Ball attaching apparatus for correcting warpage of substrate and method of attaching solder balls using the same

A ball attaching apparatus for respectively attaching solder balls onto a plurality of ball lands of a material which has mold caps formed between the ball lands. The apparatus includes an indexer on which the material is seated and fixed; a holder located above the indexer such that it can be raised and lowered; an attachment plate installed on a lower surface of the holder, having projections at positions corresponding to the mold caps of the material, and defined with grooves at positions corresponding to the ball lands of the material, in which the solder balls are placed; and eject pins arranged in the respective grooves of the attachment plate for conveying and dropping the solder balls through introduction and removal of vacuum.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Korean patent application number 10-2007-0025740 filed on Mar. 15, 2007, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a ball attaching apparatus and a method of attaching solder balls using the same, and more particularly to a ball attaching apparatus which can precisely attach solder balls to respective ball lands of a substrate even though warpage occurs in the substrate, and a method for attaching solder balls using the same.

Two key areas of semiconductor package development are miniaturization and improved electrical characteristics. A ball grid array (BGA) package is one such semiconductor package generally regarded as being capable of meeting the requirements.

The size of the entire BGA package is about the same or comparable to that of a semiconductor chip such that the mounting area of a BGA package can be minimized. A BGA package is electrically connected to the external circuits by solder balls, and this leads to improved electrical characteristics as the electric signal transmission paths of the solder balls are short.

The semiconductor chips of recent design are characterized with more miniaturization and higher functionality with more signal input and output pads. To meet this requirement, the BGA packages are manufactured as FBGA (fine pitch BGA) packages.

FIG. 1shows a cross-sectional view of a conventional FBGA package.

Referring toFIG. 1, a semiconductor chip100is attached by an adhesive109to a substrate102, which has a cavity107at a center portion thereof. The bonding pads101of the semiconductor chip100are electrically connected with the electrode terminals103of the substrate102by metal wires106, which pass through the cavity107. The upper surface of the substrate102including the semiconductor chip100, the cavity107including the metal wires106in the cavity107are molded using a molding material108. Solder balls105for mounting the FBGA package ofFIG. 1to external circuits are attached to the ball lands104provided on the lower surface of the substrate102.

The solder ball attaching process is implemented using a ball attaching apparatus by vacuum-sucking the molten solder through the eject pins of the ball attaching apparatus, positioning the eject pins on the ball lands104, and then dropping solder balls105on the ball lands104by removing the vacuum in the eject pins.

On the other hand, the substrate warpage could occur in the course of assembling a semiconductor package. Any warped substrate would cause irregular variations in the intervals between the balls lands leading to misalignment of the eject pins for dropping the solder balls and the ball lands to which the solder balls are to be dropped, as such solder balls cannot be precisely attached onto the ball lands formed on a warped substrate.

FIG. 2shows the types of warpage occurring in a substrate, andFIG. 3is shows the problematic attachment patterns of solder balls due to warpage of a substrate.

Referring toFIG. 2, the substrates could be warped during assembly of a semiconductor package into different shapes of a “smiling” type, a “crying” type, a “gull” type, and an “inversed gull” type. These types of warpage produce intolerant variations in the gaps between the ball attaching apparatus and the ball lands provided on the warped substrate such that a uniform distance between ejector pins and the balls lands cannot be maintained. The gaps between the ball attaching apparatus and the ball lands provided on the warped substrate could vary from a small value less than 300 μm to a large value greater than 900 μm.

Accordingly, due to misalignment of the ball attaching apparatus and the ball lands on a warped substrate, a solder ball could miss a ball land or an extra solder ball could be attached to a ball land or a solder ball could be miss-positioned on a ball land due to the fact that the solder balls are not precisely dropped onto the ball lands formed on a warped substrate during a solder ball attaching process.

For example, now referring toFIG. 3, the solder balls used in a BGA package may have a height of 400˜450 μm, and the warpage of a substrate occurs in the smiling type. In this example, the gap between a set of eject pins and the ball lands near an edge of the warped substrate could be less than 300 μm, as such it would be impossible to secure the sufficient gap required for proper attachment of the solder balls.

As the solder balls are pressed by eject pins during a ball attaching process (due to the insufficient gaps as explained above), the eject pins may not drop the solder balls on the intended ball lands but instead carry away the solder balls to a different location causing the “missed ball” phenomenon and the “extra ball” phenomenon as shown inFIG. 3.

Toward the center portion of the substrate warped in a “smiling” type, the gap between eject pins and the ball lands may be greater than 900 μm, which is at least two times larger than the height of solder balls. This will lead to the “mis-positioned ball” phenomenon (seeFIG. 3), in which the solder balls are amispositioned on the ball lands when attaching the solder balls.

The variations in the gap between the ball attaching apparatus and the ball lands on a warped substrate cannot ensure proper attachment of solder balls on the ball lands such that it will cause defects and lower the productivity yield.

SUMMARY OF THE INVENTION

An embodiment of the present invention is directed to a ball attaching apparatus which can precisely attach solder balls to respective ball lands of a substrate regardless of warpage occuring in the substrate and a method for attaching solder balls using the same.

In one embodiment, a ball attaching apparatus for respectively attaching solder balls onto a plurality of ball lands of a material which has mold caps formed between the ball lands comprises an indexer on which the material is seated and fixed; a holder located above the indexer such that it can be raised and lowered; an attachment plate installed on a lower surface of the holder, having projections at positions corresponding to the mold caps of the material, and defined with grooves at positions corresponding to the ball lands of the material, in which the solder balls are placed; and eject pins arranged in the respective grooves of the attachment plate for conveying and dropping the solder balls through introduction and removal of vacuum.

The holder and the attachment plate further includes a tensioning part which has a dial gauge for constantly maintaining tension between the mold caps of the material and the projections of the attachment plate.

A lower end of the eject pin is positioned higher than a lower surface of the attachment plate.

The lower end of the eject pin is positioned higher than the lower surface of the attachment plate by 0.1˜0.2 mm.

The projection has a height which is determined such that a summed height of the projection and the mold cap formed on the material permits a distance of 0.02˜0.03 mm between an upper end of the solder ball to be attached and the eject pin.

The projection has a height which is proportional to that of the solder ball which is attached such that a distance between an upper end of the solder ball and the eject pin is 0.02-0.03 mm.

The groove has a width which is greater than that of the solder ball to be attached.

The material comprises a substrate.

The indexer has material fixing unit for fixing the material using vacuum.

The holder has a stopper for adjusting a height of the holder.

The holder further has a driving unit for controlling raising and lowering of the holder.

In another embodiment, there is provided a method for attaching solder balls using a ball attaching apparatus including an indexer on which a material is seated and fixed, a holder located above the indexer such that it can be raised and lowered, an attachment plate installed on a lower surface of the holder, having projections at positions corresponding to mold caps of the material, and defined with grooves at positions corresponding to a plurality of ball lands of the material, in which the solder balls are placed, and eject pins arranged in the respective grooves of the attachment plate for conveying and dropping the solder balls through introduction and removal of vacuum, the method comprising the steps of seating and fixing the material which has the plurality of ball lands and is formed with the mold caps between the ball lands, on the indexer; bringing the projections formed on the attachment plate into contact with the respective mold caps of the material by lowering the holder and then pressing the mold caps to correct warpage of the material; and attaching the solder balls onto the ball lands provided to the material by removing vacuum in the eject pins.

In the step of bringing the attachment plate into contact with the mold caps of the material, a gap between the eject pin and the material is constantly maintained by a pressure applied to the mold caps by a tensioning part which includes the attachment plate and a dial gauge disposed in the holder.

A lower end of the eject pin is positioned higher than a lower surface of the attachment plate.

The lower end of the eject pin is positioned higher than the lower surface of the attachment plate by 0.1˜0.2 mm.

The projection has a height which is determined such that a summed height of the projection and the mold cap formed on the material permits a distance of 0.02˜0.03 mm between an upper end of the solder ball to be attached and the eject pin.

The projection has a height which is proportional to that of the solder ball which is attached such that a distance between an upper end of the solder ball and the eject pin is 0.02˜0.03 mm.

The groove has a width which is greater than that of the solder ball to be attached.

The material comprises a substrate.

The indexer has material fixing unit for fixing the material using vacuum.

The holder has a stopper for adjusting a height of the holder.

The holder further has a driving unit for controlling raising and lowering of the holder.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In response to the warpage caused in a material during a manufacturing process of a semiconductor device, a ball attaching apparatus according to an embodiment of the present invention employs, inter alia, projections to press the warped material with predetermined tensioning force to even out the warped material and then attaching the solder balls to the ball lands.

As a consequence, solder balls are attached precisely to the ball lands of a material without the “missed ball” or “extra ball” or “mis-positioned ball” phenomenon (as shown inFIG. 2). Consequently, the reliability of the semiconductor packages is improved by the present invention.

As the precise attachment of the solder balls to the ball lands of the material are achieved by the present invention, at least 95% of the temporary interruptions triggered by a quality control inspector inspecting for improper solder ball attachments due to the substrate warpage are eliminated. The present invention also simplifies the FBGA package manufacturing process since the step of passing an FBGA package through an external visual inspection (EVI) equipment for inspecting the proper attachment of solder balls becomes unnecessary.

Hereafter, a ball attaching apparatus in accordance with an embodiment of the present invention will be described with reference toFIGS. 3 through 6.

FIG. 4is a view showing a ball attaching apparatus in accordance with an embodiment of the present invention;FIG. 5is an enlarged view for the part ‘A’ ofFIG. 4; andFIG. 6is a view illustrating a tensioning part provided to the ball attaching apparatus in accordance with the embodiment of the present invention.

Referring toFIGS. 4 and 5, a ball attaching apparatus400in accordance with an embodiment of the present invention includes an indexer410on which a material450is seated, a movable holder420which is raised and lowered for attachment of solder balls456, an attachment plate422which is attached to the lower surface of the holder420, and eject pins426which convey the solder balls456.

The material450can be understood as a substrate having a plurality of ball lands454and mold caps452. The indexer410has a material fixing unit412for securely holding the material450on the indexer410using vacuum.

The holder420includes stoppers430which are positioned adjacent the edges of the upper surface of the holder420to adjust the heightwise position of the holder420, and a driving unit440which is disposed on the center portion of the upper surface of the holder420to control raising and lowering of the holder420. The driving unit440is also composed of a driving cylinder (not shown) and a driving circuit (not shown).

The attachment plate422has grooves425and projections428. The grooves425are defined on the lower surface of the attachment plate422at positions, which correspond to the ball lands454of the material450. The grooves are formed with an opening on the upper surface of the attachment plate422so that the elector pins426are disposed in the hole. The grooves425have a width, which is greater than that of the solder balls456, to prevent interference with the solder balls456to be attached. The projections428are formed on the lower surface of the attachment plate422at positions, which correspond to the mold caps452of the material450.

As shown inFIG. 5, the eject pins426are located in the respective grooves425through the holes on the upper surface of the attachment plate422at the positions corresponding to the ball lands454of the material450. The eject pins426function to convey and drop the solder balls456through introduction and removal of vacuum therein and therefrom. In order to prevent interference of the eject pins426with the solder balls456to be attached, the lower ends of the eject pins426are positioned higher than the lower surface of the attachment plate422. The gap ‘a’ between the lower end of the eject pin426and the lower surface of the attachment plate422is 0.1˜0.2 mm.

The projection428has a height which is determined such that a summed height of the projection428and the mold cap452formed on the material450permits a distance ‘b’ of 0.02˜0.03 mm between the upper end of the solder ball456to be attached and the eject pin426. That is, the projection428has a height, which will be proportionally adjusted to the height of the solder ball456to maintain the distance ‘b’.

Referring toFIG. 6, the holder420and the attachment plate422has therein a tensioning part460, which includes a dial gauge462, such that the tension between the mold caps452of the material450and the projections428of the attachment plate422can be constantly maintained when attaching the solder balls456.

Hereinbelow, a method for attaching solder balls using the ball attaching apparatus400in accordance with an embodiment of the present invention will be described with reference toFIGS. 4 through 6.

As shown in the drawings, the material450having the plurality of ball lands454and the mold caps452is loaded and fixed on the indexer410, which has the material fixing unit412.

The solder balls456to be attached to the ball lands454of the material450are held in a state in which they are vacuum-sucked to the respective eject pins426.

The holder420is lowered by the driving unit440, which is installed at the center portion of the holder420. The projections428, which are formed on the attachment plate422, are brought into contact with the mold caps452of the material450, and thereupon, press the mold caps452of the material450with predetermined tensioning force determined by the tensioning part460provided in the holder420, so that the warpage of the material450is corrected. The predetermined tensioning force is set in the dial gauge462provided in the tensioning part460so that the degree of tension, which is applied by the projections428to the mold caps452, can always be constant.

Therefore, when conducting a ball attaching process, the warpage of the material450is corrected, and the entire ball lands454of the material450can be positioned at the same height, whereby it is possible to precisely attach the solder balls456onto the entire ball lands454.

Then, the vacuum in the eject pins426is removed, and the solder balls456that were sucked inside the eject pins426are dropped precisely onto the respective ball lands454of the substrate450.

As is apparent from the above description according to an embodiment of the present invention, since the projections formed on an attachment plate press the mold caps provided at corresponding positions on a material with a predetermined tensioning force, the warpage of the material is corrected so that the solder balls are attached precisely.

Therefore, the present invention prevents the “missed ball” or the “extra ball” or the “mis-positioned ball” phenomenon occurring during a ball attaching process due to warpage of a substrate, and thereby improves the semiconductor package reliability.

Moreover, as the solder balls are precisely attached to the ball lands of the material according to the present invention, the temporary interruptions caused due to a quality control inspector catching the instances of the imprecisely attached solder balls due to warped substrates can be eliminated by at least 95%. Further, since it is not necessary to pass an FBGA package through external visual inspection (EVI) equipment for inspection of the solder balls, the present invention affords simpler processes.