Patent Application
20070075430
1. Field
Integrated circuit packaging.
2. Background
Integrated circuit chips or die are typically assembled into a package that is soldered to a printed circuit board. A chip or die may have contacts on one surface that are used to electrically connect the chip or die to a package substrate and correspondingly an integrated circuit to the package substrate. Accordingly, a suitable package substrate may have corresponding contacts on one surface. One way a number of contacts of a chip or die are connected to contacts of a package substrate are to a solder ball contacts in, for example, a controlled collapse chip connect (C4) process. The package substrate typically also has a number of contacts on an opposite surface that are used to electrically connect the package substrate to a printed circuit board. One way this may be done is through solder connections such as a ball grid arrays (BGAs).
Current industry practice is to replace traditional lead-based solder joints with lead-free solder joints. Lead-free solder joints typically have inferior shock performance relative to their leaded counterpart. As future packaging technology is driven towards finer pitch as package size shrinks and input/output (I/O) count increases, there is a concern that lead-free solder joints may not provide adequate shock performance in these applications (for example, less than 0.8 millimeters (mm) in pitch size for BGA applications).
Features, aspects, and advantages of embodiments will become more thoroughly apparent from the following detailed description, appended claims, and accompanying drawings in which:
In the embodiment shown in
When lead-free solders (e.g., tin-silver-copper (Sn—Ag—Cu)) are melted on a metallic substrate, such as copper or nickel contact pads (e.g., contact points), in microelectronic packaging, the solders react with the substrate to form brittle intermetallic compounds (IMC) as a reaction product or interfacial layer that is part of the solder joints. Representatively, a lead-free solder for a BGA application is Sn—Ag—Cu (Ag is 0.3 to 0.4 wt. % and Cu is ˜0.5 wt. %) may be formed using 230 to 250 C as peak reflow temperature. Typical IMCs are Cu6Sn5 and/or Cu3Sn for a copper substrate (copper contact pad or point) and Ni3Sn4 for a nickel substrate (contact pad or point) as well as Ag3Sn IMCs that form in bulk solder.
Under shock loading conditions, it is typically estimated that the strain rates that solder joints experience are of the order of 102 per second. This strain rate spans across dynamic and impact loadings. Under the strain rate, metallic materials exhibit so-called strain-rate sensitivity. In order words, metallic materials become stronger with increasing strain rate, according to the following relationship
σ=C({dot over (ε)})m|e,T
The strain rate sensitivity is quite small at low homologous temperature but can be significant at high homologous temperatures to which solder materials are typically subjected during operation. For example, with m of 0.2, strain rate of 102/second increases yield strength to 250 percent of quasi-static yield strength. Because of this, under shock loading conditions, plastic deformation is generally suppressed and inherently ductile solder materials tend to become more and more brittle. Therefore, little or no plastic deformation is available to dissipate and/or absorb the incoming shock energy. With the shock energy transmitted to weaker intermetallic compound interfacial layers, solder joints typically exhibit a brittle fracture behavior along the IMC interfaces formed at joint regions under shock loading conditions.
In one embodiment, an intermetallic compound (IMC) is formed including an interfacial reaction product between (1) a solder material and a material of contact point and (2) a reaction species selected to improve the shock resistance of the IMC and a material of the contact point. In one embodiment, the reaction species is a Rare Earth element. Rare Earth elements tend to be extremely reactive. The chemical reactivity is believed due to the large negative-free energy from the formation of oxides/nitrides/hydrides. Due to their reactivity, Rare Earth elements will form an intermetallic compound with metals typically used in the metal finish of a contact point. In other words, a Rare Earth element will preferably form an intermetallic compound with copper, nickel, silver or gold rather than, for example, tin, after reflow. Rare Earth/contact metal IMCs have higher tensile ductility and fracture toughness than prior IMCs. The higher and fracture toughness will, in turn, mitigate brittle interfacial fracture during shock loading, resulting in improved shock performance and improved joint integrity/reliability. Thus, in one embodiment, a Rare Earth/contact metal IMC is formed with enough Rare Earth element(s) to increase the tensile ductility and fracture toughness of an IMC relative to an IMC formed without the Rare Earth element(s) present. A representative amount of Rare Earth element(s) is on the order of more than 0.1 weight percent to 10 weight percent of the IMC.
Suitable Rare Earth elements include Scandium (Sc), Yttrium (Y), Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Promethium (Pm), Samarium (Sm), Europium (Eu), Gadolinium (Gd), Terbium (Th), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thullium (tm), Ytterbium (Yb), and Lutetium (Lu).
There are various ways of implementing various Rare Earth element(s) as an intermetallic compound. Rare Earth elements can be introduced (e.g., doped) into solder material, such as lead-free solder material using standard ingot metallurgy. For example, Rare Earth element(s) can be introduced (e.g., in elemental form) in an amount up to three weight percent. The melting of the ingots should be conducted in a vacuum to minimize Rare Earth oxidation during processing. The ingots may then be used to form solder balls or paste that may be used, for example, in an integrated circuit package environment, such as to connect a chip to a package substrate or the package (chip and package substrate) to a printed circuit board.
An alternative to introduce Rare Earth elements in an intermetallic compound is introducing Rare Earth element(s) into a solder paste. For example, Rare Earth elements as a powder can be combined with a solder powder and the combined powder may be used to form the paste. Alternatively, a Rare Earth element powder can be mixed with conventional solder powder and mechanically-alloyed to form an alloyed powder. During a reflow of the paste, the Rare Earth element will preferably react with metals of the contact point.
In another alternative, Rare Earth element(s) can be introduced into solder flux. For example, Rare Earth powder can be mixed with or mechanically-alloyed with conventional flux to produce a Rare Earth element-doped flux. The flux may be introduced on a contact point prior to the introduction of the solder balls or paste. During reflow, the Rare Earth element(s) present in the flux will react with metal of the contact point. In yet another alternative, the Rare Earth element(s) may be coated on the contact point prior to introducing a solder material or a solder flux.
In the above discussion, an intermetallic compound is described including a reaction product between a Rare Earth element and a metal of the contact point. In another embodiment, a method is described wherein a species is introduced or doped into a solder material and the intermetallic compound or IMC is formed as an interfacial reaction product between the solder material and a contact point. The species introduced to the solder material, rather than reacting with a metal of the solder or contact point, will instead be present in a non-reacted sense in the IMC to improve the tensile ductility of the intermetallic compound as the solder joint.
In one embodiment, a species that tends to improve the ductility and (impact) toughness of an intermetallic compound and a solder joint is boron. When a solder joint is doped with an appropriate amount of boron, the improvement in ductility and toughness may be due to a number of potential benefits. For example, boron tends to segregate to imperfect, high-energy region (grain boundaries and interfaces) to promote bonds with current element. This segregation results in an increase of cohesive strength since the previously “weaker” regions of grain boundaries and interfaces approach the strength of the bulk. It is believed the fracture mode can change from intergranular to transgranular.
In addition to increasing the intrinsic toughness of grain boundaries and interfaces, boron may also limit environmental embrittlement. Moisture-induced hydrogen embrittlement of grain boundaries can occur in polycrystalline materials, such as Ni3Al, Ni4Mo, etc. Boron doping tends to minimize embrittlement for the suggested reason that boron atoms in grain boundaries inhibit the diffusion of hydrogen atoms due to a repulsive interaction between them. Boron doping may also improve the ductility and toughness of an intermetallic compound and a solder joint through grain size refinement. Boron doping is known to retard grain growth at elevated temperatures (e.g., during reflow or at high operating temperatures). In general, grain refinement results in strength enhancement and may lead to improved shock resistance of interfacial layers of intermetallic compound.
Boron may be introduced (doped) into solder material in various ways. For example, small amounts of boron, such as parts per million levels up to one weight percent, can be added into a solder ingot using conventional ingot metallurgical processes. In one embodiment, boron levels of one weight percent or less, are preferred as higher concentrations could result in the formation of borides that may be generate detrimental effects of mechanical properties. The boron-doped ingot is used as a starting material for a subsequent solder ball and powder processes using conventional procedures. Boron can, alternatively, be added to paste by adding a small of boron powder during powder mixing.
Boron may also be utilized in an electroless process. For example, in an electroless nickel deposit, a layer of nickel-phosphorous is typically introduced as a diffusion barrier. Boron may be substituted for phosphorous (e.g., to form a layer of nickel-boron) or as a surface finish on a layer of nickel-phosphorous.
In the preceding detailed description, reference is made to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.
