Same-Size Die-Stacking Options | DDAF, WBC, Spacers

Same size die stack­ing is dif­fer­ent from oth­er die stack­ing in that the bond­line thick­ness must be a min­i­mum of 60µm — 80µm high to ensure enough wire­bond-loop height.

In short, there are four proven ways to achieve this:

  • Dum­my-Die sil­i­con inter­pos­er (a space-con­sum­ing, expen­sive and redun­dant method not dis­cussed here as it is a mature tech­nol­o­gy)
  • Spac­er-filled (PMMA) pastes
  • Dic­ing Die-Attach Film (DDAF)
  • Wafer Back Side Lam­i­na­tion (WBL)
Ways to stack same-size dies
Ways to stack same-size dies

Spac­er-filled (PMMA) pastes:
The spac­er-filled pastes, do work, and PMMA is OK at the high­er tem­per­a­tures. The point is that the PMMA spac­ers hold the bond­line – and pre­vents it from col­laps­ing under the weight of the sub­se­quent die. In my expe­ri­ence, it should not be used to define the bond­line thick­ness – the bond­line thick­ness should be set by the die-place­ment machine. Should the die-place­ment tool come in con­tact with one or two spac­ers (as there is always a slight dis­tri­b­u­tion in spac­er sizes), it will com­press the PMMA quite eas­i­ly and the remain­der of the spac­ers will sup­port the die dur­ing sub­se­quent die place­ments.

Although I am quite famil­iar with this process, I don’t believe it is the future as the die paste write pat­tern (ie. a snowflake design is the best pat­tern) is still a bot­tle­neck in the process, and each die requires a paste dis­pense and must still be placed indi­vid­u­al­ly. Fur­ther­more, the total Cost of Own­er­ship (COI) of the PMMA-filled spac­er-pastes is def­i­nite­ly more expen­sive than either of the oth­er two options.

Dic­ing Die-Attach Film (DDAF):
Dic­ing Die-Attach Films (DDAF) are in my opin­ion the future of this mar­ket. The biggest chal­lenge with this prod­uct as I see it is that no sin­gle com­pa­ny pos­sess­es a long his­to­ry and expe­ri­ence with the pro­duc­tion of such a prod­uct. These prod­ucts are the result of a com­bi­na­tion of die-attach chem­istry and wafer dic­ing film capa­bil­i­ties.

His­tor­i­cal­ly, die-attach com­pa­nies are experts in the under­stand­ing, design and for­mu­la­tion of the adhe­sive por­tion – the die attach paste, glue or adhe­sive. The lat­est tech­nol­o­gy of 2nd gen­er­a­tion epox­ies, bis­maleimides, PEAM-based adhe­sives, and hybrids of these chemistries push­es the lim­its of pre­vi­ous tech­nol­o­gy epox­ies to achieve pre­vi­ous­ly unachiev­able JEDEC and reli­a­bil­i­ty lev­els. The mar­ket lead­ers in these areas are Ablestik (now part of Henkel) and Hitachi – which togeth­er make up more than 75% of the organ­ic die attach mar­ket. The prob­lem with these com­pa­nies is that they have no film­ing exper­tise or expe­ri­ence, so they have to go exter­nal­ly to work with a part­ner to devel­op this.

On the oth­er hand are the Wafer Dic­ing Film com­pa­nies whom have a long expe­ri­ence and his­to­ry with film, tapes and the film­ing process. What they pos­sess in film­ing and tap­ing capa­bil­i­ties, they lack in die-attach chem­istry know-how. Most wafer dic­ing tape man­u­fac­tur­ers pro­pose a 1st gen­er­a­tion epoxy-based DDAF solu­tion, and as not­ed above, lead­ing-tech­nol­o­gy die-attach pastes use new tech­nolo­gies and hybrid chemistries to achieve high­er JEDEC lev­els. Fur­ther­more, wafer dic­ing tape com­pa­nies are his­tor­i­cal­ly only famil­iar with non-con­duc­tive die-attach adhe­sives, but the con­duc­tive die-attach pastes today rep­re­sent the larg­er and big­ger poten­tial of the mar­ket as a whole. Wafer Dic­ing Tape com­pa­nies will need to cov­er a lot of ground if they hope to devel­op this mar­ket on their own. The win­ner of this new DDAF mar­ket is far from clear.

CAPLINQ can now pro­pose its AWD120 which is a non-UV Dic­ing Die Attach Film (DDAF) which is used for same-size die stack­ing as well as to remove bot­tle­necks from exist­ing die-attach process­es. This prod­uct is a non-con­duc­tive die attach mate­r­i­al suit­able for back-side lam­i­na­tion onto the wafer, but as of today, it is only avail­able in 20µm thick­ness­es – so we can­not today offer the 75µm film need­ed to ensure enough wire­bond-loop height for same-size die stack­ing. I will write again when we will be able to offer such a prod­uct.

Wafer Back­side Coat­ing (WBC) or Lam­i­na­tion:
Wafer Back­side Coat­ing (WBC) with B-stage tech­nol­o­gy is anoth­er wor­thy approach which, though sub-opti­miz­ing the process by still requir­ing two dis­tinct mate­ri­als: Wafer Dic­ing Tape and Die-Attach Paste, at least keeps the experts in their respec­tive fields. For this prod­uct, CAPLINQ rep­re­sents, sells and pro­motes Shin-Etsu Chem­i­cal SFX-513S wafer back­side coat­ing die attach mate­r­i­al. This non-con­duc­tive, B-stage die-attach is an epoxy-sil­i­cone chem­istry hybrid which B-stages in 10 mins @ 120°C and is then sta­ble at 25°C for up to 6 months.

With the stan­dard filler load­ing, it has a Young’s Mod­u­lus (E) of 1.8 GPa (1800 MPa) at 25°C and retains 50 — 80MPa at 150°C, which is high enough for the sub­se­quent wire bond­ing process. It has passed MSL1 260°C and is cur­rent­ly in pro­duc­tion with a num­ber of large mem­o­ry man­u­fac­tur­ers. A tech­ni­cal pre­sen­ta­tion of the SFX-513S is avail­able by con­tact­ing us.

For more infor­ma­tion of UL clas­si­fi­ca­tion or poly­imide tapes, please vis­it us or con­tact us for more details.

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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