Don Moore and Gloria Studley
Flip chip assembly is gaining 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 simplicity and reliability of conventional underfills with the added advantage of reworkability. 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 nearing completion and their commercialization underway, the task shifts to designing and perfecting rework equipment and techniques. The goal is to physically remove the offending flip chip that is underfilled with one of the new reworkable materials, and replace it with a good die.
In response to the development of these reworkable underfills, some manufacturers of surface mount rework stations are converting their machines for flip chip placement capability in order to capitalize on the new market potential for flip chip rework. These machines suffer from being designed to handle large circuit boards and large components and thus lack the fine precision capability 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. Surface mount rework machine conversions also have other limitations which we will later address.
Semiconductor Equipment Corporation, a flip chip bonder manufacturer, is taking a different approach. The company has developed a rework attachment option (Model 870) for its low cost standard Model 850 flip chip bonder. This approach gives the user a less expensive alternative to new equipment purchase while providing all the capabilities for precision placement required for flip chip replacement and underfilling with the new materials.
The Rework Process
The rework process begins with heating the substrate evenly to a temperature below the melting point of solder. The chip undergoing rework is then spot heated to melt the solder connections and break down the underfill. The chip is gripped mechanically and then twisted or sheared away from the circuit. Any residual solder and underfill are cleaned off the substrate. Once cleanup of the substrate is complete, a new chip can be aligned, bonded, reflowed, and underfilled.
To successfully remove the defective flip chip from the substrate, the process must be compatible with the assembled board and the components attached to it. It requires a machine that is equipped with a stage for bottom-side heating of the substrate to temperatures up to 200°C, with the most common temperature range being 125° to 150°C.
The stage should also have a hold-down device for the substrate during removal of the chip. In those cases where the substrate is fragile or exceptionally thin, such as with PC cards, custom fixturing may be required. The substrate holder must be of solid design to completely support the substrate, but not the “universal” design found on most surface mount rework systems. These are almost like a big vise for holding boards that measure from two inches square up to over a foot square. These “universal” holders fail to hold the substrate flat.
Spot Heating System
To heat the top of the chip, the spot heating system must be easily positioned over the chip and must confine substrate heating to the flip chip site. The heating system must accommodate die from 1 to 30mm². The temperature must be maintained by a controller that can store and run thermal profiles.
Much of S.E.C.’s development work used the Model 430 profiler, which is incorporated into the S.E.C. 870 rework attachment. It 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. A separate computer and monitor are used for graphic presentation of profiles,. The profiler reproduces the same thermal cycling used during the initial reflow of the flip chip in 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.
After 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. This would adjust to accommodate different chip sizes while supporting 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 suction hold to remove the chips.
However, in trials carried out at S.E.C. on samples of the new reworkable underfill materials, we found that such a grabbing tool was cumbersome and slow to operate and that a hand tool worked just fine for removing the chip from the substrate. A hand tool is adequate for removing all flip chips currently in use. Hand tool removal has been used for years in hybrid microelectronics rework to repair epoxy-bonded die.
Site Redressing Requirements
Cleanup after chip removal removes any underfill residue and excess solder on the substrate. This must be done without damaging the pads and adjacent components on the substrate. A microscope on the Model 870 attachment facilitates viewing the area undergoing rework. It can also be used for precise alignment of the spot heating nozzle to the chip prior to removal.
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 Dremelâ tool equipped with a stiff horsehair flattened brush to mechanically remove the adhesive residue from the die site. These manufacturers had tested several different types of brush styles and materials – including pig’s hair – in arriving at their selection at their selection of this brush. The brush must 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 placement.
Eliminating Messy Tools
However, again in trials conducted at S.E.C., we found that using the Dremelâ tool and brush tended to be messy. Some debris remained for cleanup, and over – brushing could occur. We determined that scraping the softened underfill off the heated substrate with a Teflon tool immediately after the chip is removed is faster and works better than brushing.
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. Such a tool is included with the company’s rework attachment.
Regardless of the clean-up method, excess solder should be wicked off, and a final cleaning made using the solvent. 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
After site cleanup, the substrate is ready for the replacement chip. In the case of S.E.C.’s offering, this is accomplished using the Model 850 bonder portion of the company’s flip chip bonder-rework combination package. The waffle pack containing new chips is placed on a pedestal, which, in turn, is placed on the system’s X – Y precision slide table. A target chip in the waffle pack is manually aligned to the bonder’s vacuum pickup head with the aid of the system’s sliding table and extend-retract cube beam splitter viewer. The viewer is retracted, the pickup cycle is initiated and the head makes contact with the target replacement chip and lifts it from the container.
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. A 40 x 40 vacuum chuck holds the substrate steady and keeps it coplanar 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 kilograms, 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.
For Further Information
Semiconductor Equipment Corporation