Methods for vacuum assisted underfilling

Methods for applying an underfill with vacuum assistance. The method may include dispensing the underfill onto a substrate proximate to at least one exterior edge of an electronic device attached to the substrate. A space between the electronic device and the substrate is evacuated through at least one gap in the underfill. The method further includes heating the underfill to cause the underfill to flow into the space. Because a vacuum condition is supplied in the open portion of the space before flow is initiated, the incidence of underfill voiding is lowered.

BACKGROUND

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 expoxy, 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.

SUMMARY OF THE INVENTION

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 electrical 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.

DETAILED DESCRIPTION

Generally, the embodiments of the invention are directed to a vacuum-assisted process for underfilling an electronic device mounted on a substrate by an array of solder balls. Underfill is dispensed or otherwise provided (e.g., in either a liquid or solid form) in one or more lines around the edges of an unheated electronic device, which is mounted to an unheated substrate by means of an array of reflowed solder balls. Preferably, at least one gap is left in the one or more lines of underfill and, preferably, if the space between the electronic device and substrate is very small, there is a space between the underfill and the exterior edges of the electrical device. The substrate is transported into a vacuum chamber, before significant capillary underfilling (and air or gas entrapment) occurs, and a vacuum is applied to evacuate the space. While the vacuum is being applied, the gap, or gaps, in the one or more lines of underfill allows air to flow out from under the device through the gap(s), to establish a vacuum condition (i.e., a pressure less than atmospheric pressure) under the electronic device between the electronic device and the substrate. An alternative, less preferred process, is to provide no gap in the underfill and to rely upon the air trapped under the device to bubble through the underfill when the device is placed under vacuum. Under either process, while the vacuum condition is being maintained, the electronic device and substrate are heated to cause the underfill to completely flow under the electronic device into the spaces between the reflowed solder balls. Underfilling in the presence of the vacuum condition means any void entrapped in the underfill will be partially evacuated of gases commensurate with the level of the applied vacuum. The vacuum pressure applied must not be lower than the vapor pressure of the underfill, otherwise the underfill will boil and the process will become less stable. The vacuum chamber is then vented. Any voids present in the underfill will now collapse because of the evacuated condition and become filled with underfill. The underfilled electronic device and the substrate are then moved out of the vacuum chamber.

The embodiments of the invention also apply to other interconnect technologies, in addition to solder bumps, for creating conductive joints between the electronic device and the substrate, such as copper pillars and other thermal compression bonding interconnect technologies.

With reference toFIG. 1, an assembly10includes a substrate12, such as a printed circuit board, and an electronic device14that is mounted to a surface16of the substrate12. In representative embodiments, the electronic device14may be a flip chip, chip scale package (CSP), ball grid array (BGA) or package on package assembly (PoP), for example. Likewise, the substrate12may be a printed circuit board (PCB), electronic chip or wafer, for example, or any substrate or interposer used in semiconductor packaging of electronic devices

With reference toFIGS. 1,1A, and3A, the electronic device14has a footprint on the substrate12such that the substrate12is exposed adjacent to each of the side or exterior edges18,20,22,24of the electronic device14. Solder joints26mechanically and electrically connect the electronic device14with the substrate12. A space28is defined between the electronic device14and substrate12and a portion of the space28is open (i.e., unoccupied) and unfilled by the solder joints26that may have a representative form of solder balls. At each of the exterior edges18,20,22,24, a gap27is defined between the electronic device14and the substrate12. The gap27communicates with the space28. Preferably, for small gaps24(e.g., less than 200 microns), a space43on the surface of substrate12exists between the underfill30and corresponding device edges18,20,22and24.

An underfill30is used to fill the space28between the electronic device14and the substrate12, as shown inFIG. 1A. In one example, the underfill30is a curable non-conductive silicon dioxide particle filled epoxy that is fluid when applied to the substrate12and flows by capillary action. Other types of underfill can be used including those that are solid at room temperature or are frozen. Underfills are typically filled with small particles of glass, for example to provide the desired properties in the cured underfill. When cured and hardened, the underfill forms a strongly bonded, cohesive mass.

With reference toFIG. 2, a procedure for vacuum underfilling in accordance with an embodiment of the invention is described. In theFIG. 2embodiment, a liquid underfill is dispensed onto the substrate. Instead of dispensing the underfill30in a liquid form, the underfill30could be applied in a solid form in position to buy a pick and place machine, for example, as mentioned above. In block52, liquid underfill30is dispensed onto the substrate12. The underfill30may be applied as one or more continuous lines (FIG. 3A) proximate to one or more exterior edges18,20,22,24of the electronic device14. Preferably, underfill30does not touch edges18,20,22, or24so that surface43is not covered by the underfill30until full vacuum is applied. Typically, the dispensed amount of underfill30is equal to the volume of the open space28under the electronic device14plus the fillet31(FIG. 1A) that forms along the perimeter of the device14after the underfill operation has been completed. The substrate12is unheated when the underfill30is applied and a gap42(FIG. 3A) is preferably present in the underfill30so that an air path to the open portion of space28through the gap42is maintained. As discussed above, the less preferred method is not to leave a gap42or open space43, and to rely on air trapped under the electronic device14to bubble through the underfill30.

The underfill30may be applied to the substrate12using multiple different types of dispensers and in multiple different ways. For example and although the invention is not so limited, a series of droplets of underfill30may be dispensed onto the surface16of the substrate12from a moving jetting dispenser that is flying above the surface16.

In block54, the underfill30is cooled when dispensed onto the substrate12. In one embodiment, the substrate12is cooled, for example, by one or more thermoelectric coolers to a temperature below room temperature and the underfill30cools shortly after application to approximately the temperature of the substrate12. Alternatively or in addition to cooling the substrate12, the underfill30may be cooled in the dispenser before being dispensed onto the substrate12. In one embodiment, the underfill30is cooled to a temperature in the range of 0° C. to 10° C. Cooling increases the viscosity of the underfill30, which further prevents or reduces capillary flow into the open portion of the space28between the electronic device14and the substrate12before vacuum is applied

In block56, the unfilled portion of space28is evacuated to a sub-atmospheric pressure through the gap42in the underfill30or space43to establish a vacuum condition (i.e., a pressure less than atmospheric pressure) in space28. Or, if no gap has been provided, or if the open space43is not maintained, the gas will bubble through the underfill30. To create the vacuum, in one embodiment, the substrate12, which carries the electronic device14and the underfill30, is moved into a vacuum chamber, sealed inside the chamber, and the vacuum chamber is evacuated to a sub-atmospheric pressure. In one embodiment, a suitable sub-atmospheric pressure for the vacuum is greater than or equal to 25 inches of Hg (about 95 Torr) to 26 inches of Hg (about 100 Torr). In any event, the sub-atmospheric pressure is limited such that the physical properties of the underfill are not significantly or detrimentally modified.

Any suitable technique may be used for moving the substrate12into and out of the vacuum chamber, and conventional vacuum systems are familiar to a person having ordinary skill in the art. The substrate12is preferably transferred into the vacuum chamber before the occurrence of capillary underfilling (and air or gas entrapment) or before the underfill30is allowed to touch any of surfaces18,20,22,24thereby maintaining surface43be uncovered with underfill30.

In block58, after the vacuum chamber is evacuated, and while the vacuum condition is being maintained, the underfill30is heated to a temperature in excess of room temperature, for example to a temperature in a range of 30° C. to 120° C. The underfill30may be heated by heating the substrate12, the electronic device14or both and in any desired sequence to direct flow. In response to the heating, the underfill30flows by capillary action through the narrow gap27from each of the exterior edges18,20,22,24into the space28and around the reflowed solder balls. Because the open portion of the space28is evacuated, the underfill30can flow across the space28such that any void entrapped in the underfill30will be evacuated of gases to the vacuum level.

In block60, after sufficient time has been provided for complete capillary flow to have occurred, then the vacuum condition is removed and atmospheric pressure is restored. For example, the vacuum chamber may be vented to provide the atmospheric pressure condition. Under the influence of atmospheric pressure, any voids present in the underfill30will collapse because of their evacuated state of sub-atmospheric pressure and become filled with underfill30(FIG. 3C). The substrate12is then transferred from the vacuum chamber to a curing oven and the underfill30is cured.

With reference toFIGS. 4A-4Cand in alternative embodiments, the underfill30may be applied proximate to the exterior edges18,20,22,24of the electronic device14as a series of disconnected regions (FIG. 4A) with multiple gaps61. InFIG. 4B, the gaps61disappear as the underfill30is heated after evacuating the open portion of spaces28and43to a vacuum condition. InFIG. 4C, the underfill30flows beneath the device14.

With reference toFIGS. 5A-5Eand in alternative embodiments, the underfill30may be applied proximate to one or more of the exterior edges18,20,22,24of the electronic device14in one or more passes. In this case,FIG. 5Ashows a line of underfill applied along each of the four edges of the device, with a gap62and space43present at each corner between each pair of exterior edges18,20,22,24. InFIG. 5B, the underfill30is heated after evacuating the space28through the gaps62to a vacuum condition. InFIG. 5C, the underfill30, in the heated state, flows beneath the device14.

In an alternative embodiment and as shown inFIG. 5D, the underfill30could be provided as lines using an L pass along exterior edges18and24of the electronic device14, preferably providing space43. In this case, a gap is present along the exterior edges20and22. In another alternative embodiment and as shown inFIG. 5E, the underfill30could be provided as lines using a U pass along exterior edges18,20,22of the electronic device14preferably providing space43, but not along exterior edge24of the electronic device14. In another alternative embodiment and as shown inFIG. 5F, the underfill30could be provided as a line using an I pass along exterior edge20of the electronic device14, preferably providing space43, but not along exterior edges18,22, and24. As probably the least preferred alternative embodiment and as shown inFIG. 5G, the underfill30could be applied as lines along all four edges18,20,22and24and in an overlapping manner with no gaps defined at the corners. In this case, the air, or gas, trapped under the electronic device14will bubble through the underfill30when the vacuum is applied.

The lines of underfill, in addition to being applied in the preferred method from a non-contact jetting valve, such as the DJ 9000 sold by Nordson ASYMTEK of Carlsbad, Calif., could alternatively be applied as solid preforms of epoxy. The solid preforms are placed on the substrate12and then melted upon the application of heat. The solid preforms could be placed into position by a pick and place machine or mechanism.

Gas or air66can be trapped under the underfill30when the underfill is provided on the substrate. Air that is trapped under the underfill30when the underfill30is applied or laid along the edge of the electronic device14may vent underneath the electronic device14after the vacuum is applied and the underfill30is heated it induce capillary flow. The vented air may become trapped under the electronic device14as air pockets, which may lead to the formation of voids in the underfill30. Ensuring space43is maintained until the full vacuum is applied mitigates this trapped air from venting under the electronic device14.

In accordance with alternative embodiments of the invention, the substrate12may include an obstacle positioned on the surface16proximate to at least one exterior edge18,20,22,24of the electronic device14. In a representative embodiment, the obstacle may be formed as a linear body. The obstacle is located between the location of the dispensed underfill30and the adjacent exterior edge18,20,22,24of the electronic device14.

The obstacle serves as an impediment over which the underfill30must flow before flowing toward the exterior edge18,20,22,24of the electronic device14and into the open portion of the space28, during the procedure for vacuum underfilling shown inFIG. 2thereby maintaining space43. The liquid underfill30(or a majority thereof) is able to flow over the obstacle, and the obstacle has only a negligible or minor effect on the flow and flow rate of the underfill liquid. However, air or gas pockets are generally incapable of surmounting the obstacle or are vented as the underfill30flows over space43before reaching the gap27. As such, the obstacle removes air or gas pockets from the underfill. Therefore, this embodiment helps reduce or eliminate trapped gas under the electronic device14during the vacuum-assisted underfilling operation.

As the distance between the dispensed underfill30and the exterior edge18,20,22,24of the electronic device14increases, the ability of trapped gas66under the underfill30to reach the gap27decreases. If air66is trapped under the underfill30and the underfill30is laid adjacent to the exterior edges18,20,22,24of the electronic device14(i.e., in contact with the electronic device14), the air66trapped under the underfill30may be vented under the electronic device14when the vacuum is applied and the underfill30is heated. Air that vents under the electronic device14may become trapped under the electronic device14. Therefore, the underfill30should be positioned on the substrate12far enough away from the exterior edge18,20,22,24of the electronic device14so as to avoid venting under the electronic device14. When the underfill30is positioned far away from the exterior edge18,20,22,24of the electronic device14, the substrate12may be tilted so as to help induce the underfill30to flow toward the exterior edge18,20,22,24and under the electronic device14when the underfill30is heated. The overall purpose is to prevent air66trapped under the underfill30from venting under the electronic device14, with the underfill30then flowing around the air so as to form a bubble under the electronic device14. The use of the obstacle, as in the present embodiment, effectively achieves the same result, as the underfill30is spaced apart from the exterior edges18,20,22,24of the electronic device14by a distance required for the placement of the obstacle.

With reference toFIGS. 6A-7Bin which like reference numerals refer to like features inFIGS. 1-5Gand in accordance with an alternative embodiment, the obstacle may be a dam68formed on the surface16of the substrate12. The dam68may have a top wall72raised above the surface16of the substrate12and side walls70ascending from the surface16to the top wall72. As discussed above, the surface16receives the dispensed underfill30. Consequently, the dam68is located on the same surface16that receives the dispensed underfill30and on which the electronic device14is mounted and between underfill30and the exterior edges of18,20,22,24. A height of the dam68is sufficiently low so that the underfill30may flow over the dam68when the assembly10is heated to a given temperature. The height of the dam68is low enough not to impede the underfill flow after heating. Although the underfill30may flow over the dam68, the air66is unable to surmount the wall70or the air vents through the underfill as it flows over space32toward external edges18,20,22,24and, therefore, the air does not flow under the electronic device14.

The dam68may be formed of a legend ink, such as that which is typically used on PC boards for visible markings or letters. Alternatively, a damming material such as that which is typically used for dam and fill operations could be employed. More generally, the damming material could be any thixotropic material, meaning any material that does not flow once it is deposited on the surface16of the substrate12.

Although the side walls70and the top wall72of the dam68form two right angles in the representative embodiment, the side walls70and/or the top wall72may be inclined, contoured, and/or curved. Alternatively, the two side walls70may converge at an angle, such that the top of the dam68forms a peak or a crest rather than a wall that is parallel to the surface16of the substrate12. Moreover, a width of the dam68, including the dimensions of the side walls70or the top wall72, may vary.

With reference toFIGS. 8A-9Bin which like reference numerals refer to like features inFIGS. 6A-7Band in accordance with an alternative embodiment, the obstacle may be a channel74formed in the substrate12and recessed below the surface16of the substrate12. The channel74may be formed by a router, for example. As discussed above, the surface16receives the dispensed underfill30. Consequently, the channel74is located on the same surface16that receives the dispensed underfill30and on which the electronic device14is mounted. The channel74may have a base78positioned at a distance below a level of the surface16and side walls76descending from the surface16to the base78. The channel74may obstruct or impede the underfill30from flowing to external edges18,20,22,24, prior to heating the underfill. As shown inFIGS. 9A and 9B, after vacuum is applied and the underfill30is heated, the underfill30flows into and/or over the channel74before flowing toward the at least one exterior edge18,20,22,24of the electronic device14and into the open portion of the space28. However, the air66trapped under the underfill30is trapped in the channel64; once the air66flows into the channel64, it is unable to surmount the sidewalls76. Any remaining air vents through the underfill30before the underfill30reaches the gap27. In this way, the channel74helps to prevent the air66from flowing under the electronic device14. The depth of the channel74should be sufficiently shallow so that substantially all of the liquid underfill30may flow through or over the channel74. However, the depth of the channel74, and thus the heights of the side walls76, may vary.

Although the side walls76and the base78of the channel74form two right angles in the representative embodiment, the walls76and/or base78may be inclined, contoured, and/or curved. Alternatively, the two side walls76may converge at an angle, such that the channel74lacks a planar base. Moreover, a width of the channel74, including the dimensions of the side walls76or the base78, may vary.

In an alternative embodiment, the obstacle may include combined features of the dam68and the channel74. For example, the dam68may be immediately followed by the channel74on the substrate12, such that the underfill30flows over the dam68and through the channel74before flowing toward the exterior edge18,20,22,24of the electronic device14.

In the representative embodiment, a single obstacle is shown extending around an entire periphery of the electronic device14. However, in alternative embodiments, one or more obstacles may extend along any combination of the one or more exterior edges18,20,22,24. Moreover, the obstacles may be longer or shorter than the lengths of the one or more exterior edges18,20,22,24. Preferably, a positioning of the one or more obstacles will correspond to the positioning of the dispensed underfill30, such that all of the underfill30must flow over the obstacles in order to reach the exterior edges18,20,22,24of the electronic device14.

With reference toFIGS. 10 and 11and in accordance with an alternative embodiment, the surface16of the substrate12having an original composition and wettability may be modified to mitigate the trapping of air under the underfill30is originally dispensed. In this embodiment, the substrate12may be plasma treated so as to change the wettability of the surface on which the underfill30is dispensed. The plasma treatment process may be activated by methods known to those of ordinary skill.

With particular reference toFIG. 10, the substrate12may also be plasma treated so as to activate a surface layer94of the substrate12. Such activation may alter a chemical composition, and, thus, physical characteristics of the surface layer94of the substrate12so as to change its wettability. The surface layer94of the substrate12has a thickness t1. The plasma activation does not add a layer to the substrate12; rather, it modifies the layer94with thickness t1of the preexisting substrate12.

In an embodiment, the plasma treatment decreases the wettability of the layer94of the substrate12. By rendering the surface layer94of the substrate12less wettable, less air may be trapped and air that is trapped under the underfill30, when the underfill30is positioned on the substrate12, may more easily escape from beneath the underfill30when the vacuum is applied. The surface layer94with decreased wettability may have more surface imperfections through which the air may escape than the original surface16of the substrate14. In this way, the trapping of air under the electronic device14during the vacuum-assisted underfill operation may be reduced by the reduction of trapped air66under the underfill30.

In another embodiment, the plasma treatment increases the wettability of the layer94of the substrate12. Less air is entrapped under underfill30deposited on the plasma-treated surface having an increased wettability than on a non-plasma treated surface because air may be more easily displaced as the underfill30is applied. By reducing the initial trapping of air under the underfill30, the trapping of air under the electronic device14during the vacuum-assisted underfill operation may also be reduced.

With particular reference toFIG. 11, plasma deposition may be used to deposit a very thin, glass-like layer90or film on the surface16of the substrate12. The layer90has a thickness t2, and, thus, a height of the plasma-treated substrate is increased (as compared to a height of the original substrate12) by height t2. The plasma-treated surface may be so smooth and flat that there are fewer surface imperfections, such as depressions, in which air can be trapped. As such, conducting vacuum-assisted underfilling on the plasma deposited layer90helps prevent air or gas from being trapped under the underfill30. By reducing the initial trapping of air under the underfill30, the trapping of air under the electronic device14during the vacuum-assisted underfill operation may also be reduced. In an embodiment, a combined plasma treatment method may be employed, in which the glass-like layer90is deposited on the substrate12and then is activated so as to further increase wettability.

In an embodiment, a combination of the methods provided above may be employed to help prevent the entrapment of air bubbles under the electronic device14. For example, the top surface16of the substrate12may be plasma treated so as to increase the wettability of the top surface16and/or to deposit a glass-like layer90on the substrate12. Such plasma treatment will help prevent air from being trapped under the underfill30when it is provided on the substrate12. In addition, an obstacle, such as a dam68or a channel74, may be provided on the plasma-treated substrate12so as to block any air66that may have been trapped under the underfill30from flowing under the electronic device14during the vacuum-assisted underfill operation or to prevent the underfill30from flowing over space43prior to heating the underfill30in the vacuum

With reference toFIG. 12, a system110for use in vacuum underfilling is configured to dispense amounts of the underfill30on the substrate12upon which the electronic device14is mounted by reflowed solder balls, or another interconnect technology, and is separated from the substrate12by the space28. The space28has an open portion that is not occupied by the conductive joints26, which in this case are in the form of reflowed solder balls.

A controller120, which is electrically coupled with a motion controller118and a dispenser controller116, coordinates the overall control for the system110. Each of the controllers116,118,120may include a programmable logic controller (PLC), a digital signal processor (DSP), or another microprocessor-based controller with a central processing unit capable of executing software stored in a memory and carrying out the functions described herein, as will be understood by those of ordinary skill in the art.

The system110preferably includes a cooling device133and a cooling device135that is coupled with the dispenser132. The cooling device133is configured to cool the substrate12such that the underfill30cools when dispensed onto the substrate12. The cooling device135is configured to cool the underfill30such that the underfill30is cooled before dispensing onto the substrate12. The cooling devices133,135are preferred, and optional, and may be respectively operated by a temperature controller139under the control of controller120to reduce the temperature of the substrate12to below room temperature and/or to reduce the temperature of a portion of the dispenser132to below room temperature.

The system110includes a dispenser132, which may be a jetting dispenser, used to dispense the amounts of the underfill. Downstream from the dispenser132, the system110further includes a vacuum chamber154configured to permit access for inserting and removing each assembly10and configured to provide a sealed condition in which an interior space of the vacuum chamber154is isolated from the surrounding atmospheric-pressure environment. A vacuum pump160is coupled with the interior space of the vacuum chamber and is configured to evacuate the interior space as operated by the controller120. A vent174is used under the control of the controller120to admit gas to the interior space to raise the chamber pressure. The controller120supplies motion instructions to the motion controller118to operate a transfer device122used to move the substrate12, which is carrying the underfill30, into the vacuum chamber154.

A heater166is disposed inside the vacuum chamber154and is configured to be powered by a temperature controller169linked with the controller120. Heat is transferred from the heater166to each substrate12. In one embodiment, the temperature of the substrate12and underfill on the substrate ranges from 30° C. to 120° C.

In use, the substrate10is moved to a location beneath the dispenser132and underfill is dispensed or otherwise applied. In the representative embodiment, the controller120sends commands to the motion controller118to cause the transfer device122to move the dispenser32and the controller120sends commands to the dispenser controller116to cause the dispenser32to dispense the underfill in one or more lines around the exterior edges18,20,22,24of the electronic device14. The substrate12is not heated during the dispensing operation. Preferably, at least one gap is left in the one or more lines of underfill32and preferably the underfill30is not in contact with the exterior edge18,20,2224. For a jetting dispenser132, the dispenser controller16triggers the jetting of droplets at appropriate times during the movement such that the droplets will impact at a desired location on the substrate12. Each dispensed droplet contains a small volume of the underfill, which is typically controlled with high precision by the dispenser controller16.

In one embodiment, the cooling device133may be used to cool the substrate12so that the underfill30cools to a temperature below room temperature upon contact with the substrate12. Alternatively, the cooling device135coupled with the dispenser132may be used to cool the underfill30before dispensing.

After the dispensing operation is completed and before significant capillary underfilling (and air or gas entrapment) occurs, the controller120sends commands to the motion controller118to cause the transfer device122to transport the assembly10and dispensed underfill30on the substrate12into the vacuum chamber54. Once the assembly10and dispensed underfill30on the substrate12are isolated inside the vacuum chamber54from the ambient environment, the controller120causes the vacuum pump160to evacuate the interior space inside the vacuum chamber154. While the vacuum is being applied, each gap allows a vacuum condition (i.e., a pressure less than atmospheric pressure) to be established under the electronic device14between the electronic device14and the substrate12or, if there is no gap, then the gas bubbles through the underfill to create a vacuum condition under the electronic device14.

When a suitable vacuum pressure exists inside the vacuum chamber154and with the vacuum condition being maintained, the controller120causes the temperature controller169to operate the heater166, which heats the substrate12, electronic device14, and the underfill30. The elevated temperature encourages the underfill30to flow over the substrate space43and into the open portion of the space beneath the electronic device14. The underfill30completely flows under the electronic device14and into the spaces between the reflowed solder balls. Underfilling in the presence of the vacuum condition means any void entrapped in the underfill will be partially evacuated of gases. After flow ends, the controller120sends commands to the motion controller118to cause the vent174to admit gas to the vacuum chamber154so that the pressure inside the vacuum chamber154is returned to atmospheric pressure. Any voids present in the underfill30collapse because of the evacuated condition and become filled with underfill30. The substrate12with the underfilled electronic device14is transferred out of the vacuum chamber154to, for example, a curing oven (not shown).