Semi-sintering die-attach paste processing: Air vs Nitrogen vs Vacuum

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:

1. Pre­vent cure-inhi­bi­tion for oxy­gen-sen­si­tive cure systems
2. 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.