News Column

Researchers Submit Patent Application, "Methods for Vacuum Assisted Underfilling", for Approval

July 24, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors Babiarz, Alec J. (Encinitas, CA); Quinones, Horatio (San Marcos, CA); Ratledge, Thomas L. (San Marcos, CA); Zhao, Jiangang (Concord, CA), filed on March 17, 2014, was made available online on July 10, 2014.

The patent's assignee is Nordson Corporation.

News editors obtained the following quote from the background information supplied by the inventors: "The invention relates generally to methods for applying an underfill between an electronic device and a substrate.

"It is typical for an electronic device, such as a flip chip, chip scale package (CSP), ball grid array (BGA) or package on package assembly (PoP) to include a pattern of solder bumps that, during mounting, are registered with pads on a substrate, or joined using another type of interconnect technology such as copper pillars or other types of thermal compression bonding interconnects. The substrate can be a printed circuit board, electronic chip or wafer, for example. The solder is reflowed by heating and, following solidification, solder joints connect the electronic device and the substrate. Underfill, which may be an epoxy, may be used to fill the open space between the electronic device and the substrate that remains between the reflowed solder balls. The underfill protects the solder joints against various adverse environmental factors, redistributes mechanical stresses due to shock, and prevents the solder joints from moving under strain during thermal cycles.

"In the process of underfilling, voids may be formed due to the following reasons, but not limited to, uneven surface topography in the gap between the electronic device and substrate, fluid flow rate race conditions as underfill flows around a solder connection, different wetability conditions on the substrate, air in the underfill, or air induced from the dispensing process. Because the voids are unfilled by underfill, unsupported solder joints adjacent to voids may not be adequately protected against cold flow when exposed to strain from thermal expansion during operation or to mechanical shock caused by dropping the assembled end product, such as a cell phone, that includes the underfilled electronic device. Voids at solder joints prevent the solder bump from being in held in a state of hydrostatic compression and strain restraint, which may increase solder joint fatigue and thereby increase the probability of solder joint cracking.

"Therefore, improved methods are needed for applying an underfill that reduces the probability of forming voids in the underfill."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "In one embodiment, a method is provided for distributing an underfill into the space between the reflowed solder balls which connect an electronic device to a substrate. The method includes providing the underfill onto the substrate near to at least one exterior edge of the electronic device with at least one gap in the underfill, providing an air path to the space between the electronic device and the substrate and then evacuating that space through the gap, or gaps, to provide a vacuum condition in the space. After evacuating the space, the underfill is heated above room temperature to cause capillary flow of the underfill to the exterior edge, or edges, and into the space between the electronic device and substrate and around the reflowed solder balls. The underfill can be provided as a material which is solid at room temperature and is positioned by pick and place equipment onto the substrate, and thereafter becomes liquid at elevated temperatures, or as a liquid material that can be dispensed onto the substrate by, for example, a valve or dispenser.

"Another embodiment of the invention is directed to a method of providing an underfill on a substrate upon which electronic device is mounted by electrically conductive joints and is separated from the substrate by a space. The space has an open portion that is unoccupied by the conductive joints. The method includes providing the underfill onto the substrate proximate to at least one exterior edge of the electronic device, and evacuating the space to provide a vacuum condition in the open portion of the space between the underfill and external edges of the electronic device. After evacuating the space to a vacuum condition, the underfill is heated to a temperature above room temperature to cause flow of the underfill to at least one exterior edge and into the open portion of the space, thereby allowing any air trapped under the underfill itself to vent before reaching the external edge of the electrial device and the gap between the electrical device and substrate

"Other embodiments of the invention are directed to methods of blocking air that has been trapped under the underfill from flowing under the electronic device. In one such method, an obstacle is placed between the edge of the electronic device and the underfill prior to applying the vacuum. After applying the vacuum condition, the underfill is heated to a temperature above room temperature to cause flow of the underfill over the obstacle and from at least one exterior edge into the open portion of the space. Forcing the underfill to flow over an obstacle, helps block the air trapped under the underfill from flowing under the electronic device and allows the trapped air to vent prior to reaching the gap under the electrical device

"Yet another embodiment of the invention is directed to a method of exposing a surface of the substrate to a plasma so as to change the wettability of the substrate prior to providing the underfill onto the substrate proximate to at least one exterior edge of the electronic device. This plasma treatment reduces the opportunity for air to be trapped under the underfill. The method further includes evacuating the space to provide a vacuum condition, in the open portion of the space. After evacuating the space to a vacuum condition, the underfill is heated to cause flow of the underfill toward at least one exterior edge and into the open portion of the space. Since the plasma treatment of the substrate reduces the entrapment of air under the underfill, an amount of air trapped under the electronic device during the underfill operation may also be reduced.

"Similar to the plasma treatment method, a glass-like film may be deposited on the substrate so as to provide a more perfectly smooth and flat surface. This flat surface has fewer depressions or imperfections in which air can be trapped when the underfill is positioned on top of the glass-like film. As entrapment of air under the underfill is reduced, an amount of air trapped under the electronic device during the underfill operation may also be reduced.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

"The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with a general description of embodiments of the invention given above, and the detailed description given below, serve to explain the principles of the embodiments of the invention.

"FIG. 1 is a side view of an electronic device mounted to a substrate by an array of solder balls and with underfill provided along a side edge of the electronic device.

"FIG. 1A is a side view similar to FIG. 1 in which the underfill has moved into the open space between the electronic device and substrate that is unoccupied by the solder balls.

"FIG. 2 is a flow chart of a procedure for vacuum underfilling in accordance with an embodiment of the invention.

"FIGS. 3A-C are diagrammatic top views illustrating a sequence for vacuum underfilling beneath an electronic device mounted on a substrate in accordance with an embodiment of the invention.

"FIGS. 4A-C are diagrammatic top views similar to FIGS. 3A-C in accordance with another embodiment of the invention.

"FIGS. 5A-C are diagrammatic top views similar to FIGS. 3A-C in accordance with yet another embodiment of the invention.

"FIGS. 5D, 5E and 5F are diagrammatic top views similar to FIG. 5A in which the underfill is provided on the substrate with, respectively, an L pattern, a U pattern, and an I pattern.

"FIG. 5G is a diagrammatic top view similar to FIG. 5A in which the underfill is provided on the substrate with no gaps.

"FIG. 6A is a diagrammatic top view similar to FIG. 5A in which a dam is positioned between at least one side edge of the electronic device and the underfill.

"FIG. 6B is a side view of the dam positioned between at least one side edge of the electronic device and the underfill.

"FIGS. 7A and 7B are side views of the underfill flowing over the dam at different time sequences.

"FIG. 8A is a diagrammatic top view similar to FIG. 5A in which a channel is positioned between at least one the side edge of the electronic device and the underfill.

"FIG. 8B is a side view of the channel positioned between the side edge of the electronic device and the underfill.

"FIGS. 9A and 9B are side views of the underfill flowing over the channel at different time sequences.

"FIG. 10 is a cross-sectional view of a plasma-treated substrate according to an embodiment of the invention.

"FIG. 11 is a cross-sectional view of a plasma-treated substrate according to another embodiment of the invention.

"FIG. 12 is a schematic representation of a vacuum underfilling system in accordance with an embodiment of the invention."

For additional information on this patent application, see: Babiarz, Alec J.; Quinones, Horatio; Ratledge, Thomas L.; Zhao, Jiangang. Methods for Vacuum Assisted Underfilling. Filed March 17, 2014 and posted July 10, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=6674&p=134&f=G&l=50&d=PG01&S1=20140703.PD.&OS=PD/20140703&RS=PD/20140703

Keywords for this news article include: Legal Issues, Nordson Corporation.

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Source: Politics & Government Week


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