Underfill Adhesive Thermal Expansion Analysis: CTE Testing, Methods, and Standards

Underfill adhesive thermal expansion analysis determines the coefficient of thermal expansion (CTE) and glass transition temperature (Tg) that govern the reliability of flip-chip, BGA, and chip-scale-package assemblies during thermal cycling. By quantifying expansion behavior below and above Tg using thermomechanical analysis (TMA), packaging engineers can match underfill properties to solder joints and substrates, minimize thermo-mechanical stress, and validate materials against international standards. This article explains the technical background, test samples, measured parameters, standard test methods, and typical applications of underfill thermal expansion characterization.

Technical Overview

Underfill adhesives are low-viscosity, silica-filled epoxy systems dispensed beneath flip-chip dies, ball grid array (BGA) components, and chip-scale packages (CSP) to mechanically couple the die to the substrate. Their core function is to redistribute thermomechanical strain away from fragile solder interconnects, dramatically extending fatigue life during power and temperature cycling. Thermal expansion analysis is the principal technique for verifying whether an underfill can perform this role.

The engineering problem is one of mismatch. Silicon expands at roughly 2.6 ppm/°C, eutectic and SAC solders at approximately 21–25 ppm/°C, and organic laminates at 14–17 ppm/°C below their own transition regions. During thermal excursions, these differential strains concentrate at solder joints. A well-formulated underfill constrains the assembly with a CTE close to that of the solder, typically 20–45 ppm/°C, converting a severe shear condition at the joints into a distributed, manageable stress field.

Because underfills are cross-linked thermosets, expansion is not linear across the entire temperature range. Below the glass transition temperature (Tg), the material is glassy and exhibits a low expansion coefficient, designated α1. Above Tg, segmental mobility increases and expansion rises sharply to α2, frequently 3–6 times higher than α1. Thermal expansion analysis therefore must report α1, α2, Tg, and the full dimension-versus-temperature curve. Filler loading, degree of cure, and moisture history all influence these values, which is why standardized testing per ASTM E831, ISO 11359-2, and IPC TM-650 is essential for material qualification and finite-element modeling inputs.

Testing Samples

Cured Bulk Underfill Specimens

The reference configuration for TMA testing is a fully cured, void-free plaque of the underfill itself. Liquid material is cast or molded into a shimmed cavity and cured according to the supplier's schedule, for example 150°C for one to two hours. Specimens are then cut or machined to the required geometry with flat, parallel faces.

Liquid Capillary-Flow Underfills

Capillary-flow products must be converted to solid test pieces before measurement. The laboratory casts the adhesive, replicates the production cure profile (snap cure, thermal cure, or extended bake), and verifies degree of cure by residual-enthalpy DSC before TMA testing, since an under-cured specimen will show artificially high expansion and a drifting baseline.

No-Flow and Reflow-Curable Underfills

No-flow underfills cure simultaneously with the solder reflow profile. Specimens are prepared by running the uncured resin through a simulated reflow with a representative peak temperature (235–260°C), followed by any post-cure specified by the formulator, so that the tested state matches the as-assembled condition.

Assembled Components and Cross-Sections

For failure analysis or process verification, small sections cut from populated boards can be measured directly to observe the composite expansion of underfill plus solder plus laminate, though results are qualitative and supplement, rather than replace, bulk-specimen data.

Testing Items

A complete thermal expansion characterization of an underfill adhesive typically reports the following parameters:

• CTE α1 — linear coefficient of thermal expansion below Tg (ppm/°C), the primary qualification value
• CTE α2 — linear coefficient of thermal expansion above Tg (ppm/°C), indicating behavior during overheating or rework exposure
• Glass transition temperature (Tg) by TMA — determined from the inflection point of the expansion curve
• Full dimensional-change-versus-temperature curve (ΔL/L0) across the scanned range
• Cure-induced volume or linear shrinkage, derived from the difference between first and second heating runs
• Expansion hysteresis and residual stress relaxation between consecutive thermal cycles

Testing Methods

Thermomechanical Analysis (TMA)

TMA is the definitive technique for underfill expansion measurement, performed to ASTM E831 and ISO 11359-2. A flat quartz expansion probe applies a small static force (typically 0.01–0.05 N) to the specimen while a furnace ramps temperature at 3–5°C/min under nitrogen purge. Suitable specimens are 3–6 mm in lateral dimension and 2–6 mm thick with parallel faces. The scan commonly covers 25°C to 250°C, and a second heating run is standard to separate irreversible cure and stress-relief effects from true reversible expansion. IPC TM-650 method 2.4.41.5 provides the electronics-industry implementation for polymeric packaging materials.

Differential Scanning Calorimetry (DSC)

DSC to ASTM E1356 or ISO 11357-2 supplies an independent Tg measurement from the heat-capacity step and quantifies residual cure exotherm, confirming that TMA specimens were fully cured before expansion data were collected.

Dynamic Mechanical Analysis (DMA)

DMA to ASTM E1640 or ISO 6721-11 detects Tg from the mechanical damping (tan δ) peak. Because package stress depends on stiffness as well as expansion, DMA Tg and modulus data are frequently reported alongside TMA CTE results.

Instrument Calibration and Quality Control

Length calibration and temperature calibration using indium and zinc melting standards, plus verification with a certified reference material such as sapphire, are performed in accredited third-party laboratories prior to each test campaign to ensure traceable, defensible data.

Applications

Thermal expansion data for underfill adhesives directly support design, qualification, and manufacturing decisions across the electronics packaging industry:

• Material qualification of capillary, no-flow, and molded underfills against IPC-SM-817 requirements
• Flip-chip and wafer-level CSP packaging, where CTE matching to SAC solder joints governs joint fatigue life
• BGA and corner-bond reinforcement in handheld, wearable, and mobile devices
• Stacked-die, fan-out, and 2.5D/3D module packaging requiring multi-material stress management
• Automotive and power electronics, where −40°C to 125°C cycling per JEDEC JESD22-A104 demands stable α1 and high Tg
• Inputs to finite-element thermo-mechanical simulation models used to predict solder joint life and delamination risk
• Incoming inspection and supplier lot-to-lot comparison for contract manufacturers
• Failure analysis of delamination, popcorning, and solder fatigue in returned assemblies

FAQ

What is the ideal CTE for an underfill adhesive?

The target α1 is generally 20–35 ppm/°C, closely matching SAC solder (approximately 21–23 ppm/°C). Values far below this over-stress the solder during heating, while values far above it leave the joints unprotected. The optimum is always a balance of CTE, Tg, and elastic modulus considered together.

Why does CTE increase above Tg, and why does it matter?

Above Tg the cross-linked network gains segmental mobility and free volume increases, so expansion jumps to α2 values of 80–200 ppm/°C. If Tg sits within or below the operating temperature range, the underfill loses both dimensional stability and mechanical coupling. Standard practice is to keep Tg at least 20–30°C above the maximum sustained service temperature.

Which standards govern underfill thermal expansion testing?

The primary methods are ASTM E831 and ISO 11359-2 for CTE and Tg by TMA, IPC TM-650 2.4.41.5 for electronic materials, and ASTM E1356 / ISO 11357-2 (DSC) plus ASTM E1640 / ISO 6721-11 (DMA) for corroborating Tg. Overall underfill material qualification is referenced to IPC-SM-817, with reliability demonstrated through thermal cycling per JEDEC JESD22-A104 and IPC-9701.

How does silica filler content affect the results?

Raising fused-silica loading lowers α1 and α2 and raises Tg because the filler itself expands at only about 0.5 ppm/°C. The trade-off is increased viscosity and reduced flow speed for capillary underfills, so formulators optimize loading against processability, and testing verifies whether the delivered balance matches the datasheet.

What specimen size and preparation are required?

TMA specimens are typically 3–6 mm across and 2–6 mm thick, with two flat, parallel, perpendicular faces cut from a fully cured plaque. Any voids, bubbles, or non-parallel faces introduce serious errors, so specimen quality is inspected before testing and thickness is measured at multiple points.

Why are two heating runs performed?

The first run may show additional cure or stress relief that inflates apparent expansion. The second run reflects the fully stabilized material and is normally the reported result, with the difference between runs documented as an indicator of residual cure or process-induced stress.