Processing options for semi-sintering paste

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:

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.

High Ther­mal Con­duc­tiv­i­ty Die Attach Options: Die Size vs. Ther­mal Conductivity

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.

About Chris Perabo

Chris is an energetic and enthusiastic engineer and entrepreneur. He is always interested in taking highly technical subjects and distilling these to their essence so that even the layman can understand. He loves to get into the technical details of an issue and then understand how it can be useful for specific customers and applications. Chris is currently the Director of Business Development at CAPLINQ.

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