Same Size Die Stacking | Wafer Back Side Coating (WBC) Options

In the last post about Same Size Die Stack­ing Options, we dis­cussed the var­i­ous ways to do same size die stack­ing of die, a typ­i­cal con­fig­u­ra­tion for mem­o­ry die stacks that have the same dimen­sions. In this post, we would like to elab­o­rate on the Wafer Back­side Coat­ing (WBC) options for same-size die stacks.

As can be seen in the pho­to below, there are also two vari­a­tions on the Wafer Back­side Coat­ing Options also. The first fig­ure on the left, there is the “no flow over wires” option. In this option, using Shin-Etsu SFX-513S the entire back­side of the die is not coat­ed and the wire­bond­ing is either not affect­ed dur­ing place­ment of the 2nd die, or the wire­bond­ing is done in a sub­se­quent stage in the process. In the sec­ond image on the right, using either Shin-Etsu SFX-524A Shin-Etsu SFX-526Aor the Wafer Back­side Coat­ing mate­r­i­al does flow over the wires, and there­fore needs to be liq­uid enough to pass over the wires dur­ing die place­ment and not cause any wire damage.

Advan­tages of No Flow Over Wire (NFOW) Technique:

• Epoxy Mold­ing Com­pound (EMC) com­plete­ly encap­su­lates the wire with no risk of Coef­fi­cient of Ther­mal Expan­sion (CTE) mis­match between Die Attach (DA) and EMC
• Die Attach Oper­a­tion can be done sep­a­rate­ly from Wirebonding

Dis­ad­van­tages of No Flow Over Wire (NFOW) Technique:

• A non-uni­form wafer back­side coat­ing is applied to each die — requir­ing sten­cil application

Advan­tages of Flow Over Wire (FOW) Technique:

• A uni­form wafer back­side coat­ing is applied to each die, sten­cil print­ing or spin-coat­ing is possible
• Entire Wafer can be coated
• In-line process of die-attach, wire­bond­ing, die-attach, wire­bond­ing is preserved

Dis­ad­van­tages of Flow Over Wire (FOW) Technique:

• Epoxy Mold­ing Com­pound (EMC) does not com­plete­ly encap­su­lates the wire, so there is a risk of Coef­fi­cient of Ther­mal Expan­sion (CTE) mis­match between Die Attach (DA) and EMC
• Die Attach Oper­a­tion can be done sep­a­rate­ly from Wirebonding

The dif­fer­ence between Shin-Etsu SFX-524A and Shin-Etsu SFX-526A is that Shin-Etsu SFX-524A con­tains no sol­vent and is there­fore bet­ter suit­ed for spin-coat­ing, whear­eas Shin-Etsu SFX-526A has sol­vent which helps it hold its shape bet­ter after sten­cil printing.

For more infor­ma­tion regard­ing Shin-Etsu Die Attach SFX-513S, Shin-Etsu SFX-524A or Shin-Etsu SFX-526A please vis­it us or con­tact us for more details.