Patent Application
20110048782
1. Field of the Invention
The present invention relates generally to a solder pad structure, and in particular, to a solder pad structure with high bondability to a solder ball.
2. The Prior Arts
Recently, electronic products are developed smaller and smaller. Correspondingly, openings of electronic points on substrates, for example those openings for solder ball pads or bump pads, as well as the solder balls or bumps filled in the openings are also configured smaller and smaller. For example, a pitch of a ball grid array (BGA) solder pad is now configured with a size of 0.4 mm, or even 0.3 mm, comparing with the previous typical sizes thereof about 1 mm. Further, a typical solder mask (SM) solder pad opening is diminished from a previous size about 350 μm to a current size under 200 μm. Similarly, a typical flipchip solder pad opening is also diminished from a size about 100 μm to about 70 μm. However, this trend also causes a great difficulty on the packaging process.
Bondability between a solder ball and a solder pad is generally positive proportional with a contact area of the solder ball and the solder pad. As such, when the sizes of the solder ball and the solder pad become smaller and smaller, the bondability therebetween decreases correspondingly. A lower bondability between the solder ball and the solder pad only causes dropping off, peeling off, crack of the solder ball from the solder pad after packaged when suffering external forces. This may cause disconnection of electronic points, and opening of the circuit thereof. As such, upon the configuration of small size solder pad and solder ball, how to improve the bondability therebetween becomes an important concern in this art.
For example, electronic components for handheld devices are often required to be tested by dropping test, so as to guarantee the final products will not be damaged due to failures caused by unconscious dropping on the ground. The failures hereby often are cracks happened to the solder balls when impacted by the dropping. An IC chip in an encapsulation process is often processed by implanting a solder ball after being assembled to the substrate. Thereafter, the assembler then disposes the encapsulated component on a PCB solder pad with assembly equipment. A solder ball of the IC chip is melted and then welded to the PCB by a reflow process. In general, solder pads of a PCB often are greater than that of IC substrates, and therefore the problems of reliability of the interfaces between the solder balls and the solder pads are more likely happened at the IC substrate side. In this manner, when an electronic product is impacted, a crack is often happened at an inter-metallic compound (IMC) layer configured by the IC substrate and the solder pad, for example, typically a cell phone dropped on ground often cannot be turned on.
Generally speaking, in order to obtain a better bondability between the solder ball and the solder pad, a solder resist opening (SRO) is usually enlarged. However, as the entire component is desired to be smaller, the size of the SRO is strictly restricted. Further, an enlarged SRO often requires more precious image transferring and alignment exposure, which requires excessive equipment and higher cost. Furthermore, in order to prevent the IMC layer from being thickened due to the reflow processes, a thin layer of palladium is sometimes introduced between a nickel layer and a gold layer to depress the thickening of the IMC layer. However, the introducing of the palladium layer requires modification of the plating equipment. Moreover, palladium is very expansive metal element, thus the use of palladium increases the production cost.
A primary objective of the present invention is to provide a solder pad structure with a high bondability to a solder ball. The present invention provides a larger contact area with the solder ball so as to increase the bondability according to the principle that the bondability is positive proportional with the contact area therebetween. The present invention provides a solder pad having a certain geometric shape, so as to increase the contact area and strengthen the structure thereof, and thus increasing the bondability with the solder ball.
Another objective of the present invention is to provide a solder pad structure with high bondability to a solder ball. The solder pad structure configures a T bone structure and embeds the same into the solder ball so as to resist stress caused by the dropping of the electronic product.
For achieving the foregoing objectives, the present invention provides a solder pad structure with high bondability to a solder ball. The solder pad structure has a certain geometric shape so as to increase a contact area with the solder ball filled therein, so as to improve the bondability with the solder ball. In a circuit board, a solder pad opening is configured by a solder resist layer surrounding a circuit layer, in which the circuit layer within the solder pad opening is defined as a solder pad. In such a way, when a solder ball is filled in the solder pad opening, besides walls of the solder pad opening, the certain geometric shape of the solder pad provides an extra contact area with the solder ball, and thus improving the bondability for the solder ball.
The present invention will be apparent to those skilled in the art by reading the following detailed description of a preferred embodiment thereof, with reference to the attached drawings, in which:
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
The present invention provides a solder pad structure which with a high bondability to a solder ball. The solder pad structure has a certain geometric shape so as to increase a contact area with the solder ball filled therein, so as to improve the bondability with the solder ball. In a circuit board, a solder pad opening is configured by a solder resist layer surrounding a circuit layer, in which the circuit layer within the solder pad opening is defined as a solder pad. In such a way, when a solder ball is filled in the solder pad opening, besides walls of the solder pad opening, the certain geometric shape of the solder pad provides an extra contact area with the solder ball, and thus improving the bondability for the solder ball. Different geometric shapes of the solder pad are to be illustrated in the following embodiments shown with different cross-sectional views.
In order to produce the desired the geometric shape 16a of the T shape or the anchor shape, a first plating metal 18a, 18b and a second plating metal 18c are plated on the solder pad 14 in sequence. Then, a selective etching is performed with the first plating metal 18a, 18b serving as an etching mask, so that the second plating metal 18c is etched and configured to have an inward recess. In such a way, the solder pad 14 to have the certain geometric shape 16a with a cross-sectional view of a T shape or an anchor shape. Please note, according to an aspect of the embodiment, for the convenience of selective etching, areas of the first plating metal 18a, 18b, and the second plating metal 18c are preferably smaller than the solder pad 14a.
In more details, the solder pad 14 is preferably a BGA solder pad. The first plating metal 18a, 18b is copper or nickel alloy, and the second plating metal 18c is copper. As such, when being flash etched, the first plating metal 18a, 18b made of nickel alloy can serve as an etching mask and remains its original form after the etching process, and configures a cliff (“E”) shaped T bone as shown in
Although the present invention has been described with reference to the preferred embodiments thereof, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the present invention which is intended to be defined by the appended claims.
