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26 March 2014

Rework of Underfilled Flip Chips

Posted in Application Notes

Equipment: Model 850 Flip Chip Placement System with Model 870 Rework Option

 

Application: Rework of underfilled flip chips
Equipment: Model 850 Flip Chip Placement System with Model 870 Rework Option

 

 

Process: The process of reworking underfilled chips begins with heating the substrate evenly to a temperature below the melting point of solder.  The chip undergoing rework is then spot heated to melt the solder connections and break down the underfill.  The chip is gripped mechanically and then twisted or sheared away from the circuit.  Any residual solder and underfill is cleaned off the substrate.  Once cleanup of the substrate surface is complete, a new chip can be aligned, bonded, reflowed and underfilled.

 

Machine Discussion: To successfully carry out this process, a machine needs to be equipped with a stage that can be heated up to 200°C. (the most common temperature range for this type of application is 120-150°C.),  The stage should also have a hold-down device for the substrate during removal of the chip. 

To heat the die, the machine needs to have a spot heating system that can be easily positioned over the substrate and isolate the heating to the die being reworked.  It is important not to overheat nearby devices.  The heating needs to be maintained by a controller that can store and run thermal profiles; the temperatures and heating times should be similar to those used by the ovens in which the chips were originally reflowed.

To remove the chip, a gripping device is needed.  The device must align properly to the chip and support it to prevent damage during the removal process.  The clamping area of this device should be adjustable to accommodate several different chip sizes.

Cleanup after chip removal is required to remove any underfill residue and excess solder,  We have used two different methods to cleanup.  One method is to use a dremmel tool with a brush.  This method tends to be messy as the debris needs to be cleaned up and over brushing can occur.  It may be necessary to wick off excess solder that cannot be removed by the brush and dremmel tool.  Another method involves scraping the softened underfill off the heated substrate with a teflon tool immediately after the chip is removed.

A microscope is useful in the cleanup process to view the area undergoing rework.  It can also be utilized for precise alignment of the spot heater to the chip prior to removal.

After cleanup, a new chip can be placed on the substrate.  This process involves picking up the chip, dipping it in flux, aligning and placing it on the substrate (see the Solder Flip Chip Application Bulletin). 

Once the chip is placed it needs to be reflowed.  The same spot heating capability that the system uses to remove the chip should also be to reflow the solder.  A manual dispensing system can be used to apply underfill after the chip is soldered.  In the case of no flow underfill is dispensed by a manual dispenser prior to placing the chip.

 

Machine Features: The Model 850 Flip Chip Placement System with the Rework Option has the following features which make it a good choice for reworking underfilled flip chips. The basic Model 850 includes a beam splitter alignment system with no video display, a die tray holder for die in waffle packs, a fluxing station (motorized or manual) and a micrometer adjusted stage, A four-inch heated workholder capable of 350°C. is available. 

The Rework Option includes a hot gas spot heating system with interchangeable nozzles.  The hot gas system (600° C. max) uses low gas velocity (needle valve with flow indicator) to localize the heating.  The temperature controller will run thermal profiles, which can be saved in memory.  The hot gas system is mounted on the Model 850.  A Leica Stereo Zoom 4 Microscope with Illuminator and mounting is available.

 

Required Equipment: The following equipment from Semiconductor Equipment Corporation is required to perform this application:
 
  • Model 850 Semi-automatic Flip Chip Placement System
  • Die Tray Pedestal
  • Manual Fluxing Station or Motorized Fluxing Station
  • Model 855 heated 4 x 4 workstage
  • Special 4x4 vacuum workstage chuck for specific devices
  The following equipment from Semiconductor Equipment Corporation is optional to perform this application:
 
  • Microscope with Illuminator
  • Hot Gas Spot Heating with Z-motion
  • Accessories Kit
  • Dispenser

 

Contact: Lester Salvatierra
Sales Manager
Phone: (805) 529-2293 x 21
Fax: (805) 529-2193
E-mail: Click Here