Product category:
PCB Assembly Equipment and Tools
News Release from: Ekra | Subject: X5-WAF
Edited by the Electronicstalk Editorial
Team on 15 November 2001
Inline printer puts BGA bumps on 300mm
wafers
Ekra has released the X5-WAF, a new inline wafer bumping system.
Ekra has released the X5-WAF, a new inline wafer bumping system The X5-WAF is a stencil printing system which can print 300mm diameter wafers precisely with solder paste
This article was originally published on Electronicstalk on 14 Nov 2001 at 8.00am (UK)
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In electronics manufacturing the term "advanced packaging" stands for cost reduction and ongoing miniaturisation for electronics components.
An important contribution in terms of cost reduction is the so-called wafer bumping, a process where the dies or flip-chip/ball grid arrays (BGA) are bumped with solder paste.
Wafer bumping replaces the costly and elaborate process of bonding with gold wire.
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Among the bumping processes the printing of small solder alloys offers the greatest potential for cost savings available today.
The bumping procedure consists of three working steps: printing, reflow and cleaning of the wafer.
Also, automatic inspection of the paste deposits (before reflow) and/or of the solder bump (after reflow) are often seen as controlling the process for maintaining high production yields.
The requirement of a continuously high yield is essential for the choice of the manufacturing equipment.
Construction and design should be highly robust as well as precise.
The model X5-WAF from Ekra is an advanced inline printing system for wafers up to 300mm diameter.
The line is based on a high-precision screen/stencil printer and small wafer-handling robots on each side.
The robot is suited as well for the transfer of the wafers to and from the printer.
After pre-aligning a wafer from one of the input cassettes, the robot loads a shuttle which transfers the wafer into the printer.
After printing, it is then passed through to the robot on the output side.
The wafer can then be loaded onto a reflow oven or into an output cassette.
Such fully automated in-line configurations are commonly used in back-end assembly environments with high throughput rates.
The emergence of 300mm diameter wafers in these applications has created additional challenges for back-end assembly and wafer fabs.
These wafers are far more fragile than 200mm diameter or smaller wafers, and their value can easily exceed $1000 per wafer.
Equipment that is able to process these large substrates must be carefully designed to avoid potential problems.
Cell configurations are more common in front-end environments.
These cells (also called cluster tools) are a collection of handling, processing and measuring machines tightly integrated together to implement a specific process step.
Tracking and handling of work in progress (WIP) is facilitated by requiring that wafers must always be returned to the same slot/cassette after processing in a cell.
For the strict handling needs of 300mm diameter wafers, this is a requirement.
The cassette for 300mm diameter wafers - called a FOUP (front opening uniform pods) - is a complex mechanism that accompanies its wafers throughout their production process.
Fully loaded with 25 wafers the FOUPS weigh approximately 40kg, too heavy for manual handling.
The openers for the FOUPs (or load ports) are sophisticated machines that present a standardised mechanical interface to automated material handling systems (AMHS).
The new cell-based wafer printing system from Ekra employs - as a basic configuration - a sophisticated wafer-handling robot surrounded by printer, FOUP opener(s) and a pre-aligner.
Equipped with a dual end-effecter, the robot can typically unload and load a wafer and move to the next processing station within 9s.
After removing the wafer from the FOUP and pre-aligning it, the robot directly and accurately places the wafer in a fixed print nest.
As opposed to inline (or pass through) systems, the wafer is loaded and unloaded from the same side of the printer.
The accuracy of a wafer printing process is a critical factor in wafer bumping applications.
Currently the print process is capable of bump-to-bump pitches of down to 150um, and development activities are now targeting 100 microns.
In order to achieve maximum height-to-diameter ratios, the pads are overprinted with solder paste.
As a result, the edge-to-edge spacing of the printed paste deposits can be as small as 30-50um and the stability of the reflow process is strongly influenced by the alignment of this pattern on the wafer.
The XG5 printer from Ekra fulfils all those requirements with an alignment accuracy of +/-12.5um over a 300 x 300mm area.
The chance of the wafer-to-stencil misalignment exceeding a range of +/- 6.3um is less than 2 out of 1 billion prints with this printer.
In-situ inspection of the print quality and stencil opening after the printing provides important feedback to ensure a constant, robust process.
Additional machines, such as an oven, automatic inspection station, or a second printer, can be integrated into this basic configuration.
The total cycle time for a print cell is approximately 1min/wafer for many applications.
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