US 20080017410 A1
A method for forming a plated microvia interconnect. An external dielectric layer (EDL) is mounted on a substrate in direct mechanical contact with a conductive element thereon. An opening in the EDL exposes the conductive element and create a microvia in the EDL. A sidewall and bottom wall surface of the microvia are treated to promote adhesion of copper and are plated with a layer of copper that includes a copper layer on a copper seed layer and is in direct mechanical and electrical contact with the conductive element. A wet solder paste is deposited on the layer of copper to overfill a remaining portion of the microvia. The solder paste is reflowed to form a solder bump in and over the remaining portion of the microvia to form the plated microvia interconnect. A stiffener is attached to the EDL using a first adhesive.
1. A method for forming a plated microvia interconnect, comprising:
mounting an external dielectric layer (EDL) on a surface of a substrate such that the EDL is in direct mechanical contact with a conductive element comprised by the surface of the substrate;
forming an opening in the EDL to expose the conductive element and create a microvia in the EDL;
treating a sidewall surface and a bottom wall surface of the microvia to promote adhesion of copper to the sidewall surface and the bottom wall surface;
plating the sidewall surface and the bottom wall surface of the microvia to form a layer of copper on the sidewall surface and the bottom wall surface, wherein the layer of copper is in direct mechanical and electrical contact with the conductive element, wherein said plating comprises: depositing a copper seed layer on the sidewall surface and the bottom wall surface of the microvia; and plating a copper layer on the copper seed layer such that the layer of copper comprises the copper seed layer and the copper layer;
depositing a wet solder paste on the layer of copper to overfill a remaining portion of the microvia;
reflowing the solder paste to form a solder bump in and over the remaining portion of the microvia to form the plated microvia interconnect; and
attaching a stiffener to the EDL using a first adhesive to adhere the stiffener to the EDL.
2. The method of
visually inspecting the microvia to determine whether the solder bump is missing.
3. The method of
4. The method of
coupling a chip to the substrate by attaching the chip to the solder bump.
5. The method of
disposing an encapsulation material on the EDL, wherein the encapsulation material is in direct mechanical contact with the first adhesive.
6. The method of
disposing a cover plate over the stiffener using a second adhesive to adhere the cover plate to the stiffener.
This application is a divisional application claiming priority of Ser. No. 10/281,463, filed on Oct. 25, 2002; which is a divisional of U.S. Pat. No. 6,492,600, issued Dec. 10, 2002.
1. Technical Field
The present invention relates to interconnect structures, and more specifically relates to a receptor pad for use in a chip carrier package.
2. Prior Art
Forming electrical connections between components remains an ongoing challenge in the electronics industry. As sizes decrease, more precision and smaller interconnections are required when attaching components to circuit boards, planar surfaces, substrates, and the like (collectively referred to herein as “landing areas”). While soldering techniques are widely available to connect components, the ability to provide landing areas with small and reliable receptor pads becomes increasingly difficult.
One particular application that utilizes high density interconnections involves integrated circuit (IC) chip packaging. An IC chip package comprises a relatively small IC device encapsulated in a larger package, which is more suitable for use in the industry. The “larger” IC chip package includes external connectors (e.g., a ball grid array) suitable for electrical communication with a traditional circuit board. Conversely, the smaller IC device, which comprises much smaller connectors, resides within the IC chip package on a landing area or laminate. Thus, the IC chip package must provide a relatively small landing area as well as a system for internally routing signals between external package connectors and internal IC device connectors.
In order to achieve this redirection of signals, a landing area comprised of a circuitized substrate or laminate is provided having a set of internal (high density) receptor pads. Circuit lines within the substrate route the signals from external connectors, which are in communication with external devices, to the receptor pads on a landing area, which connect with the IC device. Connection between the landing area and IC device is generally achieved by soldering. Solder techniques are well known in the art and examples are found in U.S. Pat. No. 5,597,469 issued on Jan. 28, 1997 to Caret' et al., and assigned to International Business Machines.
As noted, given the trend towards smaller IC devices, the circuitized substrate must provide a high number of receptor pads in a very small surface area. This is becoming more difficult to achieve with present design techniques. Specifically, because solder cannot wet down onto non-metal and/or organic materials, receptor pads must be designed with an adequate metal surface to ensure attachment. If such a surface is not provided, solder often fails to properly wet the pads and becomes inadvertently removed from the pad during subsequent reflow and wash processes. The most common pad structure to ensure adequate metal surface comprises a “dog bone” structure that utilizes a flat pad attached to an adjacent via. Unfortunately, these structures take up a lot of surface area.
Thus, a need exists to provide a receptor pad that can reliably receive solder without requiring an extensive amount of surface area. All of the aforementioned references are hereby incorporated by reference.
The present invention provides a method for forming a receptor pad on a laminate, comprising the steps of: providing a circuitized substrate that includes a surface having a conductive element; mounting an external dielectric layer (EDL) on the surface; forming an opening in the EDL to expose the conductive element and create a microvia; treating an interior side wall surface of the microvia to promote copper adhesion; and electroplating the microvia with copper.
Once the microvia is electroplated, a resist process is used to define and finalize the receptor pad. Thereafter, a wet solder paste may be deposited on the receptor pads followed by a reflowing and washing step to create a reliable solder bump.
The invention also comprises a laminate having a receptor pad formed thereon, comprising: a circuitized substrate having a conductive element on a surface; an EDL mounted on the circuitized substrate, the EDL having an opening positioned above the conductive element to form a microvia; and an electroplated layer deposited within the microvia.
It is therefore an advantage of the present invention to provide a microvia structure that can be used as a receptor pad and directly receive a solder deposit.
It is therefore a further advantage of the present invention to provide higher density surface mounting technology by eliminating the need for dog bones and the like.
It is therefore a further advantage of the present invention to provide a microvia that allows solder to reliably wet thereon.
The purpose of the foregoing and other aspects and advantages will be better understood from the following detailed description of the invention with reference to the drawings, in which:
Referring to figures,
The integrated circuit chip package 10 includes a chip 12, a laminate 14, connections 16 that interconnect the laminate 14 with the chip 12, a cover plate 20, a stiffener 24, adhesives 26 and 28, an encapsulation material 22, and a ball grid array (BGA) structure 18. While this preferred embodiment deals generally with the electrical interconnect between laminate 14 and chip 12 within a chip package, it is understood that the structure and methods described herein could be used on any planarized surface that provides component interconnections. Moreover, the figures are provided primarily for explanation purposes, and are not necessarily drawn to scale.
Referring now to
Referring now to
Because a solder deposit or bump 46 (supplied during subsequent reflow processes) will not reliably wet (i.e., remain attached) directly to the interior side walls of the EDL 34 opening, this invention utilizes a metallically plated microvia to provide a surface that will reliably receive and maintain solder bump 46. Without the plating, solder will not reliably flow over and adhere to the EDL walls, particularly if the EDL is comprised of an organic material. However, similar to the solder bump, a reliable system for attaching the electroplated layer 44 to the interior surface of the EDL 34 opening must likewise be implemented. Accordingly, the present invention provides a treated interior side wall surface 42 that is used to ensure adhesion of the plating 44. For the preferred embodiment, the interior side wall surface 42 is treated in any known manner that will promote copper adhesion. For example, the interior wall 42 may include a surface that is “roughened” to enhance the adhesion of the electroplating 44 to the EDL 34. Roughening may be achieved with, for example, a mechanical or a chemical process such as mechanical scrubbing, epoxy etching or plasma sanding.
Referring now to
Next, the interior surfaces of the microvias are electroplated with copper. This may be achieved with a multi-step “plate-up” process that includes the application of a copper seed layer, followed by a full plating operation. The application of the seed layer may be accomplished with an electroless strike process that applies copper seeding to the treated side wall surfaces. Full panel electroplating with acid copper can then be used to finish the plating. This plating process may be accomplished with a bath process utilizing dip tanks, or any other known plating methods. As noted above, an electroless plating methodology could likewise be used. The result is a microvia (as well as the laminate surface) lined with metal plating. While the thickness of the plating may vary depending upon the particular application, this preferred embodiment contemplates a thickness of about 1 mil on the side walls and 0.7 mils on the bottom.
Alternatively, the microvia could be filled with a conductive material as described above.
Finally, the structure of the receptor pads are formed and finalized with a resist process to etch the pad. This process creates discrete pads on the laminate surface and eliminates copper from the laminate surface where it is not desired. Any known photo resist process to define the pads may be used, including the plate up and etch method described, or by using an additive or semi additive pattern plating process using electroless copper plating. Because the resultant microvia sidewalls have metal rather than bare epoxy, solder paste can wet down to the pad eliminating unreliable connections.
Once the pad is complete, a solder paste may be applied to the receptor pads to provide solder bumps. One method for applying solder paste to the receptor pads involves a “flip chip” screen printing process. This process utilizes a solder screen printer, which is an automated tool used to deposit wet solder paste onto a card or any fine pitch, micro BGA, or chip carrier site. The screen printer utilizes a framed metal mask (stencil) with apertures in the same pattern array as the carrier. Typical BGA stencils are 8 mils thick with 30 mil diameter apertures. For this preferred embodiment, a 2 mil thick stencil with a 5-6 mil diameter aperture may be used.
The screen printer may utilize a high magnification vision system to align up the carrier with the stencil. After alignment is complete, a camera moves to the side and the carrier is automatically pushed up to the stencil thereby aligning the pads on the carrier to the stencil. Solder paste is applied to the stencil and squeegee blades or a printing head comes down in contact with the stencil, sweeping across, and depositing the wet solder onto the carrier. The carrier comes down away and out from the stencil and is removed. This process may be repeated multiple times. The carrier may then reflowed and washed and a visual inspection may be performed to inspect for missing bumps. Because the microvias are plated up (approximately 0.7 mils thickness in the bottom of the well), the result is a relatively small gap between the top of the receptor pad and the screened solder paste, which provides higher reliability.
While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.