Picture this: you open a container of epoxy resin, expecting a smooth, clear liquid, but instead, it’s cloudy, gritty, or even solid. Naturally, this will catch you by surprise. This phenomenon, called epoxy resin crystallization, isn’t a flaw—it’s a natural quirk of high-quality resins. Better yet, it’s fully reversible, ensuring that assembly adhesives remain top performers in electronics manufacturing.
In this blog, we’ll explain what epoxy resin crystallization is, why it happens, and how it ties into the reliability of industry-leading assembly adhesives epoxies, like LOCTITE 3875 used for thermal management in transistor, rectifiers and memory chips.
Let’s break it down into practical tips for handling this behavior.
What Is Epoxy Resin Crystallization?
Think of crystallization like water turning to ice. Epoxy resin undergoes a phase change from a liquid to a solid or semi-solid state, often looking cloudy, hazy, or packed with crystals. These denser crystals sink to the bottom of the container or cling to the sides, giving the resin a sandy or milky appearance. The good news? Just like thawing ice, you can restore crystallized epoxy to its original liquid form without losing any of its properties.
This reversibility is a key reason why non conductive adhesives, renowned for their precision in semiconductor packaging, remain a go-to choice. Crystallization might look odd, but it doesn’t compromise the performance of these high-reliability adhesives.
Signs of Crystallization
Crystallization can show up in a few forms:
- Cloudiness or haziness: The resin turns foggy or milky white.
- Free-floating crystals: Tiny particles float in the liquid.
- Crystal masses: Dense clumps settle at the bottom or along container walls.
- Complete solidification: The entire container may solidify into a hard mass.
If you’re using CAPLINQ’s LINQBOND PM-1231, a favorite for its excellent aging and moisture resistance, you might notice these signs in the resin component (Part A). Rest assured, this is normal and easily managed.
Why Does Crystallization Happen?
Epoxy resins, including those in Henkel’s electronics assembly adhesives, are super-cooled liquids. At room temperature, they stay liquid even though they’re below their freezing point. This super-cooled state makes them prone to crystallizing when conditions nudge them toward their natural solid form, much like super-chilled water forming ice when disturbed.
Several factors can trigger crystallization in Henkel’s epoxies:
1. High Purity
Electrically conductive adhesives, like LOCTITE® ABLESTIK 2902, are formulated with high-purity resins to ensure consistent, reliable performance in applications like microchip bonding. Purer resins, with a narrow molecular weight distribution, are more likely to crystallize. Impurities can act like antifreeze, lowering the freezing point and reducing crystallization. So, if you see crystals in your epoxy–not to be confused with thaw voids, it’s a sign of the premium quality packed into such products.
2. Low Viscosity
Many adhesives, such as LOCTITE ECCOBOND F113SC, are engineered with high Tg and low viscosity for precise dispensing and flow in complex semiconductor assemblies. Low-viscosity resins crystallize more easily because their molecules move freely, aligning into crystals faster. Higher-viscosity resins, with more molecular resistance, are less prone to this behavior.
3. Temperature Fluctuations
Temperature plays a big role. Extreme cold (like sub-zero storage) or thermal cycling (shifting between warm and cool, like day-to-night changes) can spark crystal formation. Once a “seed crystal” forms, it encourages more crystals to grow. So materials might arrive crystallized if shipped in a cold truck.
4. Additives and Fillers
Fillers like silica or alumina enhance properties like thermal conductivity. These fillers are optimized for performance. However, certain fillers can act as seeds for crystal growth. Even a scratch on the container’s surface can trigger crystallization.
5. Moisture and Storage Conditions
While rare, moisture or poor storage can influence crystallization. Epoxies are designed for durability, but storing them in humid environments or leaving containers unsealed can introduce variables that affect their behavior.
How to Reverse Crystallization
Spot crystals in your two-part assembly or die attach adhesive? No problem—here’s how to fix it:
Warm It Up: Gently heat the resin (Part A only) to 40-50°C for a few hours using a water bath, oven, or heating equipment. Monitor the temperature to avoid overheating, which could harm the resin. This is not applicable for One part epoxies which is the majority of die attach. Temperature would commence the curing process, which is something we don’t want.
Inspect for Residual Crystals: After heating, check the container’s sides, bottom, and cap for any lingering crystals. These could reseed crystallization if not fully melted.
Clean the Container: Prevent re-crystallization by cleaning the bottle cap, neck, or dispensing equipment with isopropyl alcohol (IPA) or acetone after use. This removes resin residue that could form seed crystals.
Store Properly: Keep Henkel epoxies at a stable 20-25°C to avoid temperature swings. Avoid extreme cold or humidity.
Important Note: Only heat the resin component (Part A) of a two-part system. Heating a single-component epoxy could cause premature curing. One part epoxies should follow a specific thawing process. If in doubt, reach out to CAPLINQ’S technical support for tailored advice.
Preventing Crystallization
Prevention is better than a cure. Here’s how to keep your Henkel epoxies crystal-free:
- Stable Storage: Store epoxies like LOCTITE ECCOBOND F113SC in a climate-controlled environment at room temperature unless specified otherwise.
- Temperature-Controlled Shipping: Ensure epoxies are shipped in temperature-controlled conditions, especially in winter.
- Good Housekeeping: Seal containers tightly and clean dispensing equipment regularly to avoid seed crystal formation.
- Use Fresh Stock: Freshly manufactured epoxies are less likely to show crystallization.
Why Crystallization Isn’t a Dealbreaker
Crystallization might seem like a hiccup, but it’s a hallmark of the high-purity resins. The same qualities that make two part electrically conductive adhesives ideal for precision electronics also make them prone to crystallizing. With simple steps to reverse or prevent it, you can keep your workflow on track.
We have to repeat that One part and Two part Epoxies should be treated differently. One part epoxies crystallize almost by nature since they have to be stored at -40°C. The thawing process and steps have to be carefully followed to avoid crystallization and all the other issues that can occur from insufficient thawing.
To make things crystal clear:
Epoxy resin crystallization is just a trait of high-performance resins, and our epoxies are no exception. By understanding its causes and solutions, you can confidently handle this behavior and keep your production going. The next time you see a cloudy resin container of any kind you’ll know it’s a sign of the purity and precision that lend to the end properties of the material.
For more details on assembly adhesives or expert advice on crystallization, contact our technical support team.
Disclaimer: This blog is a general guide for epoxy resins. Always consult product data sheets and safety data sheets (SDS) for application-specific instructions.
