Patent Application
20090057378
In the manufacture of integrated circuits, forming interconnections at a pitch of 100 μm pitch or less has being been one of challenges for next generation package technology. Conventional chip attachment for controlled collapse chip connection (C4) modules is based on the reflow of solder bumps that are pre-formed on a substrate electrode pad. To pre-form the solder bumps, stencil printing techniques may be used to dispense high viscosity solder paste onto the electrode pads through a mask. Unfortunately, for electrode pads having a pitch of 100 μm or less, solder bridges are easily formed due to the narrow gaps that exist between adjacent electrode pads. The solder bridges form an undesired electrical coupling between two or more electrode pads, leading to electrical short circuits.
Another technique used to pre-form solder bumps is electroplating, however, this process is complex and expensive due to the need for a photomask and etching processes. Accurately controlling alloy compositions in ternary or higher-order alloy systems can also present problems, especially for small amounts of alloying element in lead-free solders.
Micro solder ball mounting techniques have been developed, however, they are also costly because of the increased number of solder balls needed in finer pitch applications. This technique also requires a pitch of 100 μm or more. Finally, an arrayed solder ball transferring method or a molten solder jetting method has been developed, but such processes are very immature for high volume manufacturing with limited applications.
Accordingly, improved methods of forming electrical interconnections are needed to address bridging issues that occur on electrode pads having pitches of 100 μm or less.
Described herein are systems and methods of forming interconnections between metal bumps on an integrated chip and metal pads on a substrate. In the following description, various aspects of the illustrative implementations will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some of the described aspects. For purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the illustrative implementations. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative implementations.
Various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present invention, however, the order of description should not be construed to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation.
By way of background,
Turning to
As described above, one critical issue with the method described in
To overcome issues found in conventional processes, implementations of the invention provide a self-organizing solder paste that can form solder interconnections for fine pitch interconnections of less than 100 μm. The self-organizing solder paste of the invention consists of micro solder particles dispersed in an organic flux. The solder paste is molten at reflow temperatures and is wetting on solid interconnection structures. In some implementations, two different types of solder particles may be dispersed in the organic flux to form a solder alloy interconnection.
A solder paste may be formed by combining solder particles and a flux. The solder particles are generally dispersed throughout the flux and tend to randomly travel within the flux at elevated temperatures due to the local convention of liquids in a solder paste.
As will be known to those of skill in the art, flux is a substance that facilitates soldering by chemically cleaning the metals to be joined. For instance, flux may be used to remove and prevent oxidation from the metal surfaces being interconnected, such as the metal bump, the metal pad, and the solder particles. Flux is generally an inert substance at room temperature but becomes strongly reducing at elevated temperatures, thereby preventing the formation of metal oxides. Flux also acts as a wetting agent in soldering processes. Additionally, flux seals out air, which prevents further oxidation.
In implementations of the invention, the flux used to form the solder paste is an organic flux based on a synthetic rosin. In alternate implementations, a synthetic resin may be used. The use of an organic flux enables the solder paste to remove oxidation from the solder particles as well as the metal bumps and metal pads that are being interconnected. Generally, the organic flux will react with and remove oxidation layers at elevated temperatures of around about 100° C. to 200° C. The solder paste may further contain various additives that are well known in the art, including but not limited to surfactants and activators.
The solder particles dispersed in the organic flux may include any metal typically used in solder compositions. For instance, base metals that may be used in the solder particles include, but are not limited to, tin (Sn), indium (In), bismuth (Bi), and zinc (Zn). Furthermore, alloying metals that may be combined with the base metal include, but are not limited to, copper (Cu), nickel (Ni), cobalt (Co), silver (Ag), gold (Au), titanium (Ti), aluminum (Al), lanthanum (La), cerium (Ce), iron (Fe), manganese (Mn), gallium (Ga), germanium (Ge), antimony (Sb), tantalum (Ta), and phosphorous (P). The alloy metal may be added to improve microstructure, mechanical, and thermal properties of the solder particle. In implementations of the invention, the weight percent (wt %) of solder particles in the solder paste may range from around 10 wt % to around 50 wt %, depending on the pitch of the metal pads and the volume of the dispensing solder paste.
The solder paste may include solder particles with different compositions that are dispersed throughout the organic flux. The use of more than one type of solder particle can produce in-situ solder alloys during reflow. For instance, the use of tin-containing solder particles with silver-containing solder particles may produce a SnAg eutectic alloy.
In accordance with implementations of the invention, the mean diameter of the solder particles may range from around 0.1 μm to around 10 μm, but will generally range from around 0.1 μm to around 5 μm. In some implementations, a larger diameter may be used as long as the solder particle is smaller than the gap that exists between adjacent electrode pads to prevent the occurrence of solder bridging. The small size of the solder particles used in the solder paste of the invention relative to conventional solder particles aids in the coalescing of solder on the metal structures and helps minimize the occurrence of solder bridges.
In accordance with implementations of the invention, the self-organizing solder paste of the invention may be applied over an array of metal bumps, such as an array of copper bumps, and a reflow process may be carried out to fabricate an individual solder bump over each copper bump. This process may be carried out without the use of a mask or stencil printing techniques.
The following description, which references
It is believed that the self-organizing mechanism of the solder paste of the invention is based on a series of wetting, spreading, and coalescing processes. For instance, at a temperature that is at or above the melting point of the solder, the solder particles become molten and continue to travel through the flux. As shown in
Next, as shown in
ΔG=γ3V/R
In the above equation, γ represents the surface energy of the molten solder particle, V represents the molar volume of the solder particle, and R represents the radius of the particle. As shown, the interface Gibbs free energy (ΔG) decreases as the radius of the particle increases. Accordingly, two solder particles can be easily combined to form a larger particle, thereby decreasing the interface Gibbs free energy.
Similarly, the Laplace pressure within a particle is given by:
Δp=γ2/R
Here, γ again represents the surface energy of the molten solder particle and R represents the radius of the particle. As with the interface Gibbs free energy, the Laplace pressure (Δp) decreases as the radius of the particle increases. Accordingly, two solder particles can be easily combined to form a larger particle, thereby decreasing the internal Laplace pressure. It is therefore believed that the high Laplace pressure within smaller molten solder particles causes them to be further attracted to the spreading molten solder, which has a relatively lower internal Laplace pressure. Furthermore, as known to those of skill in the art, fluxing generally occurs from higher pressure to lower pressure.
The self-organizing solder paste of the invention may be used on a variety of substrates and with a variety of metal bumps. For instance, the solder paste may be used on organic package substrates and motherboards, ceramic package substrates and motherboards, and on silicon substrates. In further implementations, other types of substrates not mentioned here but known in the art may be used with the solder paste of the invention.
At least one of the substrates includes metal bumps formed on its surface. Any metal bumps may be used as long as the melting temperature of the metal is higher than the temperatures used during the chip attachment process (e.g., the reflow temperature). A metallic surface finish may be used on the metal bump structures to prevent surface contamination and to improve solder wetting. Examples of such metallic surface finishes include gold, gold-nickel alloys, silver, and tin.
Examples of metal bumps that may be used include stud bumps, balls, wires, microvias, and metal pads. The shape of the metal bumps may vary depending on the specific application in which they are used or formed.
The process 600 begins by providing a first substrate having an array of metal pads (process 602 of
The solder paste is dispensed over the entire metal pad-containing surface of the first substrate without the use of masking and/or stencil techniques. In other words, a single, blanket layer of solder paste is formed on the first substrate that is substantially or completely continuous.
The process 600 continues by providing a second substrate having an array of metal bumps to be interconnected with the first substrate (606). The metal bumps may be formed of any metal that is conventionally used to form metal pads such as copper. Next, the second substrate is pressed into the solder paste on the first substrate (608). The second substrate is oriented such that its metal bumps are within the solder paste and each metal bump is aligned with a corresponding metal pad on the first substrate. The second substrate is brought into close proximity with the first substrate, generally leaving a small gap between the metal bumps and their corresponding metal pads. In various implementations, this small gap may range from around 1 μm to around 50 μm. The gap provides space for the solder particles in the solder paste of the invention to self-organize into solder bumps between the metal pads and the metal bumps. The size of the gap controls the bond line thickness.
A conventional chip placing module may be used to join the second substrate with the first substrate. In some implementations, a spacer may be used to control the size of the gap between the metal pads and the metal bumps. By controlling the size of the gap, the spacer ensures space exists for the solder particles to form into solder bumps and the spacer controls the bond line thickness.
Once the second substrate is properly positioned and aligned, a reflow process is carried out to melt the solder particles and allow them to self-organize into solder bumps (610). As mentioned above, during a reflow process, the temperature of the solder paste is elevated to a level that is above the melting point of the solder particles but below the melting point of the metal bumps and the metal pads. In implementations of the invention, the temperature of the reflow process may range from 100° C. to 500° C. and the reflow process may be carried out for a time duration that falls between around 30 seconds and 900 seconds.
In accordance with implementations of the invention, the time and temperature profile of the reflow process is controlled such that the solder particles melt and appropriately self-organize into solder bumps. The specific time and temperature profile used will depend on the composition of the solder particles in the solder paste of the invention and may further depend on the type of substrate used. In implementations of the invention, the peak reflow temperature will fall between around 100° C. and around 400° C. For lead-free solder particles, the peak reflow temperature will typically fall between around 200° C. and around 300° C. For specially designed low temperature, lead-free solder particles, including but not limited to BiIn, SnIn, BiInZn, SnInZn, SnBi, and SnZnIn, the peak reflow temperature will typically fall between around 100° C. and around 200° C. For specially designed high temperature, lead-free solder particles, including but not limited to SnAu, ZnSn, and AlSn, the peak reflow temperature will typically fall between around 300° C. and around 500° C. The substrate materials used will depend on their ability to withstand the temperatures used during the reflow process, and include materials such as silicon, ceramic, and organic substrates.
In implementations of the invention, the time duration of the reflow process may range up to 15 minutes or longer, depending on the specific composition of the solder particles and the type of substrate used. For lead-free solder particles, the time duration will typically fall between around 3 minutes and around 10 minutes. For specially designed low temperature, lead-free solder particles, the time duration will typically fall between around 0.5 minutes and around 5 minutes. And for high temperature, lead-free solder particles, the time duration may range up to 15 minutes or more.
In some implementations, the temperature of the solder paste may be varied over the time duration, for instance, the temperature may be slowly elevated until it reaches a peak temperature. In further implementations, after reaching the peak temperature, the solder temperature may then be slowly decreased until the end of the time duration. The time and temperature profile used in implementations of the invention tend to minimize or prevent to formation of solder bridges between adjacent metal pads.
As shown in
The non-solder materials of the solder paste may then be evaporation or they may remain on the solder bump after reflow, as shown in
In implementations where a mixture of solder particles with different compositions is used, a reflow temperature should be chosen that is higher than the melting point of at least one of the compositions. When the solder particles of at least one composition are molten, they are able to form alloys having much lower melting temperatures. For example, when molten tin solder particles (with a melting point of 232° C.) contact solid sliver solder particles (with a melting point of 961° C.), a SnAg eutectic alloy having a melting temperature of 221° C. may be formed.
A substantial percentage of the solder particles in the solder paste are used in forming the solder bumps. In some implementations, substantially all of the solder particles in the solder paste are used in forming the solder bumps.
It should be noted that in alternate implementations, the self-organizing solder paste may be initially deposited on the second substrate having the metal bumps. The first substrate having the metal pads may then be brought into contact with the solder paste to form interconnections with the second substrate.
Accordingly, an in-situ chip attachment process using a self-organizing solder paste has been disclosed. The self-organizing solder paste of the invention couples interconnect structures having a fine pitch of 100 μm or less without pre-solder bumping. The chip attachment process described herein simplifies the chip attachment process by eliminating the need for masking or stenciling processes, thereby providing a significant cost reduction for various applications.
The above description of illustrated implementations of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific implementations of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications may be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific implementations disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
