Printer-hosted processes for solder ball placement are now widely used for package technologies ranging from BGAs using ball diameters above 750um to the latest wafer-level chip scale packages (WL-CSP) demanding 250um diameter.
This broadening spectrum of applications brings more choices in terms of stencil design rules and production methodologies.
Semiconductor OEMs,
wafer processing houses and back-end packaging specialists need
processes for solder ball attachment that are fast, robust, and
cost-effective. To keep pace with other packaging processes performed
either side of solder ball attachment, wafer bumping specialists are
typically able to accept a cycle time of between 60 seconds and four
minutes. The cycle time depends on the size of the wafer, the number of
interconnects required, and the type of IC being assembled. In other
processes, such as those where substrates are bumped in strips, a cycle
time of around 10 seconds may be required.
The solder ball attachment process has two elements, comprising a fluxing phase followed by solder ball placement. Current processes are compatible with wafers in standard diameters up to 300mm, and will also accept substrates presented as strips or in a standard carrier such as an Auer boat. Singulated substrates may be processed one at a time.
Alternatively, a number of units may be aligned simultaneously for fluxing and solder ball attachment using a virtual-panel substrate support technology that aligns each substrate individually.
Using two precision screen printing platforms delivers equal accuracy and repeatability for fluxing and ball placement.
A precision emulsion screen is used to deposit flux at uniform thickness and volume. Emulsion screen technology is used to maintain a tightly gasketed seal against the surface of wafer or substrate, preventing smearing of the flux even after many hundreds of cycles without cleaning the screen. The resulting high beat rate and low usage of cleaning consumables help to boost effective productivity. Fluxing is followed by solder ball placement, using a metal stencil that combines a laminated stand-off layer.
The solder ball placement machine is fitted with a purpose-designed solder ball transfer head. The width of the transfer head exceeds the active area of the stencil. Internally-machined channels provide a low-friction solution to continuously direct solder balls to the surface of the stencil apertures. The transfer head is driven at a constant speed across the active area of the printer. As the transfer head moves over the stencil, in direct contact with its surface, a positive placement force is exerted to push the solder balls through the apertures and thereby populating each fluxed interconnect site. The linear speed of the transfer head is adjusted to optimize yield and throughput. The total instances of defects, such as unpopulated sites or damaged balls, is typically less than 0.01% of placed solder balls in a well-adjusted process.
The solder ball storage capacity of the transfer head is usually sufficient to sustain at least one hour of continuous operation. Upon completion of the solder ball placement stage the wafer or substrate is unloaded from the screen printer, and may then be inspected and stored for subsequent reflowing of the solder balls to complete the attachment process.
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