A new method uses laser-cut polyimide stencils and fixtures for low-volume jobs.

As area-array device vendors and customers switch to Pb-free, EMS and other assemblers are faced with situations in which either:

•    The product “owner” has an RoHS exemption and, therefore, has not switched to a Pb-free process, and the device comes only in a Pb-free alloy, or

•    The product “owner” has switched to an RoHS-compliant design, and the area-array device is not yet available in a Pb-free version.

In either case, assuming that the die is rated for the correct processing temperature, and the risk analysis of the correct alloy parts has been completed, devices need to be reballed. While several device manufacturers do not recommend reballing because of the number of thermal excursions the devices will ultimately undergo, solder balls have been successfully reattached to every major device type.

For cases in which the need to reball up to 100 devices in a few business days is imperative, there has not been an optimal process that provides for a fast, low marginal cost method of reballing. In the case of solder ball preforms – an otherwise efficient method for reballing – it takes several weeks for new patterns to be tooled up. Mechanical fixturing methods, while boasting low incremental material costs, take several weeks for new fixtures to be fabricated.

A new, quickturn reballing technique that relies on laser-cut polyimide stencils and fixtures is designed to place new solder balls onto area-array devices in small volumes (<100 devices). Unlike mechanical fixtures that require machining, the tooling can be fabricated on the same day the mechanical specifications are transferred to electronically manipulated files.

First, as with prior reballing processes, existing solder balls are removed from the device and the sites properly dressed. Flux is applied to the part bottom. A polyimide stencil corresponding to the size of the balls and part pads is aligned over the part. While the fixture keeps the stencil affixed to and aligned with the part, the properly sized solder balls are then poured into the apertures. The assembly is then reflowed, cleaned and inspected. Figure 1 outlines the process flow.


Pad dressing can be performed by one of several methods. Solder balls can be removed from the package by such methods as solder wick, solder vacuum systems and by using a solder pot. A solder vacuum tool (Figure 2) may slow removal but reduces the chances of pad damage. Solder wicking, while faster than the vacuum system, can cause lifted pads or damage soldermask if not done properly. If a soldering pot is used, the equipment must maintain a consistent temperature. To prevent solder pot contamination when using this technique, it is important to know the alloy type of the balls.


After the balls have been removed by any of these techniques, it is good practice to leave some solder on the pads to facilitate reballing. After removing the balls, clean the ball side of the package with isopropyl alcohol to reduce flux contamination. In addition, inspect the solder mask between the pads to ensure its integrity. This inspection should be performed using a stereo microscope or other optical inspection.

The next step is to ensure the correct replacement solder balls are used. Verify the polyimide fixtures and stencils have clean holes. Finally, confirm the “tacky flux” is appropriate for the process.

Dispense and spread tacky flux onto the bottom of the part. Make sure the flux has the following characteristics:

•    It must be a paste or “tacky” or “sticky” flux, as the tack property and long activation time are essential for ball reattachment.

•    It should contain a mild to medium activity organic acid.

•    It should be water-soluble to simplify the cleanup process.

Next is solder ball population into the polyimide stencil. The stencil is aligned with the pads on the device underside, and solder balls are poured into the various apertures of the stencil using the fixture to “capture” excess balls (Figure 3). An ESD-safe soft bristled brush is used to assist in “sweeping” balls into apertures. Excess solder balls are collected in the fixture’s “catch basin,” which permits recycling them into the original solder ball container.


As with all soldering processes, reflowing with the correct temperature profile is a critical ingredient. Every package type may require a different thermal profile setting. Variables impacting the profile include the package material, package mass, solder alloy and package size. At this point, place the device and fixture into the hot air source and begin the reflow heat cycle (Figure 4).


After retrieving the reballed BGAs from the reflow source, the water soluble flux, because of its aggressive nature, must be removed from the area-array device substrate. A rigorous, closed-loop batch cleaning process will help remove residues. This ensures that no matter the end use of the reballed devices, they are returned to the manufacturing floor with confidence of no future reliability issues related to ionic contaminants or residues. Ensure ionic cleanliness specifications are met for all reballed devices. If greater cleanliness testing confirmation is required, ion chromatography (IC) is the most common tool for assessing device cleanliness. The most common test method is IPC-TM-650 2.3.28, which can confirm the presence of various ionic species such as chlorides or other halides.

It is always a good idea to ensure plastic packages have not been “overexposed” in terms of their susceptibility to moisture absorption. It is smart practice to follow J-STD-033 guidelines with respect to controlling the moisture absorption into plastic packages. If the exposure time or MSD level is unknown, a safe way to proceed is by baking the devices for 48 hrs. at 125°C.

Finally, inspect the reballed device for contamination, missing balls and flux residue. These inspections should be performed using a stereo microscope or other optical inspection tool.

All reballed components should be marked so that the ball alloy can be easily identified. To mark the part, a simple colored dot can be applied to the exterior of the package with a heat-resistant pen (so that the marking can still be seen after the device has been reflowed). Laser etching the correct alloy symbol is an alternative.

Bibliography

• IPC-7711A, "Rework, Repair and Modification of Electronic Assemblies," October 2003.
  
  
• IPC-TM-650, "Test Methods Manual," 2.3.28, Ionic Analysis of Circuit Boards, Ion Chromatography method, May 2004.
  
  
• J-STD-033B, "Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices," January 2006.

Bob Wettermann is president of BEST Inc. (solder.net); bwet@solder.net.

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