KR19990029705A - Solder ball placement and laser reflow 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, means for 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 accurate guidance of the ball onto the pad by the small ball placement means. One ball is placed on a pad comprising a flow layer that falls into each hole in the template. The solder balls located on the pad are then exposed to the laser through the laser head and allow the solder balls to quickly melt on the substrate pad. The molten balls are then cooled quickly. This laser reflow method with a template eliminates the use of a reflow oven, which is a bulky device included in the packaging of the BGA device, increases the accuracy of ball placement and eliminates the problem of ball bridging.

Description

Solder ball placement and laser reflow 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 use of laser technology for the reflow of ball grid array (BGA) devices in a 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.

A 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 laid down on one substrate with a pre-applied flow. This BGA assembly is then passed to the 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 up to 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, a chip 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 a high density BGA configuration with pads with 10 mil pad diameter and 20 mil pitch, a 12 mil diameter solder ball 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 the mixing 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 ball using a vacuum suction head causes a high rejection rate 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.

In addition to the problems described above, the devices for the various stages of ball displacement and reflow are large in size and expensive. Therefore, there is a need to improve the packaging process by designing a concept of ball placement to prevent movement and bridging of the solder balls while minimizing the size and cost of the devices involved in the process.

It is an object of the present invention to provide an accurate method of 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 reflow treatment in BGA assembly lines.

Another object of the present invention is to eliminate the need for a reflow oven when soldering solder balls on a BGA device.

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

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

3 is a schematic illustration to show positioning of the laser head for laser reflow of solder balls;

4 is a flow chart for explaining the steps involved in the laser reflow process according to the present invention;

* Code Description

20 ... substrate 22 ... template

24 ... ball suction head 27 ... joint

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 ball placement means. One ball is placed on a pad comprising a flow layer that falls into each hole in the template. The solder balls located on the pad are then exposed to the laser through the laser head and allow the solder balls to quickly melt on the substrate pad. The molten balls are then cooled quickly. This laser reflow method with template eliminates the use of a reflow oven, a bulky device included in the packaging of the BGA device, increases the accuracy of ball placement and eliminates the problem of ball bridging.

1 shows a schematic description of a ball arrangement process according to the present invention. After the flow is applied to the substrate 20 according to a conventional method, 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 ball flows 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. As the preferred heat resistant material material at this time, stainless steel is preferable. 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 about 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 solder ball and quickly melted under the laser beam to form a bond between the metals. Once the laser beam is off, the molten solder ball cools rapidly at high cooling rates. 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 such laser reflow operations.

The laser reflow may be performed at room temperature or in a nitrogen environment.

The exposure time and intensity are different for different solder balls, and can be determined by conventional experiments.

3 shows the optical fiber alignment of the laser head with solder balls 28 located in the template 29 cavity 27 over the flow layer 32. The optical fiber is accommodated in the housing 34 as long as the position of the optical fiber 36 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 appropriately performed 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. After laser soldering, it can be heated in a reflow oven or hot plate 48 for a short time to surface the soldered balls. The visual inspection step is preferably carried out using a camera such as a CCD (capacitor charger) camera.

The present invention uses a combination of two separate techniques to yield synergistically higher results in high density BGA device packaging. 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 positioned in the template on the substrate, a second technique, laser reflow operation, is used, which produces melting of the solder balls and effective soldering. The template is then removed and the apparatus is moved directly to the cleaner to provide flow residues, eliminating the use of bulky reflow ovens and also reducing the time needed to complete the packaging process.

Claims (7)

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 solder ball placement and laser reflow system for packaging a ball grid array of IC chips into a substrate having a pad array comprising a continuous step in which the solder balls are melted and soldered somewhat on the pads. The method of claim 1, The system further comprises a camera, A solder ball placement and laser reflow system, wherein a further step of visual inspection by the camera is performed after solder ball placement. The method of claim 1, A solder ball placement and laser reflow system, wherein said laser head is a matrix laser head. The method of claim 1, And the laser head delivers an Nd: YAG laser beam. The method of claim 1, Wherein the laser soldering step is performed under a nitrogen environment. The method according to any one of the preceding claims, Soldering ball placement and laser reflow system, characterized in that the additional heating surface treatment of the soldering ball is performed with a reflow oven or heating plate after the laser soldering step. Means for template alignment, Means for placing solder balls, and Apparatus for solder ball placement and laser reflow of a ball grid array package of an IC chip to a substrate having a pad array for interconnection comprising a laser head adapted to direct a laser beam to the solder ball.