by Don Moore, President, Semiconductor Equipment Corporation
by Gloria Studley, Design Engineer, Semiconductor Equipment Corp.
US Tech, July/August 2000
|Flip chip assembly technology is poised to gain greatly increased acceptance in the electronics industry thanks to the introduction of the new reworkable underfill materials from such companies as Loctite, Emerson & Cuming and IBM. Like their underfill predecessors, the new formulations create a uniform and void-free underfill layer that protects the active surface of the die while improving the reliability performance of flip chip devices by distributing stress away from the solder interconnects.
The new materials provide the processing and reliability capabilities of conventional underfills with the added advantage of rework ability. They are specifically designed to minimize the need to scrap entire boards with high cost devices bonded on them because testing has determined that a flip chip is defective.
With development of these new formulations at or near completion and their commercialization underway, now the work involving these new formulations shifts to designing and perfecting equipment and techniques. The goal: to be able to physically remove the offending flip chip that is underfilled with one of the new reworkable materials and replace it with a good one.
In responding to the development of these reworkable underfills, some manufacturers of surface mount rework stations are trying to convert their machines for flip chip placement capability in order to capitalize on the new market potential for flip chip rework. These machines suffer from the fact that they were designed to handle large circuit boards and large components and, as such, lack the fine precision capability that’s needed when working with flip chips. They do not provide the finely controlled spot heating, viewing magnifications and precision bond load that are needed. The equipment also tends to have large footprints, and is very expensive. In essence, these designs have been influenced by soldering rather than microelectronic considerations. There are also other equipment issues with the use of these surface mount rework machine conversions, which we will get into a little later.
Semiconductor Equipment Corporation, a flip chip bonder manufacturer, is taking a different approach. The company has developed rework attachment option (Model 870) for its low cost ($30,000) standard Model 850 flip chip bonder. The company’s new rework option sells for about $9000. This approach gives the user a less expensive alternative to equipment purchase while providing all the capabilities for precision placement required for flip chip replacement and underfilling with the new materials.
The Rework Process
Spot Heating System
Much of S.E.C.’s development work was done using the Model 430 profiler which is being incorporated into the S.E.C. 870 rework attachment. The S.EC. rework attachment features a movable (up and down) interchangeable hot gas jet nozzle that delivers precise, stable gas flow rates (600°C max.) to a specific area as well as a temperature controller for running thermal profiles which can be controlled and saved on the controller. For graphic presentation of profiles, a separate computer and monitor are used. The profiler reproduces the same thermal cycling used during initial reflow of the flip chip via an oven. Using the S.E.C. regimen, hot gas is applied to the rework site for less than 30 seconds with a gas temperature of approximately 450°C at a flow rate of approximately 3 liters per minute. The key to precisely heating a flip chip is to use a fairly high gas temperature at a fairly low flow rate.
fter the defective chip has been heated, it is ready to be removed. Our initial development plans called for putting a specially designed pneumatic chip grabber on the company’s Model 870 rework attachment – one that would be adjustable to accommodate different chip sizes that would give support to the chip to prevent damage during the removal process. A prototype of such a device was made that would provide the torque necessary to break the fillet’s adhesion to the board. This is especially important, since current vacuum type systems do not have enough hold to remove the chips. However, in trials carried out at S.E.C. on samples provided by manufactures of the new reworkable underfill materials, it was determined that such a grabbing tool was cumbersome and slow to operate and that, instead, a hand tool worked just fine for removing the chip from the substrate.
A hand tool will in fact suffice for all flip chips currently in use, and such use is in line with techniques that have been employed for years in hybrid microelectronics rework to repair epoxy – bonded die.
Site Redressing Requirements
Various methods have been examined for redressing the site. In working with the manufacturers of the new reworkable underfills, we found that in their underfill development work they had tried using, among other things, a Dremmel tool equipped with a stiff horsehair flattened brush to mechanically remove the adhesive residue from the die site. These manufactures had tested several different types of brush styles and materials – including pig’s hair – in arriving at their selection of this brush. The brush needed to be held so as to exert a minimum amount of pressure on the board, and moved slowly across the die site to allow removal of all residual adhesive, concentrating first on the fillet – which had the greatest amount of adhesive – then moving to the center of the die site once the fillet had been cleaned. Excess brushing increased the chances of board damage.
Isopropyl alcohol was used to clean the area for ease of inspection. A Teflon® – tipped vacuum wand for removing excess solder also was tried. In general, these methods worked reasonably well in redressing the site in preparation for chip replacement.
Eliminating Messy Tools
The Teflon scraper does not damage the board or pads in any way. The debris itself comes off in a pliable mass; as it is scraped, it plows up into a pile which can be flicked off or brushed aside, and does not stick to the tool, since it is Teflon. This technique removes most of the underfill debris. The remaining solder and underfill are removed by solder wicking, using a hand – held soldering iron and solder wick. This leaves the site virtually clean and all that is then needed is an application of solvent.
Replacing the Chip
The chip is then dipped into a flux tray (manual or motorized) that has been placed on the sliding table. After coating the chip’s solder bumps with the bond pads on the substrate located in the micrometer – adjusted X – Y and theta workstage with a 40 x 40 vacuum chuck holds the substrate steady and keeps it co – planar to the chip.
The cube beam splitter viewing system is a vital equipment feature for flip chip bonding, presenting real – time views of the chip’s bumps and substrate bond pads superimposed on each other. Adequate lighting of both the flip chip bumps and bond pads is attained with separate, adjustable fiber optic illuminators.
When alignment has been achieved, the viewer is retracted and the placement cycle is initiated: the chip on the pickup head is automatically lowered onto the bond site under sufficient bond load (50 gm to 2 kilos, depending on size and quantity of bumps on the chip) to properly seat the chip.
The chip’s solder bumps are now ready for reflowing, to be followed by underfilling. Reflow may be accomplished with the 40 x 40 heated stage (capable of 350°C) and hot gas spot heating nozzle system with thermal profiling capability provided on the 870 rework attachment. Underfilling with new reworkable underfills, or any other formulations for that matter, can be accomplished using one of two methods. One method, which takes a little practice, is to use a manual dispenser and allow the material to chip placement.
After reflowing and underfilling the assembly is then removed for inspection and testing. The usual practice is to x – ray the reworked device, subject it to electrical testing or do both.
|As appeared in US Tech – July/August 2000|