Customs Harmonization Code (HTS#) for SAC 305 and SAC405 Solder Spheres

8001.20.00.10: Unwrought tin: Tin alloys containing, by weight, 5 percent or less of lead

Customs Harmonization Code (HTS#) for Sn63Pb67 Solder Spheres

8001.20.00.90: Unwrought tin: Tin alloys containing, by weight, more than 25 percent of lead

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Surface Resistance multiplied times the thickness of the material in centimeters, equals the volume resistivity.

Volume Resistivity vs. Volume Conductivity

Volume Resistivity is also called bulk resistivity because it is the inherent resistance of a semiconductive material independent of shape or thickness. Very simply, resistivity is the inverse of conductivity. Volume Resistivity is measured in Ohms-cm, or spoken as “ohms-centimeter” and Volume Conductivity is measured in S/cm, or spoken as “Siemen per centimeter”. Therefore, a Volume Resistivity of 500 ohm-cm is equal to a Volume Conductivity of 1/500 or 0.002 S/cm.

Volume Resistivity vs. Surface (or Sheet) Resistivity

Surface Resistivity (also called Sheet Resistivity) is expressed as Ohms/sq, spoken as “ohms-per-square”. It is a useful measurement when measuring a layer or thin film of a semiconductive material such as LINQSTAT as the surface resistance of a given material will change depending on the thickness of the layer. Surface Resistance (ohms-per-square) multiplied times the thickness of the material in centimeters, equals the volume resistivity (ohms-cm). Likewise, the Volume Resistivity divided by the thickness in centimeters equals the surface resistivity.

Therefore, looking at the LINQSTAT VCF materials, the surface resistivity of each material can be determined based on the thickness of the film and the volume resistivity (<500 ohm-cm)

• VCF-2xxxxS-Series: 500 ohm-cm / 2mil (0.00508cm) = 98,425 ohms/square
• VCF-4xxxxS-Series: 500 ohm-cm / 4mil (0.01016cm) = 49,213 ohms/square
• VCF-8xxxxS-Series: 500 ohm-cm / 8mil (0.02032cm) = 24,606 ohms/square

Why does CAPLINQ give “less than” values (ie. <500 Ohm-cm) instead of Typical Values?

LINQSTAT Volume Conductive Film is a carbon-loaded polyethylene. As such, there are necessarily some differences in the conductivity over the entire surface. The data that CAPLINQ provides is meant to be a minimum, repeatable value that can be used in the design of the final product.

For more information on LINQSTAT VCF-Series, please contact us or visit the CAPLINQ website today to find out more, or for more information.

PIT2SD, a 2-mil double-sided polyimide tape is often used to mount wafers on a carrier for through wafer etching and it works very well.

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Certification Classification

For tapes and other thin materials, the certification classification typically used is UL94 VTM-0. The UL94 V-0 code is used for a self supporting material. The problem that arises with a very thin material is that it may not burn but it will shrink or distort as the flame is directed to the sample and if it cannot withstand a ten second application before in shrinks to the holding clamp, so it will not be classified as a 94V material. Cases like this are what UL 94VTM is used for. You take a sheet 200mm X 50m and form a cone with it to perform the flame test as shown in the figure.  Many thin materials that will not burn but will shrink to the clamp and need VTM testing.

Vertical Testing of Thin Materials (VTM-0, VTM-1, VTM-2)

This test is used for materials that are thin, or are too flexible or may distort, shrink or flex during ordinary vertical testing. Procedure: An 8×2 in specimen is rolled longitudinally around a 1/2 in diameter mandrel and taped on one end. When the mandrel is removed the specimen forms a cone. The cone is supported in a vertical position and a flame is applied to the bottom of the specimen. The flame is applied for three seconds and then removed until flaming stops at which time the flame is reapplied for another three seconds and then removed. Two sets of five specimens are tested. The two sets are conditioned under different conditions.

Vertical Flame Test for Thin Materials

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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.

Below is the recommended cleaning procedure:

Step 1: Do 10 shots of Melamine at 175°C for 300s
Step 2: Do 3-5 shots of Mold conditioner at 175°C for 180s
Step 3: Run 2 shots of dummy shots at 175°C for 90s @ 2 shots
Step 4: Resume production as usual

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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.

What customers do not always realize though is that UL classification must be tested for every unique film backing, thickness and adhesive combination, and the cost for any single combination is approx. $1000 + cost of material + delivery charges. It is difficult to justify the expense for the certification for each product without a significant customer request.

CAPLINQ does, however, work with an independent testing facility who can carry out this the UL Certification, but then CAPLINQ normally requests either a commitment for a certain order quantity (ie. 100 rolls), or that the customer pay for the testing. In order to share the burden of testing, we offer to pay for all the material, administration and shipping charges if the customer pays $1000 toward the testing.

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Silicone and Acrylic adhesive applied polyimide films are used for various coil wrappings including Tesla Coils, gold-finger masking, high-temperature masking, splicing, temporary bonding of thermocouples for oven profiling, insulating circuit boards, and other applications that require both the properties of the polyimide film as well as the adhesive properties provided by either silicone or acrylic.

CAPLINQ also offers non-adhesive polyimide film (PITN-Series). This product has been successfully tested as a release film in a welding press that fuses PTFE-coated (Teflon-coated) composite conveyor belting (made of glass-fabric). This process previously used a silicone impregnated glass, but some customers do not like the silicone release cloth burn-off and therefore this alternative method, using adhesive-free polyimide film was developed.

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KJR-4013E is a flexible, moisture-cure silicone most often used as junction-coating resins in the manufacture of diodes. KJR-651E, KJR-655E and KJR-657E are rigid thermoset polyimide-silicones used as junction coating resins in the manufacture of higher power diodes and thyristors. Both are used due to their excellent adhesion to epoxy molding compounds (EMC), high humidity resistance, and excellent volume resistivity and dielectric breakdown properties.

To be able to conduct failure analysis, customers have asked if there is a process to remove the cured silicone / polyimide-silicone from the diodes and/or thyristors (decap).

One recommendation is to use sulphuric acid (H2SO4) which can normally clearly strip organic compounds.  Nitric acid (HNO3) is another option.  If the dissolving rate is slow, adding heat to the process helps very much.  Of course, for either of these processes, a fume hood is necessary all the time.

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