Determination of Coefficient of Linear Thermal Expansion for HDI Boards

Determining the coefficient of linear thermal expansion (CTE) is a critical process for ensuring the reliability and structural integrity of High Density Interconnect (HDI) printed circuit boards. This technical article provides a comprehensive guide to the testing standards, sample preparation requirements, and measurement methodologies used to evaluate the thermal expansion properties of advanced HDI substrates.

Technical Overview

The coefficient of linear thermal expansion, commonly abbreviated as CTE, is a fundamental physical property that quantifies the rate at which a material expands or contracts in response to temperature changes. For HDI boards, which utilize microvias and high-density circuitry, the CTE is a critical reliability indicator. A mismatch between the CTE of the substrate and the components mounted on it can lead to solder joint fatigue, via cracking, and eventual device failure. The measurement process, typically performed using Thermomechanical Analysis (TMA), focuses on both the in-plane (X and Y axes) and the out-of-plane (Z-axis) expansion characteristics.

Testing Samples

Sample Preparation and Dimensions

Proper sample preparation is essential for accurate CTE determination. Samples must be representative of the production material and free from visible defects such as delamination or warpage. For Z-axis expansion testing, the sample thickness should typically be between 0.50 mm and 3.0 mm to ensure the probe detects dimensional changes accurately. For X-axis or Y-axis measurements, longer rectangular strips are prepared to facilitate length change detection.

Conditioning Requirements

Prior to testing, samples usually undergo conditioning to remove moisture, as water content can influence the glass transition temperature (Tg) and expansion rates. A typical conditioning protocol involves baking the samples at 105 degrees Celsius for 24 hours, followed by storage in a desiccator until the test begins.

Testing Items

The evaluation of HDI board linear expansion involves several key parameters derived from the thermomechanical analysis curve. These parameters provide a complete picture of the material's thermal behavior.

• Alpha 1 CTE (a1): The coefficient of thermal expansion below the glass transition temperature (Tg). This value represents the expansion rate of the polymer matrix in a rigid, glassy state.
• Alpha 2 CTE (a2): The coefficient of thermal expansion above Tg. This value is typically higher than a1, as the polymer transitions to a soft, rubbery state, leading to accelerated expansion.
• Glass Transition Temperature (Tg): The temperature at which the slope of the expansion curve changes significantly. TMA determines Tg based on the intersection of the extrapolated a1 and a2 slopes.
• Total Z-Expansion: The total percentage change in thickness over a specified temperature range, critical for assessing via reliability.

Testing Methods

Thermomechanical Analysis (TMA)

The primary method for determining the linear expansion coefficient of HDI boards is Thermomechanical Analysis (TMA). This technique measures dimensional changes in a material as a function of temperature under a controlled atmosphere. A probe applies a minimal force to the sample surface, and as the sample expands or contracts, the probe movement is recorded with high precision.

Test Procedure Standards

The standard procedure generally follows IPC-TM-650 Method 2.4.24 or similar international standards like ASTM E831. The process involves placing the sample on a quartz platform and positioning the probe. The instrument is heated at a constant rate, typically 5 degrees Celsius or 10 degrees Celsius per minute, often under a nitrogen purge to prevent oxidation. The resulting plot of displacement versus temperature yields the expansion coefficients.

Specifics for Z-Axis vs. In-Plane Measurement

For Z-axis measurement, a flat-ended probe rests on the surface of the sample. For in-plane measurements (X and Y axes), a specialized sample holder is used to measure the change in length. In HDI design, Z-axis CTE is particularly scrutinized because it dictates the strain on plated through-holes and microvias during thermal cycling.

Applications

CTE data is indispensable for various stages of electronics manufacturing and quality assurance.

• Material Selection: Engineers use CTE data to match substrate materials with specific components, such as Ball Grid Array (BGA) packages, to minimize thermomechanical stress.
• Reliability Prediction: Z-axis expansion data helps predict the risk of via barrel cracking and inner layer delamination during reflow soldering processes.
• Process Optimization: Manufacturers analyze a1 and a2 values to adjust lamination parameters and drilling processes, ensuring the integrity of the dielectric layers.

FAQ

Why is Z-axis CTE more critical for HDI boards than X/Y axis?

HDI boards rely heavily on vertical interconnects like microvias and plated through-holes. During heating, the Z-axis expansion of the dielectric material is typically much higher than the expansion of the copper barrel. This differential strain can cause cracks in the via plating, leading to open circuits. Therefore, controlling Z-axis CTE is vital for preventing structural failure.

How does moisture affect the CTE measurement?

Moisture absorption can act as a plasticizer, lowering the glass transition temperature (Tg) and potentially altering the expansion rate. It may also cause a step change in the curve due to evaporation during heating. Therefore, drying the sample before testing is crucial for obtaining accurate thermal properties.

What is the difference between CTE and TMA Tg?

CTE refers to the rate of expansion (measured in ppm/degree Celsius), while TMA Tg is a specific temperature point where the material property changes from a glassy state to a rubbery state. The TMA test measures both; it calculates the CTE values (alpha 1 and alpha 2) and identifies the Tg point where the expansion rate shifts.