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Flip Chip Spray Fluxing: Method and Apparatus

IP.com Disclosure Number: IPCOM000118511D
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 4 page(s) / 83K

Publishing Venue

IBM

Related People

Slesinger, KS: AUTHOR

Abstract

Application of solder flux is a critical process in the assembly of Controlled Collapsible Chip Connection (C4) chips to printed circuit boards. Flux acts as an adhesive, holding chips in proper position until they can be soldered permanently to the board. It also minimizes metallic oxidation which can occur at soldering temperatures, improving reliability of the electrical connection. Volume control is important: it must be enough to fully cover the entire chip area but not so much that the post-reflow residue impacts subsequent chip underfill operations.

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Flip Chip Spray Fluxing:  Method and Apparatus

      Application of solder flux is a critical process in the
assembly of Controlled Collapsible Chip Connection (C4) chips to
printed circuit boards.  Flux acts as an adhesive, holding chips in
proper position until they can be soldered permanently to the board.
It also  minimizes metallic oxidation which can occur at soldering
temperatures, improving reliability of the electrical connection.
Volume control is  important:  it must be enough to fully cover the
entire chip area but not  so much that the post-reflow residue
impacts subsequent chip underfill  operations.

      Chip placement must be performed while the flux is wet
or it will not be tacky enough to hold chips in location during
soldering.  However, the no-clean flux (EN-92; a low viscosity, low
solids formulation consisting primarily of isopropyl alcohol)
developed for this application, dries very quickly (in seconds).
Therefore, fluxing must be performed immediately before chip
placement.  As a result, a new spray fluxing method was developed and
integrated into an automated chip placement system.

      As shown in Fig. 1, liquid flux is stored in a syringe style
reservoir.  A stepper motor advances the plunger at a user-defined
distance per unit time, providing a consistent flow rate of flux to
an ultrasonic spray nozzle pointed upward.  When power is applied to
the nozzle, it "erupts", spraying a cloud of atomized flux upwards
(Fig. 2).  Height of the cloud is a function of flow rate and
ultrasonic power.  Flux is deposited onto the C4 chip by programming
the placement robot to move the chip over the nozzle just prior to
plac...