Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-06-08
2001-11-13
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S250000, C361S760000, C361S777000, C228S180210
Reexamination Certificate
active
06316736
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to printed circuit boards, and more particularly to a printed circuit board having improved resistance against solder bridging and component decentering/tombstoning.
2. Disclosure Information
A conventional printed circuit board (PCB) generally includes a substrate
10
with conductive circuit traces
30
and mounting pads
14
thereon, as shown in
FIGS. 1-3
. Once the traces and pads have been disposed on the substrate, a thin, solder-resistant layer of material, often called a “solder mask”
16
, is applied to the surface of the PCB. Typically the solder mask covers substantially all of the circuit traces, but not the mounting pads; this is accomplished by forming apertures
18
in the solder mask wherein each aperture generally conforms in shape with and is situated concentrically about a respective mounting pad. For example, if a given mounting pad is rectangular, its corresponding solder mask aperture will likewise be rectangular, with the edges
22
of the aperture evenly spaced apart from the corresponding edges
28
of the mounting pad by a given amount or gap P. This gap P between the mounting pad edges
28
and aperture edges
22
is referred to as “solder mask pullback”; for screenprinting, the pullback P is typically about 10-20 mils, whereas for liquid photoimageable solder mask (LPISM) the pullback P is typically 0-5 mils. As illustrated in
FIGS. 1-3
, this pullback of the mask from the mounting pad creates a “trough”
32
about each pad.
After the solder mask is applied to the PCB surface, the mounting pads and any other circuit structures exposed through the apertures may be tinned, followed by solder paste being applied thereto, typically by screen-printing or deposition. Solder paste is typically a slurry of metallic solder particles (e.g., eutectic tin/lead), flux, and other components. After the paste has been printed/deposited, and the PCB has been populated by placing electronic components thereon with the component terminations atop their corresponding mounting pads, the populated PCB is subjected to reflow processing.
During reflow, the metallic component of the solder paste melts and forms solder joints, while the flux and other effluent components of the paste are liberated and often flow across the solder mask surface about each solder joint. Various gases and vapors are also ejected from the molten paste during reflow and solidification. It is well known that when flux and other effluents flow across the solder mask, they provide solder-wettable paths along which solder balls may be transported. This presents a serious problem, because these migratory solder balls may end up solidifying in such a way as to short out two or more adjacent mounting pads; or, enough of the solder or paste may be wicked away from a pad such that the solder that is left thereon is insufficient to form an acceptable joint (or any appreciable joint at all), resulting in an open circuit, or a weak joint if one forms at all. Additionally, if flux or other effluents wick up underneath the component
24
, the component may “skate” or “float” on the flux/effluents and become unacceptably decentered with respect to the mounting pads. Furthermore, gases escaping from any paste which wicks up under a component may escape so violently as to promote or worsen the tendency for the component to decenter or tombstone.
Therefore, it would be advantageous to provide a way of channeling away and containing the flux and other effluents liberated during reflow, in order to preclude the flux/effluents from promoting or exacerbating the aforementioned problems of solder ball migration and component decentering and tombstoning.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of prior art approaches by providing a PCB having improved resistance against solder bridging and component decentering/tombstoning. The PCB comprises: (a) a substrate having a top surface; (b) at least two mounting pads disposed on the top surface in matched relation with terminations of an electronic component; and (c) a solder mask disposed on the top surface and having at least two apertures therethrough, wherein each aperture generally conforms in shape with and is arranged about a respective one of the mounting pads. Each aperture has at least one inner aperture edge generally within a projected footprint of the electronic component and at least one outer aperture edge generally outside the footprint. Each aperture includes a notch in one or more of the at least one outer aperture edge, wherein each notch extends generally outward from its respective mounting pad.
It is an object and advantage of the present invention that the solder mask aperture notches provide a reservoir into which solder paste effluents (such as flux) may flow during reflow. This facilitates the collection and containment of effluents and/or solder balls during reflow, thereby reducing the tendency to form effluent/flux paths across the solder mask surface along which solder balls could migrate to form solder bridges.
Another advantage is that the present invention acts to prevent flux/effluents/solder balls from wicking underneath the component during reflow, thus minimizing or eliminating the tendency toward component decentering and tombstoning.
Yet another advantage is that the solder mask notches of the present invention are easy and inexpensive to implement, and may be used compatibly and beneficially with existing solder mask/PCB technology.
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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Jairazbhoy Vivek Amir
McMillan Richard Keith
Paladini Albert W.
Shelton Larry I.
Visteon Global Technologies Inc.
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