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.

Wafer Backside Coating for Same Size Die Stacking: SFX-513S, SFX-524A and SFX-526A

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.

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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

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.

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A direct equivalent to 3M Velostat (read the article on generic brands here), Linqstat VCF is a very low cost (up to 50% cheaper than 3M Velostat), volume conductive film for EMF shielding.

What’s the difference between volume conductive and just conductive?

Like it’s 3M Velostat equivalent, LINQSTAT VCF is a carbon impregnated black polyethylene film. The carbon impregnation gives it a volume conductivity that fends off EMF, but cannot be used to conduct electricity. Though an electrically conductive medium (such as tin or silver) could also be used for EMF shielding, these materials are many times more expensive and just simply not necessary for EMF Shielding or ESD (electro-static discharge) prevention.

The most cost-effective solution is with LINQSTAT VCF is 72″ wide x 150′ long rolls

The conductivity is not affected by aging or humidity and with a volume resistivity of <500 Ohms/cm and a surface resistivity of <31,000 ohms/cm² (click here for technical datasheet in Adobe PDF) the LINQSTAT VCF is an excellent conductor for a plastic.

How thick is thick enough?

Though the LINQSTAT VCF is available in various thicknesses, the normal thickness of 4mil (100µm or 0.004″) is recommended For extra toughness, heavy duty 8mil (200µm or 0.008″) thickness can be used, but the thickness implies more material which is more expensive, and shipping costs roughly double since the weight of the material is also doubled. If you really want to use 8mil thicknesses in some areas, then we recommend to simply put two layers of 4mil thickness instead. It’s much more cost-effective.

How else can I can it more cost-effective?

Certainly for large covering jobs like EMF shielding a room in your house, you can think of this like EMF wallpaper; the wider the sheets, the better. Standard rolls are available in 36″(0.91 m) width and 72″ (1.83 m) widths. If you’re wondering how the heck you can handle a 72″ wide roll, let me reassure you that these are supplied in what’s known as a V-sheet. A V-sheet is simply a sheet that is 72″ wide folded over on itself. Therefore a 72″ wide roll is actually folded over on itself and supplied as a 36″ wide roll.

Secondly, the length of the roll matters. Standard lengths are 150′ (46m) and 750′ (229 m). Therefore instead of ordering 5 rolls of 150′ length, a single roll of 750′ length will do. To summarize the above information, it is more cost-effective to order a single roll of V-sheet 72″ wide by 750′ long LINQSTAT VCF than to order 10 rolls of 36″ wide by 150′ long rolls.

Where else can it be used?

As discussed here, LINQSTAT VCF can be used to line walls, windows and floors, but because it can be heat-sealed, it is also great for wrapping electrical wires, telephone cords and computer cords too. For these applications, a special tubing version is available.

Contact us or visit us at www.caplinq.com for more information.

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