Patent Application
20150001706
Embodiments pertain to electronic packaging of integrated circuits. Some embodiments relate to solder bonds for packaged integrated circuits.
Electronic systems often include integrated circuits (ICs) that are connected to a subassembly such as a substrate or motherboard. The ICs can be packaged and inserted into an IC package that is mounted on the subassembly. This is sometimes referred to as second level interconnect (SLI) or higher level. As electronic system designs become more complex, it is a challenge to meet the desired size constraints of the system. One aspect that influences the overall size of a design is the spacing required for the interconnection of the contacts of the IC packages. The IC packages may use ball grid array (BGA) interconnection that attaches solder balls to the IC packages before being bonded into an assembly. As the spacing is reduced, the current methods used to attach solder bumps or passive components to the IC packages becomes more challenging. Thus, there are general needs for devices, systems and methods that address the spacing challenges for contacts of ICs yet provide a robust and cost effective design.
The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims.
Conventional ball attach processes use either flux or paste to hold the solder balls until a reflow process where the solder balls are attached metallurgically. However, conventional ball attach processes are not able to meet the demands for smaller packaging, such as fine pitch substrates having a pitch of 300 micrometers (300 μm) or less between connections for soldering. Injection molded soldering (IMS) has been developed to allow solder-bump formation with fine pitch package substrates.
In an IMS process, solder bumps are formed using a solder mask. The solder mask is typically formed of a flexible material, such as polyimide. The shape of the solder bumps is determined by the shape of the cavity in the solder mask. The solder bump size can be changed by changing the mask thickness or changing the cavity size. Solder bump pitch is determined from the pitch of the cavities in the solder mask. A problem can occur in solidification of the solder bumps. Protrusions can form in the solder bumps giving the solder bumps an irregular shape. The protrusions may have a needle-like shape or a platelet shape, and can cause gaps in the solder bond of the IC package, or undesired bridging between the solder bonds.
The protrusions are an intermetallic compound (IMC) such as a compound of silver and tin (Ag3Sn). Analysis has shown that Ag3Sn IMCs are more likely to form when there is a slow rate of cooling of the molten solder. The process of Ag3Sn IMC formation can be referred to as nucleation and the protrusions typically occur near the surface of the solder bumps due to the lower temperature at the surface.
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In some examples, the solder additive decreases undercooling of the molten solder (or increases the rate of cooling) to provide an increased solidification temperature of the molten solder and reduce formation of the silver compound. In certain examples, the additive is a nucleating agent. In some examples, the additive includes zinc. In certain examples, the additive includes a zinc concentration in a range of two to five percent by weight (2-5 wt %). In certain examples, the additive includes a zinc concentration of 3 wt %. In some examples, the additive includes iron (Fe). In certain examples, the additive includes an iron concentration of 0.3 wt %. In certain examples, the molten solder includes a nucleating agent to retard the formation of IMCs. Instead of, or in addition to, raising the solidification temperature, adding an anti-nucleating agent may delay formation of the IMCs until the solder reaches a solidification temperature.
As explained previously, it is desirable to increase the rate of cooling of the solder to prevent the protrusions from forming in the solder balls. The rate of cooling of the solder bumps may be slowed by the material of the solder mask. The solder can include an additive that increases the rate of cooling of the solder. In some examples, the solder mask is a metallic mask (e.g., titanium). The metallic mask may increase dissipation of heat and help to cool the solder faster; thereby reducing the precipitation of IMCs. Also typical masks used in IMS (e.g., polyimide) have tendency to warp, which can also lead to deformations in solder bumps. In addition to enhancing cooling, titanium or a similar material will have lower warpage leading to fewer deformities in the solder bumps.
In some examples, the method 100 of
The device 300 may be included in a system that includes a printed circuit board 330 (PCB). The IC package substrate 315 may be bonded to the PCB using a plurality of solder connections, and the solder connections have the pitch spacing of the solder bumps (e.g., 300 μm or less). In some examples, the IC package 305 includes a second IC package substrate (340) arranged above the IC 310. A second IC package (not shown) containing a second IC may be arranged above the IC package 305 shown and bonded to the second IC package substrate (340) using solder bumps (345) to create a three-dimensional IC packaging solution.
The methods, systems and devices described minimize formation of protrusions in the solder bumps created by an injection molded solder process. This allows an injection molded solder process to reliably meet the packaging goals of fine-pitch IC packaging.
An example of an electronic device using semiconductor chip assemblies and solders as described in the present disclosure is included to show an example of a higher level device application for the present invention.
An electronic assembly 410 is coupled to system bus 402. The electronic assembly 410 can include any circuit or combination of circuits. In one embodiment, the electronic assembly 410 includes a processor 412 which can be of any type. As used herein, “processor” means any type of computational circuit, such as but not limited to a microprocessor, a microcontroller, a complex instruction set computing (CISC) microprocessor, a reduced instruction set computing (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, a graphics processor, a digital signal processor (DSP), multiple core processor, or any other type of processor or processing circuit.
Other types of circuits that can be included in electronic assembly 410 are a custom circuit, an application-specific integrated circuit (ASIC), or the like, such as, for example, one or more circuits (such as a communications circuit 414) for use in wireless devices like mobile telephones, personal data assistants, portable computers, two-way radios, and similar electronic systems. The IC can perform any other type of function.
The electronic device 400 can also include an external memory 420, which in turn can include one or more memory elements suitable to the particular application, such as a main memory 422 in the form of random access memory (RAM), one or more hard drives 424, and/or one or more drives that handle removable media 426 such as compact disks (CD), flash memory cards, digital video disk (DVD), and the like.
The electronic device 400 can also include a display device 416, one or more speakers 418, and a keyboard and/or controller 430, which can include a mouse, trackball, touch screen, voice-recognition device, or any other device that permits a system user to input information into and receive information from the electronic device 400.
The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.
To better illustrate the methods and apparatuses disclosed herein, a non-limiting list of examples is provided below.
Example 1 can include subject matter (such as a method, means for performing acts, or a machine readable medium that can cause the machine to perform acts) including positioning a solder mask on an integrated circuit (IC) package substrate, wherein the solder mask includes cavities that extend to the IC package substrate, applying molten solder to the flexible solder mask to fill the cavities of the solder mask with solder, and removing the solder mask to expose solder bumps on the IC package substrate, wherein the molten solder includes silver and an additive to reduce formation of a silver compound that causes deformation of solder bumps, and wherein the solder bumps have a pitch of 300 micrometers (300 μm) or less.
In Example 2, the subject matter of Example 1 optionally includes applying molten solder that includes an additive that includes zinc to reduce formation of a silver compound.
In Example 3, the subject matter of one or any combination of Examples 1 and 2 optionally includes applying molten solder having a zinc concentration in a range of 2-5 wt %.
In Example 4, the subject matter of one or any combination of Examples 1-3 optionally includes applying molten solder having an iron concentration of 0.3 wt %.
In Example 5, the subject matter of one or any combination of Examples 1-4 optionally includes applying molten solder having a nucleating agent as the additive to reduce formation of a silver compound.
In Example 6, the subject matter of one or any combination of Examples 1-5 optionally includes applying molten solder that includes tin and an additive to reduce formation of a silver-tin compound.
In Example 7, the subject matter of one or any combination of Examples 1-6 optionally includes applying molten solder having an additive that decreases undercooling to provide an increased solidification temperature of the molten solder and reduces formation of the silver compound.
In Example 8, the subject matter of one or any combination of Examples 1-7 optionally includes positioning a metallic mask on the IC package substrate.
In Example 9, the subject matter of one or any combination of Examples 1-8 optionally includes arranging an IC on the IC package substrate, wherein the solder bumps provide electrical continuity to connection pads of the IC.
In Example 10, the subject matter or one or any combination of Examples 1-9 optionally includes arranging an IC on a first IC package substrate and arranging a second IC package substrate above the IC. The method further optionally includes forming solder bumps on both the first and second IC package substrates, wherein the solder bumps include the additive to reduce formation of a silver compound that causes deformation of the solder bumps.
Example 11 can include subject matter, or can optionally be combined with one or any combination of Examples 1-10 to include subject matter (such as an apparatus), including an IC, an IC package including a first IC package substrate and the IC bonded to the first IC package substrate, a plurality of solder bumps formed on the first IC package substrate to provide electrical continuity to connection pads of the IC, wherein the solder bumps have a pitch spacing of 300 μm or less, and wherein solder of a solder bump includes silver and an additive to reduce formation of a silver compound that causes deformation of the solder bump.
In Example 12, the subject matter of Example 11 can optionally include solder including a zinc concentration in a range of 2-5 wt % as the additive to reduce formation of a silver compound.
In Example 13, the subject matter of one or any combination of Examples 11 and 12 can optionally include solder including an iron concentration of 3 wt % as the additive to reduce formation of a silver compound.
In Example 14, the subject matter of one or any combination of Examples 11-13 optionally includes solder including a nucleation agent as the additive to reduce formation of a silver compound.
In Example 15, the subject matter of one or any combination of Examples 11-14 optionally includes solder including tin and an additive to reduce formation of a silver-tin compound.
In Example 16, the subject matter of one or any combination of Examples 11-15 optionally includes solder including an additive that increases a solidifying temperature of molten solder and reduces formation of a silver compound during cooling of the molten solder.
In Example 17, the subject matter of one or any combination of Examples 11-16 optionally includes a second IC package substrate. The IC is optionally bonded to a top surface of the first IC package substrate and a bottom surface of the second IC package substrate, and the plurality of solder bumps may be arranged on a bottom surface of the first IC package substrate and a top surface of the second IC package substrate.
Example 18 can include subject matter, or can optionally be combined with one or any combination of Examples 1-17 to include subject matter (such as a system), including an IC, a printed circuit board (PCB), and IC package that includes an IC package substrate. The IC is bonded to the IC package substrate and the IC package substrate is bonded to the PCB using a plurality of solder connections. The solder connections may have a pitch spacing of 300 μm or less, wherein the solder of a solder connection includes silver and an additive to reduce formation of a silver compound that causes deformation of the solder bump
In Example 19, the subject matter of Example 18 optionally includes solder including zinc as the additive to reduce formation of a silver compound.
In Example 20, the subject matter of one or any combination of Examples 18 and 19 optionally includes solder including a nucleating agent as the additive to reduce formation of a silver compound.
In Example 21, the subject matter of one or any combination of Examples 18-20 optionally includes solder including an additive that decreases undercooling to provide an increased solidification temperature of the molten solder and reduces formation of the silver compound.
Example 22 can include subject matter, or can optionally be combined with any portion or combination of any portions of any one or more of Examples 1 through 21 to include subject matter, that can include means for performing any one or more of the functions of Examples 1 through 21, or a machine-readable medium including instructions that, when performed by a machine, cause the machine to perform any one or more of the functions of Examples 1 through 21.
Each of these non-limiting examples can stand on its own, or can be combined in various permutations or combinations with one or more of the other examples.
