The single biggest risk associated with storage at higher than the recommended storage conditions is a reduction of flow rate. Epoxy molding compounds when blended already have the resin/hardener combination into one blend — and this is what we call a “B‑stage” epoxy molding compound. B‑stage refers to a stage between the unmixed (A‑stage) and the cured (C‑stage) of the curing cycle.
Final Epoxy Properties Unaffected by Shorter Flow
In its B‑stage, it is therefore important to keep the material refrigerated to slow down the curing process. The colder the material is kept, the slower the process takes place.
The most important reason to slow down this process is to be able to ensure that the material can be processed at the customer location. If the material is not kept under proper storage conditions, the flow rate of the material will decrease and there is a risk that the material does not fill into the cavities of the mold properly resulting in a defect known as “incomplete fill”.
What is very important to note is that regardless of the flow rate of the material, for all intents and purposes the final properties of the cured material are unaffected. This means that whether the flow rate is still at 100% of its initial value, or at 50% if the material is then post-mold cured, the Tg, CTE, moisture resistance, and all other critical properties remain the same.
This being said, “incomplete fill” is also a defect, so the shelf-life of the material is given such that the epoxy mold compound is within the flow rate specification right until the end of the shelf-life. The way we measure this shelf life is to keep materials stored at various temperatures and then we test the flow rate over a period of many days to see the effect on the flow rate.
As you can see in the attached chart for a “temperature-sensitive epoxy”, as expected, the flow rate data drops drastically at room temperature (23C). At 5C however, we have a significantly improved shelf life — and have less than a 20% reduction even after 50 days. At ‑10C storage, we appear to have almost no reaction taking place, though we need more data to be able to comment more accurately.
Please visit us at www.caplinq.com to learn more about our whole range of molding compounds including our semiconductor-grade epoxy molding compounds, our fiberglass-reinforced industrial molding compounds, and our optically clear epoxy mold compounds (CMC). If you have any other questions about B‑stage or shelf-life or UL-approvals for epoxy mold compounds, please feel free to leave a comment below or don’t hesitate to contact us.