Patent Application
20050277226
This application is related to printed circuit boards (PCB) having an integrated circuit chip or chips affixed thereto, and, more particularly, to a PCB having a flip chip affixed thereto.
As the printed circuit art becomes more and more complex, and as smaller and smaller footprints are desired or required, the use of flip chips, which lend themselves to miniaturization, i.e., smaller footprint, and which enable the use of more complex circuitry for interconnection therewith, in becoming more and more in demand and, consequently, more popular. In a typical flip chip arrangement, the flip chip having a plurality of input/output (I/O) solder bumps thereon is attached to metal pads, usually copper pads, on the printed circuit board by soldering it thereto. The procedure requires the use of a solder mask or masks to prevent intrusion of the molten solder onto the metal traces of the PCB, which would otherwise render such circuit traces invalid or inoperable by inevitably short circuiting the traces.
As the number of I/O bumps increases in density PCBs and the dimensions of the PCB and the flip chip decreases, there is a growing need for PCBs having higher and higher densities, resulting in finer and finer pitches between the metal pads on the PCB, which are limited by the presence of solder masks.
One problem that arises with decreased size, more coupled circuitry, and higher circuit densities arises from the solder itself. Eutectic tin-lead solder, which is the universally used solder in the manufacture of PCBs, has a tendency to run when being applied, and often to run over the exposed metal parts of the PCB. This tendency is typical of lead bearing solders. As a consequence of the running characteristic it is necessary to use solder masks on the PCB when soldering the flip chip thereto. It can readily be appreciated that the use of solder masks, which occupy a portion of the available space, creates a physical limit to the minimum dimensions of the assembly that can be achieved, and to the maximizing of density.
The present invention is based upon the use of lead-free solder in the fabrication of PCB assemblies. It has been found that certain lead-free solders, such as tin-silver or tin-silver-copper do not have the tendency to run or wet forward aggressively on copper or certain other metal surfaces as in the manner of lead containing solder, yet such solders form reliable metallic bonds with the copper traces. It is not necessary, therefore, to use solder masks to block or prevent running, and thus that particular limitation toward miniaturization and maximizing density is removed.
In an illustrative embodiment of the invention, a solder mask free printed circuit board has, thereon a plurality of metal pads, preferably of copper, forming the metal traces of the PCB, and there is no solder mask or masks separating them and shielding them from running solder. As a consequence, the pitch, i.e., the distance between adjacent metal pads, can be greatly reduced, thereby allowing a more compact PCB with the possibility of greater complexity of circuitry and greater density thereof. The solder used to join the flip chip to the PCB is a lead-free solder, preferably of a tin-silver alloy, which has a minimal or non-existent tendency to run between adjacent the metal pads or traces.
The various features and principles of the present invention will be more readily apparent from the following detailed description, read in conjunction with the accompanying drawings.
In the drawings hereinafter, the several arrangements depicted therein are not drawn to scale, several elements having exaggerated dimensions relative to other elements, and in all cases are intended as diagrammatic representations of the actual apparatus.
The assembly 11 of
In
D=a+b+2c=75μ+50μ+100μ=225μ (1)
which represents the minimum feasible dimensions where, for example, lead based solder is used.
D=e+2f+g=75μ+2×50 μ+50μ=225μ (2)
where c is the minimum distance between two metal pads.
An illustrative embodiment of the invention is shown in
D=a+g=75μ+50μ=125μ (3)
where g is the minimum distance between two metal pads.
It is clear, from this, that density of the circuitry on the PCB can be nearly doubled over that of present day PCB assemblies. Further, even with an increase in density, there can be, too, an increase in miniaturization inasmuch as the invention clearly makes possible the need for less space on the PCB for a given circuit pattern or traces.
The foregoing has been illustrative of the present invention in an illustrative embodiment thereof. These principles and features may be readily applied to other arrangements not herein disclosed that may occur to workers in the art, without departure from the spirit and scope of the invention.
