B-Stage Epoxy Mold Compound Systems

B‑Stage Epoxy Molding Compound Cross-Linking

By def­i­n­i­tion, once a resin and hard­en­er are mixed, the sys­tem becomes a B‑stage epoxy sys­tem. This def­i­n­i­tion how­ev­er, is too vague for our pur­pos­es. What we need is a bet­ter expla­na­tion of this effect, and what it means in terms of han­dling a B‑Stage epoxy sys­tem such as epoxy mold com­pounds.

Sim­ply put, a B‑stage epoxy sys­tem is one where­in the cross-link­ing reac­tion (ie. the cur­ing, or hard­en­ing phase) is ini­ti­at­ed, but is not yet com­plete. The rate of this reac­tion is depen­dent on two fac­tors: time and tem­per­a­ture. At high­er tem­per­a­tures (ie epoxy mold­ing con­di­tions), this hard­en­ing reac­tion occurs very quick­ly. At low­er con­di­tions, this reac­tion occurs more slow­ly. The gen­er­al rule of thumb for epoxy mold com­pounds is that every 10°C either dou­bles or halves the rate of reac­tion.

The prob­lem with this gen­er­al rule is that it is over­sim­pli­fied. Yes, the rate of reac­tion is affect­ed sig­nif­i­cant­ly by expo­sure to high­er tem­per­a­tures. How­ev­er, at low­er tem­per­a­tures — espe­cial­ly at typ­i­cal epoxy stor­age tem­per­a­tures of 5–8°C, this reac­tion occurs at such a slow rate that it is bare­ly mea­sur­able.

Fur­ther­more, for epoxy mold com­pounds for­mu­lat­ed with cat­a­lysts – these low­er stor­age tem­per­a­tures are far below the min­i­mum acti­va­tion tem­per­a­ture (some­times called the “kick-off” tem­per­a­ture) is nev­er reached. This acti­va­tion tem­per­a­ture is essen­tial for achiev­ing full cure, and with­out it, the com­plete reac­tion or cross-link­ing nev­er ful­ly takes place.

It is impor­tant to under­stand there­fore, which prop­er­ties are affect­ed dur­ing this B‑stage stag­ing process, to which extent these prop­er­ties are affect­ed, and what effect these changes have on final mate­r­i­al prop­er­ties.

As it con­cerns one-part epox­ies and epoxy mold com­pounds, expo­sure to high­er than rec­om­mend­ed stor­age tem­per­a­tures over time affects two spe­cif­ic mate­r­i­al prop­er­ties: min­i­mum achiev­able vis­cos­i­ty and spi­ral flow.

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

Please vis­it us at www.caplinq.com to learn more about our whole range of epoxy mold com­pounds includ­ing our Trans­fer Mold Epoxy Com­pounds and our Com­pres­sion Mold Epoxy Com­pounds. If you have any oth­er ques­tions about how to process and cure epoxy mold­ing com­pounds 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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