Effects of Epoxy Mold Compound Viscosity, Flow and Filler on Wire Sweep

Recent­ly, a cus­tomer com­plained that he had a hard time cor­re­lat­ing the wire bond sweep with mate­r­i­al prop­er­ties like spi­ral flow, filler con­tent, filler size, etc. He was using mold flow analy­sis to pre­dict wire sweep, but the results didn’t seem to make sense to him.

The rea­son for this is because these prop­er­ties can be mutu­al­ly exclu­sive, and com­bi­na­tions of these prop­er­ties and result in vast­ly dif­fer­ent properties.

Let me start with a basic exam­ple to make my point. Imag­ine you have a mold com­pound that is 80% filled by weight with spher­i­cal sil­i­ca with an aver­age par­ti­cle size of 100um. If we did noth­ing else but swap out this filler a sil­i­ca nanopar­ti­cle with an aver­age par­ti­cle size of 0.1um, what would be expect in mate­r­i­al prop­er­ty differences?

Though both par­ti­cle sizes may pre­dict sim­i­lar wire sweep, in real­i­ty, the nanopar­ti­cle-filled mold com­pound would be so thick that it wouldn’t be able to flow at all. Epoxy mold com­pound for­mu­la­tions often have mul­ti­ple dif­fer­ent sizes (from nanopar­ti­cles to sev­er­al hun­dred microns) and shapes of filler includ­ing spher­i­cal, angu­lar and oth­er. These com­bi­na­tions are done exact­ly to opti­mize pack­ing den­si­ty, vis­cos­i­ty and flow.

Like­wise, spi­ral flow itself is not a mea­sure of vis­cos­i­ty. It is a mea­sure of, well… flow. Spi­ral flow mea­sures the dis­tance a mold com­pound will flow when exposed to mold tem­per­a­ture and pres­sure. All oth­er things being equal, a mate­r­i­al with a low­er vis­cos­i­ty will flow fur­ther than one with a high­er vis­cos­i­ty. But all things are rarely equal. If the low vis­cos­i­ty mate­r­i­al has a short­er gel time, than it may flow fur­ther while it is still liq­uid, but will stop flow­ing soon­er than the mate­r­i­al with the longer gel time.

Epoxy mold­ing com­pound for­mu­la­tion is cer­tain­ly as much of an art as a sci­ence. Add epoxy mold con­fig­u­ra­tion, part geom­e­try, lay­out and part-count to the com­plex­i­ty and the puz­zle becomes even more com­plex. So what is an engi­neer to do when con­front­ed with this infor­ma­tion? The short answer is col­lect infor­ma­tion and test.

What data should be collected?

Start by col­lect­ing the fol­low­ing data on the epoxy mold­ing compound:

• Spi­ral Flow
• Gel Time
• Filler Type
• Filler Size
• Filler Shape
• Vis­cos­i­ty

Once you have this, ask the epoxy mold com­pound man­u­fac­tur­er to explain the trade-offs made in each of the for­mu­la­tions and then ask for sam­ples to test based on your requirements.

Please vis­it us at www.caplinq.com to learn more about our whole range of epoxy and oth­er mold­ing com­pounds includ­ing our epoxy mold com­pounds for semi­con­duc­tors, indus­tri­al-grade mold com­pounds, and opti­cal­ly clear opto­elec­tron­ics-grade epoxy mold com­pounds. If you have any oth­er ques­tions about epoxy mold­ing com­pound pel­let sizes, please feel free to leave a com­ment below or don’t hes­i­tate to con­tact us.