November- December 1998
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Automated Solutions for Solder Ball Attachment
-By Vern Solberg
-Contributing Editor
Chip-scale and chip-size ball grid array devices are rapidly moving from the development and experimental stage of manufacturing into the more efficient level of mass production. The machines that have been developed for CSP processing are also improving in both quality and efficiency. A key factor in the assembly process for chip-scale BGA packages is the attachment of the solder balls, a process step performed at the end of the manufacturing cycle. The ball attachment process requires accurate placement of very small solder alloy spheres and reliable reflow soldering. One of the difficulties that companies specializing in ball attachment systems faced during machine development was the automated handling of the spheres specified for CSP. While wider pitch (1.0 mm to 1.6 mm) BGA devices are typically furnished with a relatively large diameter ball contact, the fine-pitch CSP family of devices has adapted a much smaller (0.30 mm to 0.40 mm) diameter contact.
CSP Standards
Both the JEDEC JC- 11 and EIAJ EE- 13 committees have agreed on general guidelines for chip-scale package ball sizes. Although the package profile standards allow for a wide contact pitch variation as(shown in the table), the "thin profile" devices (< 1.20 mm) are specified with a nominal sphere diameter of 0.30 mm. The "low profile" CSP, on the other-hand, will adapt a larger ball diameter, increasing in size with each contactpitch variation. This provision was allowed because many of the thickerbodied devices are manufactured using ceramic or other rigid interposer materials.
For the majority of chip-scale BGA applications, suppliers are furnishing precision, pre-formed solder spheres and offering several alloy combinations. The spheres are filtered and graded to meet exacting roundness specifications and tolerances. The 0.30 mm diameter solder ball size, for example, is within +/-10 µ. Suppliers maintain that for solder balls less than 0.30 mm in diameter, close tolerances will be more difficult.
Both semi-automated and fully-automated solder ball attachment machines are available. The user can purchase a stand-alone machine for integration into a production line, or work with a supplier to implement flux application, ball placement and reflow-solder processes into a single system.
| Ball Size Standards for Low and Thin-Profile Packages | ||
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Contact Pitch | TFBGA Thin Profile (<1.20 mm) | LFBGA Low Profile (1.70 mm max.) |
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0.50mm 0.65mm 0.75mm 0.80mm | 0.25-0.35mm 0.25-0.35mm 0.25-0.35mm 0.25-0.35mm | 0.25-0.35mm 0.35-0.45mm 0.35-0.45mm 0.45-0.55mm |
Current Systems
Reflow soldering may be furnished as an integral part of the system or as an inline addition to the basic ball placement system. Some of the companies that supply these machines offer a broad range of models for various user applications, while others furnish a system that meets a more specific application*.
Those currently offering systems for small-ball placement include, Daiichi Jitsugyo, Panasonic Create and Shibuya Kogyo Co. Ltd., Japan; Han Mi and Koses, Korea; Motorola, Phoenix, Ariz. and RVSI/Vanguard, Tucson, Ariz.; and Meco Equipment Engineers, B.V., in the Netherlands, which offers a fully integrated ball placement and reflow system.
The manufacturers of some mass ball placement systems noted above currently offer models and variations for 0.30 mm diameter sphere attachment these systems include:
Panasonic Create's BGA Placer uses vacuum to pick up the array of balls from a reservoir, lowers the array into a flux bath, then transfers the spheres to the contact sites of the package. Placement accuracy specified is +/- 0.10 mm and vision inspection capability is optional.
The Model BPlOS-S has a total cycle time of 19 seconds and a 50 mm x 50 mm work area with a ball diameter range of 0.30 mm to 0.50 mm.
Koses' Ball Attach Machine is a fully automated, magazine-to-magazine system, including pin-transfer flux application, ball placement and vision inspection.
RVSI/Vanguard's Fluxing and Placement Cell is available as a semi-automated or fully-automated system with magazine-to-magazine handling capability. Flux is applied to the CSP device contact site using a screen printing transfer process. The solder balls are loaded into cavities machined to match the device's contact pattern from a unique ball-feed reservoir. Without the use of vacuum, the balls are transferred to the tacky flux surface on the device, ready for reflow. The placement area for attaching solder ball contacts is a generous 75 mm x 200 mm.
The Model VAi 5200 is fully automated, it claims an 8-10 second cycle time with built-in inspection, and can accommodate strip or flex-frame carriers from a stacked magazine or conveyor.
The Shibuya Solder Ball Mounter adapts a pin-transfer technique for flux application and features a product-specific (customized) vacuum pick-up head for ball placement.
The Model SBM 100 furnishes +/0.10 mm placement accuracy. The machine requires hand loading of the CSP carrier, suitable for product development or low volume applications.
The Model SBM 300 furnishes +/0.05 mm placement accuracy, includes ball attachment and vision alignment and post placement inspection.
The Meco Solder Ball Process is an in-line, fully integrated machine that transfers the strip carriers by conveyor through the entire attachment process from ball placement through reflow. The product has been designed for both BGA and CSP applications and claims to attach balls with or without flux. Because the process is continuous, strip carriers exit the machine at 3-5 second intervals.
Several manufacturers whose equipment is not listed here (Motorola and Texas Instruments for example), have developed systems for mass ball attachment, as well. Some offer flux printing, ball placement and inspection capability.
With the exception of the fully integrated Meco system, most of the ball placement machines are adaptable to inline solder reflow systems typical of the Ultra Profile 2000 manufactured by Sikama International, Santa Barbara, Calif. One of the most compact, in-line reflow systems available, the Sikama machine adapts conduction and convection heat to uniformly reflow the solder balls.
For those specifying flux-free soldering, Scientific Sealing Technology, Downey, Calif., may have the solution. The company supplies both batch type systems (Model MV-2200) and a high throughput production machine (Model 2400) rated at 78,000 units per hour.
Single ball placement and attachment (or direct bumping) techniques are available, as well, for placing one solder ball or bump at each contact site. Currently, this application is most successful with wafer bumping rather than solder ball attachment. Ball forming systems are available from MPM/Speedline Technologies. MPM's proprietary Solder Jet technology, for example, is based on inkjet droplet technology. The inkjet method delivers a series of oxide-free droplets to the contact site until the volume deposited meets the criteria established to form the ball or bump.
Another single ball placement method is offered by Zeiss/OKM Optische in Germany. The Zeiss system dispenses a preformed 0.30 to 0.40 mm ball on the attachment site and, using a pulsed laser, reflows the solder (about one ball per second). Because these singleball systems for ball placement or bumping are controlled by software, they may prove to be both practical and economical for lower volume CSP applications.
Solder bumping with solder-paste is also popular. Bump contacts are formed using solder dot dispensing systems, as well as conventional solder-paste stencil and reflow processes. While these bumping methods have proven to be a low cost solution, the uniformity of their small solder deposits may be difficult to control.
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| The ball attachment process may employ a series of unique machines linked in-line or include the integration of two or three process steps within a single machine to provide a uniform and reliable contact array for CSPs. |
The infrastructure for chip-scale BGA package assembly continues to expand, and manufacturers who are developing equipment for solder ball placement and reflow will find a lucrative and growing market.
In addition to those companies and systems described here, an increasing number of products are available or in development throughout the industry. This magazine will provide supplier information and general features of equipment as specifications and data become available.
*See Ron Iscoff, "Solder Ball Attachment: An Equipment Overview," in the September issue of Chip Scale Review.
To contribute to Application Notes or to comment, please contact Vern Solberg at Tessera by e-mail to vern@tessera.com or 408.383.3614.