Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2001-05-24
2003-09-02
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S245000, C148S024000, C252S513000, C252S514000, C252S518100, C252S520100
Reexamination Certificate
active
06613123
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to solder and brazing materials and more particularly to variable melting point solders and brazes.
BACKGROUND OF THE INVENTION
Solder pastes are used to form joints between components and printed circuit boards (PCB) in the microelectronics industry. The solder provides both a physical and electrical bond. A conventional solder paste comprises a powder, made from a solder alloy, which is suspended in a liquid vehicle that contains a flux. The paste is dispensed onto a PCB, for example, by a screen-printing process. Components to be attached to the PCB are placed into the paste where they are held in place by the high viscosity of the paste. This assembly is heated such that the solder powder melts and coalesces to form a dense liquid whereupon it spreads over the component and PCB surface thus forming a metallurgical joint. Upon cooling the liquid solder solidifies and a solid joint is formed with the required mechanical, electrical and thermal properties. Common solder powders consist of alloyed metals such as lead (Pb) and tin (Sn) having compositions which give low melting points (i.e. eutectic compositions), which is advantageous from a processing point of view. A key characteristic of these solders is that they have a single reproducible melting point.
There are situations during the fabrication of microelectronics where stepwise soldering would be advantageous. In these cases, some components are soldered to a PCB, the assembly taken to another operation and then a third operation where additional components are soldered to a PCB. In yet another manufacturing method, components are soldered to one side of a PCB and other components are later soldered to the opposite side of the PCB.
In these cases the solder used in the first step must have a melting temperature higher than the solder used in the second step to avoid remelting of the solder from the first step. The use of more than one solder complicates, adds expense and can adversely affect the performance of the joints in an application.
U.S. Pat. Nos. 5,540,379, 5,573,602, and 5,803,340 are directed to a solder paste consisting of a low melting point and a high melting point metal (or alloy). When the paste is heated to the melting point of the low melting point powder the latter melts and while this phase is still liquid the paste is heated a second time to a temperature high enough to melt the rest of the mixture followed by cooling. These pastes are formulated with a composition such that most or all of the metal is liquid at the highest solder temperature.
These pastes use complex solder compositions with three or more alloying elements, contain lead (Pb) and high contents of low melting point elements such as bismuth (Bi) and indium (In). Since the solders are liquid at the soldering temperature and have a complex composition, the formation of low temperature phases which can segregate to particle or grain boundaries during freezing can be formed. These phases are detrimental to physical and mechanical properties of the resulting solder joint.
U.S. Pat. No. 5,229,070 teaches a solder produced by mixing a high tin (Sn) content alloy powder with a Sn powder substantially alloyed with either In or Bi. The Sn powder alloyed with Bi or In melts at a lower temperature than the high Sn content powder. The process consists of a single soldering step in which the paste is heated to a high enough temperature to melt all the components in the mixture. The primary purpose of this solder paste is to improve wetting and reduce the time of soldering. The solder is not designed for stepwise soldering and solidification at the soldering temperature does not occur.
U.S. Pat. No. 4,834,794 teaches a solder paste using a low and high melting point solder along with a reactive powder to form the paste. The objective of this patent is to provide a solder that has a low melting point and a higher remelting point. This is achieved by a reaction between the powders to form intermetallic compounds. A drawback to this type of solder relates to the fact that intermetallic compounds tend to be brittle and detrimental to the mechanical properties of the solder joint.
Thus the development of a solder which can melt the first time at a low temperature but not remelt at that temperature in subsequent solder operations (i.e. acquire a higher remelt temperature) would be of significant value in microelectronics where stepwise soldering is required or would be advantageous.
In the manufacture of light bulbs, solder joints are formed on an automated production line. It is advantageous to have a solder which melts at a low temperature so the cost of the soldering process can be minimized. However, when the light bulb is put into an application the melting point of the solder must be high enough that it does not re-melt during use. Therefore when using conventional solders, which have a single reproducible melting point, the soldering temperature cannot be lower than the service temperature. Thus the development of a solder which can melt the first time at a low temperature to reduce manufacturing costs but obtain a higher re-melt temperature during soldering such that it does not melt at the service temperature, would be advantageous.
In many automated furnace brazing applications it is desirous to have as low a brazing temperature as possible to minimize any detrimental microstructural changes that can occur in the base materials to be brazed. However, the service requirements again mean that the brazing temperature can not be lower than that experienced in the application. Thus the development of a braze material which can melt the first time at a low temperature to reduce damage to the base materials but obtain a higher re-melt temperature during brazing such that it does not melt at the service temperature, would be advantageous.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide variable melting point solders and brazes.
The present invention provides a composition for soldering, comprising:
a powder mixture including a first constituent having a first melting point, said first constituent having a preselected first mean particle size, said first constituent including a first metal or metal alloy powder; and
a second constituent including a second metal powder having a second melting point and a preselected second mean particle size, the first melting point being lower than the second melting point, said first and second mean particle sizes being selected so that upon heating to a solder temperature, such temperature being above the first melting point and below the second melting point, in-situ alloying occurs between melted first metal or metal alloy powder and the second metal powder in such a way that solidification occurs at the solder temperature with substantially no intermetallic phase formation.
The first mean particle size and said second mean particle size are selected to give a maximum rate of initial melting with minimal in-situ alloying prior to melting, and a maximum rate of in-situ alloying and solidification at the solder or brazing temperature.
The present invention also provides a composition for soldering, comprising:
a Pb—Sn eutectic powder mixed with a substantially pure Pb powder, wherein a bulk composition of Sn in the mixture is in a range from about 5 wt % to about 21 wt %, said Pb powder including particles having a first mean particle size and said Pb—Sn eutectic powder including particles having a second mean particle size, said Pb powder and Pb—Sn eutectic powder having mean particle sizes selected so that upon heating to a solder temperature, such solder temperature being above the Pb—Sn eutectic melting point and below the melting point of said substantially pure Pb powder, in-situ alloying occurs between melted Pb—Sn eutectic powder particles and Pb powder particles in such a way that solidification occurs at the solder temperature with substantially no intermetallic phase formation.
In this aspect of the invention the bulk comp
Corbin Stephen F.
McIsaac Douglas J.
Qiao Xin
Mai Ngoclan
Schumacher Lynn C.
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