Patent Application
20090289101
The present invention generally relates to assembly of electronic components, and more particularly relates to a method for surface mount attaching of electronic components with an improved ball grid array (BGA) solder attach process.
Surface mount is a conventional electronic component processing technique widely-employed to attach electronic components such as semiconductor devices to receptacles such as a printed circuit board (PCB). Many electronic components are prepared for surface mount processing by a conventional attachment of a solder ball grid array (BGA) to the electronic component package. Thereafter, for example, the BGA-prepared electronic component can be mounted on a PCB or mounted into a package-on-package (POP) semiconductor assembly, wherein the electronic component in a semiconductor package is surface mounted on or over a bottom package of the POP assembly, wherein the top package is connected to a substrate of the bottom component in the POP assembly, the substrate providing the interconnection therebetween and the substrate of the bottom component acting as a receptacle for both the bottom and top electronic components and forming the means for later attachment of the POP to another receptacle component.
During such attachment process, it is necessary to expose the package-on-package assembly to high temperatures to reflow the BGA solder balls. Yet, electronic components can warp during the temperature excursion (i.e., heating the electronic package, exposing it for a time to the high temperature, and thereafter cooling the electronic component) due to a mismatching of different thermal expansion coefficients (i.e., Coefficients of Thermal Expansion (CTEs)) of the individual materials inside the electronic component. This CTE mismatch between the individual materials inside the electronic component will cause bending or warpage of the electronic component when the POP assembly cools after heating. High warpage or warpage in the direction of the top and bottom components may not match, causing the solder in the solder balls to not “wet” on the bottom pad (i.e., make contact therewith when in the liquid form), thereby maintaining the form of a solder ball attached to the top component without connecting to the bottom pad. This results in an electrical failure where there is one or more open connections between the electronic component and the bottom pad. This failure is particularly common for larger electronic components (e.g., electronic components having a surface perimeter larger than 10 mm×10 mm).
Accordingly, it is desirable to provide a method for surface mount attachment of electronic components which compensates for the warpage induced by CTE mismatch during high temperature excursion during assembly. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
A method is provided for solder reflow. The method includes the steps of providing a receptacle having receptacle pads formed on an upper surface and placing a component on the receptacle, the component having a ball grid array of solder balls attached thereto. The component is placed on the receptacle in a manner which aligns the solder balls with the receptacle pads on the receptacle. The method further includes the steps of placing a weight having a predetermined size and a predetermined mass on top of the component to form a stack of the receptacle, the component and the weight, and reflowing the stack to attach the component to the receptacle by exposing the stack to high temperature to reflow the solder balls.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to illustrate various embodiments and to explain various principles and advantages in accordance with the present invention.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The following detailed description of an embodiment or embodiments of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description of the embodiments.
Surface mount is a processing technology which can be used to attach a prepared component to a receptacle in a highly manufacturable manner, wherein the surface mount component is prepared with an array of solder balls (called a ball grid array) attached to a bottom surface thereof. The receptacle has receptacle pads or ball pads formed on an upper surface of the receptacle to align with the solder balls when the component is placed on the receptacle. Surface mount processing technology can be used for mounting components to different receptacles, such as printed circuit boards, substrates and substrates with bottom components mounted thereon. This latter process (mounting a top component over a bottom component on a single substrate) is termed package-on-package or POP mounting. Referring to
As can be seen in the cross-sectional view 100, the top component 116 has a larger surface perimeter than the bottom component 114 so that the solder balls 118 of the BGA of the top component 116 can be attached to the substrate 112. The top component 116 can be surface mounted to the substrate 112 by submitting the POP stack 102 to a conventional solder reflow process. First, solder paste or flux is applied to the solder balls 118 on the top component 116 and/or to the ball pads 120 on the substrate 112. Next, the top component 116 is positioned above the substrate 112 and the solder balls 118 are aligned with the ball pads 120 using a pick-and-place machine. Next, the POP stack 102 is exposed to a high temperature (typically 220° C. to 260° C.) sufficient to reflow the solder of the solder balls 118 and thereby forming an integrated solder joint between the top component 116 and the ball pads 120. This reflow step for a BGA component such as the top component 116 is typically termed a BGA wetting process. The POP stack is then cooled to ambient temperature (approximately 25° C.), after which additional conventional processing is performed to complete the POP assembly.
The larger the top component 116, the more likely an OPEN connection 202 will occur from warpage force generated by the temperature excursion from ambient temperature to the high reflow temperature and back to ambient temperature during the POP assembly fabrication process. The OPEN failure is particularly common for a top component 116 having a perimeter larger than 10 mm by 10 mm.
Referring to
The predetermined mass depends on elements of the top component such as the interconnect and materials of the top component 116, and can be as small as 100 mg or as large as 100 g. For example, the predetermined mass can be calculated from a measurement of the non-coplanarity of an OPEN connection 202 (
The weight 304 may have a predetermined size substantially equal to the size of the top component so that the same component storage trays may be used for storage and pick-and-place. Alternatively, the weight 304 may have a lip 306 formed thereon as shown in
Referring to
The outer perimeter 604 of the weight 304 is determined in response to the process equipment (e.g., component storage trays and pick-and-place machines) used in placement of the weight 304 on and removal of the weight 304 from the POP stack 302 (
Referring to
Once the substrate 112 is provided 802, the top component 116 is picked 804 by a pick-and-place machine from a component storage tray. Solder paste or flux is then applied 806 to the solder balls 118 of the ball grid array of the top component 116. Alternatively, step 806 could encompass applying solder/flux to the ball pads 120 on the substrate 112. The top component 116 is then placed 308 on the substrate 112.
In accordance with the present invention, the weight 304 having a predetermined mass and size is picked 810 from a component storage tray of such weights 304 by a pick-and-place machine. As stated above, it is preferable that the predetermined size of the weight 304 be such that the same component storage trays and the same pick-and-place machine used for storage and picking, respectively, of the top component 116 and the weight 304 can be utilized to reduce manufacturing costs of the POP assembly.
The pick-and-place machine then places 812 the weight 304 on the top component 116. The POP stack 302 with the weight 304 thereon is heated in a solder reflow step 814 to a sufficient temperature to liquefy the solder of the solder balls 118. As discussed above, the weight 302 has a sufficient predetermined mass to maintain attachment of the solder balls 118 to the ball pads 120 on the substrate 112 during the reflow step 814. If the bottom component 114 has a ball grid array of solder balls attached thereto and is placed on the upper surface of the substrate 112 with the solder balls thereof aligned with a plurality of ball pads formed on the substrate prior to step 804 where the top component 116 is also placed on the substrate 112, the reflow step 814 would simultaneously attach the bottom component 114 and the top component 116 to the substrate 112 by reflowing the solder balls of the bottom component 114 and the solder balls 118 of the top component 116. In this manner, additional manufacturing steps could be excluded, thereby saving both manufacturing time and manufacturing costs.
After reflow 814, the POP stack 302 is allowed to cool 816 to ambient temperature. The weight 304 remains on the POP stack 302 during cooling 816 to assure solid formation of a solder connection between the ball pads 120 on the substrate 112 and the top component 116. After cooling 816, the weight 304 is removed 818 by a weight removal machine which will pick up the weight 304 by magnetic force (if the weight 304 includes metal) or a similar mechanism. The weight 304 is then returned to a component storage tray for reuse in subsequent POP assembly processing. The reuse of the weights 304 provides additional cost savings in implementation of the methodology of the embodiment of the present invention as a unique weight 304 does not have to be provided for each POP stack 302.
Thus it can be seen that an improved method for BGA wetting and solder attach for surface mount electronic components has been provided for POP assembly wherein a weight 304 having a predetermined size and a predetermined mass compensates for the warpage induced by CTE mismatch during high temperature excursions (e.g., reflow 814 and cooling 816) during the assembly process.
While at least one exemplary embodiment concerning a package-on-package fabrication process has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. For example, the process of the present invention could provide an advantageous method for surface mounting electronic components on a printed circuit board. It should also be appreciated that the exemplary embodiments are only examples and are not intended to limit the scope, applicability, or configuration of the present invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in the exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
