Metal fusion bonding – With means to juxtapose and bond plural workpieces – Including compliant cushioning medium
Reexamination Certificate
2000-03-22
2001-06-05
Ryan, Patrick (Department: 1725)
Metal fusion bonding
With means to juxtapose and bond plural workpieces
Including compliant cushioning medium
C228S049500, C269S067000, C269S068000, C269S069000, C269S902000, C269S903000
Reexamination Certificate
active
06241141
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to tool fixtures and soldering systems, and more particularly to a self-leveling, self-aligning fixture for precision reflow soldering applications.
BACKGROUND
There are many reflow soldering applications that require precise alignment and coplanarity between components for proper electrical and mechanical bonding. One of the final steps in the production of a printed circuit board (PCB) such as a PCMCIA card, for example, is the reflow soldering of the PCB to an input/output connector (I/O connector). A typical I/O connector may be single or double-sided and has a plurality of fragile leads or pins, each of which corresponds to a respective connector trace at the edge of a PCB card. To reflow solder these components together, a small amount of flux is placed on the connector traces on the PCB to form solder pads. The I/O connector is then placed or slipped on the end of the PCB to form a temporary connector/PCB assembly and the two components are then reflow soldered to create a finished assembly.
Reflow soldering of connector/PCB assemblies has been accomplished by many different methods. One common method for double-sided connectors involves sandwiching the assembly between two hot bars, or thermodes, using pressure and heat to reflow solder the leads to the PCB. Alignment of the pins on the I/O connector with the corresponding solder pads on the PCB and coplanarity between the I/O connector and PCB are critical to ensure good mechanical and electrical coupling at every connector trace and compliance with industry standard specifications. The specifications for personal computers, laptops and other electronic products, for example, require that the edges of the connector and PCB be parallel and within a certain distance of one another in order for the assembly to be mechanically packaged in the final product.
Such specifications, however, are sometimes difficult to achieve because PCBs often become warped by heat and mechanical forces the board experiences during processing. This creates problems during reflow soldering since warpage of the board can prevent proper alignment of the thermode contact surface with the connector/PCB assembly. Misalignment can result in an uneven pressure profile at the surface to be soldered, or work surface, creating either too much pressure at some soldering sites and/or too little pressure at other sites. If the pressure exerted by the thermodes is too large, the leads may be squeezed toward each other and solder may flow over multiple leads, causing a short circuit. Conversely, if the pressure is too small in certain areas, the resulting bonds may be too weak, resulting in an open circuit or even mechanical failure.
One solution known in the art to prevent misalignment of the connector/PCB assembly with the thermodes has been to place a pivot at the top of the thermode head. However, undesirable skidding can occur in this arrangement if one edge of the thermode contacting surface initially contacts the work area between two I/O connector pins, consequently engaging a lead and pushing the connector out of alignment with the PCB during its rotation. Such skidding can cause damage to the connector leads, misalignment problems including offset between the leads and corresponding connector traces, and movement of the connector out of the plane of the PCB.
Another solution aimed at addressing these skidding problems has been to provide thermodes connected to two leaf spring flexures, usually thin blades of stainless steel. In this thermode head, the thermode pivots around a point at the center of the contact surface of the thermode. While this may reduce skidding, such devices are somewhat complicated and expensive to manufacture. In addition, the resilience of the material used in the leaf spring flexures can be affected by heat during the reflow cycle, which over time can reduce the reliability and repeatability of the reflow soldering process.
In view of the deficiencies in the known processes, those skilled in the art would desire an improved system for reflow soldering electrical components, which can accommodate for any misalignment or non-coplanarity between the connector and connector/PCB assembly and allow for leveling of the heating element without skidding.
SUMMARY OF THE INVENTION
The present invention provides a novel self-aligning fixture having a fixture tray and table for reflow soldering electrical components such as a printed circuit board (PCB) and a corresponding input/output (I/O) connector. A connector/PCB assembly is placed on a fixture tray in the self-aligning fixture which rotates into place to correctly align the work surface of the connector/PCB assembly with the thermode contacting surface prior to reflow soldering. The fixture tray includes a body and first and second cylindrical rods extending from opposite sides of the body. The fixture tray is removably mounted in an upper plate of a tray table having a window adapted to rotatably support the tray and first and second grooves adapted to accommodate the fixture tray rods. The rods allow the fixture tray to self-level and rotate into alignment when the thermode contacting surface is pressed onto the connector/PCB assembly.
In the case of applications requiring simultaneous reflow soldering of opposite sides of a component such as a double-sided I/O connector to a PCB, bearing shafts are attached to the upper plate of the tray table and are resiliently mounted in corresponding shaft housings extending through a lower plate of the table. This allows vertical motion of the upper plate with respect to the lower plate and provides clearance for the fixture tray to rotate and self-level with the upper thermode contacting surface prior to contact between the bottom work surface of the connector/PCB assembly with the lower thermode contacting surface.
In operation, as the upper thermode is lowered and contacts the top work surface on the connector/PCB assembly, the fixture tray rotates to bring the work surface into full contact with the thermode contacting surface. The upper thermode continues to press into the assembly, pressing the upper plate of the tray table downward until the bottom work surface of the connector/PCB assembly makes contact with the lower thermode contacting surface and the proper pressure between the connector/PCB assembly and the thermodes is attained. The thermodes are then heated and the PCB is reflow soldered to the connector.
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Hughes Technology Products Division brochure entitled “Hughes Model HTT-SLT-2000 PCMCIA Station”, 1994.
Christie Parker & Hale LLP
Ryan Patrick
Stoner Kiley
Unitek
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