Patent Application
20050121310
Embodiments of the present invention may relate to controlling the flow of materials over a substrate. More particularly, embodiments of the present invention may relate to reducing the flow of an underfill material used to attach a silicon die to a substrate.
Integrated circuits are typically assembled into packages (or package assemblies) that are mounted to a printed circuit board. The package may include a substrate that has solder balls or other types of contacts that are attached to the circuit board. An integrated circuit is mounted to the substrate. The integrated circuit (IC) may be connected to corresponding surface pads of the substrate with solder bumps in a process commonly referred to as controlled collapsed chip connection (C4). The substrate typically has routing traces, vias, etc. that electrically connect the integrated circuit to the solder balls.
Packages may contain an underfill material that is formed between the integrated circuit and the substrate. The underfill material structurally reinforces the solder bumps and improves the life and reliability of the package. The underfill material is typically dispensed onto the substrate in a liquid or semi-liquid form. The liquid underfill then flows between the integrated circuit and the substrate under a capillary action. The liquid underfill is eventually cured into a solid state. The underfill process fills the space between the integrated circuit and the substrate to structurally reinforce all of the solder bumps.
The foregoing and a better understanding of the present invention will become apparent from the following detailed description of arrangements and example embodiments (and the claims) when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written and illustrated disclosure focuses on disclosing arrangements and example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and the invention is not limited thereto.
The following represents brief descriptions of the drawings in which like reference numerals represent like elements and wherein:
In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing figure drawings. Further, in the detailed description to follow, example sizes/models/values/ranges may be given although the present invention is not limited to the same. Where specific details are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that the invention can be practiced without these specific details.
In the following description, the terminology substrate may be used to describe a material to support a die or integrated circuit. This substrate may or may not include various layers, including a solder resist layer. That is, these various layers including the solder resist layer may be considered as part of the substrate.
Embodiments of the present invention may provide a method of forming a package assembly that involves obtaining a substrate and identifying a die placement area and a keep out area of the substrate. A protective area of the substrate may be identified between the die placement area and the keep out area A mask may be provided over the substrate to allow selective sputtering over the substrate. That is, areas corresponding to the protective area may receive little or no sputtering due to the mask, whereas areas corresponding to the die protective area may receive sputtering (such as by O2 plasma or H2 plasma). Thus, the area corresponding to the protective area may have a smoother roughness than the area corresponding to the die placement area. Underfill material may subsequently be applied at the die placement area. The area having the smoother roughness (at the protective area) may prevent overflow of the underfill material to the keep out area.
Various arrangements and embodiments of the present invention will now be described with respect to the respective figures.
The package may include an underfill material 15 that is located between the die 10 and the substrate 20. The underfill material 15 may be an epoxy that strengthens the solder joint reliability and the thermo-mechanical moisture stability of the IC package.
The package may have hundreds of solder bumps 12 arranged in a two dimensional array across the bottom of the die 10. The underfill material 15 may be applied to the solder bump interface by dispensing an uncured epoxy material along a side of the die 10. The underfill material 15 may then flow between the solder bumps 12 based on capillary action. The underfill material 15 is dispensed in a manner that covers all of the solder bumps 12. Other methods of applying the underfill material may also be performed.
The solder bumps may carry electrical current between the die 10 and the substrate 20. The substrate 20 may include an organic dielectric material. The package 30 may also include a plurality of solder balls (not shown) attached to a bottom surface of the substrate 20. These solder balls can be reflowed to attach the package 30 to a printed circuit board (not shown).
The substrate 20 may also contain routing traces, power/ground planes, vias, etc. (not shown) that electrically connect with the solder bumps 12. The die 10 may be encapsulated by an encapsulant (not shown). Additionally, the package 30 may incorporate a thermal element (not shown) such as a heat slug or a heat sink to remove heat generated by the die 10.
The flow of the underfill material 15 may first contact the trench 72 and the underfill material 15 can spread out in a direction of the length of the trench 72. The underfill material 15 that flows across (or breaches) the trench 72 may contact the dam 74 that will further direct flow along the direction of the length of the dam 74. The underfill material 15 that breaches the dam 74 may contact the dam 76. The dam 76 can further deflect flow along the length of the dam 76 as well as along the length of the trench etched within the dam 76.
Other structures and methodologies for protecting the flow of overfill material will now be described. These methodologies and structures may relate to surface roughness. A rougher surface topology may be important for smoothly filling the underfill material 15 under the die 10 by capillary action. That is, the capillary action of the underfill material 15 performs better for a rougher surface than a smoother surface. O2 plasma may be used (at a substrate supplier, for example) to roughen the surface as well as for cleaning organic residue. However, when the O2 plasma may be applied over the surface of the solder resist 60 or the substrate 20, the surface topology is relatively homogeneous. Substrates with a homogenous surface topology may have inconsistent underfill filet formation from substrate to substrate after dispensing and curing the underfill material.
Embodiments of the present invention may provide selective roughening (i.e., no or less roughening at a protective area close to the keep out area) on the solder resist/substrate. The selective roughening may produce a consistent fillet area to avoid improper fillet formation. This may help avoid issues such as overflow to the keep out area, dendrites, Insufficient Die Edge Coverage (IDEC) and butterfly shape filleting (top view) that was not completely filed in the gap between a die (IC) and a substrate.
Embodiments of the present invention may utilize selective sputtering (or selective chemical etching) on the surface of the substrate 120 (or more specifically on the surface of the solder resist 160). The selective sputtering may be applied to (or over) the protective area 150 to avoid the underfill material from overflowing to the keep out area. This may be accomplished without sacrificing a higher density design. Selective sputtering may be accomplished using masking materials to avoid sputtering on a particular area. Selective sputtering may also be accomplished by printing (or providing) some materials (e.g. resist, ink or other organic material) on the particular area and sputtering in a normal manner. Thus, only the printed material is sputtered at the selective area rather than the substrate/solder resist. The original surface of the substrate/solder resist is sputtered normally.
On the other hand,
Various operations may then occur before die placement. For ease of illustration, not all of these operations are shown.
In block 314, the die may be placed onto the die placement area of the substrate. The assembly may be heated to perform reflow in block 316 and flux may be washed (or defluxed) in block 316. The assembly may then be pre-baked in block 318. In block 320, the underfill material may be provided (or injected) between the die and the substrate. Pre-bake and curing may occur in block 322. The assembly may be electrically tested in block 324. Upon proper testing, the assembly may be appropriately sold or used.
Embodiments of the present invention may optimize the underfill process to eliminate or reduce the occurrence of underfill related defects (such as overflow to the keep out area, underfill dendrites and/or butterfly shape filleting that was not completely filled in the gap between a die (IC) and a substrate). This may also allow less space between the die area and the keep out area. Thus, high density capacitors (Die Side Chip (DSC) capacitors) may be placed closer to the die area. This may also provide a cost savings because no additional metal traces are necessary such as in solder resist trenches. Additionally, embodiments of the present invention may help avoid issues related to the printing pre-solder paste for the die side chip capacitors to avoid pre-solder stretch.
While embodiments of the present invention have been described with respect to O2 plasma being used for sputtering, other techniques are also within the scope of the present invention. For example, H2 plasma may also be used. Additionally, chemical etching may also be used. Still further, embodiments of the present invention may also achieve a smoother surface topology by using additional sputtering (or additional chemical etching).
Additionally, embodiments of the present invention may be provided within one or more component packages, such as integrated circuit packages, which can be physically and electrically coupled to a printed circuit board to form an electrical assembly. The term electrical assembly may be part of an electronic system. An electronic system may be any product including an electronic assembly. Examples of electronic systems include computers (e.g. desktop, laptop, hand-held, server, etc.), wireless communications devices (e.g. cellular phone, cordless phones, pagers, etc.), computer-related peripherals (e.g., printers, scanners, monitors, etc.), entertainment devices (e.g. televisions, radios, stereos, tap and compact disc players, video cassette recorders, MP3 (Motion Picture Experts Group, Audio Layer 3) players, etc.), and the like.
Additionally, while embodiments have been described with respect to integrated circuits mounted on an integrated substrate, embodiments of the present invention are not limited only to such applications, as it may be used for other types of electronic packages and other types of components, such as passive components, hybrid modules, printed circuit boards, mezzanine boards, and for any other type of electrical structure requiring underfill.
Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments. Furthermore, for ease of understanding, certain method procedures may have been delineated as separate procedures; however, these separately delineated procedures should not be construed as necessarily order dependent in their performance. That is, some procedures may be able to be performed in an alternative ordering, simultaneously, etc.
Although embodiments of the present invention have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the foregoing disclosure, the drawings and the appended claims without departing from the spirit of the invention. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.
