Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-08-19
2001-02-27
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C361S777000
Reexamination Certificate
active
06194667
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to receptor pad structures for chip carriers, and in particular, non-floating BLM C-4 receptor pads for organic chip carriers preferably utilizing eutectic solder.
Basically, the chip carriers employ eutectic solder consisting of either a tin/lead plated solder, or it is adapted to be solder-injected or flip chip screen printed on the chip carrier C4 (controlled collapsed chip connect) receptor pads. In essence, the eutectic solder facilitates the use of existing C4 technology in conjunction with organic chip carriers, and wherein C4 bumps consists of Ball Limiting Metallurgy (BLM) chrome and chrome copper phased and evaporated 3.5% by weight of tin and 96.5% by weight of lead. In order to be able to produce solder joints between the eutectic solder on the chip carrier and the C4 bump, it is ordinarily the practice to deposit a controlled amount of eutectic solder on the C4 receptor pad. Basically, this amount of solder deposition is empirically determined and is predicated on various parameters including: chip size and the number of C4 bumps; the chip pitch; the C4 ball diameter; the C4 receptor pad area (normally determined by the copper trace width of the circuit lines and solder mask opening); and, finally, the encountered organic chip carrier warpage, which may be predicated on temperature differentials between the chip carrier surfaces. Ordinarily, each one of these of parameters, in turn, is derived through a set of specified secondary parameters, and upon all of the parameters being optimized, the eutectic solder is deposited on the organic chip carrier so as to form high reliable solder joints. In contrast therewith, when the above-referenced parameters are not properly optimized, the eutectic solder creeps up to the BLM (i.e. the C4 bump) and detaches the latter from the chip. Upon this occurrence, the solder joint will fail to maintain or lose its reliability, and the system will fail.
Currently, organic chip carrier C4 receptor pads create floating BLM which, occasionally, even in the instances of deposition of equal amounts of solder on various pads configurations, result in the obtaining of various or differing bump heights. Thus, the solder volume distributes itself over the pad area, and inasmuch as the pad areas are frequently of different sizes and shapes, this results in the creation of unequal bump heights. Consequently, even though the solder bumps are flattened prior to the placement of the chips on the printed circuit board, the tall bumps will rise faster which creates chip skewing or tilting, thereby adversely affecting the integrity and functional reliability of the formed solder joints. Moreover, as the chip settles into the molten solder during solder reflow, the tall bump solder will creep around the C4 bump and can easily reach the BLM, resulting in the adversely influencing BLM floating. Current technology utilizing the openings or windows of various insulating material masks with regard to the formation of solder joints have failed to solve the above encountered problem in ensuring the reliability of the solder joints. In essence, the current state-of-the-technology, has not been found an effective or fully satisfactory method of obviating the problems encountered in attempting to provide uniform and controlled formations of solder joints with regard to receptor pad structures.
DISCUSSION OF THE PRIOR ART
Machuga, et al., U.S. Pat. No. 5,523,920 discloses a printed circuit board including elevated bond or receptor pads, and wherein a mask constituted of an insulating material includes a mask opening or window which only exposes areas not encompassing the runner sections of circuit traces on a printed circuit board arranged below the mask. Moreover, the bond pads which are integrally formed with the runner sections at the opening include an upper surface having a lip overlying the insulating material mask. This will not enable the desired equalization of the deposition and distribution of solder on the receptor or bond pads.
Carey, et al., U.S. Pat. No. 5,075,965 discloses a controlled collapse chip connect (C4) method of joining an integrated circuit board to a microelectronic circuit card. There is no disclosure of providing a window or opening in an insulating material mask in a manner analogous to that described in the present invention for controlling the deposition and distribution of solder on receptor pads.
Barrow, U.S. Pat. No. 5,706,178 discloses a plurality of elliptically-shaped solder pads on the surface of a package substrate, wherein the pads are covered to a specific surface extent by a solder mask, and only portions of the pads are exposed through mask openings or windows which are essentially circular in nature. That type of masking structure does not facilitate the controlled deposition and distribution of eutectic solder on a receptor pad arrangement analogous to that contemplated by the present invention.
Fujita, et al., U.S. Pat. No. 4,336,551 discloses a thick-film printed circuit board, and wherein the wiring conductors include connecting sections for connection to solder electrodes of a semiconductor chip. In that particular instance, the holes formed in the pads will cause self-centering of the solder balls, and wherein each pad is located on top of the solder mask. This has nothing in common with the inventive concept and the solution to the above-mention problem encountered in the formation of reliable solder joints.
SUMMARY OF THE INVENTION
Accordingly, in order to solve the problem encountered in the prior art with regard to satisfactorily depositing solder, such as eutectic solder, on the receptor pads of chip carriers, particularly in providing equal distributions of the solder so that all pad areas are equalized irrespective as the type and size of receptor pad configuration, the present invention sets forth the novel utilization of a solder mask constituted of an insulating material, and which incorporates specifically sized and shaped solder openings or windows wherein, in a particular instance, the C4 receptor pad is essentially at the intersection of a copper trace and the solder mask window. Pursuant to a particular aspect, the solder mask opening is of a polygonal configuration, whereas in other instances, as described hereinbelow, the C4 receptor pad design may be essentially of a socalled “band aid” or circular configuration in which a circular pad is located below and within a round solder mask opening or window. The circular pad extends within and beyond the solder mask window or opening through the intermediary of the copper trace, thereby enabling the formation of equal solder areas on all pads and eliminating the previously encountered BLM floating problem leading to potentially unreliable connections and resulting chip failures.
Accordingly, it is an object of the present invention to provide a receptor pad design which equalizes all pad areas in the uniform deposition and distribution of solder.
Another object of the present invention resides in the provision of a solder mask incorporating an opening of generally polygonal shape in which the receptor pad is created by the intersection of a copper trace and solder mask.
Still another object of the invention is to provide for the utilization of a solder mask wherein a circular pad is located within a round solder mask opening or window, and in which the circular pad is extended within and beyond the window by the copper trace of a circuit line.
REFERENCES:
patent: 4336551 (1982-06-01), Fujita et al.
patent: 4706167 (1987-11-01), Sullivan
patent: 4775573 (1988-10-01), Turek
patent: 5075965 (1991-12-01), Carey et al.
patent: 5313021 (1994-05-01), Sajja et al.
patent: 5386087 (1995-01-01), Lee et al.
patent: 5428505 (1995-06-01), Sakemi et al.
patent: 5483421 (1996-01-01), Gedney et al.
patent: 5523920 (1996-06-01), Machuga et al.
patent: 5706178 (1998-01-01), Barrow
patent: 5795818 (1998-08-01), Marrs
patent: 5801446 (1998-09-01), DiStefano et al.
patent: 5812379 (1998-09-01), Barrow
patent: 5
Jimarez Miguel Angel
Neira Reinaldo Anthony
Gaffin Jeffrey
International Business Machines - Corporation
Norris Jeremy
Pivnichny John R.
Scully Scott Murphy & Presser
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