When it comes to high thermal conductivity die-attach options, an engineer is presented with four basic options:
- Electrically Conductive Die Attach Pastes
- Semi-Sintering Die Attach Pastes
- Silver Sintering Pastes
- Solder Pastes
Why use semi-sintering pastes?
For many applications, semi-sintering die-attach pastes present an interesting option. As you can see in the figure below, semi-sintering (often also called hybrid sintering) combines the advantage of a die-attach paste with that of a silver-sintering paste.
On the one hand, it offers much higher thermal conductivity that a standard electrically conductive die attach paste cannot achieve and balances this with the ability to sinter at 200°C without pressure. This allows you to use the same die-attach equipment without needing to move to the equipment needed for silver-sintering.
Processing semi-sintering die-attach paste
Once engineers have decided to use semi-sintering pastes, the question turns to process. Reflow ovens typically come in three flavours:
- Standard oven (with simply means there is air in the environment)
- Nitrogen-purge oven (purges the air that contains moisture or contaminants)
- Vacuum or low-pressure oven (removes the air from the environment)
The main reasons for Nitrogen introduction during cure are to:
- Prevent cure-inhibition for oxygen-sensitive cure systems
- Prevent the Cu leadframe from oxidizing more under high temperature.
The first point is aimed toward UV and anaerobic cure materials, but has little to do with epoxy-based systems which include semi-sintering materials such as the LOCTITE® ABLESTIK ABP 8068TA or LOCTITE® ABLESTIK ABP 8068TB.
The second point also doesn’t affect the semi-sintering products either as these require a Ag or Au/PPF plating on the die pad.
Further comments from Product Development on the influence of AIR and N2 on sintering performance of LOCTITE® ABLESTIK ABP 8068TB type:
“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
So that’s for the air vs nitrogen-purge ovens. Now concerning the vacuum oven vs. a nitrogen-purge oven. The first question is why a vacuum or low-pressure oven is even being considered. Vacuum drying ovens are most often used for silver-sintering pastes as it helps remove the solvents more effectively. This causes silver-sinter to void less which reduces the thermal conductivity and creates reliability issues caused by excess voiding.
For solvent-based pure sintering materials like LOCTITE® ABLESTIK SSP 2020 this can be an advantage, though pure sintering materials are not typically suitable for semiconductor die-attach applications that require higher MSL levels.
For semi-sintering processes, the vacuum oven will indeed improve (reduce) potential voiding issues by removing the solvent in a controlled way. Because semi-sintering has a resin that fills these voids anyway, voiding is typically not perceived as a problem for resin-based semi-sintering type die-attach pastes.
As such, there is no real advantage to using a vacuum or low-pressure drying oven, though there is no known disadvantage to using one either. Either standard reflow ovens or nitrogen-purge ovens can be used just as well.
In short, semi-sintering pastes have a wide processing window and can be processed using a standard reflow oven, or one with a nitrogen purge or a low-pressure drying oven.
Please visit our website to learn more about die attach pastes and view our entire range of die attach materials including die attach pastes, die-attach films, wafer backside coatings and of course our semi-sintering die-attach pastes and specifically LOCTITE® ABLESTIK ABP 8068TB. You can also contact us if you have any questions about semiconductor packaging or relevant applications.