Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
1999-01-08
2002-01-22
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S233200, C228S248100
Reexamination Certificate
active
06340113
ABSTRACT:
BACKGROUND OF THE INVENTION
The technical field of this invention is soldering and, in particular, soldering methods and compositions that resist conductive joint fracture and/or facilitate automated soldering of complex electronic components, such as chips to printed circuit boards, to other chips, or to other substrates. The invention further concerns solder compositions that exhibit compliance, i.e. the ability to give easily with strain, and thus resist conductive joint fracture.
Modem electronic devices are typically formed by soldering electronic components together. Such devices can include logic or memory chips on printed circuit boards, multichip modules or complex integrated circuits. In these devices, numerous electrical connections must be formed between a component and its mating substrate. The conventional approach, especially with simple electronic components such as resistors, capacitors, transistors and small chips, has been to pass leads from the component through holes in the substrate and then to fill the hole with solder to secure the connection. However, as the device structures become denser and chips become smaller and more complex, the use of leads has become more and more cumbersome.
A more recent approach to joining electronic components is “surface mounting” in which metal regions or tabs on the components are aligned and soldered to corresponding metallized pads on the substrate. In these devices, numerous connections must be formed between the component and conductive circuitry on the substrate (a circuit board). The conducting paths on a substrate may form a tight network or array of connecting points. Thus, accuracy and precision in placement of the conducting pads on the substrate is essential to proper function of the resulting device. In one common approach, often referred to as “ball grid array” assembly, the metallized regions of the component or the mating pad of the substrate are prebumped with solder balls prior to assembly. The assembly is then heated so that reflow of the solder occurs, forming a permanent physical and electrical connection at each soldered point in the array.
Unfortunately, the reflow of molten solder is not always reliable. The solder can spread further than desired, causing short-circuits between adjacent conductive lines. In addition, the solder can migrate from the desired location creating a solder-starved joint or one in which the electrical connection has failed entirely. As the density of interconnective lines becomes greater and greater, the problem of solder reliability becomes increasingly more difficult to solve.
Moreover, most modem electronic solder connections are thin, high aspect ratio, joints. (The aspect ratio is a measure of the area of a solder joint divided by its thickness.) It is increasingly common for electronic components to be joined to printed circuit boards and the like by thin surface-mounting solder joints having a thickness of less than about 150 micrometers (less than 0.006 inches). In this regime, the solder joint is plastically constrained and can develop triaxial (hydrostatic) stresses several times greater than the average tensile strength of the bulk solder material. In addition, stresses arise from the thermal cycling of electronic circuits as the device incorporating such electronics is turned on and off. The solder joint experiences the full shear resulting from changes in component dimensions with temperature. Thus, even when solder joints are initially satisfactory, the nature of the joint itself becomes a critical point for device failure over time.
In addition to the physical limitations of conventional solder compositions in effecting reliable electronic connections, more and more concern has been expressed in recent years over the use of lead as one of the principal components in conventional solders. The toxicity of lead and the human body's limited capacity to reverse lead poisoning, has fueled a public health movement to curtail any unnecessary use of lead. The nature of automated soldering processes is such that significant amounts of lead are released as vapors into the processing facility. These vapors necessitate protective measures for personnel and the scrubbing of air before it can be released to the ambient environment. Moreover, since every electronic device eventually fails and must be discarded, the use of heavily leaded solders places an additional burden on waste disposal programs.
There exists a need for better methods and compositions for joining electronic components which can reduce the likelihood of conductive joint failure during the initial soldering process, and/or inhibit subsequent joint fatigue and/or fracture. Methods and compositions that can improve the efficiency of assembling ball grid arrays and the like would provide a solution to a problem that has troubled the automated manufacturing of complex electronic devices for a long time. Likewise, soldering methods and compositions which reduce triaxial stresses and, hence, the potential for joint fatigue, solder fracture or other electronic failure of thin, high aspect, solder joints, generally, would satisfy a long felt need in the art.
Moreover, solder compositions which can reduce the amount of lead necessary to form a reliable electric connection, and/or reduce the release of lead during manufacturing or waste disposal, would also address a long unsolved problem.
SUMMARY OF THE INVENTION
Soldering methods and compositions are disclosed that provide electrical connections between surfaces with reduced likelihood of short circuits or solder-starved joints. In addition, solder compositions are disclosed that exhibit compliance, i.e. the ability to give easily with strain, and thus resist conductive joint fracture. These solder compositions preferably form “metallic foam” joints upon heating.
Because the solder compositions of the present invention can fuse together rapidly without necessarily becoming entirely liquid, the invention reduces the likelihood of reflow-related solder joint faults. They can also reduce the potential for thermal fatigue and other solder joint failures in electronic devices following fabrication because the porous solder joints relieve plastic constraints and lower the average tensile joint stress. The compositions further provide proper standoff distance, the distance between the component and the substrate, allowing greater contact of the joint with the component. In other words, the compositions, by producing joints with proper standoff, prevent slumping of the joints away from the component.
In one aspect of the invention, solder compositions are employed which are composed of particles of a first metal coated with a second metal. The metals are chosen such that their individual melting points are higher than the melting points of the alloy or alloys formed when they are combined. Upon heating of such coated particles, melting occurs at the interfaces between the core materials and their coatings. The liquid so formed causes various particles to fuse together in a porous metal foam that provides a compliant electronic connection capable of withstanding thermal cycling with significantly lower failure rates. This soldering technique is particularly advantageous when applied to soldering of grid arrays and similar structures that facilitate mounting of chips to printed circuit boards, other chips or substrates, generally. The present invention can also be useful in reducing the total amount of lead and/or other toxic components present in solder compositions.
More generally, the solder compositions of the invention are formed from two distinct metallic components. The components can be elemental metals or metal salts. The components can be coated one upon the other or they can be physically separated. For example, an alternative to the bimetallic particles described above can be formed by particles of first metal surrounded by salt solution or suspension of the second metal. Again, the metals are chosen such that their individual melting points are higher th
Avery Donald H.
Ranieri John P.
Dunn Tom
Stoner Kiley
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