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.

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.