by Don Moore, President, Semiconductor Equipment Corporation
Given the Extensive use of BGA packaging in under-the-hood and telecommunications applications, repair capabilities will
continue to be in demand.
Although BGAs (ball grid arrays) provide density and yield advantages, they also provide the assembler with rework obstacles. With solder bumps hidden from view, reflow cannot be visually verified. Conventional cameras and microscopes do not permit visual alignment of bumps with pads or checking for backward placement. Visual inspection after BGA replacement is not possible, and after replacement, the BGAs solder bumps cannot be altered to eliminate bridges, insufficient solder, voids, opens, poor wetting, and misregistration. BGA-specific rework devices help the manufacturer clear some of these obstacles.
Vision problems associated with component placement and alignment have been effectively solved. One method is through the use of a cube beam splitter tied to a camera equipped with a motorized zoom lens and a direct viewing screen on which the operator can simultaneously view the PCB pads and the solder bumps on the BGA. By superimposing the two images, misalignment can be seen and easily corrected before the BGA is placed on the pads and solder reflow is started.
Rework Heat Techniques
The ideal way to reattach a BGA is to replicate the original reflow by uniformly heating the board and the package. Rework equipment manufactures are divided as to how best to achieve these conditions at localized defect areas without creating heat stress defects on other portions of the assembly.
The hood heating technique utilizes a hood-shaped nozzle, or shell, which fits over the BGA and is sealed against the board, creating a controlled environment beneath the array. Heated gas injected into the shell flows horizontally under the BGA to heat the solder bumps and exits through exhaust ports, which direct heat away from neighboring devices. A thermocouple measures the exhaust gas temperature to monitor the reflow state of the solder and to determine the proper time to install the replacement array. Since any significant temperature differential results in warpage and joint defects, this technique relies on close correlation between the nozzle exhaust temperature and the gas temperature under the device.
Another approach makes use of an upper head-sweeping, multi-nozzle assembly and backside heater. A slide in/out BGA alignment system is an option for reworking PLCCs and other fine-pitch devices. Combining the cube splitter beam with this computer-controlled rework system provides accurate BGA placement and controlled localized heat. Four gas nozzles sweep heated air at a low velocity directly on top of the BGA in an even pattern. Nozzle patterning is programmable to accommodate varying BGA sizes, and the entire nozzle head assembly is raised or lowered either by computer control or by an operator.
Supporting the sweeping nozzles is a backside heater just under the PCB at the BGA location, which forces heated air through a distribution screen on top of the nozzle aimed at the target area on the underside of the board. The turbulent flow of the hot air evenly sweeping over and through the BGA closely duplicates the uniform heating conditions of a reflow oven. During the replacement process, the temperature of the target area and the BGA is gradually evenly raised by the backside heater and nozzles to a preheat or soak stage of approximately 125 º C. After soaking for about one minute, the temperature is rapidly ramped up to bridge the device to reflow at 200 º to 210 º C.
Throughout localized BGA replacement soldering, the balance of the assembly is not heated above 125 º C, preventing heat stress and crystallization of solder joints on other components. This heating profile also avoids subjecting the assembly to a time-consuming, 12-hr. drying/baking process and prevents popcorning of plastic devices on the board.
A macro capability in the rework system´s computer can integrate a group of process commands to create programs that prompt the operator through the process. A profile of all process parameters can be stored in the computer’s data files to ensure repeatability of the heating process.
BGA Rework Peculiarities
The overriding consideration in rework concerns the disparate thermal characteristics of BGAs. Ceramic BGAs tend to heat evenly, and transmit heat to the board quite well and are relatively easy to rework. Plastic BGAs heat and transmit heat relatively poorly and are more successfully repaired with techniques like the sweeping nozzle method.
In one IBM ceramic BGA design, high-reflow-temperature solder balls are held in place with standard-reflow-temperature solder. During rework at the lower temperature, some of the solder balls may remain on the board, and some go with the package upon its removal, both of which must be manually cleaned. Another difficult IBM package makes use of a small aluminum cover soldered over the BGA, which usually pops off during rework.
New BGA Designs
After languishing for 10 years, surface mount is today’s technology of choice. The use of BGAs is expected to flourish. Examples of this emerging technology include Motorola’s combination BGA-flip chip SLICC (slightly larger than IC carrier), IBM´s multilayer C4PBA package, and fine-line microlaminate pin grid arrays. Motorola, Tessera Inc., Mitsubishi Electric Corp., and Sandia National Laboratories are all pushing the micro-miniaturization of BGAs.
The new flex circuit BGAs with their compliant elastometric layers offer protection against mechanical forces applied during socketing for test and burn-in. These BGAs should better accommodate TCE stress and differential contractions during and after soldering and should, therefore, require less rework. Conventional rigid board BGAs, however, will continue to suffer the solder-attach cracking failures and die-cracking caused by TCE mismatches. Given the extension of BGA packaging in under-the-hood and telecommunications applications, repair capabilities will continue to be in demand. As the designs and uses of BGAs continue to evolve, so will capabilities of BGA-specific rework equipment.
|Reprinted with permission from Circuits Assembly, December, 1996
© 1996 Miller Freeman Inc. All Rights Reserved.