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)
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.