Test Methods for Thermal Shrinkage Rate of FPC Insulation Films

The thermal shrinkage rate of FPC insulation films is a critical parameter for ensuring the dimensional stability and reliability of flexible printed circuits during high-temperature manufacturing processes. Accurate testing of this property helps manufacturers prevent defects such as warpage, delamination, and circuit misalignment. This article provides a comprehensive technical guide to the testing standards, sample preparation, and calculation methods used in the industry.

Technical Overview

Flexible Printed Circuits (FPC) rely heavily on insulation films, such as Polyimide (PI) and coverlay materials, to maintain mechanical flexibility and electrical isolation. However, these polymer materials are susceptible to dimensional changes when exposed to heat. The thermal shrinkage rate is a key indicator of the material's thermal stability, representing the percentage change in dimensions after being subjected to specific temperature conditions.

Excessive thermal shrinkage can lead to significant quality issues in downstream processes like SMT (Surface Mount Technology) assembly or during the lamination process. If the insulation film shrinks more than anticipated, it can cause misalignment of circuit layers, cracking of solder masks, or wrinkling of the coverlay. Therefore, precise testing of the thermal shrinkage rate is essential for incoming material quality control (IQC) and new product development.

Testing Samples

The selection and preparation of samples are vital for obtaining accurate test results. Different materials exhibit distinct behaviors under thermal stress.

Polyimide (PI) Base Films

Polyimide films are the most common substrate for FPCs. These films usually exhibit anisotropic shrinkage, meaning the shrinkage rate may differ between the Machine Direction (MD) and the Transverse Direction (TD). Samples must be cut to precise dimensions, typically strips of specific length and width, ensuring the edges are smooth and free from nicks that could induce stress concentrations.

Coverlay Films

Coverlay films, which consist of a polyimide film coated with an adhesive layer (epoxy or acrylic), present a more complex case. The adhesive layer may cure or flow during heating, influencing the overall dimensional stability. Samples for coverlay testing must be representative of the actual production roll, and protective release films must be removed prior to testing.

Adhesiveless Flexible Copper Clad Laminates (FCCL)

For double-sided or multilayer FPCs, the interaction between the copper foil and the insulation film affects dimensional stability. While this article focuses on insulation films, testing the bare film is often a prerequisite to understanding the baseline behavior of the laminate material.

Testing Items

When conducting a thermal shrinkage test, several specific parameters must be monitored and recorded to ensure the data is relevant to the manufacturing environment.

• Dimensional Change in Machine Direction (MD): Measures shrinkage along the length of the material roll.
• Dimensional Change in Transverse Direction (TD): Measures shrinkage across the width of the material roll.
• Moisture Content Impact: Assessing whether pre-baking is required to remove absorbed moisture that could skew results.
• Post-Heat Treatment Stability: Observing if the dimensions remain stable after the heat source is removed.

Testing Methods

The standard procedure for testing thermal shrinkage involves exposing the sample to a defined temperature profile and measuring the dimensional change. The industry widely references standards such as IPC-TM-650 or JIS C 6471 for these procedures.

Sample Preparation and Marking

Samples are typically cut to a standard size, for example, 150mm x 15mm. Two reference marks are made on the sample at a specified distance (e.g., 100mm) using a precision marker or a micro-hardness indenter. It is crucial to measure the initial distance (L0) between these marks with high precision, using tools like a digital microscope or a toolmakers microscope.

Heat Treatment Process

The sample is placed in a hot air circulation oven or a heated press. The temperature and duration depend on the material specifications and the intended application. Common test conditions involve exposure to temperatures ranging from 150°C to 200°C for 30 to 60 minutes. For materials intended for lead-free soldering processes, higher temperatures around 260°C for short durations (e.g., 10 seconds) may be used to simulate reflow conditions. The sample must be placed flat without constraints to allow free shrinkage.

Calculation of Shrinkage Rate

After the heat treatment, the sample is allowed to cool to room temperature in a desiccator or a controlled environment. The final distance (L1) between the reference marks is then measured. The thermal shrinkage rate is calculated using the following formula:

Shrinkage Rate (%) = [(L0 - L1) / L0] x 100%

If the result is positive, the material has shrunk. If negative, it indicates expansion, which is rare for cured insulation films under normal processing conditions. The test is usually repeated for multiple samples (e.g., 3 to 5 specimens) to determine the average value and standard deviation.

Applications

Data obtained from thermal shrinkage testing is utilized across various stages of FPC production and design.

• Artwork Scaling: Designers use shrinkage data to scale the photoplots (Gerber files) to compensate for anticipated material shrinkage during lamination.
• Process Control: Manufacturers set limits on allowable shrinkage to prevent registration errors during the imaging and etching processes.
• Material Selection: Engineers compare different suppliers or material grades to select the film with the best thermal stability for high-density interconnect (HDI) applications.

FAQ

Why is pre-baking sometimes necessary before testing?

Materials like Polyimide are hygroscopic and absorb moisture from the environment. When heated, this moisture evaporates rapidly, potentially causing bubbles or temporary expansion, which interferes with the accurate measurement of true thermal shrinkage. Pre-baking removes this moisture, ensuring the results reflect the intrinsic thermal properties of the polymer.

What is the typical acceptable shrinkage rate for FPC films?

While specifications vary by application, high-quality Polyimide films used in FPC manufacturing typically exhibit a thermal shrinkage rate of less than 0.05% to 0.1% at standard lamination temperatures. For fine-line circuitry, tighter control is required, often necessitating films with shrinkage rates below 0.02%.

Does the thickness of the film affect the shrinkage rate?

Generally, the inherent thermal properties are consistent across different thicknesses of the same grade. However, thinner films may appear to have higher shrinkage due to reduced mechanical rigidity, making them more susceptible to handling stress during the test. Thicker films may retain internal stress differently, potentially affecting the rate.