Patent Application
20140175160
Integrated circuit mounting.
Surface mount technology is a method for configuring electronic circuits in which components are mounted directly onto a surface of a substrate typically a printed circuit board (PCB). One technique involves ball grid array (BGA) packaging in which an integrated circuit package is used to mount devices such as a microprocessor to a printed circuit board through soldering. Electrical signals from the circuit device to the printed circuit board may be connected through balls of solder stuck to the bottom of the package. These solder balls are connected to contact pads on the printed circuit board by placing the solder balls in contact with respective ones of the contact pads and heating the structure to reflow the solder. One technique involves applying a paste that is a mixture of solder powder and a solder flux to the contact pads of the printed circuit board and then contacting the solder balls of the package to the paste followed by a reflow process. The solder ball and the solder powder of the paste join together in a common solder bond.
One issue that has arisen with the thermal connection of a BGA package to a printed circuit board is warpage of the package and/or board in response to the thermal process to heat the solder to a sufficient reflow temperature. It is believed that the package and possibly the printed circuit board warp in the presence of such heat because of coefficient of thermal expansion mismatches associated with the different layers of the package and/or board.
The warpage of the package and/or board can cause the solder balls of a package to separate from the contact pad to which it is aligned. The separation becomes problematic when the paste, that was originally on the pad, itself pulls away from the pad with the solder ball. With a tin (Sn) faced solder ball and a tin (Sn) solder powder in the solder paste, the solder paste tends to favor adhering to the solder ball over a contact pad of a different material (e.g., a copper contact pad). Upon heating, this can lead to what is referred to as a non-wet open (NWO) solder joint defect observed after reflow. With halogen free solder paste, NWO defects can be particularly high (e.g., as high as or greater than 80 percent yield loss), due to a general decrease in the aggressiveness of the fluxing action to remove surface oxides from solder metal.
Solder paste is generally a mixture of a solder powder and solder flux. Solder flux is generally made with rosin or resin, activators, viscosity controlling additives, chemicals, stabilizers and solvents. It has been determined that the tendency of solder paste to release from a contact pad in favor of adhering to a solder ball is affected by a tackiness, measured in grams-force (gf) units, of the solder flux. At temperatures necessary to heat the solder paste to reflow the solder, the prior art solder flux and consequently the prior art solder paste decrease in tackiness. As the solder paste loses its tackiness or softens, the solder paste will tend to lose its adherence for a contact pad and separate or pull away with the separating solder ball. Generally, the fluxing agents will remove the oxides present on the solder balls sooner in the processing cycle relative to the contact pads, leading to better adhesion to the solder balls, causing preferential separation from the contact pads.
A representative temperature range for a solder reflow process is a range from room temperature to 250° C.-270° C. Upon warpage, it has been determined that separation happens in a temperature range on the order of 120° C. to 190° C. which corresponds to a typical flux activation temperature. It has been found that if the solder paste can maintain its tackiness at least through a portion of the flux activation temperature, the paste will resist a tendency to separate or pull away from a contact pad in the presence of warpage, as the fluxing agent will have sufficient physical contact with the contact pad to allow the oxides to be removed and a joint to form.
Accordingly, in one embodiment, a solder paste is disclosed that seeks to maintain a tackiness through at least a portion of a flux activation temperature range. The property of tackiness in a solder paste is primarily the contribution of a rosin material in the solder flux of the paste. Thus, one way to maintain tackiness of a solder paste through at least a portion of the flux activation temperate range is through a solder flux that includes a rosin material having a property to maintain a less than ten percent drop in tackiness from an initial tackiness value of 20 gram-force (gf) to 120 gf over a temperature regimen of 20° C. to 200° C.
In one embodiment, a tackiness of a solder flux is determined by the temperature (softening temperature) at which secondary bonds between cross-linked chains in the flux (in the rosin or resin of the flux) break and cause the flux to spread and drop in tackiness. In one embodiment, in rosin material selected for a solder flux includes a softening temperature of 150° C. to 190° C. By modulating the softening temperature and a molecular weight of a rosin or resin mixture, a tackiness of the solder flux may be maintained over a temperature range associated with a flux activation temperature where a solder paste might otherwise separate from a contact pad in the presence of warpage of the package and/or board.
In one embodiment, a solder flux includes one or more rosins that have a molecular weight in the range of 300 g/mol to 600 g/mol. Representative rosins include, but are not limited to, rosin esters, hydrogenated rosin resins, dimerized rosin resins and modified rosin resins. One particular class of rosins is phenolic modified rosin resins. Representative of this class are phenolic modified rosin esters including Pentalyn™ 793 HV-M resin, commercially available from Eastman.
In one embodiment, a solder flux includes 10 percent to 80 percent by weight of a rosin material. A suitable flux includes one that is a mixture of the rosin with additional raw materials including, but not limited to, one or more of organic acids, amines, solvents, activators and other additives. Suitable organic acids include, but are not limited to, mono-, di-, tri-carboxylic acid having between two and 20 carbon atoms. Examples of suitable organic acids include, but are not limited to, glycolic acid, oxalic acid, succinic acid, malonic acid and the like or their combinations. Suitable amines include primary, secondary and tertiary amines including four to 20 carbon atoms. Representative examples of suitable amines include, but are not limited to, butyl amine, diethylbutyl amine, dimethylhexyl amine and the like or their combinations. Suitable solvents include a wide variety of solvents as known in the solder flux industry. Suitable activators include halogenated and non-halogenated activators.
In one embodiment, a composition of a solder paste includes a mixture of a flux such as described and solder powder. A representative range for solder flux in such a mixture is on the order of 10 weight percent to 40 weight percent and for solder power 60 weight percent to 90 weight percent. The solder powder can include solder metal, including metal alloys, having particle sizes ranging from 30 nanometers to 50 micrometers. Representative alloys include tin-rich alloys such as Sn—XAg—XCu, Sn—XCu, Sn—XAg. Other low melting point powders can be included that encompass a reflow temperature in the range of 150° C. to 300° C.
A paste composition such as described will provide a temperature stable tackiness ranging from 20 gf to 120 gf, with a change in tackiness of less than 10 percent over a temperature range of 20° C. to 200° C.
Prior to solder reflow, solder ball 220 of package 210 is brought into contact with solder paste 250. During solder reflow, one or both of package 210 and printed circuit board 230 can warp. Warpage can cause solder ball 220 to separate from the solder paste 250 and contact pad 240. The separation force is indicated by arrows 260 and 270. Because solder paste 250 includes an Sn-type solder powder, the solder paste will favor adherence to solder ball 220. Where solder paste 250 is a material such as described above, with, for example, sufficient tackiness through at least a portion of the paste activation temperature, a portion of solder paste 250 may pull away from contact pad 240 in response to separation forces (arrows 260 and/or 270), but the adhesion of the paste to contact pad 240 can be maintained (and preferably matched to the adhesion between solder paste 250 and solder ball 220) as indicated in the illustration on the right side of the figure where a profile of solder paste 250 shows matched adhesion of adhesion of the paste between solder paste 250 and solder ball 220.
Depending on its applications, computing device 300 may include other components that may or may not be physically and electrically coupled to board 302. These other components include, but are not limited to, volatile memory (e.g., DRAM), non-volatile memory (e.g., ROM), flash memory, a graphics processor, a digital signal processor, a crypto processor, a chipset, an antenna, a display, a touchscreen display, a touchscreen controller, a battery, an audio codec, a video codec, a power amplifier, a global positioning system (GPS) device, a compass, an accelerometer, a gyroscope, a speaker, a camera, and a mass storage device (such as hard disk drive, compact disk (CD), digital versatile disk (DVD), and so forth).
Communication chip 306 enables wireless communications for the transfer of data to and from computing device 300. The term “wireless” and its derivatives may be used to describe circuits, devices, systems, methods, techniques, communications channels, etc., that may communicate data through the use of modulated electromagnetic radiation through a non-solid medium. The term does not imply that the associated devices do not contain any wires, although in some embodiments they might not. Communication chip 306 may implement any of a number of wireless standards or protocols, including but not limited to Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE 802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE, GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, derivatives thereof, as well as any other wireless protocols that are designated as 3G, 4G, 5G, and beyond. Computing device 300 may include a plurality of communication chips 306. For instance, a first communication chip 306 may be dedicated to shorter range wireless communications such as Wi-Fi and Bluetooth and a second communication chip 306 may be dedicated to longer range wireless communications such as GPS, EDGE, GPRS, CDMA, WiMAX, LTE, Ev-DO, and others.
In further implementations, another component housed within computing device 300 may contain a microelectronic package incorporates a primary core surrounding a TSV or non-TSV integrated circuit die that inhibits package warpage.
In various implementations, computing device 300 may be a laptop, a netbook, a notebook, an ultrabook, a smartphone, a tablet, a personal digital assistant (PDA), an ultra mobile PC, a mobile phone, a desktop computer, a server, a printer, a scanner, a monitor, a set-top box, an entertainment control unit, a digital camera, a portable music player, or a digital video recorder. In further implementations, computing device 300 may be any other electronic device that processes data.
In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. The particular embodiments described are not provided to limit the invention but to illustrate it. The scope of the invention is not to be determined by the specific examples provided above but only by the claims below. In other instances, well-known structures, devices, and operations have been shown in block diagram form or without detail in order to avoid obscuring the understanding of the description. Where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the figures to indicate corresponding or analogous elements, which may optionally have similar characteristics.
It should also be appreciated that reference throughout this specification to “one embodiment”, “an embodiment”, “one or more embodiments”, or “different embodiments”, for example, means that a particular feature may be included in the practice of the invention. Similarly, it should be appreciated that in the description various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of the invention.
