Epoxy Mold Compounds (EMC) Curing

How Epoxy Molding Compounds (EMC) cross-link, harden and cure

In order to under­stand the shelf-life, expi­ra­tion date, and prod­uct fit­ness for use it is impor­tant to under­stand the basics of epox­ies and the mech­a­nism through which they get hard (a process know­ing as cur­ing or cross-link­ing). The term “epoxy” is quite broad in scope and encom­pass­es a wide range of tech­nolo­gies. With the excep­tion of a spe­cif­ic group of epox­ies that cure with them­selves, all epox­ies — and specif­i­cal­ly epoxy mold­ing com­pounds – cure through the mix­ing of two com­po­nents: an epoxy resin and a hardener.

Depend­ing on the resin/hardener com­bi­na­tion used (and the num­ber of com­bi­na­tions of resins and hard­en­ers is tru­ly stag­ger­ing), the prop­er­ties of the end prod­uct are dif­fer­ent. One impor­tant prop­er­ty, par­tic­u­lar­ly in the case of epoxy mold­ing com­pounds is the Tg. The Tg or “glass tran­si­tion tem­per­a­ture” of an epoxy mold­ing com­pound is the tem­per­a­ture above which epoxy prop­er­ties, specif­i­cal­ly the coef­fi­cient of ther­mal expan­sion (CTE) and the dynam­ic mod­u­lus (E), change.

Low Tg mate­ri­als require less heat to ful­ly cure (in some cas­es this may even be ambi­ent tem­per­a­ture) to obtain max­i­mum mate­r­i­al prop­er­ties, but as a result, have a low­er tem­per­a­ture at which these prop­er­ties begin to degrade. High Tg mate­ri­als on the oth­er hand require a much high­er tem­per­a­ture to ful­ly cure, but then pro­vide much high-tem­per­a­ture sta­bil­i­ty, mean­ing that their prop­er­ties are much more sta­ble to much high­er temperatures.

Tra­di­tion­al­ly, for­mu­lat­ing for a prod­uct that had a glass tran­si­tion tem­per­a­ture in the range of 120C to 160C, allowed you to use resin/hardener com­bi­na­tions that gave the epoxy mold com­pound very good room tem­per­a­ture sta­bil­i­ty before it was processed, and then cured very quick­ly when mold­ed at tem­per­a­tures between 160°C and 180°C (typ­i­cal mold­ing temperatures).

In the past 15 years, appli­ca­tions for epoxy mold com­pounds have grown to include those that require tem­per­a­ture sta­bil­i­ty above 180°C, which required the devel­op­ment of epoxy mold­ing com­pounds that had Tg’s above 180°C. The down­side to this achieve­ment is that mate­ri­als with such a high Tg required cur­ing tem­per­a­tures much high­er than 180°C to achieve max­i­mum mate­r­i­al properties.

Of course, epoxy mold com­pound users could either not expose their prod­ucts to such high tem­per­a­tures, or their equip­ment was not capa­ble of mold­ing above their tra­di­tion­al 180°C temperatures.

In order to accom­mo­date for this dis­crep­an­cy, epoxy mold com­pound man­u­fac­tur­ers intro­duced cur­ing cat­a­lysts to their for­mu­la­tions. These cat­a­lysts essen­tial­ly allowed the cur­ing to take place faster while achiev­ing sim­i­lar glass tran­si­tion tem­per­a­tures at low­er mold­ing con­di­tions. If you have been fol­low­ing the sto­ry thus far, you will real­ize that it was the intro­duc­tion of these cat­a­lysts, that on one hand allowed man­u­fac­tur­ers to pro­duce epoxy mold­ing com­pounds with a high­er Tg, but on the oth­er allowed these com­pounds to be cured at low­er mold­ing tem­per­a­tures. Fig­ure 1 shows a con­tour plot that out­lines the Tg devel­op­ment of a typ­i­cal epoxy mold com­pound vs cure temperature.

Con­tour plot of Tg (°C) devel­op­ment vs.cure tem­per­a­ture for EMC

This point is impor­tant to under­stand in the con­text of this arti­cle, specif­i­cal­ly as it con­cerns epoxy mold­ing com­pounds and “one-part” epox­ies. Cus­tomers famil­iar with epoxy mold com­pounds often believe that this is a “one-part” epoxy as it is pre­sent­ed as a sin­gle, homoge­nous prod­uct, either in gran­u­lar form or in a pressed pel­let form.

In fact, most one-part epox­ies, includ­ing epoxy mold­ing com­pounds are a class of prod­ucts that is “pre­mixed and cooled”, mean­ing that they are mixed and man­u­fac­tured ahead of time and deliv­ered to cus­tomers after they have been mixed. The indus­try term for this con­di­tion is called B‑stage. Though I am not aware of the term’s ori­gins, the term is used to describe epoxy sys­tems that are beyond their “A‑Stage” (ie unmixed) and their C‑stage (ie com­plete­ly cured).

In its B‑stage there­fore, epox­ies – by def­i­n­i­tion — are mixed but are not yet ful­ly cured. Here­in lies the crux of the issue. What hap­pens to epox­ies that have been ful­ly mixed, but have not yet been exposed to the tem­per­a­tures required to achieve max­i­mum mate­r­i­al prop­er­ties? Specif­i­cal­ly, as it relates to this arti­cle, how quick­ly does this reac­tion occur? What prop­er­ties are affect­ed pri­or to full cure and what are the effects of these prop­er­ty changes on the final mate­r­i­al properties?

Please stay tuned for the next part in this series to explore the behav­ior of epoxy mold­ing compounds.

Please vis­it us at www.caplinq.com to learn more about our whole range of epoxy mold­ing com­pounds (EMC) includ­ing our semi­con­duc­tor-grade epoxy mold com­pounds, fiber­glass-rein­forced indus­tri­al-grade mold com­pounds, and opti­cal­ly clear epoxy mold com­pounds (CMC) for emit­ters and detec­tors. If you have any oth­er ques­tions about how epoxy mold com­pounds cure, please feel free to leave a com­ment below, or don’t hes­i­tate to con­tact us.

About Chris Perabo

Chris is an energetic and enthusiastic engineer and entrepreneur. He is always interested in taking highly technical subjects and distilling these to their essence so that even the layman can understand. He loves to get into the technical details of an issue and then understand how it can be useful for specific customers and applications. Chris is currently the Director of Business Development at CAPLINQ.

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