Understanding Gelation and Cure Behavior in Semiconductor Packaging

You uncap a syringe of epoxy expecting a smooth, free-flowing resin, only to find it’s thicker than usual, stringy, or already semi-solid. For production teams and process engineers, this raises an immediate concern.

In high-performance encapsulants used for semiconductor packaging, changes in material behavior are not always caused by defects. Often, they reflect the natural chemical progression toward gelation. For operators, the key challenge is distinguishing between premature curing caused by handling or storage issues and the intended chemical reactivity of the formulation. Understanding the difference between gelation and curing behavior is key to maintaining stable process windows, improving yield, and ensuring reliable semiconductor packaging performance.

In this article, we unpack what happens during gelation and curing of encapsulants, explain why viscosity monitoring matters, show how to track these transitions, and outline practical steps for handling unexpected material behavior in semiconductor packaging production.

Gelation: When the Flow Stops

Gelation signals the transition from a viscous liquid to a viscoelastic semi-solid, caused by the formation of a continuous polymer network. Think of an agar–water mixture turning into gelatin; it still contains liquid, but no longer flows. In packaging applications, gelation marks the last moment when components can be repositioned or when flow into tight geometries is still possible.

The timing of gelation depends on formulation chemistry, including material type, filler loading, filler surface treatment, catalyst, and resin backbone. Missing the gelation window can lead to incomplete mold filling or misalignment during assembly. To illustrate these variations, we summarized the gel times of different epoxy molding compounds across applications in the table below.

The gel time of epoxy molding compounds is typically measured at 175 °C, which is the standard molding temperature. Based on the table, the gel time of EMCs usually falls within 20–60 seconds. Short gel times (≤30) seconds are suited for passive components, optocouplers, and through-hole devices,  where short runners and quick gel lock wires and leads in place. Slightly longer, 30–35 seconds, balances flow and wire-sweep control for discrete SMT packages, power modules, and TO/IGBT devices. Medium gel times of 35–45 seconds are common for IC leadframes and BGA/LGA packages, which require longer flow paths but still need controlled gelation to prevent sweep. The upper range, 46–60 seconds is preferred for long-flow IC leadframes and molded underfill substrates, where extended flow and leveling over complex features are essential

Similarly, gel time is one of the key properties to consider when selecting encapsulants and underfills for potting and optically clear applications. In contrast, for glob top, dam-and-fill, underfills, and wafer-level packaging, properties such as thixotropic index, viscosity, and work life are more critical.

In CAPLINQ’s potting and optically clear encapsulant materials, the balance between gel time and application-specific performance ensures proper flow, filling, and long-term reliability. Potting materials typically take several minutes to hours to start solidifying. This longer gel time gives enough time for the material to fully flow and fill large or complex packages before it begins to harden. In contrast, optically clear materials are designed for much faster gelation under elevated temperatures to balance clarity and throughput. These shorter gel times support efficient encapsulation of optical or LED devices while ensuring minimal bubble entrapment and high transparency.

Curing: From Semi-Solid to Structural Integrity

While gelation is the point of no return for repositioning, curing is what gives the material its final hardness and strength. Like baking a cake, gelation is the setting of the batter, while curing is the full bake. Incomplete curing in semiconductor applications leads to delamination, weak die attach, or underfill instability. Most epoxy systems cure within 1–2 hours at 150 °C, while others use a staged cure process outlined in their technical data sheets. Following these recommendations helps achieve maximum crosslinking and ensures optimal material performance.

To understand the degree of crosslinking as a primary indicator of curing, let’s take a look at OPTOLINQ OLS-1000HF, a two-part optically clear epoxy encapsulant designed for the encapsulation of LED lamps and displays.

Figure 1: Differential scanning calorimetry (DSC) curves of uncured and partially cured OLS-1000HF recorded at a heating rate of 10 °C/min under nitrogen flow (50 mL/min, exothermic direction upward). The plot shows the exothermic peaks corresponding to the curing reactions, allowing comparison of the thermal behavior before and after partial curing.

In the figure, the black shaded region shows the DSC curve of the uncured liquid resin, while the red outline represents the DSC curve of a partially cured resin (130 °C for 1 h).  Uncured liquid resin was tested to establish the baseline dynamic reactivity window and the total enthalpy (ΔH_total) for the degree-of-cure calculations. 

The degree of cure in resins is determined by dividing the residual enthalpy of the cured sample by the total heat of cure of the uncured resin.

Degree of Cure (DoC) = Residual Enthalpy /Total Enthalpy Degree of Cure

(DoC) = ΔH_residual /ΔH_total

After curing at 130 °C for 1 hour, OLS-1000HF achieved a degree of cure of 98.18 %, meaning nearly all reactive groups have crosslinked. The small residual exotherm (ΔH_res = 5.25 J/g) appearing at high temperature(160–210 °C) confirms that a minor amount of unreacted resin remains. The shift of T_onset to ≈ 150 °C and appearance of a clear Tg ≈ 139 °C indicate that the network has become largely rigid, with curing now limited by diffusion rather than reaction rate. In practical terms, the material is fully hardened after molding, and any additional post-cure would maximize Tg and thermal stability.

Most industries consider a 95% degree of cure acceptable, with post-curing or baking used to maximize electrical and thermomechanical properties. To achieve full cure, parameters such as temperature, hold time, and ramp-up conditions are specified for each material in its technical data sheet. However, since end users apply these materials in different package types, geometries, and part sizes, curing parameters can vary. For this reason, the provided curing conditions of encapsulation materials should be treated as guidelines, and users are encouraged to validate and optimize them for their specific application.

The DSC scan of an uncured resin is a valuable tool for defining baseline curing conditions. At a standard heating rate of 10 °C/min, a practical rule of thumb is:

1. Initial Cure Temperature – Set the cure temperature about 10–20 °C above the DSC onset temperature. This ensures the reaction remains chemistry-controlled and progresses at a practical rate.
   
2. Verification with DSC – After the initial cure, run another DSC scan to check if the residual enthalpy (ΔH_res) meets the target.
   
3. Post-Cure Adjustment – If ΔH_res is still too high, apply a post-cure at the current glass transition temperature (Tg) +25–35 °C, hold for 15–60 minutes (preferably under N₂ atmosphere), and recheck ΔH_res and Tg.
   

This stepwise approach translates dynamic DSC results into real production curing profiles, ensuring a cure that is fast, complete, and safe for epoxy resin systems used in semiconductor packaging and other electronic applications.

Viscosity as a Process Indicator

During material processing and end-use applications, viscosity is one of the most critical properties that determines whether a material can be effectively applied, dispensed, or flowed into tight geometries. Materials with high initial viscosity typically have a shorter working life because the resin is already closer to its gel point. As a result, the system “locks up” faster and provides less time for proper application. On the other hand, materials with low initial viscosity usually offer a longer processing window but increase the risk of slump or bleed. If viscosity is too low, gravity and capillary forces can dominate, leading to sagging, uncontrolled spreading, or even filler–resin separation before curing takes place.

The balance between workability and stability is largely determined by viscosity. However, the ideal viscosity range varies depending on the process and application, since each one is influenced by different physical forces, dispensing methods, and performance or optical requirements. These factors ultimately dictate the rheology needed for reliable results.

Because viscosity directly impacts how materials behave during application, monitoring its changes becomes critical in real production settings. During processing, viscosity-based measurements are essential for process engineers as they define the handling window and indicate when flow or alignment operations must be completed. Key parameters such as pot life, working life, and gel time are all linked to viscosity changes.

By contrast, cure time is not governed by viscosity but by the extent of chemical crosslinking or the degree of cure. Once the resin gels, viscosity ceases to be a meaningful measure, as the material may continue curing while already immobile. Cure time is therefore tracked using properties, such as glass transition temperature (Tg), modulus, hardness, or residual enthalpy. A resin is considered fully cured when it achieves its intended network structure and mechanical integrity, regardless of whether viscosity has changed.

Read More: Difference between pot life, working life, and gel time.

The distinction is clear: viscosity-based measurements define the process window, while degree-of-cure metrics ensure final reliability and performance of the packaged device.

While viscosity can be an indicator of the degree of curing, it’s important to remember that this only holds true up to the gelation point. Before the gel point, increasing viscosity corresponds with increasing crosslink density and curing. However, beyond this, the polymer transitions from a liquid to a gel, and viscosity measurements become inaccurate (as viscosity approaches infinity). After gelation, the degree of crosslinking and curing is now reflected by other properties such as modulus or glass transition temperature.

If you’re experiencing unpredictable viscosity or curing behavior, CAPLINQ’s technical team can help. Explore our epoxy selector guides or contact us for customized support in advanced packaging applications.

Disclaimer: This blog is a general guide for epoxy resins. Always consult product technical data sheets (TDS) and safety data sheets (SDS) for application-specific instructions.