Flip Chip Assembly in Air Very low oxygen levels in reflow allow good yields over a wide range of conditions

Flip Chip Assembly in Air

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

Universal Consortium Flip Chip Test Substrate

Air Reflow Profiles Profile A was Also Run with 1000 ppm Oxygen

Paste Fluxes and Liquid Fluxes

Very Poor Soldering

Poor Soldering

No Electrical Failures but Poor Soldering

No Electrical Failures but Poor Soldering

Good Soldering

Fluxes that Exhibited Good Soldering

Jet Dispensing of Liquid Fluxes for Flip Chip Assembly

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

Jet Dispensing of Liquid Fluxes for Flip Chip Assembly

Dispense Pattern for Jetted Flux

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

List of Liquid Fluxes That Were Evaluated

Air Reflow Profiles Fluxes Showed No Preference

Residue and Some Bridging with Liquid Fluxes

Alpha Metals 9171 Flux Residue Varies from “Excessive” to “Very Little”

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.