Sintering and Semi Sintering Pastes

TDS says that I should cure in N2 Oven. Can I cure in a conventional one?

Many customers prefer to cure in Nitrogen, to protect components from oxidation. However,our testing data packages state that cure in Air or Nitrogen is mostly OK. Justification data is rather mathematical, but it tends to show that die shear strength is not significantly affected by the curing atmosphere.

• Looking at all test results so far, we believe that our hybrid sintering materials can sinter well under both AIR and N2 atmosphere. We see some variation depending on the application which can go both ways.
• The O2 in AIR will typically support the sintering process by decomposing the lubricant barrier on the Ag filler at higher temperature faster and this way accelerate the Ag particles to contact and diffuse.
• The flow of N2 will help to remove the volatiles in the system, such as solvent and low molecular resins, and this way support the sintering process. To decompose the lubricant under N2 condition may require longer sintering process to achieve full sintering.

The main reason for using cure under Nitrogen is to prevent the Cu lead frame from oxidizing more under high temperatures. Practically, this has nothing to do with our semi-sintering materials as long as they use Ag of Au/PPF plating on die pad.

What effects Gan on SiN and the substrate will have on the process?

Silicon Nitride is an alternative to Silicon. The wafer / chip is normally fabricated from Silicon Nitride. This is mechanically fairly similar to silicon (it’s a bit harder & more brittle), so die attach that works with silicon is typically good on SiN.

GaN (Gallium Nitride) is a semiconductor layer that is grown on top of a silicon or SiN wafer / chip surface. So for die attach it is similar to attaching a Si or SiN chip.

Semi sintering materials need a layer of Ag or Au on both the chip & the substrate to give a good bond. This means that you should have a noble Back side metallization (BSM) on the die for the sintering to take effect. Bare Cu on the substrate can be OK for some “non-critical” applications but not recommended.

Will curing of a semi-sintering paste work in a vacuum oven?

Short answer is, "probably, but we cannot recommend this". We don't except benefits for sintering semiconductor die attach pastes.
Long answer is going to be, well, long so we change our format to take the entire column span... Let's begin by saying that we have no specific data about curing semi-sinter adhesives in vacuum and this is mostly anecdotal experience:

• The “chemistry” of the adhesive resin does not require oxygen (or any other gas) to be present during the cure process.
• There is a belief that oxygen may help to remove the coatings on the fine Ag filler, and therefore promote efficient sintering. Early sintering products were quite sensitive to this effect. Our latest formulations show little dependency on oxygen content, so this mechanism may now be “history”.
• The solvent (about 4% by weight) needs to be removed in a controlled way, to eliminate the risk of void formation. A high vacuum will cause the solvent to evaporate quickly, and it could even boil in some circumstances.
• Some components in the epoxy resin can evaporate in the early stages of cure (before they have a chance to react & cross-link). This evaporation will be accelerated in a vacuum so the end properties of the adhesive may be changed. This may not be significant, but we have no data – therefore we recommend to start with curing at normal pressures, and then evaluate the effects of vacuum carefully.

The vacuum oven will indeed improve (reduce) potential voiding issues, but this is typically not perceived as a problem for resin based ABP 8068TB product types. For solvent based pure sintering materials like SSP 2020 this can be an advantage BUT these products are typically not suitable for semiconductor die attach passing certain MSL levels, so this is irrelevant.