Reworking Underfilled Flip Chips

By Don Moore and Gloria Studley

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 standard Model 850 flip chip bonder and its low cost new rework option.  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.

Figure 1.  S.E.C. Model 850 Flip Chip Bonder with accompanying Model 870 hot gas rework profiler designed for use with flip chips underfilled with new reworkable formulations.

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 needs to 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-one that can be heated up to 200º, with the most common temperature range being 125º to 150ºC.  The stage should also have a hold-down device for the substrate during removal of the chip.  Note that in those cases where the substrate is fragile or exceptionally thin such as with PC cards-custom fixturing may be required.  The substrate holder needs to 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 two inches square and 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 machine needs to have a spot heating system that can be easily positioned over the chip and that will confine any heating of the substrate to the flip chip's site.  The heating system needs to be accommodate die from 1 to 30mm?.  The temperature needs to be maintained by a controller that can be store and run thermal profiles.

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 (600ºC 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 450ºC 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 - 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

Cleanup after chip removal is required to remove 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 utilized 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 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.

Figure 2.  Close - up view of the gas nozzle on the S.E.C. Model 870 rework attachment delivering hot gas to the top of a defective flip chip underfilled with one of the new reworkable formulations in preparation for its removal.