Electric heating – Metal heating – By arc
Reexamination Certificate
2001-08-13
2003-07-15
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06593545
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating a leadframe and a leadframe package.
2. Description of the Related Art
Leadframe packages are well known to those of skill in the art. A leadframe package was fabricated from a leadframe. The leadframe included a plurality of leads. The leadframe further included tiebars, sometimes called dam bars, which held the leads together during fabrication of the leadframe package. Ultimately, the tiebars were trimmed to singulate the leads from one another.
In a standard wirebond leadframe package, the electronic component, sometimes called a die or chip, was attached to a die pad of the leadframe. The bond pads of the electronic component were then electrically connected to the leads of the leadframe by bond wires. The wirebonder, which formed the bond wires, determined the location on the leads to which the bond wires were attached and there was no requirement to define bond pads on the leads.
More recently, flip chip leadframe packages are being formed with electronic components flip chip mounted to leadframes. To form a flip chip leadframe package, the bond pads of the electronic component were bumped, i.e., solder balls were formed on the bond pads of the electronic component.
The electronic component was inverted such that the solder balls were in contact with the leads of the leadframe. The assembly was heated to reflow the solder balls thus forming solder bumps between the bond pads of the electronic component and the leads to the leadframe. The solder bumps, sometimes called bumps or solder joints, supported the electronic component on the leadframe while at the same time formed the electrical connections between the bond pads and the leads of the leadframe.
To form the solder bumps reliably, wettable pads on the leadframe were defined. The wettable pads were solder wettable. During formation of the solder bumps, the solder wet only the wettable pads on the leadframe. This resulted in consistent and reliable solder bump formation. If wettable pads were not defined on the leadframe, the solder would wet the leads uncontrollably, i.e., would run up the leads, resulting in inconsistent formation or failure of the solder bumps.
To form the wettable pads on the leadframe, photoresist was applying over the entire leadframe, the leadframe being formed of a wettable material such as copper. The photoresist was then selectively exposed to ultraviolet radiation thus selectively curing the photoresist. Then cured or uncured photoresist was removed depending upon the type of photoresist used, e.g., negative or positive photoresist, leaving the tips of the leads of the leadframe covered.
The entire leadframe except for the cover tips was plated with a non wettable material such as nickel. The photoresist was then stripped from the tips and the tips defined the wettable pads.
Alternatively, a solder mask was applied, e.g., in a ring, and cured to form a non wettable barrier between the tips of the leads and the remaining leadframe. The tips defined the wettable pads.
As should be readily apparent, using either of the conventional techniques, defining the wettable pads on the leadframe was relatively labor intensive, time-consuming and thus expensive.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the present invention, a leadframe includes a lead. The lead includes a wettable pad and a wettable lead surface. A non wettable barrier separates the wettable pad from the wettable lead surface, the non wettable barrier being formed from a modified portion of the leadframe.
Advantageously, solder wets only the wettable pad during formation of a solder bump between the wettable pad and a bond pad of an electronic component flip chip mounted to the leadframe. This results in consistent and reliable solder bump formation.
In accordance with an alternative embodiment, a method of forming a leadframe includes modifying a wettable region of the leadframe to form a non wettable barrier. A wettable pad is defined with the non wettable barrier.
In one embodiment, the non wettable barrier is formed in a single automated step with a laser. In this manner, the labor intensive, time-consuming, and expensive selective plating or solder mask application to the leadframe of the prior art is eliminated.
REFERENCES:
patent: 6143981 (2000-11-01), Glenn
patent: 6227436 (2001-05-01), Nishikawa et al.
patent: 6378758 (2002-04-01), Haba
Greenwood Jonathon Gerrit
Juskey Frank Joseph
Amkor Technology Inc.
Gunnison McKay & Hodgson, L.L.P.
Heinrich Samuel M.
Hodgson Serge J.
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