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 Production and Testing of Flex PCB Prototypes: Ensuring Quality and Reliability

Production and Testing of Flex PCB Prototypes: Ensuring Quality and Reliability


Flexible circuits are made from flexible insulating materials. such as polyimide or polyester films. offering high reliability and excellent flexibility for printed circuits. They feature high wiring density, lightweight, thin thickness, and good bending properties. Due to their widespread applications. the prototype service for flexible circuits has gained recognition from customers. GESFLEX has also discussed “Why Choose Flex PCB Prototype?” So, as an FPC manufacturer, what testing of flex PCB prototypes do we conduct on FPC prototypes to ensure quality? Today, this article will detail the testing procedures. conducted after prototyping flex PCBs to ensure quality and reliability.

Environmental Testing for flex circuit prototyping

After prototyping a flex PCB, GESFLEX undertakes various testing of flex PCB prototypes to ensure their quality and reliability. These include a dimensional inspection to verify specifications. visual inspection to detect defects, and electrical testing to confirm circuit integrity. Mechanical tests assess flexibility, while adhesion and thermal profiling ensure durability. Environmental and chemical resistance tests simulate real-world conditions. and reliability tests predict long-term performance. Let us now explain the objectives of each test one by one and the criteria.

To better complete the FPC Prototyping Understand the drawbacks of FPC

As flexible circuits gain widespread use. we recognize their benefits like lightweight design, excellent bending capabilities. versatile routing, and space efficiency. However, understanding their drawbacks is crucial before discussing testing methods. The flexibility that makes FPCs advantageous also increases the complexity of manufacturing processes. involving numerous steps such as cutting, drilling, copper plating, and etching. Factors like material quality, tool wear, human error. and environmental changes can introduce defects during production.

Common FPC defects fall into two main categories: pad defects and trace defects. Pad defects occur at metal pads and include issues like missing or protruding metal outlines. non-metallic residues, and surface contamination. Trace defects involve electrical issues. such as shorts, open circuits, surface scratches, and foreign objects. Therefore, testing of flex PCB prototypes focuses on identifying and resolving these issues. to ensure the reliability and functionality of FPCs before they enter full-scale production.

Testing Scope

Our testing of flex PCB prototypes primarily involves single-sided, double-sided, and multi-layer boards. This encompasses flexible copper-clad laminates (FCCL) utilizing polyimide (PI) or polyester (PET) substrates. including both adhesive (3L-FCCL) and non-adhesive (2L-FCCL) types.

Purpose of Testing

The purpose for evaluating the visual quality of FPCs. It provides criteria for accepting or rejecting product appearance defects. aiming to enhance manufacturing techniques and reduce unnecessary waste and environmental pollution.

Additionally, specific individual metrics requested by customers are separately assessed.

Basic Testing Standards

Visual Inspection:

  • The FPC surface should be free from visible scratches, deformations, or oxidation.
  • The edges of the FPC should be neat, smooth, and free from burrs.
  • Solder pads and pins on the FPC should be intact, without soldering defects such as voids or bridges.

Substrate Film Appearance: Defects in the appearance of the substrate film. where conductors are absent, fall within allowable ranges detailed in the table. No concavities, creases, wrinkles, or foreign objects affecting usability are permitted.

Cover Layer Appearance: Defects in the appearance of cover films and coating layers. allowable ranges include no concavities, creases, wrinkles, or delamination affecting usability.

Dimensional and Tolerance Inspection:

  • Verify if the overall dimensions of the FPC meet design specifications.
  • Check if the positions and spacing of components on the FPC conform to design requirements.
  • Ensure that the hole sizes and pitch on the FPC match the design specifications.
Mechanical Testing for flex circuit prototypiing

Deviations in Connection Pads and Cover Layers: Deviations in connection pads. and cover layers should be within ±0.3mm for dimensions. below 100mm and within ±0.3% of the dimension for sizes above 100mm.

Adhesive and Coating Flow: The flow extent (f) of adhesives and coating layers should be less than 0.2mm. At connection pads, combined with deviations in the cover layer and punch deviations. the minimum annular width (g) must be ≥0.05mm.

Electrical Testing

Electrical Performance Inspection:

  • Validate the conductivity of the FPC through continuity testing.
  • Verify if the insulation performance of the FPC meets requirements through insulation testing.
  • Confirm the stability of the impedance of the FPC through impedance testing.

Reliability Inspection:

  • Assess the heat resistance of the FPC through high-temperature testing.
  • Evaluate the moisture resistance of the FPC through humidity testing.
  • Test the vibration resistance of the FPC through vibration testing.
  • Verify the impact resistance of the FPC through impact testing.

Discoloration: Conductor discoloration under the cover layer. after a humidity resistance test of 40°C 90% relative humidity for 96 hours. must still meet requirements for voltage resistance, bending resistance, fold resistance, and solderability.

Missing Coating: Testing for missing coatings should adhere to solderability requirements. ensuring no tin adhesion on conductors where coatings are missing.

Poor Plating Bond: Plating layers should not exhibit delamination. The width (W1), length (L), and processed conductor width (W). affected by poor plating bond should not compromise reliability in the contact area.

Environmental Inspection:

  • Ensure compliance of the FPC with environmental requirements. including the absence of harmful substances.
  • Check the environmental friendliness of the soldering process used for the FPC, such as the use of lead-free solder.

Prototype Inspection:

  • Conduct comprehensive inspection of the initial batch of FPCs. to ensure conformity with design requirements.
  • Evaluate the stability of the production process for the FPCs. to meet the requirements of mass production.

The above represents some basic aspects of FPC inspection standards. Specific standards and requirements can be adjusted and supplemented based on actual conditions. Adhering strictly to FPC inspection standards ensures quality and reliability. enhancing product competitiveness and market acceptance.

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Standard Inspection Procedures

Currently, there are three primary quality inspection methods for FPCs:

Visual Inspection:

Visual inspection remains the traditional method for PCB inspection. widely used by most small to medium-sized production manufacturers in China. Inspectors under high-light conditions use naked eyes. or magnifying glasses to directly detect defects.

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Probe Scanning:

Probe scanning, also known as electrical testing, falls under contact measurement. and is extensively used for online inspection. Inspectors place probe tools at both ends of the circuit to conduct quality checks using electrical current. This method offers fast detection speed. and robust capabilities for detecting short circuits and open circuit defects. However, due to potential secondary damage from contact. and the need for equipment adjustments for different products. this testing process can be cumbersome and requires skilled operators.

AOI Inspection:

AOI (Automated Optical Inspection) utilizes industrial cameras and image processing algorithms. to analyze images for defects without contact. This non-destructive testing method offers consistent inspection environments and is operator-friendly. making high-precision AOI technology a focal point for both research and manufacturing. This article primarily outlines AOI inspection strategies.

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Visual Inspection Strategies

2D Matrix + Variable Magnification Optical Microscope:

Given the small defects in FPCs. such as scratches on the sub-micron level. high-resolution matrix cameras can capture images effectively. Variable magnification optical microscopes adjust resolution. and field of view based on product accuracy. minimizing image acquisition frequency to enhance inspection efficiency. Utilizing coaxial light sources for concave-convex and flat surfaces offers distinct reflective characteristics. highlighting surface indentations and mechanical damage. such as scratches for subsequent defect capture and positioning.

2D Line Scan Camera + Motion Platform:

Considering the system’s high resolution requirements and FPC shape factors. a high-resolution line scan industrial camera combined with a high-precision mechanical motion platform (rails, servo motors, PLC controllers, etc.). achieves smooth FPC movement control for constructing high-precision acquisition systems. Coupled with high-intensity linear light sources. and special circular lenses to form narrow-band light. this setup effectively addresses uneven lighting-induced shadow issues, ensuring high-quality image acquisition.


In conclusion, rigorous adherence to FPC inspection standards ensures our products’ quality. Visual checks identify surface flaws. while dimensional and component spacing verification ensures design accuracy. Electrical tests confirm conductivity and stable impedance, guaranteeing performance consistency. Robust reliability assessments, including heat, moisture, vibration, and impact tests, validate durability. Environmental compliance ensures products are free from harmful substances. using eco-friendly processes like lead-free soldering. These measures not only uphold quality. but also enhance market acceptance and customer satisfaction. For top-tier FPC solutions meeting your standards. contact GESFLEX today for tailored, innovative circuit solutions.

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