KR19990029704A - Soldering ball placement system for ball grid array packaging of IC chips - Google Patents

Abstract

One template for solder ball placement on a BGA package consisting of means for template arrangement, solder ball placement and a laser head is aligned with a connection pad located on a substrate surface with a pre-applied flow. The aligned template allows for accurate guidance of the ball onto the pad by the small positioning means. One ball is placed on the pad away into each hole in the template. The balls located on the pad are then exposed to a laser through the laser head for a short time to allow the balls to partially melt. This short exposure to the laser results in a smooth adhesion to the pad of the substrate or a tag of the balls. Once the balls are tackled, these balls are not easily moved and can be delivered to the reflow oven.

Description

Soldering ball placement system for ball grid array packaging of IC chips

The present invention is directed to the use of solder in the electrical connection between IC devices and printed circuit boards. In particular, the present invention relates to the precise placement of solder balls on a ball grid array (BGA) during the production process.

Ball grid array (BGA) packaging of integrated circuit (IC) devices is becoming increasingly important in IC device production. In BGA packaging, IC chips are typically mounted on a padded copper substrate, where a flow is applied on the substrate followed by positioning of the solder balls. The solder ball is then soldered onto the pad in a reflow oven.

The conventional method of solder ball placement is to use a vacuum suction head with an appropriate suction hole arrangement to pick-up the solder ball. The balls sucked into the head in the proper arrangement are lowered onto one substrate with a pre-applied flow. This BGA assembly is then passed to a reflow oven to solder the solder ball.

The current trend in IC chip production is to integrate more and more ICs on the chip. The higher the density of the IC, the greater the number of interconnections required on the same chip size. Thus, there have been a number of BGA packages in high density pads and solder balls. In such a high density BGA package, the number of interconnect pads per chip is 1,000-2,000 higher compared to low density BGA packages with less than 400 pads for chips with the same surface area. For high density packages with more than 400 pads per chip, the pitch (the distance between the solder balls) and the solder ball size should be reduced accordingly. For example, for chips with a pad diameter of 25 mils and a pitch of 50 mils, a low density BGA configuration of 400 pads or less uses a 30 mil diameter solder ball. For example, for high density BGA configurations with pads with 10 mil pad diameter and 20 mil pitch, low 12 mil diameter solder balls should be used instead.

This reduced pitch and ball size are problematic for soldering. Since the solder ball is small and lightweight, air turbulence or minute warpage of the substrate for only one minute causes the ball to shift. Because of the pitch required in high density arrangements, bridging occurs, which results in a mixture of two solder balls to form one connection during the soldering process in the reflow oven. Once bridging occurs, the entire package must be rejected. As a result, the conventional method of positioning the balls using a vacuum suction head causes high rejection rates due to the bridging. The second problem arises from the transfer of packaging from the ball placement location to the reflow oven. Even if the solder balls are positioned correctly, the movement required to deliver the package to the reflow oven will cause the balls to move. Therefore, there is a need to improve the packaging process by designing the concept of ball placement to prevent movement and bridging of the solder balls.

It is an object of the present invention to provide a method of small solder ball placement in the packaging of a BGA device.

It is another object of the present invention to reduce the occurrence of solder ball bridging in BGA assembly processing.

1 is a schematic illustration of a ball placement step guided by a template according to the present invention;

2 is a schematic illustration of a laser tag step in accordance with the present invention.

3 is a schematic illustration to illustrate positioning of the laser head for the tag of solder balls.

4 is a flow chart for explaining the steps included in the laser tag system according to the present invention.

* Code Description

20 ... substrate 22 ... template

24 ... ball suction head 27 ... cavity

28 ... solder ball 29 ... template

30 ... laser head 32 ... flow layer

34 ... housing 36 ... optical fiber

42 ... ball suction head

The present invention provides a system for solder ball placement on a pad of a BGA package substrate consisting of means for template arrangement, means for solder ball placement and a laser head. One template is aligned with a connection pad located on the substrate surface with a pre-applied flow. The aligned template allows accurate guidance of the ball onto the pad by the small placement ball means. One ball is placed on the pad away into each hole in the template. The balls located on the pad are then exposed to a laser through the laser head for a short time to allow the balls to partially melt. This short exposure to the laser results in a smooth adhesion to the pad of the substrate or a tag of the balls. Once the balls are tagged, these balls are not easily moved and can be delivered to the reflow oven.

1 shows a schematic description of the first stage of the ball placement process according to the invention. After the flow is applied to the substrate 20 in accordance with conventional methods, a template 22 of high density arrangement is placed on the substrate. A ball suction head 24 with solder balls is aligned on the substrate using the template as a guide. When the arrangement in the ball suction head is aligned with the template arrangement, the vacuum at the ball suction head is released and the solder balls flow down onto the substrate. The ball suction head is a conventional one commonly used in BGA packaging and the template arrangement is adapted for high density devices. The template may be a wire mesh having a mesh size having a required density. The template may be made of a heat resistant material such as stainless steel or aluminum. Stainless steel is preferable as a preferable heat resistant material material at this time. The mesh wire is thick enough to prevent solder balls from rolling past the wire. For example, for a solder ball having a diameter of 12 mils, the thickness of the mesh wire is approximately 12 mils, as indicated by reference numeral 26 in FIG. 3, to prevent the ball from rolling away from the cavity 27. In addition, the surface area of the cavity is preferably somewhat larger than the diameter of the solder ball to facilitate placement by the ball suction head and to prevent the ball from easily attaching to the wire mesh. The preferred area is 15-35% larger than the solder ball cross-sectional area.

2 illustrates how a matrix laser head is used to tag solder balls. The ball suction head is moved away from the substrate after placement of the ball and the laser head 30 is placed on the template and the solder ball. The solder balls are provided with a short laser exposure to cause local melting, which in turn results in the tag of solder balls onto the substrate. The laser head is preferably a gyroscope head or matrix type head composed of a series of optical fibers arranged in the same arrangement as the ball arrangement. Lasers such as neodymium: yttrium-aluminum-garnet (Nd: YAG) lasers are suitable for the tag operation above. The exposure time and strength depends on the size of the solder ball.

FIG. 3 shows the optical fiber alignment of the laser head with solder balls 28 located in template 29 cavity 27 over flow layer 32. The optical fiber is received in the housing 34 as long as the optical fiber 26 is fixed to match the position of the pad and the position of the solder ball. The wire match to the template has a height 26 approximately equal to the solder ball diameter.

4 is a flow chart illustrating a process according to the present invention. After application of the flow 40, the template is aligned with the substrate followed by a ball placement with the ball suction head 42. After step 42, a visual inspection 44 is suitably performed with a camera, such as a CCD (capacitor charging device) camera, to ensure that the balls are in place, followed by a laser tag 46. . Steps 42-46 are preferably performed with the package fixed to minimize any disturbance to the ball once the balls are placed on the substrate.

The final step 48 is to deliver a package to the reflow oven for soldering the balls.

The present invention uses a combination of two separate techniques for high density BGA device packaging. Applying only one technique will result in unsatisfactory results, but a combination of the two techniques with appropriate purity will result in synergistically exceeding expected additional gains. This first technique involves the use of a template to release and position the ball from a conventional ball suction head. Once the balls are properly placed in the template on the substrate, a second technique, laser tagging, is used, which results in local melting of the solder balls. This allows the balls to be smoothly attached or tagged to the substrate. The template is then removed and the device is passed to a reflow oven for proper soldering. The result is a BGA package provided with high quality solder to solder balls. The defects due to bridging of the solder balls are therefore minimal.

Claims (5)

Means for template alignment, A template having an array of cavities that match the pad array, Means for placing solder balls, and A pad comprising a laser module having a laser head adapted to direct at least one laser beam to the solder ball, Align the cavities of the template to the pad array with pre-applied flow, Using the template as a guide to align the array of solder balls with the array of pads, aligning means for solder ball placement to the substrate, Release the aligned solder balls onto the pad in a pre-applied flow such that one solder ball is located in each cavity of the template, Align the laser head to the substrate, and A soldering ball placement system for packaging ball grid arrays of IC chips into a substrate having a pad arrangement, wherein the solder ball comprises a continuous step of melting and tagging the solder balls somewhat on the pads. 2. The solder ball placement system of claim 1, wherein the system further comprises a camera, and wherein an additional step of visual inspection is performed after solder ball placement. The solder ball placement system of claim 1, wherein said laser head is a matrix laser head. The solder ball placement system of claim 1, wherein the laser head delivers a Nd: YAG laser beam. Means for template alignment, Means for placing solder balls on the pad, and And an apparatus for solder ball placement of a ball grid array package of IC chips into a substrate having a pad array for interconnection comprising a laser head adapted to direct a laser beam to the solder ball.