Flip Chip Assembly in Air

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Flip Chip Assembly in Air
Antonio Prats March 2002
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Flip Chip Assembly in Air

• Very low oxygen levels in reflow allow good
  yields over a wide range of conditions
• Unfortunately, nitrogen can be expensive in
  certain regions
• Reflow in nitrogen can increase the chance of
  tombstoning with small 0201 and 0402 components
• It would be nice to have an air reflow process
  for flip chip assembly

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Universal Consortium Flip Chip Test Substrate
Trench and Pads
Trench and Pads
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Air Reflow ProfilesProfile A was Also Run with
1000 ppm Oxygen
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Paste Fluxes and Liquid Fluxes
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Very Poor Soldering
Alpha Metals CF2400, Hot Air
3.7 mil
Heraeus TF38, Cool Air
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Poor Soldering
Indium FC-NC-LT-C, Hot 1010 ppm Oxygen
Exposed Gold
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No Electrical Failures but Poor Soldering
Almit BM1-RMA-NC, Cool Air
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No Electrical Failures but Poor Soldering
Indium TAC023, Hot Air
Exposed Gold
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Good Soldering
2.6 mil
Almit BM1-RMA-NC, Hot 985 ppm Oxygen
Indium TAC023, Cool Air
Kester R903, Cool Air
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Fluxes that Exhibited Good Soldering
Hot Profile Peak 228C, end of soak 168C Cool
Profile Peak 210C, end of soak 152C
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Jet Dispensing of Liquid Fluxes for Flip Chip
Assembly
Antonio Prats March 2002
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Jet Dispensing of Liquid Fluxes for Flip Chip
Assembly

• In the past, spray fluxing of water-soluble or
  no-clean liquid fluxes did not offer a robust
  assembly process
• It is time to re-evaluate liquid fluxes and look
  at the Asymtek Dispense Jet process

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Jet Dispensing of Liquid Fluxes for Flip Chip
Assembly
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Dispense Pattern for Jetted Flux
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Jet Dispensing of Liquid Fluxes for Flip Chip
Assembly

• Some Fluxes Volatilize Quickly
• Only one leaves behind a tacky film (Alpha Metals
  9171)
• The rest leave a non-tacky film that does not
  hold a flip chip in place
• Other Fluxes Do Not
• These fluxes remain fluid through die placement

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List of Liquid Fluxes That Were Evaluated
Volatile fluxes are those which dried before
the die could be placed. The other fluxes
remained liquid for die placement.
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Air Reflow ProfilesFluxes Showed No Preference
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Residue and Some Bridging with Liquid Fluxes
Indium FC-NC-LT-B, 1010 ppm Oxygen
Almit BM1-RMA-5000, cool air
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Alpha Metals 9171Flux Residue Varies from
Excessive to Very Little
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Summary

• A good liquid flux will
• Promote good soldering without solder bridges
• Hold the die in place through reflow
• By remaining liquid or
• By volatilizing and leaving behind a sufficiently
  tacky film
• Not leave excessive amounts of residue
• We should be able to dispense less flux, and see
  what effect that has upon the amount of residue
• At this time, we have no information on underfill
  compatibility with these fluxes. This will be
  very important.