Metal treatment – Compositions – Fluxing
Reexamination Certificate
1998-10-29
2001-04-10
Jenkins, Daniel (Department: 1742)
Metal treatment
Compositions
Fluxing
C075S252000
Reexamination Certificate
active
06214131
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to low-temperature solder processes. More particularly, this invention relates to solder pastes used in electronic assembly.
2. Description of the Related Art
Solder paste is widely used in the electronics industry. Solder paste is a combination of a flux carrier and a metallic solder alloy in a powder form. At room temperature the solder paste is compliant enough so that it can be made to conform to virtually any shape. At the same time, it is “tacky” enough that it tends to adhere to any surface it is placed into contact with. These qualities make solder paste useful for both surface mount soldering and for forming solder balls, sometime called solder bumps, on electronic components such as ball grid array (BGA) packages.
During soldering, the flux carrier reacts with and removes oxides from all surfaces involved in the soldering process including the solder pads, solder bumps (discussed in detail, below) and the surfaces of the individual solder spheres that form the solder alloy powder. Once the solder powder begins to melt then molten solder balls coalesce into a whole liquid body. This process is called “reflow”. The reflowed solder contacts and wets the solder pads, and, once cooled, solidifies to form a complete solder joint.
Good “wetting” of the solder pad by the reflowed solder is necessary for the formation of a strong bond. Wetting is strongly dependent on the metallurgical reaction between solder and soldering surface, and on the efficiency of the solder paste flux. Wetting starts whenever the molten solder is in contact with clean, oxide-free surface. Therefore, the temperature that solder spheres making up the solder powder starts to melt and the time that the solder is held above the temperature for the flux reaction are important factors for ensuring good wetting and a strong solder joint.
A mixed solder paste for improved wetting is known and described in U.S. Pat. No. 5,382,300 entitled, “Solder Paste Mixture”. This mixed solder paste includes mixing eutectic tin-lead solder with various other metals and alloys in order to improve the wetting and strength of the solder joints.
In general, the surface mount soldering process involves placing the electrical contact of an electronic component or substrate, a small amount of solder paste, and a solder-wettable pad on a printed circuit board (PCB) in close proximity. They are then heated until the solder reflows, forming an electrical connection between the solder-wettable pad and the electrical contact of the electronic component. Once the solder has cooled, it forms both an electrical and a mechanical connection between the electronic component and the PCB. This process has numerous advantages over other methods of interconnection. First, a large number of components can be interconnected simultaneously. Second, the process is highly repeatable and relatively low cost and is easily adapted for mass production.
The surface mount soldering process typically begins by stencil printing solder paste onto the solder-wettable pads of a PCB. Once the solder paste is on the solder-wettable pads, the electronic components to be soldered are aligned and set into place on the PCB with the electrical contacts of the electronic components in contact with the solder paste. The solder paste holds the electronic components in place during the heating and reflow process.
Solder bumps may be formed on the solder-wettable pads of an electronic component or a PCB using a method termed contained paste deposition (CPD) described in U.S. Pat. No. 5,539,153 assigned to the assignee of this invention. CPD provides for effective “micro-stenciling” of substances. Using a CPD process, a mask is used to micro stencil solder paste onto the solder-wettable pads. The solder paste is then reflowed and forms into solder spheres, each wetted to a solder-wettable pad. The solder sphere is cooled, forming the solder bump. The mask may then be removed.
While many types of solder paste can be used to perform surface mount soldering and to form solder bumps, the conventional solder paste used contains an eutectic tin-lead solder alloy powder containing 63 percent (%) tin, by weight, and 37% lead, by weight, (63Sn—37Pb). 63Sn—37Pb solder alloy has a melting temperature of 183 degrees Celsius (° C.). Typically, the soldering reflow process temperature peaks 20° C. to 30° C. above the melting temperature of the solder alloy (peak reflow temperature). This ensures the solder on the whole PCB melts completely, flows properly, and wets solder-wettable surfaces adequately, thus assuring quality solder joints. For the 63Sn—37Pb solder alloy, the peak reflow temperature typically is approximately 210° C.-220° C. Such high temperatures can induce considerable strain in a multilayer PCB that the electronic components are usually soldered to. High temperatures can also damage temperature-sensitive electronic components as they are being soldered. Consequently, more expensive components or extra assembly processes are often required in order to avoid damaging the PCB and the electronic components.
In addition, the melting temperature of the 63Sn—37Pb solder alloy may make its use undesirable in some step soldering processes. Step soldering processes are processes in which, in a subsequent operation, electronic components are soldered to a PCB that already has some electronic components soldered to it. Step soldering processes might be used, for example, to solder components on a second side of a PCB that already has components soldered to the first side. For purposes of this description, the electronic components soldered to the PCB prior to the subsequent operation will be called “original components” and the solder alloy holding the original components to the PCB will be called “first solder.”
In step soldering processes, the integrity of the solder connections between the original components and the PCB may be compromised if the first solder melts during the subsequent soldering operation. The likelihood that melting the first solder will compromise the integrity of the solder connections is increased both the original components are relatively heavy, and when components are soldered to both sides of the PCB. This increase in the likelihood of compromised solder connections is because the tackiness of solder paste and the surface tension of molten solder is often enough to hold relatively light components in place during a soldering reflow operation, even when the relatively light components are inverted. The tackiness of the solder paste and the surface tension of the molten solder, however, may be overcome by gravity with relatively heavy inverted components. An example of a relatively heavy electronic component is a ball-grid-array package. If both the first solder and the solder used in the subsequent soldering operation are 63Sn—37Pb solder alloy, the first solder will almost always melt during the subsequent soldering operation.
To avoid melting the first solder, the reflow temperature of the solder paste used in the subsequent soldering operation should be at least 25° C. lower than the melting temperature of the first solder. The 25° C. difference between the melting temperature of the first solder and the reflow temperature of the solder paste in the subsequent soldering operation allows two conditions to be met. First, the subsequent soldering operation can occur at 20° C. above the melting point of the solder alloy used in the subsequent soldering operation to ensure quality solder joints. Second, the subsequent soldering operation can occur at a safe 5° C. below the melting point of the first solder. If the first solder is the common 63Sn—37Pb solder alloy with a melting temperature of 183° C., then the solder paste used in the subsequent soldering operation should have a reflow temperature below 158° C.
Some low-temperature reflow solder pastes containing low-temperature melting solder alloys are known in the art. For example, eutectic bismuth-tin (58Bi—42Sn) s
Agilent Technologie,s Inc.
Jenkins Daniel
Mayer Marc R.
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