Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1997-06-25
2001-10-16
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S261000, C361S760000, C361S767000
Reexamination Certificate
active
06303872
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electronic circuit assemblies. More particularly, the present invention relates to electronic circuit assemblies having surface mount devices with anti-tombstoning solder joints.
2. Disclosure Information
A typical surface mount device (SMD) is mechanically and electrically attached to a printed circuit board (PCB) substrate by providing mounting pads on the substrate, depositing solder paste onto these pads, placing the SMD on the solder paste/mounting pads such that its terminations are aligned with their respective mounting pads, reflowing the solder through the application of heat, and allowing the solder to cool so as to form a solder joint connecting each termination with its respective mounting pad.
Once the solder paste deposition atop each mounting pad melts, pressure forces and surface tension forces from the molten solder begin acting at the interface between the solder joint outer surface and the SMD, thereby exerting forces and moments upon the SMD. These forces are illustrated in
FIG. 1
, where P
1
and P
2
represent the pressure (suction) forces and F
1
and F
2
represent the surface tension forces acting by the solder joint upon the SMD, and where W represents the weight of the device. It is well known that during the reflow process the solder depositions melt at slightly different times, such that solder joints at one end of the SMD may melt and begin exerting forces on the SMD before solder joints at another end. This uneven application of forces often causes the SMD to undergo two related phenomena known as “tombstoning” and “decentering”.
Tombstoning occurs when a net moment is exerted on the SMD by melted solder joints at one end of the device such that the device rotates—first about the device's center of mass, and then about a fulcrum point G, located at the outer edge of the bottom termination adjacent the molten solder joint, when the fulcrum point touches down upon the top surface of the adjacent mounting pad—whereupon the SMD “stands up” on that end. This causes the device to detach from the unmelted or partially melted solder paste at the opposite end. Decentering occurs when a net force is exerted on the device in a direction toward the molten solder joint, thus tending to pull the opposite end of the device off of its unmelted or partially melted solder paste. Device decentering typically occurs before tombstoning does, and it exacerbates the tombstoning problem by detrimentally changing the angles made by F
1
and F
2
with their respective lever arms. Both tombstoning and decentering can cause the device to be mechanically and electrically disconnected from the mounting pads on the end where the solder paste melted last. As indicated in
FIG. 1
, forces P
1
and F
1
contribute toward tombstoning and decentering.
Prior art attempts at addressing the problems of tombstoning and decentering have focused on making the various depositions of solder paste on a PCB melt more simultaneously. However, this is extremely difficult to accomplish with conventional reflow soldering equipment and processes. It is desirable, therefore, to provide a way of reducing tombstoning and decentering which does not rely on making the solder depositions melt more simultaneously.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of the prior art by providing an electronic circuit assembly having anti-tombstoning solder joints comprising: a substrate having at least one mounting pad thereon; an SMD having at least one termination; and at least one solder joint connecting each termination to its respective mounting pad. Each solder joint is convex in shape, having a continuous and substantially circular arcuate outer profile and covering substantially all of the mounting pad and substantially all of a top portion of each termination. Each mounting pad has a predetermined length and each solder joint has a predetermined volume of solder selected such that a positive net anti-tombstoning moment is exerted upon the SMD when at least one solder joint is in a molten state.
It is an advantage that the effectiveness of the present invention does not rely on near-simultaneous melting of the solder depositions on an electronic circuit assembly.
It is another advantage that the present invention reduces tombstoning while providing flexibility in the spacing apart of mounting pads located at opposing ends of the SMD.
These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow.
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patent: 5468919 (1995-11-01), Shiozaki et al.
patent: 5504277 (1996-04-01), Danner
patent: 5508562 (1996-04-01), Horie et al.
patent: 5569880 (1996-10-01), Galvagni et al.
patent: 2-54989 (1990-02-01), None
patent: 6-21604 (1994-01-01), None
Jairazbhoy Vivek Amir
McMillan, II Richard Keith
Paladini Albert W.
Visteon Global Tech.
Visteon Global Tech.
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