Patent Application
20080239660
The subject matter described herein relates generally to the field of electronic devices and more particularly to flux precoated solder preform for thermal interface materials.
Electronic components, including integrated circuits, may be assembled into component packages by physically and electrically coupling them to a substrate. During operation, the component package may generate heat that can be dissipated to help maintain the circuitry at a desired temperature. Heat sinks, heat spreaders, integrated heat spreaders (IHS) and other heat dissipating elements may be attached to the package via a suitable thermal interface material.
Semiconductor manufacturing processes may involve spraying liquid flux material onto the backside of a die by a flux dispenser followed by a solder thermal interface material (STIM) placement by pick and place (P&P) equipment, which may be followed by a second flux spray onto the STIM surface by a flux dispenser. Prior to these operations, set-up procedures are conducted to ensure an accurate flux weight is dispensed by the flux dispenser. However, due to change in process conditions, flux viscosities, and micrometer setting stability issues, flux weight changes during lot processing, which may cause variations in the amount of flux dispensed and flux overspray onto the substrate and other components. In addition, the set up procedures for flux weight measurement require an additional 30 to 40 minutes per lot processing time which reduces efficiency.
Inaccurate flux measurements may cause STIM voiding, which commonly occurs at the bulk thermal interface material (TIM) or Die-STIM-Integrated heat spreader (IHS) interfaces. Such voiding is mainly due to flux amount and solvent loss causing high viscous residual flux, which causes material entrapment in the liquid STIM and residual flux material out gassing during a high temperature STIM reflow process. Thus, controlling liquid flux amount and coverage on the STIM prior to reflow process may reduce high voiding at the STIM-die-IHS interfaces.
Described herein is a flux precoated solder preform for thermal interface materials which may be used in electronic system such as, e.g., computing systems. In the following description, numerous specific details are set forth to provide a thorough understanding of various embodiments. However, it will be understood by those skilled in the art that the various embodiments may be practiced without the specific details. In other instances, well-known methods, procedures, components, and circuits have not been illustrated or described in detail so as not to obscure the particular embodiments.
In some embodiments the thermal interfaces described herein may be implemented to transfer heat from surfaces of electronic components such as, e.g., integrated circuits (ICs). In alternate embodiments the thermal interfaces described herein may be implemented to transfer heat in any setting where heat is to be conducted from one surface to another. For ease of explanation, the example of cooling an IC will be described.
IC die 120 generates its heat from internal structure, including wiring traces. Heat generated by IC die 120 may be dissipated by a heat dissipation assembly 150. In some embodiments, heat dissipation assembly 150 may include a heat sink to dissipate heat into the ambient environment. The heat sink may be active, i.e., it may utilize one or more fans to dissipate heat, or passive, i.e., it may rely on convection to dissipate heat. In some embodiments, heat dissipation assembly 150 may include a heat pipe assembly that utilizes a fluid such as, e.g., water or oil, to dissipate heat generated by the integrated circuit die 120. The heat dissipation assembly 150 may, in one embodiment, include a “heat spreader.”
A thermal interface material 130 is disposed between the integrated circuit die 120 and the heat dissipation assembly 150 to establish a thermal pathway between the integrated circuit die 120 and the heat dissipation assembly 150. In some embodiments, thermal interface material 130 comprises at least one of an indium alloy, an indium-tin alloy, an indium-silver alloy, a boron-nitride compound, or a lead-tin alloy. Thermal interface material may include a polymer base such as, e.g., a grease, a gel, or a phase change material (PCM).
In some embodiments a barrier layer 140 may be disposed between the thermal interface material 130 and the heat dissipation assembly 150. Barrier layer 140 may be formed from a material such as, e.g., nickel, which inhibits intermetallic interaction between the heat dissipation assembly 150 and the thermal interface material 130. In some embodiments, barrier layer 140 may be formed as a separate structural element, which may be positioned between thermal interface material 130 and heat dissipation assembly 150. In some embodiments, barrier layer 140 may be coated onto a surface of either (or both) of thermal interface material 130 or heat dissipation assembly 150, e.g., by nickel plating, dipping, brushing, coating, or depositing a layer of nickel onto the surface.
The subject matter described herein reduces solder TIM defects such as voiding, non wetting caused by three main factors. The first factor is non uniform flux spray on the die and solder TIM during process. When flux thickness is not uniform across the die and the solder TIM, thus, some areas may not be covered by flux, which may result in voiding and non wet during reflow due to poor fluxing activity in areas without flux coverage. The second factor is that flux solvent loss causes increased viscosity of residual flux material. For example,
As described herein, the uniformity of flux thickness may be improved by apply thin coated flux (i.e., 0% to 50% solids) on an indium or any other solder TIM. Increasing the uniformity of flux thickness reduces flux coverage problems, and entrapment of residual flux material during reflow, which in turn reduces solder TIM defects.
In some embodiments, flux may be sprayed onto the backside of a die and the solder TIM. Using activated no clean flux or rosin mild activator (RMA) flux improves cleaning of the solder TIM oxide and interfacial IMC tarnishes formed prior to and during reflow. A fluxing reaction shown in Equation 1 may be thermally induced and fluxing activity is maximized at solder reflow temperature. Equation 1a indicates reaction of metal oxide with flux base component such as, e.g., abietic acid to form metal abietate such as, e.g., Indium-gold abietate (a base flux component undergoes isomeric transformations at different temperatures). Metal oxide is further reduced with flux activators as shown in equation 1b to prevent re-oxidation during reflow for maximum solder TIM defects reduction.
MOn+2nRCOOH→M(RCOO)n+nH2O (a)
MOn+2nHX→MXn+nH2O (b) Equation 1
In some embodiments, flux coated STIM may include the following:
A: Flux solid contents: activators, rosin, surfactants and other additives that make up solid contents in the range (i.e. 0% to 50% by weight, with remaining material made up of solvents) to permit application of thin layers of flux on the TIM preform. This will ensure uniform monolayer of flux thickness on solder TIM and flexibility in amount of solids and solvents for any application irrespective of STIM thickness or size.
The added component can come from a group of slow evaporating solvents (a) propylene/ethylene glycol ethers and acetates, diethers (diglyme), (ex propylene glycol methyl ether, tripropylene glycol methyl ether, di-ethylene glycol n-butyl ether, dipropylene glycol methyl ether, ethylene glycol n-butyl ether, diethylene glycol n-butyl ether, diethylene glycol methyl ether), primary amyl acetate, cellosolve acetate, diisobutyl ketone, diacetone alcohol, butyl cellosolve, butyl cellosolve acetate, glycol diacetate, carbitol acetate, butyl carbitol acetate, (b) an azeotrope of the mixture of (a) with alcohols or other solvents for ex. butyl carbitol solvent & ethylene glycol.
Acid activators can come from a group of adipic acid, pimelic acid, citric acid, malonic acid, succinic acid, glutaric acid and others.
B: Rheological additives (including ethoxylated amines, ethanolamines surfactants and other additives such as polyethylene glycols derivatives, or ricinoleic acid and oleic acid) to improve easy flow, wettability and flux material coverage to the solder TIM preform after drying. This prevents dried (coated) flux material from cracking during solder TIM preform handling and during placement onto the die. The material also improves flux tackiness on the die and IHS lid after placement and sufficiently less tacky for TIM preform pick and place
C: Flux compositions melting temperatures: Linear di-carboxylic organic acids activators at low, medium and high temperatures (e.g., Pimelic acid, which has a melting point of 105 C; adipic acid, which has a melting point of 152 C; Succinic acid, which has a melting point of 187 C) and or covalent halide activators at low, medium and high melting temperatures (e.g., ethanolamine Hcl, which has a melting point of 84 C; di methylamine Hcl, which has a melting point of 170 C; and di ethylamine Hcl, which has a melting point of 227 C) for improved stability of activators at STIM soaking temperatures and melting temperatures for enhanced cleaning of Indium oxides and for non wet improvement for different category of solder TIM.
The amount of covalent halide activator (i.e., less than 1% of Cl-expressed as mass percent per rosin content) improves wettability and clean the oxide layers on the solder TIM surface and residual oxide layers on the intermetallics from base materials containing Au and Ni. The amount will not cause corrosion or other integrated shelf life issues since covalent halide is very stable at ambient temperature.
Due to isomeric transformations of rosin at different temperatures, selection of rosin temperatures depend on other flux components and solder TIM used. For example—Levopimaric acid C20H30O2, which has a melting point of 150-152 C; Palustric C20H30O2, which has a melting point of 162-167 C; Abietic C20H30O2, which has a melting point of 172-175 C and Pimaric C20H30O2, which has a melting point of 218-219 C. Rosin melting temperature may be selected to reduce flux entrapment in the solder TIM during reflow and for improved fluxing activity.
Solvents such as IPA, which has a melting point of 82 C, aliphatic ketone such as acetone, which has a melting point of 55-65 C; and glycol ether of which has a melting point of 140-220 C to provide superior solvency action for rosin, activators and other flux additives for a balanced viscosity prior to flux application on the preform.
Table 1 illustrates various Solder alloy compositions which may be used in flux precoated TIM.
Electrical power may be provided to various components of the computing device 802 (e.g., through a computing device power supply 806) from one or more of the following sources: one or more battery packs, an alternating current (AC) outlet (e.g., through a transformer and/or adaptor such as a power adapter 804), automotive power supplies, airplane power supplies, and the like. In one embodiment, the power adapter 804 may transform the power supply source output (e.g., the AC outlet voltage of about 110VAC to 240VAC) to a direct current (DC) voltage ranging between about 8VDC to 12.6VDC. Accordingly, the power adapter 804 may be an AC/DC adapter.
The computing device 802 may also include one or more central processing unit(s) (CPUs) 808 coupled to a bus 810. In one embodiment, the CPU 808 may be one or more processors in the Pentium® family of processors including the Pentium® II processor family, Pentium® III processors, Pentium® IV processors available from Intel® Corporation of Santa Clara, Calif. Alternatively, other CPUs may be used, such as Intel's Itanium®, XEON™, and Celeron® processors. Also, one or more processors from other manufactures may be utilized. Moreover, the processors may have a single or multi core design.
A chipset 812 may be coupled to the bus 810. The chipset 812 may include a memory control hub (MCH) 814. The MCH 814 may include a memory controller 816 that is coupled to a main system memory 818. The main system memory 818 stores data and sequences of instructions that are executed by the CPU 808, or any other device included in the system 800. In one embodiment, the main system memory 818 includes random access memory (RAM); however, the main system memory 818 may be implemented using other memory types such as dynamic RAM (DRAM), synchronous DRAM (SDRAM), and the like. Additional devices may also be coupled to the bus 810, such as multiple CPUs and/or multiple system memories.
The MCH 814 may also include a graphics interface 820 coupled to a graphics accelerator 822. In one embodiment, the graphics interface 820 is coupled to the graphics accelerator 822 via an accelerated graphics port (AGP). In an embodiment, a display (such as a flat panel display) 840 may be coupled to the graphics interface 820 through, for example, a signal converter that translates a digital representation of an image stored in a storage device such as video memory or system memory into display signals that are interpreted and displayed by the display. The display 840 signals produced by the display device may pass through various control devices before being interpreted by and subsequently displayed on the display.
A hub interface 824 couples the MCH 814 to an input/output control hub (ICH) 826. The ICH 826 provides an interface to input/output (I/O) devices coupled to the computer system 800. The ICH 826 may be coupled to a peripheral component interconnect (PCI) bus. Hence, the ICH 826 includes a PCI bridge 828 that provides an interface to a PCI bus 830. The PCI bridge 828 provides a data path between the CPU 808 and peripheral devices. Additionally, other types of I/O interconnect topologies may be utilized such as the PCI Express™ architecture, available through Intel® Corporation of Santa Clara, Calif.
The PCI bus 830 may be coupled to an audio device 832 and one or more disk drive(s) 834. Other devices may be coupled to the PCI bus 830. In addition, the CPU 808 and the MCH 814 may be combined to form a single chip. Furthermore, the graphics accelerator 822 may be included within the MCH 814 in other embodiments.
Additionally, other peripherals coupled to the ICH 826 may include, in various embodiments, integrated drive electronics (IDE) or small computer system interface (SCSI) hard drive(s), universal serial bus (USB) port(s), a keyboard, a mouse, parallel port(s), serial port(s), floppy disk drive(s), digital output support (e.g., digital video interface (DVI)), and the like. Hence, the computing device 802 may include volatile and/or nonvolatile memory.
In the description and claims, the terms coupled and connected, along with their derivatives, may be used. In particular embodiments, connected may be used to indicate that two or more elements are in direct physical or electrical contact with each other. Coupled may mean that two or more elements are in direct physical or electrical contact. However, coupled may also mean that two or more elements may not be in direct contact with each other, but yet may still cooperate or interact with each other.
Reference in the specification to “one embodiment” “some embodiments” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an implementation. The appearances of the phrase “in one embodiment” in various places in the specification may or may not be all referring to the same embodiment.
Although embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that claimed subject matter may not be limited to the specific features or acts described. Rather, the specific features and acts are disclosed as sample forms of implementing the claimed subject matter.
