

A voltage significantly higher than the normal operating voltage is applied between isolated conductors or conductive structures. The objective is to confirm that the dielectric materials can withstand electrical stress without breakdown.

FPC hipot testing is commonly used to detect:

• Insulation defects

• Leakage current paths

• Arc discharge risks

• Dielectric weaknesses

• Manufacturing contamination

• Material damage

Because flexible circuits are often used in compact electronic devices with very fine conductor spacing, hipot testing provides an additional layer of quality assurance beyond standard continuity testing.

Why Arc Discharge Occurs During FPC Hipot Testing

Arc discharge is one of the most visible failure modes during FPC testing.

When the electrical field becomes strong enough to overcome the insulation strength of air or dielectric materials, electrical energy jumps across a gap and produces a visible spark.

In flexible PCB manufacturing, arc discharge may result from:

• Insufficient conductor spacing

• Damaged coverlay insulation

• Lamination voids

• Moisture absorption

• Conductive contamination

• Improper shielding structure design

The presence of arc discharge indicates that insulation integrity has been compromised and requires immediate root cause analysis.

Case Study: Black Shielding Film Causing Arc Discharge

In one production evaluation, arc discharge was observed when the FPC cable contacted a black shielding film during FPC hipot testing.

Shielding films are commonly added to flexible circuits to improve EMI performance. However, if insulation spacing is inadequate or material alignment shifts during lamination, electrical stress may concentrate between signal traces and shielding structures.

Under high-voltage conditions, the insulation barrier may no longer be sufficient, resulting in localized arc discharge. Typical evidence includes:

•  Visible sparks

• Burn marks

• Carbonized insulation

• Failed hipot tests

• Intermittent leakage current increases

 This type of failure highlights the importance of proper shielding design and insulation margin control during FPC manufacturing.

How Flux Residue Causes Leakage Voltage During FPC Hipot Testing

Another common issue identified during FPC hipot testing is leakage voltage caused by flux residue contamination.

After soldering operations, residual flux can remain on the flexible circuit board surface if cleaning processes are not properly controlled.

Although the contamination may not affect continuity testing, it can become conductive under high electrical stress, especially in humid environments. Residual flux may contain:

•  Ionic contaminants

• Activator compounds

• Weak organic acids

• Moisture-attracting residues

During FPC hipot testing, these materials can create unintended conductive paths between adjacent conductors.

Common symptoms include:

1.  Elevated leakage current

2. Reduced insulation resistance

3. Intermittent test failures

4. Unstable electrical performance

5. Increased failure rates during environmental testing

 For this reason, cleanliness verification is an important part of the FPC manufacturing process.

Other Causes of FPC Hipot Testing Failures

Moisture Absorption

Polyimide materials naturally absorb moisture from the environment. Excess moisture can reduce dielectric performance and increase leakage current during FPC hipot testing.

Coverlay Defects

Cracks, voids, or delamination within the coverlay layer may create localized insulation weaknesses.

Copper Burrs

Improper etching control can leave microscopic copper protrusions that increase electrical field concentration and raise the risk of dielectric breakdown.

Lamination Voids

Entrapped air pockets can become initiation points for partial discharge under elevated voltage conditions.

Insufficient Creepage and Clearance

As operating voltages increase, spacing requirements become increasingly important for preventing insulation failures.

How Manufacturers Investigate Hipot Test Failures

When a product fails FPC hipot testing, manufacturers typically perform a structured investigation process.

Visual Inspection

Engineers inspect the failure area for carbonization, burn marks, shielding film interaction, surface contamination, mechanical damage.

 Insulation Resistance Measurement

IR testing helps determine whether conductive contamination or dielectric degradation exists.

Cross-Section Analysis

Microsection analysis can reveal delamination, voids, cracks, misregistration,insulation thickness variations.

Cleanliness Testing

Manufacturers may perform ionic contamination or ROSE testing to identify flux-related issues.

Repeat Hipot Verification

Controlled retesting helps determine whether the failure is repeatable and identifies the breakdown voltage level.

Best Practices for Improving FPC Hipot Testing Performance

To reduce hipot failures and improve electrical reliability, manufacturers should focus on both design and process control. Recommended practices include:

• Optimizing shielding film placement

• Increasing insulation margins in high-voltage areas

• Improving coverlay alignment accuracy

• Controlling moisture exposure before testing

• Implementing effective flux cleaning procedures

• Monitoring ionic contamination levels

• Improving etching quality to eliminate copper burrs

• Performing routine insulation resistance verification

 These measures significantly reduce the likelihood of arc discharge and leakage voltage failures during FPC hipot testing.

Conclusion

FPC hipot testing is a critical quality control process for identifying insulation weaknesses that cannot be detected through continuity testing alone. Arc discharge and leakage voltage failures are often linked to shielding film interaction, flux residue contamination, moisture absorption, insulation defects, or manufacturing process variations.

By combining robust design practices with strict manufacturing controls, FPC manufacturers can improve FPC hipot testing performance and deliver more reliable flexible circuit products to customers.